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Regime Shifts
Regime Shifts

Regime Shifts (31)

Thursday, 15 September 2016 21:04

Steppe to Tundra

Written by Juan Carlos

Steppe to Tundra

Main Contributors:

Nicole Reid, Rodrigo Martínez-Peña, Johanna Mård Karlsson

Other Contributors:

Garry Peterson, Juan Carlos Rocha

Summary

Steppe (a grassland) to tundra (mosses and shrubs growing in waterlogged soils) is a regime shift that can occur in cold terrestrial ecosystems.  Tundra and steppe regime shift is typically found where permafrost occurs.  Steppe and tundra are primarily found in the Arctic, north of the tree line, where mean temperature below 10-12oC for the warmest month (Jonasson et al. 2000). Climate change and changes in the density of large herbivores are the main drivers of regime shifts between steppe and tundra. Climate changes that reduce soil moisture can favor steppe over tundra, and vice versa. Tundra is favored by moss growth, which is more limited by water than by nutrients. Steppe is favored by grass growth, which is improved by drier soils with available nutrients. Large herbivores can shape ecosystems through their impact on vegetation species composition, soil structure, and ecological dynamics. Large herbivore trampling and grazing can slow moss growth and convert tundra to steppe vegetation. At the end of the last ice age (12,000 yr BP), human hunting greatly reduced populations of large herbivores which may have contributed to a shift from grass-dominated steppe to moss-dominated tundra. In the 21st century, climate change together with the presence of horses, bison, and musk oxen could lead to shifts between steppe and tundra vegetation.

Drivers

Key direct drivers

  • Harvest and resource consumption
  • Species introduction or removal
  • Global climate change

Land use

  • Extensive livestock production (rangelands)

Impacts

Ecosystem type

  • Tundra

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Livestock
  • Wild animal and plant products

Regulating services

  • Climate regulation

Cultural services

  • Recreation
  • Aesthetic values
  • Knowledge and educational values
  • Spiritual and religious

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values
  • Social conflict

Key Attributes

Typical spatial scale

  • Sub-continental/regional

Typical time scale

  • Years
  • Decades

Reversibility

  • Hysteretic

Evidence

  • Models
  • Paleo-observation
  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Well established – Wide agreement in the literature that the RS exists

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Links to other regime shifts

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Key References

  1. ACIA. Impacts of a Warming Arctic: Arctic Climate Impacts Assessment. Cambridge University Press. 2004.
  2. Bråthen, KA, RA Ims, NG Yoccoz, P Fauchald, T Tvereaa, VH Hausner. 2007. Induced shift in ecosystem productivity? Extensive scale effects of abundant large herbivores. Ecosystems 10:773-789.
  3. Chapin, F.S., 2005. Role of Land-Surface Changes in Arctic Summer Warming. Science, 310(5748), pp.657–660.
  4. Eroglu S., Toprak S., Urgan O, MD, Ozge E. Onur, MD, Arzu Denizbasi, MD, Haldun Akoglu, MD, Cigdem Ozpolat, MD, Ebru Akoglu, M., 2012. Far North: Plant Biodiversity and Ecology of Yakutia,
  5. Folke, C. et al., 2004. Regime Shifts , Resilience , in Ecosystem Management. Annual Review of Ecology, Evolution, and Systematics, 35(May), pp.557–581.
  6. Hollesen, JB, B Elberling and PE Jansson. 2011. Future active layer dynamics and carbon dioxide prodcution from thawing permafrost layers in Northeast Greenland. Global Change Biology, 17(2) 911-926
  7. Huntington, H. P. 2013. Chapter 18 Provisioning and cultural services. In: Meltofte, H. (ed). Arctic Biodiversity Assessment. Status and trends in Arctic biodiversity. Conservation of Arctic Flora and Fauna, Akureyri. pp. 593 - 626.
  8. Ivanova, R., 2003. Seasonal thawing of soils in the Yana River valley, northern Yakutia. , pp.7–10.
  9. Jonasson, S, TV Callaghan, GR Shaver, and LA Nielsen. 2000. Arctic terrestrial ecosystems and ecosystem function. In M. Nuttall and TV Callaghan ed. The Arctic, Environment, People, Policy 275-313 Hardwood academic Publishers Newark.
  10. Jorgenson, M.T., YL Shur, and ER Pullman. 2006. Abrupt increase in permafrost degradation in Arctic Alaska, Geophysical Research Letters. 33, LO2503
  11. Karlsson, JM, A. Bring, GD Peterson, LJ Gordon, G Destouni. 2011. Opportunities and limitations to detect climate-related regime shifts in inland Arctic ecosystems through eco-hydrological monitoring. Environmental Research Letter 6
  12. Nadelhoffer, KJ, AE Giblin, GR Shaver and JA Laundre. 1991. Effects of temperature and substrata quality on element mineralization in six arctic soils Ecology 72: 242-253
  13. Natali, SM, EAG Shuur, M Mauritz, JD Schade, G. Celis, KG Crummer, C Johnston, J Krapek, E Pegoraro and VG Salmon and EE Webb. 2015. Permafrost thaw and soil moisture driving CO2 and CH4 release from upland tundra, Journal Geophysical Research: Biogeosciences, 120, 525-537
  14. Post, E. and C Pedersen., 2008. Opposing plant community responses to warming with and without herbivores. Proceedings of the National Academy of Sciences of the United States of America, 105(34), pp.12353–12358.
  15. Rockström J. et al. 2009. Planetary Boundaries: Exploring the Safe Operating Space for Humanity. Ecology and Society 14(2),32.
  16. Schuur, E. and J. Bockheim, 2008. Vulnerability of permafrost carbon to climate change: Implications for the global carbon cycle. BioScience, 58(September), pp.701–714.
  17. Schuur, E.A.G. et al., 2015. Climate change and the permafrost carbon feedback. Nature, 520, pp.171–179.
  18. Shaver, GR and FS Chapin III. 1986. Effect of fertilizer on production and biomass of tussock tundra, Alaska, U.S.A Arctic and Alpine Research 18 3:261-268.
  19. Skre, O and WC Oechel. 1979. Moss production in a black spruce Picea mariana forest with permafrost near Fairbanks, Alaska, as compared with two permafrost-free stands. Holarctic Ecology 2:249-254.
  20. Van der Wal, R and RW Brooker. 2004. Mosses mediate grazer impacts on grass abundance in arctic ecosystem. Functional Ecology 18:77-86.
  21. Welker JM, Fahnestock JT, and Jones MH. 2000. Annual CO, flux from dry and moist arctic tundra: Field responses to increases in summer temperature and winter snow depth. Climatic Change 44(1-2),139-150.
  22. Wolff, JO. 1980. The role of habitat patchiness in the population dynamics of snowshoe hares. Ecological Monographs 50:111-129.
  23. Wrona, FJ, M Johansson, JM Culp, A Jenkins, J Mård, IH Myers-Smith, TD Prowse, WF Vincent, and P.A. Wookey
  24. Zeng, H., G Jia, and BC Forbes. 2013. Shifts in Arctic phenology in response to climate and anthropogenic factors as detected from multiple satellite time series. Environmental Research Letter 8
  25. Zimov, A.S.A. VI Chuprynin, AP Orshko, FS Chapin III, JF Reynolds, and MC Chapin, 1995. Steppe-Tundra Transition : A Herbivore-Driven Biome Shift at the End of the Pleistocene Published by : The University of Chicago Press for The American Society of Naturalists , 146(5), pp.765–794.
  26. Zimov, S.A., 2005. Pleistocene Park : Return of the Mammoth ’ s Ecosystem. Science, 308, pp.796–798.

Citation

Nicole Reid, Rodrigo Martínez-Peña, Johanna Mård Karlsson, Garry Peterson, Juan Carlos Rocha. Steppe to Tundra. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-06 10:05:53 GMT.

Invasive floating to invasive submerged plant dominance

Main Contributors:

Emily Strange

Other Contributors:

Prof. Julie Coetzee, Julie Coetzee

Summary

Man-made dams and lakes can be highly vulnerable to colonization from floating non-native invasive plants. These plants can form dense mats restricting light to the water column, damaging hydroelectric equipment as well as reducing water quality and biodiversity. The implementation of classical biological control (CBC) programs has been a widely beneficial tool in the control of floating invasive plants. CBC uses host specific natural enemies (bio-control agents) of the invasive plant to control plant populations in invaded ranges. The overall aim is to induce a regime shift into a functioning system with high native biodiversity and freshwater access. However, we propose that whilst the bio-control agents do lead to a dramatic reduction in the biomass and health of the floating plant, it can also act as a catalyst inducing a shift into a second degraded stable regime. This second regime is dominated by submerged invasive plants. The establishment of bio-control agents on the floating plants can lead to a rapid plant population crash and the release of the nutrients they locked up. The increase in space, light and nutrients enables rapid growth of invasive submerged plants. These species can alter water flow, turbidity, sedimentation and can degrade water quality and biodiversity, restrict access to freshwater and damage hydro-electrical equipment.

Drivers

Key direct drivers

  • External inputs (eg fertilizers)
  • Species introduction or removal
  • Environmental shocks (eg floods)

Land use

  • Urban
  • Small-scale subsistence crop cultivation
  • Large-scale commercial crop cultivation
  • Intensive livestock production (eg feedlots)
  • Extensive livestock production (rangelands)
  • Conservation
  • Tourism

Impacts

Ecosystem type

  • Freshwater lakes & rivers

Key Ecosystem Processes

  • Nutrient cycling
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Livestock
  • Fisheries
  • Hydropower

Regulating services

  • Water purification
  • Natural hazard regulation

Cultural services

  • Recreation
  • Aesthetic values
  • Knowledge and educational values

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Years

Reversibility

  • Unknown

Evidence

  • Models
  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Speculative – Regime shift has been proposed, but little evidence as yet

Confidence: Mechanism underlying RS

  • Well established – Wide agreement on the underlying mechanism

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Key References

  1. Charles, H., Dukes J.S. 2007. Impacts of invasive species on ecosystem services. Biological Invasions; Ecological Studies (193)
  2. Coetzee et al., 2011 Prospects for the biological control of submerged macrophytes in South Africa. African Entomology : Biological control of invasive alien plants in South Africa (1999 - 2010) : Special Issue 2
  3. Coetzee, J., Hill M.P. 2012. The role of eutrophication in the biological control of water hyacinth, Eichhornia crassipes, in South Africa. BioControl 57:247-261
  4. Hill, M.P., 2002. The impact and control of alien aquatic vegetation in South African aquatic ecosystems. African Journal of Aquatic Science 28 (1): 19-24
  5. Mack, RN., and Smith, MC.,2011 Invasive plants as catylsts for the spread of human parasites. NeoBiota 9: 13-29
  6. Martin, GD., and Coetzee, JA. Pet stores, aquarists and the internet trade as modes of introduction and spread of invasive macrophytes in South Africa. Water SA [online]. 2011, vol.37, n.3, pp. 371-380. ISSN 0378-4738
  7. McConnachie, J., de Wit,MP., Hill, M.P.,Byrne, M.J., Economic evaluation of the successful biological control of Azolla filiculoides in South Africa, Biological Control, 28 (1) ISSN 1049-9644
  8. Moorhouse and Mcdonald, 2015. Are invasives worse in freshwater than terrestrial ecosystems? Wiley Periodicals
  9. Yarrow,. M., Marin, V.H., Finlayson, M., Tironi, A., Delgado L.E. and Fishcher F. 2009. The ecology of Egeria densa Planchon (Liliopsida: Alismatales): A wetland ecosystem engineer? Revista Chilena de Historia Natural 82: 299-313

Citation

Emily Strange, Prof. Julie Coetzee, Julie Coetzee. Invasive floating to invasive submerged plant dominance. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-25 07:37:11 GMT.
Saturday, 09 May 2015 14:46

Primary Production in the Arctic Ocean

Written by Sophie Laggan

Primary Production in the Arctic Ocean

Main Contributors:

Patricia Villarrubia Gomez, Helene Albinus Søgaard, Karl Samuelsson, Sophie Laggan

Other Contributors:

Thorsten Blenckner

Summary

A shift from polar to temperate primary production (PP) patterns has been detected in the Arctic Ocean. Following a regime shift in the North Atlantic in 1995, similar structural changes are now occurring in Arctic waters. Rapid warming of atmospheric and oceanic temperatures has caused a near year-on-year decline in the extent and thickness of summer sea ice since 1979 (NSIDC 2014). Anthropogenic climatic change has extended the growing season and delayed August freeze-up through a decline in albedo reflectivity and enhanced wind-driven vertical mixing. Natural modes of variability at the lower latitudes has also led to poleward shifts of temperate marine species and caused pronounced phenological changes to primary producers. The difference in the temporal scale of these forcing mechanisms makes it hard to predict which event is causing changes to PP. It is uncertain what impact this change will have on the food web of this ecosystem.

Drivers

Key direct drivers

  • External inputs (eg fertilizers)
  • Environmental shocks (eg floods)
  • Global climate change

Land use

  • Fisheries

Impacts

Ecosystem type

  • Polar

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries
  • Wild animal and plant products

Regulating services

  • Climate regulation
  • Natural hazard regulation

Cultural services

  • Recreation
  • Aesthetic values
  • Knowledge and educational values

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Typical spatial scale

  • Sub-continental/regional

Typical time scale

  • Unknown

Reversibility

  • Unknown

Evidence

  • Models
  • Paleo-observation
  • Contemporary observations

Confidence: Existence of RS

  • Contested – Reasonable evidence both for and against the existence of RS

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Links to other regime shifts

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Key References

  1. Aagaard, K., Carmarck, E.C., 1989. The Role of Sea Ice and other Fresh Water in the Arctic Circulation, Journal of Geophysical Research, 94, C10, pp. 14485-14498.
  2. Arctic Council, 2013. Arctic Resilience Interim Report 2013. Stockholm Environment Institute and Stockholm Resilience Centre, Stockholm.
  3. Ardyna M., Babin M., Gosselin M., Devred E., Rainville L., and Tremblay A-É., 2014. Recent Arctic Ocean sea ice loss triggers novel fall phytoplankton blooms. Geophysical Research Letters, 41, 17, pp. 6207–6212.
  4. Bascompte, J., C. Melian, and Sala E., 2005. Interaction strength combinations and the overfishing of a marine food web. Proceedings Of The National Academy Of Sciences Of The United States Of America, 102, pp. 5443–5447.
  5. Bates, N.R., and Mathis, J.T., 2009. The Arctic Ocean marine carbon cycle: evaluation of air-sea CO2 exchanges, ocean acidification impacts and potential feedbacks, Biogeosciences, 6, pp. 2433-2459.
  6. Butler, C.D., and Oluoch-Kosura, W., 2011. Linking Future Ecosystem Services and Future Human Well-being, Ecology and Society,11.
  7. Chylek, P., Folland, C.K., Lesins, G., Dubey, M.K., and Wang, M., 2009. Arctic air temperature change amplification and the Atlantic Multidecadal Oscillation, Geophysical Research Letters, 36, 14, L14801, doi:10.1029/2009GL038777.
  8. Curry, J.A., Schramm, J.L., and Ebert, E.E., 1995. Sea Ice-Albedo Climate Feedback Mechanism. J. Climate, 8, 240–247.
  9. Dicks L., Almond R., and McIvor A., 2011 (eds.) Arctic Climate Issues 2011: Changes in Arctic Snow, Water, Ice and Permafrost, Arctic Monitoring and Assessment Programme (AMAP) Arctic SWIPA Overview Report. DOI: 10.1088/1748-9326/4/4/045108
  10. Falk-Petersen S., Timofeev S., Pavlov V., Sargent J.R., 2007, Climate variability and possible effects on arctic food chains: The role of Calanus. In: Ørbæk J.B.,Tombre T., Kallenborn R., Hegseth E., Falk-Petersen S., Hoel A.H. (eds.), Arctic Alpine Ecosystems and People in a Changing Environment, Springer Verlag, Berlin. 433 p.
  11. Falk-Petersen, S., Pavlov V., Timofeev S., and Sargent J.S., 2007. Climate Variability and Possible Effects on Arctic Food Chains: The Role of Calanus. In Ørbæk, J.B., Kallenborn, R., Tombre, I., Hegseth, E.N., Falk-Petersen, S., and Hoel A.H (Eds.), 2007: Arctic Alpine Ecosystems and People in a Changing Environment. pp. 147-166.
  12. Frey, K.E., Arrigo, K.R., Gradiner, R.R., Arctic Ocean Primary Productivity, 2011. Arctic Report Card: Update for 2011. www.arctic.noaa.gov/report11/primary_productivity.html
  13. Greene, Charles H., Pershing, Andrew J., Cronin, Thomas M., and Ceci, N., 2008. Arctic climate change and its impacts on the ecology of the north Atlantic. Ecology 89, 11, pp. 24 - 38.
  14. Hassan, R.M., Scholes, R., Ash, N., (eds.) 2005. Ecosystems and Human Wellbeing: Current State and Trends: Findings of the Conditions and Trends Working Group. Island Press.
  15. Hátún, H., Payne M.R., Beaugrandd G., Reide P.C., Sandøb A.B., Drangeg H., Hansena B., Jacobsena J.A., and Blochi D., 2009. Large bio-geographical shifts in the north-eastern Atlantic Ocean: From the subpolar gyre, via plankton, to blue whiting and pilot whales. Progress in Oceanography 80, 3-4, 149–162.
  16. Hátún, H., Sando, A.B., Drange, H., Hansen, B., and Valdimarsson, H., 2005. Influence of the Atlantic subpolar gyre on the thermohaline circulation. Science 309, 1841– 1844. Hjarnmann, D.Ø., Bogstad, B., Eikeset, A.M., Ottersen, G., Gjøsæter, H., Stenseth, N.C., 2006. Food web dynamics affect Northeast Arctic cod recruitment. Proceedings of Royal Society B 274, pp. 661–669.
  17. Hawkins, E., and R. Sutton, 2009. The potential to narrow uncertainty in regional climate predictions. Bulletin of the American Meteorological Society, 90, 1095-1107, doi:10.1175/2009BAMS2607.1.
  18. Heckendorn, P., Weisenstein, D., Fueglistaler, S., Luo, B. P., Rozanov, E., Schraner, M., Thomason L. W., and Peter, T., 2009. The impact of geoengineering aerosols on stratospheric temperature and ozone. Environmental Research Letters 4 045108.
  19. Henson, S.A., Dunne, J.P. and Sarmiento, J.L., 2009. Decadal variability in North Atlantic phytoplankton blooms. Journal of Geophysical Research: Oceans (1978–2012), 114, C4, DOI: 10.1029/2008JC005139.
  20. Hinzman, L.D., Bettez, N.D., Bolton, W. R., Chapin, F. S., Dyurgerov, M. B., Fastie, C. L., and Yoshikawa, K., 2005. Evidence and implications of recent climate change in northern Alaska and other arctic regions. Climatic Change,72, 3, pp. 251-298.
  21. IPCC, 2014: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  22. Kriegler, Elmar, Hall, Jim W., Helda, Hermann, Dawson, Richard and Schellnhu- bera, Hans Joachim (2009) Imprecise probability assessment of tipping points in the climate system, PNAS
  23. Kriegsmann, A., Brümmer B., 2014. Cyclone impact on sea ice in the central Arctic Ocean: a statistical study. The Cryosphere, 8, pp. 303–317.
  24. Lenaerts, J.T.M., van Angelen, J.H., van den Broeke, M.R., Gardner, A.S., Wouters, B., van Meijgaard, E., 2013. Irreversible mass loss of Canadian Arctic Archipelago glaciers. Geophysical Research Letters, 40, pp. 1-5.
  25. Li, W.K.W., McLaughlin, F.A., Lovejoy, C., and Carmack, E.C., 2009. Smallest algae thrive as the Arctic Ocean freshens. Science, 326, 539.
  26. Masson-Delmotte, V., Swingedouw, D., Landais, A., Seidenkrantz, M-S., Gauthier, E., Bichet, V., Massa C., Perren, B., Jomelli, V., Adalgeirsdottir G., Hesselbjerg Christensen, J., Arneborg, J., Bhatt, U., Walker, D.A., Elberling, B., Gillet-Chaulet, F., Ritz, C., Gallée, H., van den Broeke, M., Fettweis, X., de Vernal, A., and Vinther, B., 2012, Greenland climate change: from the past to the future. Wiley Interdisciplinary Reviews: Climate Change, 3, 5, pp. 427–449.
  27. Moore, S. E. & Huntington, H. P., 2008. Arctic marine mammals and climate change: Impacts and resilience. Ecological Applications 18, pp. 157-165.
  28. Niiranen, S., Peterson, G., Biggs, R., Rocha, J.C., and Österblom, H. Marine food webs: community change and trophic level decline. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2014-10-15 03:05:20 GMT.
  29. NSIDC 2014. National Snow and Ice Data Centre. Last accessed 04/05/2015. URL: http://nsidc.org
  30. Post, E., Mads C. Forchhammer, M.C., Bret-Harte, M.S., Callaghan, T.V., Christensen, T.R., Elberling, B., Fox, A.D., Olivier Gilg, O., Hik, D.S., T. Høye, T.T., Ims, R.A., Jeppesen, E., R. Klein, D.R., Madsen, J., McGuire, A.D., Rysgaard, S., Schindler, D.E., Ian Stirling, I., Tamstorf, M.P., Tyler, N.J.C., van der Wal, R., Welker, J., Wookey, P.W., Schmidt, N.M., and Aastrup, P., 2009. Ecological Dynamics Across the Arctic Associated with Recent Climate Change Eric Post et al. Science 325, 1355, doi: 10.1126/science.1173113.
  31. Rahmstorf, S. and Ganopolski, A., 1999. Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change 43, pp. 353 - 367.
  32. Schlesinger M.E., and Ramankutty N., 1994. An oscillation in the global climate system of period 65–70 years. Nature, 367, pp. 723–726.
  33. Shadwick, E.H, Trull, T.W., Thomas, H., and Gibson J.A.E, 2013. Vulnerability of Polar Oceans to Anthropogenic Acidification: Comparison of Arctic and Antarctic Seasonal Cycles. Scientific Reports 3, 2339 doi:10.1038/srep02339.
  34. Shimada, K., Kamoshida, T., Itoh, M., Nishino, S., Carmack, E., Mclaughlin, F., Zimmermann, S., Proshotinsky, A., 2006. Pacific Ocean inflow: Influence on catastrophic reduction of sea ice cover in the Arctic Ocean. Geophysical Research Letters. 33, L08605.
  35. Sirevaag, A., de la Rosa, S., Fer, I., Nicolaus, M., Tjernström N., McFee, M.G., 2011. Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer. Ocean Sci., 7, 335–349.
  36. Stein, R., and Macdonald, R.W., 2004. The Organic Carbon Cycle in the Arctic Ocean. Springer Science & Business Media.
  37. Straneo, F., and Heimbach, P., 2013. North Atlantic warming and the retreat of Greenland's outlet glaciers Nature 504, pp. 36–43 doi:10.1038/nature12854.
  38. Stroeve, J., Holland, M.M., Meier W., Scambos T., and Serreze, M., 2007. Arctic sea ice decline: Faster than forecast. Geophysical Research Letters, 34, L09501, doi:10.1029/2007GL029703.
  39. Tremblay, J.É., S. Bélanger, D. G. Barber, M. Asplin, J. Martin, G. Darnis, L. Fortier, Y. Gratton, H. Link, P. Archambault, A. Sallon, C. Michel, W. J. Williams, B. Philippe, and M. Gosselin (2011), Climate forcing multiplies biological productivity in the coastal Arctic Ocean, Geophys. Res. Lett., 38, L18604, doi:10.1029/2011GL048825.
  40. Wang, M., and Overland, J.E., 2012. A sea ice free summer Arctic within 30 years: An update from CMIP5 models, Geophysical Research Letters, 39, L18501, doi:10.1029/2012GL052868.
  41. Wassmann, P., Duarte, C.M. Agustí, S., Sejr, M.K., 2011. Footprints of climate change in the Arctic marine ecosystem. Global Change Biology, 17, pp. 1235–1249, doi:10.1111/j.1365-2486.2010.02311.
  42. Zhou S., Flynn P.C., 2005. Geoengineering downwelling ocean currents: A cost assessment. Climatic Change, 71, pp. 203–220. doi: 10.1007/s10584-005-5933-0.

Citation

Patricia Villarrubia Gomez, Helene Albinus Søgaard, Karl Samuelsson, Sophie Laggan, Thorsten Blenckner. Primary Production in the Arctic Ocean. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-01 12:23:18 GMT.
Monday, 23 March 2015 12:04

Arctic Benthos Borealisation

Written by Sara Andersson

Arctic Benthos Borealisation

Main Contributors:

Sara Andersson, Noah Linder, Katharina Fryers Hellquist, Linn Järnberg

Other Contributors:

Thorsten Blenckner, Juan Carlos Rocha

Summary

A regime shift occurred on the west coast of Svalbard in 1996 and 2000; the former Arctic benthos was mainly constituted by red calcareous algae and filter feeders whereas the present subarctic benthos is dominated by macroalgae. The main drivers of this shift are increases in sea surface temperature and inflow of light that are both due to global warming and changes in the North Atlantic Oscillation. Changes in benthos could impact other trophic levels, potentially affecting commercial fisheries as well as tourism. The implications for ecosystem services and human well-being are highly uncertain. Management options are mainly to reduce greenhouse gases to combat global warming and an adaptive management approach is also proposed on a local scale. 

 

Drivers

Key direct drivers

  • Global climate change

Land use

  • Fisheries
  • Tourism

Impacts

Ecosystem type

  • Polar

Key Ecosystem Processes

  • Primary production

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries
  • Wild animal and plant products

Cultural services

  • Recreation
  • Aesthetic values
  • Knowledge and educational values

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Years

Reversibility

  • Unknown

Evidence

  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Well established – Wide agreement in the literature that the RS exists

Confidence: Mechanism underlying RS

  • Well established – Wide agreement on the underlying mechanism

Links to other regime shifts

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Alternate regimes

A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.  

With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.

Arctic benthos (regime 1)

With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.

Subarctic benthos (regime 2)

After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).

 

Drivers and causes of the regime shift

The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.

The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).

 

How the regime shift works

In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)

Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.

Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions. 

Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.

 

Impacts on ecosystem services and human well-being

Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.

The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being. 

 

Management options

Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.

Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.

 

Key References

  1. Beuchel, Frank, Bjørn Gulliksen, and Michael L. Carroll. 2006. “Long-Term Patterns of Rocky Bottom Macrobenthic Community Structure in an Arctic Fjord (Kongsfjorden, Svalbard) in Relation to Climate Variability (1980–2003).” Journal of Marine Systems 63(1-2):35–48.
  2. Bischoff, B., and C. Wiencke. 1993. “Temperature Requirements for Growth and Survival of Macroalgae from Disko Island (Greenland).” Helgoländer Meeresuntersuchungen 47(2):167–91.
  3. Drinkwater, Kenneth F. 2006. “The Regime Shift of the 1920s and 1930s in the North Atlantic.” Progress in Oceanography 68(2-4):134–51.
  4. Grebmeier, Jacqueline M. et al. 2006. “A Major Ecosystem Shift in the Northern Bering Sea.” Science (New York, N.Y.) 311(5766):1461–64.
  5. Holling, C. S. 1978. Adaptive Enviromental Assessment and Management. New York: John Wiley & Sons.
  6. IPCC. 2013. Climate Change 2013: The Physical Science Basis. Cambridge, United Kingdom andNew York, NY, USA: Cambridge University Press.
  7. Johansen, H. W. 1981. Coralline Algae, A First Synthesis. CRC Press.
  8. Kortsch, Susanne et al. 2012. “Climate-Driven Regime Shifts in Arctic Marine Benthos.” Proceedings of the National Academy of Sciences of the United States of America 109(35):14052–57.
  9. Snelgrove, Paul V. R. 1999. “Getting to the Bottom of Marine Biodiversity : Sedimentary Habitats Ocean Bottoms Are the Most Widespread Habitat on Earth and Support High Biodiversity and Key Ecosystem Services.” BioScience 49(2):129–38.
  10. Viken, Arvid. 2010. “Tourism, Research, and Governance on Svalbard: A Symbiotic Relationship.” Polar Record 47(04):335–47.
  11. Weslawski, Jan M. et al. 2011. “Climate Change Effects on Arctic Fjord and Coastal Macrobenthic Diversity—observations and Predictions.” Marine Biodiversity 41(1):71–85.

Citation

Sara Andersson, Noah Linder, Katharina Fryers Hellquist, Linn Järnberg, Thorsten Blenckner, Juan Carlos Rocha. Arctic Benthos Borealisation. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-07 11:19:04 GMT.
Wednesday, 19 November 2014 15:28

Thermokarst lake to terrestrial ecosystem

Written by Juan Carlos

Thermokarst lake to terrestrial ecosystem

Main Contributors:

Hannah Griffiths, Elinor Holén, Jessica Spijkers

Other Contributors:

Reinette (Oonsie) Biggs, Juan Carlos Rocha

Summary

Thermokarst lake dominated landscapes are transforming into terrestrial ecosystems (e.g.: tundra). There is a natural fluctuation between these two ecosystems. However, the rate and scale at which those fluctuations are occurring are increasing due to permafrost melting caused by the increasing atmospheric temperatures associated with climate change. Warmer air temperature increases soil temperature, which melts permafrost (permanently frozen soils found in Arctic regions). The shift in ecosystems occurs when permafrost degradation becomes severe enough for the lakes to get permanently drained, creating the necessary conditions for vegetation to establish. The increased rate and scales of these land cover changes has extensive impacts on food and freshwater provisioning, but its greatest impact is on carbon sequestration. The melting of permafrost releases greenhouse gases, i.e. carbon dioxide (CO2) and methane (CH4), which further increase climate change, creating a powerful reinforcing feedback. 

Drivers

Key direct drivers

  • Global climate change

Land use

  • Fisheries

Impacts

Ecosystem type

  • Tundra
  • Polar

Key Ecosystem Processes

  • Soil formation
  • Nutrient cycling
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Fisheries
  • Fuel and fiber crops
  • Hydropower

Regulating services

  • Climate regulation
  • Regulation of soil erosion
  • Natural hazard regulation

Cultural services

  • Aesthetic values

Human Well-being

  • Security of housing & infrastructure

Key Attributes

Typical spatial scale

  • Local/landscape
  • Sub-continental/regional

Typical time scale

  • Months
  • Years
  • Decades
  • Centuries

Reversibility

  • Irreversible (on 100 year time scale)

Evidence

  • Models
  • Contemporary observations

Confidence: Existence of RS

  • Contested – Reasonable evidence both for and against the existence of RS

Confidence: Mechanism underlying RS

  • Well established – Wide agreement on the underlying mechanism

Links to other regime shifts

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Alternate regimes

A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.  

With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.

Arctic benthos (regime 1)

With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.

Subarctic benthos (regime 2)

After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).

 

Drivers and causes of the regime shift

The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.

The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).

 

How the regime shift works

In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)

Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.

Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions. 

Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.

 

Impacts on ecosystem services and human well-being

Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.

The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being. 

 

Management options

Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.

Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.

 

Alternate regimes

Due to thermokarst processes such as permafrost melting, numerous areas of the Arctic landscape are extremely lake-rich.  The water balance of those lakes may either be maintained due to impermeable permafrost that limits groundwater flow (Marsh 2009) or lost due to seasonal drainage, a natural characteristic of the Arctic system. However, a widespread, unnatural decline in lake abundance is being observed within the Arctic due to decades of rising atmospheric temperatures in the region that has been deteriorating the state of the permafrost (Smith et al. 2005). The lake-rich landscape of the Arctic is therefore slowly transforming into a drained, dry landscape increasingly dominated by vegetation such as shrubs and graminoids (Hinzman 2005).

 

Thermokarst lake ecosystem

 

The thermokarst lake-dominated landscape is formed as a response to some imposed disturbance, such as road construction, wildfire or climatic warming, which causes permafrost to thaw, creating irregular surface topography depressions, called thermokarsts. Those may appear as near-surface massive ice melts, allowing the surface to subside (Hinzman et al. 2005). As permafrost melts over time, those depressions become filled with melted water forming the lakes characteristic of the Arctic region.

 

Terrestrial ecosystem

 

Driven by external and internal factors, Arctic lakes are drying up at an unnatural speed within the region. This shift is accompanied by changes in vegetation, for example, increased shrub- and graminord abundance and a decrease in the cover of mosses and lichens through a broad expanse of the Arctic (Hinzman et al. 2005). The changes in vegetation may then further transform the landscape through sequential effects on foraging mammals and birds, as well as aquatic fauna and insects (Hinzman et al 2005).

 

Drivers and causes of the regime shift

 

Shift from thermokarst lakes to terrestrial ecosystem

The main external driver of this regime shift is climate change, which is caused by elevated greenhouse gas emissions (IPCC 2007). Increased temperatures warm the soil and warms the permafrost. As the permafrost thaws, greenhouse gasses such as CO2 and CH4 that are trapped in the ice are released into the atmosphere, further increasing the effects of climate change (Hinzman 2005; Karlsson et al. 2011). At a local scale, the effects of climate change are incremental. However, as this interaction is occurs over large areas, the effects are aggregated and self-reinforcing, which is significant since permafrost and seasonally frozen grounds store about 25% of the total global soil carbon stock (IPCC 2007).

At a local scale, initial thawing of the permafrost from increased atmospheric temperatures creates a dynamic landscape of depressions (thermokarst) in which lakes are formed (IPCC 2007). As permafrost become more degraded, drainage through outlet channels occurs. Since the temperature of melted water is higher than that of ice, it will exacerbate the melting of permafrost, causing another reinforcing feedback. Once the layer of frozen ground is completely penetrated the lakes can become permanently drained allowing for vegetation to be established (Marsh et al. 2008).

Shift from terrestrial ecosystem to thermokarst lake ecosystem

Heightened air- and soil temperatures melt the ice-rich soils and permafrost on which forests in the Arctic region develop, changing the physical conditions for the forest growing on top of it. When the roots of the trees get flooded, the trees die and ponds and lakes eventually replace the forest (Hinzman 2005). However, it is important to note that the shift from a terrestrial ecosystem to a lake dominated one is not as frequent as the shift from lakes to terrestrial ecosystems. The reason for this are the reinforcing feedbacks as mentioned above.

How the regime shift works

The warming of soil temperatures leads to the melting of continuous permafrost and the subsequent increase in abundance of discontinuous or sporadic permafrost. Melting of permafrost causes depressions in the soil called thermokarsts. Increased precipitation in the form of snowfall may have either an insulating or cooling effect on soil temperatures depending on several variables such a timing, duration, accumulation, and melting processes of seasonal snow cover, density, structure, and thickness of seasonal snow cover etc. (Zhang 2005). Depending upon the local surface energy balance, the thawed ground may refreeze or the permafrost can continue to degrade (Hinzman et al. 2005). On sites where continuous permafrost is thawing the active layer of soil in which melted water can move deepens allowing for the formation of thermokarst lakes (Hinzman et al. 2005). In freezing seasons, if the temperature is low enough and the hydraulic parameters on site suitable, the formation of these lakes can contribute to the re-formation of permafrost soils. Ice-rich permafrost can also prevent surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet. Decreased albedo due to changes in vegetation, the extension of snow-free and ice-free periods on terrestrial and lake surfaces and abundance, and reduction in the area occupied by glaciers and continental ice sheets in high latitudes may act as a positive feedback into atmospheric warming (Hinzman et al. 2005).

On sites where there is an abundance of discontinuous permafrost the active layer also deepens allowing for increased surface area for sub-ground water movement that increases the rate of drainage. Thermokarst lakes and ponds may begin to fill or drain depending upon the direction of the hydraulic gradient beneath the lake (Hinzman et al. 2005). Therefore it is not the depth of the active layer alone that determines the rate of drainage. However, increased drainage of lakes may prevent the reformation of permafrost from lack of available water for freezing in winter months and it can be inferred through advection of heat from decreased albedo. Once these drained lake beds become permanently drained they are likely to be re-vegetated which will change the albedo and may cause the extension of snow-free and ice-free periods and may act as feedback into increasing atmospheric temperature (Hinzman et al. 2005).

The affects of the shifts from lake to terrestrial ecosystems are aggregated at a local, regional and global scale due to the large scale and frequency at which those dynamics occur. The whole regime as described in the causal loop diagram below is a re-enforcing feedback that at its current state is leading to an overall decrease in lake dominated ecosystems and a increase in terrestrial ecosystems (Smith et al. 2005).

Impacts on ecosystem services and human well-being

Even though this regime shift occurs at the local scale, its consequences go beyond that scale because it reshapes regional hydrology (through changes in water balance and surface water- connection and fragmentation) and alters regional supplies of the ecosystem (Karlsson et al. 2011). This regime shift therefore affects provisioning ecosystem services, as it will decrease the availability of freshwater. The above-ground stored freshwater, which is of vital importance to migratory birds, fish, and other wildlife used by indigenous people, will shift to below-ground stored freshwater (Artic Science Journeys Radio Stories 2005). Globally, however, freshwater will become a more abundant ES due to reductions permafrost and subsequently ice sheets and glaciers resulting from the regime’s feedbacks to climate change. Nevertheless, this global increase in the freshwater ES is not a positive one: “Increased freshwater delivery to the Arctic Ocean from reductions in ice sheets and glaciers result in rising sea level (...)” (Hinzman et al. 2005). At a local scale traditional hunting and fishing practices will be impaired (Vincent et al. 2013).  Arctic freshwater systems provide important migratory routes for fish stock. Due to this regime shit, those routes would be greatly altered in connectivity among lakes and river channels, as well as in terms of their physical coupling to the coastal marine ecosystem (Vincent et al. 2012). Globally, fishing practices would suffer as well. Increased discharge of water into the Arctic Ocean due to draining results in increased nutrient and external organic matter inputs to the Arctic Ocean that affect primary processes at the base of the marine food web (Vincent et al. 2012). Climate regulation is heavily affected on a global scale through the processes of this regime shift: as it sets up a positive feedback cycle in which the release of methane and carbon dioxide through the newly exposed soil and melted permafrost feeds into global warming. The shift from lakes to terrestrial ecosystems heightens gully erosion, a type of erosion that occurs when water is channeled across unprotected land and washes away the soil along the drainage lines. As permafrost degrades further, it causes erosion and slumping of lake edges and stream channels

The Arctic lakes are of vital importance to indigenous people as they are a source of fresh drinking water and also provides important winter transport routes. These lakes not only allows indigenous people access to their traditional hunting and fishing areas, but also to transport goods to remote communities and industries such as mining centers (Vincent et al. 2013). The regime shift would also mean an increase in products extracted from woods such as timber commodities and fuel: “increased areas of tree growth in the Arctic could serve to (…) take supply more wood products and related employment, providing local and global economic benefits” (Hassol et al. 2004).

 

Management options

There are few management options available to maintain the thermokarst lake ecosystems due to the long time scale at which permafrost melts and the natural dynamics within this system e.g. hydraulic fluctuations or site-specific soil conditions. One management option is to continue to reduce the emissions of greenhouse gases while simultaneously investigating further opportunities for carbon offsetting schemes.

For hydroelectric reservoirs, shifting ice conditions will have both positive and negative effects, and may require adaptive changes in operating procedures, with attention to minimize negative impacts associated with ice jams and ice breakup downstream of the spillway (Vincent et al. 2013). Fisheries management plans will also need to be adapted to the changes in migration and productivity of northern fish populations with ongoing climate change (Vincent et al. 2013). These essential resources require the development of integrated freshwater management plans, which include consideration of alternate water sources as traditional supplies change in quantity or quality (Vincent et al. 2011). The Arctic will require increasing vigilance and appropriate water management strategies to avoid and minimize the impacts of changing water impacts in the future (Vincent et al. 2013).

Key References

  1. Chapman, W. L. and Walsh, J. E. (1993). Recent variations of sea ice and air temperatures in high latitudes, Bull. Amer. Meteoric. Soc. 74, 33–47. http://dx.doi.org/10.1175/1520-0477(1993)074<0033:RVOSIA>2.0.CO;2
  2. Artic Science Journeys Radio Stories. (2005). Arctic Lakes Shrink, Disappear. http://seagrant.uaf.edu/news/05ASJ/06.09.05arctic-lakes.html
  3. Clarke GKC. (1982) Glacier outburst floods from “Hazard Lake”, Yukon Territory, and the problem of flood magnitude prediction. Journal of Glaciology 28(98): 3–21.
  4. Clarke GKC. 1982. Glacier outburst floods from “Hazard Lake”, Yukon Territory, and the problem of flood magnitude prediction. Journal of Glaciology 28(98): 3–21.
  5. Hassan, R. M., Scholes, R., & Ash, N. (2005). Ecosystems and human well-being: current state and trends: findings of the Condition and Trends Working Group (p. 917). Island Press.
  6. Hassol, S. J. (2004). Impacts of a Warming Arctic. Arctic Climate Impact Assessment.
  7. Hinzman, L. D., Bettez, N. D., Bolton, W. R., Chapin, F. S., Dyurgerov, M. B., Fastie, C. L., … Yoshikawa, K. (2005). Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions. Climatic Change, 72(3), 251–298. doi:10.1007/s10584-005-5352-2
  8. IPCC (2007). Climate Change 2007: The Physical Science Basis. Cambridge University Press, New York. Nap.
  9. Karlsson, J. M., Bring, A., Peterson, G. D., Gordon, L. J., & Destouni, G. (2011). Opportunities and limitations to detect climate-related regime shifts in inland Arctic ecosystems through eco-hydrological monitoring. Environmental Research Letters, 6(1), 014015.
  10. Kirpotin, S., Polishchuk, Y., Zakharova, E., & Shirokova, L. (2008). One of the possible mechanisms of thermokarst lakes drainage in West ‐ Siberian North. International Journal of Environmental Studies, 65(5), 37–41.
  11. Magnuson, J., Robertson, D., Benson, B., Wynne, R., Livingstone, D., Arai, T., Assel, R., Barry, R., Card, V., Kuusisto, E., Granin, N., Prowse, T., Steward, K., and Vuglinski, V. (2000). Historical trends in lake and river ice cover in the northern hemisphere, Science 289, 1743–1746. DOI: 10.1126/science.289.5485.1743
  12. Marsh P, Neumann N. (2001). Processes controlling the rapid drainage of two ice-rich permafrost-dammed lakes in NW Canada. Hydrological Processes 15, 3433–3446.
  13. Marsh, P., Russell, M., Pohl, S., Haywood, H., Onclin, C. (2009). Changes in thaw lake drainage in the Western Canadian Arctic from 1950 to 2000. National Hydrology Research Centre.158, 145–158.
  14. Moore, T. R., Roulet, N. T., and Waddington, J. M. (1998) Uncertainty in predicting the effect of climatic change on the carbon cycling of Canadian peatlands, Clim. Change 40, 229–245. DOI 10.1023/A:1005408719297
  15. National Snow and Ice Data Center. All about frozen ground. Webpage. Date of access: 12/11/2013. http://nsidc.org/cryosphere/frozenground/people.html
  16. Oechel,W. C. and Vourlitis, G. L. (1997), Climate change in northern latitudes: Alterations in ecosystem structure and function and effects on carbon sequestration, in Oechel, W. C., Callaghan, T., Gilmanov, T., Holten, J. I., Maxwell, B., Molau, U., and Sveinbj¨ornsson, B. (eds.), Global Change and Arctic Terrestrial Ecosystems, Ecological Studies 124, 381–401.
  17. Prowse, T., Alfredsen, K., Beltaos, S., Bonsal, B., Duguay, C., Korhola, A., … Weyhenmeyer, G. a. (2012). Past and Future Changes in Arctic Lake and River Ice. Ambio, 40(S1), 53–62. doi:10.1007/s13280-011-0216-7
  18. Schaefer, K., Lantuit, H., Romanovsky, V.E., Schuur, E.A.G., Gärtner-Roer, I. (2012). UNEP Policy Implications of Warming Permafrost nap. ISBN: 978-92-807-3308-2
  19. Smith, L.C., Sheng, Y., MacDonald, G., Hinzman, L.D. (2005). Disappearing Arctic lakes. Science (New York, N.Y.), 308(5727), p.1429. Available at: http://www.ncbi.nlm.nih.gov/pubmed/15933192.
  20. Vincent, W. F., Callaghan, T. V., Dahl-Jensen, D., Johansson, M., Kovacs, K. M., Michel, C., … Sharp, M. (2012). Ecological Implications of Changes in the Arctic Cryosphere. Ambio, 40(S1), 87–99. doi:10.1007/s13280-011-0218-5
  21. Vincent, W. F., Laurion, I., Pienitz, R., Anthony, K. M. W., & Katey, M. (2013). Climate Impacts on Arctic Lake Ecosystems. Climatic Change and Global Warming of Inland Waters: Impacts and Mitigation for Ecosystems and Societies. 27-42.
  22. Witthaus, L., Zung, A. n.d. Threatened Arctic Lakes: Pressures from Climate Change and Resource Development. PowerPoint presentation.
  23. Zhang, T. (2005). Influence of the seasonal snow cover on the ground thermal regime: An overview. Reviews of Geophysics. 43, 1-23.

Citation

Hannah Griffiths, Elinor Holén, Jessica Spijkers, Reinette (Oonsie) Biggs, Juan Carlos Rocha. Thermokarst lake to terrestrial ecosystem . In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-07 09:25:40 GMT.
Tuesday, 21 October 2014 08:26

Coniferous to deciduous boreal forest

Written by Juan Carlos

Coniferous to deciduous boreal forest

Main Contributors:

Linda Lindström Lindström, Katja Malmborg, Lara D. Mateos

Other Contributors:

Juan Carlos Rocha, Garry Peterson

Summary

This regime shift has been well studied in the interior Alaska where the coniferous dominated boreal forest are being replaced by deciduous trees due to recent climate warming and changes in the wildlife regime. Coniferous trees thrive in cold, moist soil conditions, and enhance these conditions by accumulating a deep soil organic layer. The moisture of the soil prevents frequent fires from occurring, but when they do, the soil organic layer is rarely consumed in its entirety due to the high water content. Deciduous trees, on the other hand, thrive in nutrient rich, dry and warm soils, which are conditions that they reinforce by keeping the decomposition rate high, making the soil organic layer shallow. Fires tend to be more frequent than in coniferous dominated forests, but not as intense. A severe fire can get the system to shift from one regime to the other, while changes in climate (i.e. mainly temperature or precipitation) can change the underlying conditions to make each regime less resilient. This regime shift may affect the provisioning of wild products such as berries and game. An increase in fire frequency may also decrease air quality.  

Drivers

Key direct drivers

  • Environmental shocks (eg floods)
  • Global climate change

Land use

  • Timber production
  • Conservation
  • Tourism

Impacts

Ecosystem type

  • Temperate & boreal forests

Key Ecosystem Processes

  • Soil formation
  • Primary production
  • Nutrient cycling
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Wild animal and plant products
  • Timber
  • Woodfuel

Regulating services

  • Air quality regulation
  • Climate regulation
  • Water purification
  • Regulation of soil erosion

Cultural services

  • Recreation
  • Aesthetic values

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Typical spatial scale

  • Local/landscape
  • National (country)

Typical time scale

  • Decades
  • Centuries

Reversibility

  • Irreversible (on 100 year time scale)
  • Hysteretic

Evidence

  • Models
  • Paleo-observation
  • Contemporary observations

Confidence: Existence of RS

  • Well established – Wide agreement in the literature that the RS exists

Confidence: Mechanism underlying RS

  • Well established – Wide agreement on the underlying mechanism

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Alternate regimes

A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.  

With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.

Arctic benthos (regime 1)

With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.

Subarctic benthos (regime 2)

After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).

 

Drivers and causes of the regime shift

The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.

The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).

 

How the regime shift works

In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)

Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.

Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions. 

Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.

 

Impacts on ecosystem services and human well-being

Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.

The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being. 

 

Management options

Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.

Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.

 

Alternate regimes

Due to thermokarst processes such as permafrost melting, numerous areas of the Arctic landscape are extremely lake-rich.  The water balance of those lakes may either be maintained due to impermeable permafrost that limits groundwater flow (Marsh 2009) or lost due to seasonal drainage, a natural characteristic of the Arctic system. However, a widespread, unnatural decline in lake abundance is being observed within the Arctic due to decades of rising atmospheric temperatures in the region that has been deteriorating the state of the permafrost (Smith et al. 2005). The lake-rich landscape of the Arctic is therefore slowly transforming into a drained, dry landscape increasingly dominated by vegetation such as shrubs and graminoids (Hinzman 2005).

 

Thermokarst lake ecosystem

 

The thermokarst lake-dominated landscape is formed as a response to some imposed disturbance, such as road construction, wildfire or climatic warming, which causes permafrost to thaw, creating irregular surface topography depressions, called thermokarsts. Those may appear as near-surface massive ice melts, allowing the surface to subside (Hinzman et al. 2005). As permafrost melts over time, those depressions become filled with melted water forming the lakes characteristic of the Arctic region.

 

Terrestrial ecosystem

 

Driven by external and internal factors, Arctic lakes are drying up at an unnatural speed within the region. This shift is accompanied by changes in vegetation, for example, increased shrub- and graminord abundance and a decrease in the cover of mosses and lichens through a broad expanse of the Arctic (Hinzman et al. 2005). The changes in vegetation may then further transform the landscape through sequential effects on foraging mammals and birds, as well as aquatic fauna and insects (Hinzman et al 2005).

 

Drivers and causes of the regime shift

 

Shift from thermokarst lakes to terrestrial ecosystem

The main external driver of this regime shift is climate change, which is caused by elevated greenhouse gas emissions (IPCC 2007). Increased temperatures warm the soil and warms the permafrost. As the permafrost thaws, greenhouse gasses such as CO2 and CH4 that are trapped in the ice are released into the atmosphere, further increasing the effects of climate change (Hinzman 2005; Karlsson et al. 2011). At a local scale, the effects of climate change are incremental. However, as this interaction is occurs over large areas, the effects are aggregated and self-reinforcing, which is significant since permafrost and seasonally frozen grounds store about 25% of the total global soil carbon stock (IPCC 2007).

At a local scale, initial thawing of the permafrost from increased atmospheric temperatures creates a dynamic landscape of depressions (thermokarst) in which lakes are formed (IPCC 2007). As permafrost become more degraded, drainage through outlet channels occurs. Since the temperature of melted water is higher than that of ice, it will exacerbate the melting of permafrost, causing another reinforcing feedback. Once the layer of frozen ground is completely penetrated the lakes can become permanently drained allowing for vegetation to be established (Marsh et al. 2008).

Shift from terrestrial ecosystem to thermokarst lake ecosystem

Heightened air- and soil temperatures melt the ice-rich soils and permafrost on which forests in the Arctic region develop, changing the physical conditions for the forest growing on top of it. When the roots of the trees get flooded, the trees die and ponds and lakes eventually replace the forest (Hinzman 2005). However, it is important to note that the shift from a terrestrial ecosystem to a lake dominated one is not as frequent as the shift from lakes to terrestrial ecosystems. The reason for this are the reinforcing feedbacks as mentioned above.

How the regime shift works

The warming of soil temperatures leads to the melting of continuous permafrost and the subsequent increase in abundance of discontinuous or sporadic permafrost. Melting of permafrost causes depressions in the soil called thermokarsts. Increased precipitation in the form of snowfall may have either an insulating or cooling effect on soil temperatures depending on several variables such a timing, duration, accumulation, and melting processes of seasonal snow cover, density, structure, and thickness of seasonal snow cover etc. (Zhang 2005). Depending upon the local surface energy balance, the thawed ground may refreeze or the permafrost can continue to degrade (Hinzman et al. 2005). On sites where continuous permafrost is thawing the active layer of soil in which melted water can move deepens allowing for the formation of thermokarst lakes (Hinzman et al. 2005). In freezing seasons, if the temperature is low enough and the hydraulic parameters on site suitable, the formation of these lakes can contribute to the re-formation of permafrost soils. Ice-rich permafrost can also prevent surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet. Decreased albedo due to changes in vegetation, the extension of snow-free and ice-free periods on terrestrial and lake surfaces and abundance, and reduction in the area occupied by glaciers and continental ice sheets in high latitudes may act as a positive feedback into atmospheric warming (Hinzman et al. 2005).

On sites where there is an abundance of discontinuous permafrost the active layer also deepens allowing for increased surface area for sub-ground water movement that increases the rate of drainage. Thermokarst lakes and ponds may begin to fill or drain depending upon the direction of the hydraulic gradient beneath the lake (Hinzman et al. 2005). Therefore it is not the depth of the active layer alone that determines the rate of drainage. However, increased drainage of lakes may prevent the reformation of permafrost from lack of available water for freezing in winter months and it can be inferred through advection of heat from decreased albedo. Once these drained lake beds become permanently drained they are likely to be re-vegetated which will change the albedo and may cause the extension of snow-free and ice-free periods and may act as feedback into increasing atmospheric temperature (Hinzman et al. 2005).

The affects of the shifts from lake to terrestrial ecosystems are aggregated at a local, regional and global scale due to the large scale and frequency at which those dynamics occur. The whole regime as described in the causal loop diagram below is a re-enforcing feedback that at its current state is leading to an overall decrease in lake dominated ecosystems and a increase in terrestrial ecosystems (Smith et al. 2005).

Impacts on ecosystem services and human well-being

Even though this regime shift occurs at the local scale, its consequences go beyond that scale because it reshapes regional hydrology (through changes in water balance and surface water- connection and fragmentation) and alters regional supplies of the ecosystem (Karlsson et al. 2011). This regime shift therefore affects provisioning ecosystem services, as it will decrease the availability of freshwater. The above-ground stored freshwater, which is of vital importance to migratory birds, fish, and other wildlife used by indigenous people, will shift to below-ground stored freshwater (Artic Science Journeys Radio Stories 2005). Globally, however, freshwater will become a more abundant ES due to reductions permafrost and subsequently ice sheets and glaciers resulting from the regime’s feedbacks to climate change. Nevertheless, this global increase in the freshwater ES is not a positive one: “Increased freshwater delivery to the Arctic Ocean from reductions in ice sheets and glaciers result in rising sea level (...)” (Hinzman et al. 2005). At a local scale traditional hunting and fishing practices will be impaired (Vincent et al. 2013).  Arctic freshwater systems provide important migratory routes for fish stock. Due to this regime shit, those routes would be greatly altered in connectivity among lakes and river channels, as well as in terms of their physical coupling to the coastal marine ecosystem (Vincent et al. 2012). Globally, fishing practices would suffer as well. Increased discharge of water into the Arctic Ocean due to draining results in increased nutrient and external organic matter inputs to the Arctic Ocean that affect primary processes at the base of the marine food web (Vincent et al. 2012). Climate regulation is heavily affected on a global scale through the processes of this regime shift: as it sets up a positive feedback cycle in which the release of methane and carbon dioxide through the newly exposed soil and melted permafrost feeds into global warming. The shift from lakes to terrestrial ecosystems heightens gully erosion, a type of erosion that occurs when water is channeled across unprotected land and washes away the soil along the drainage lines. As permafrost degrades further, it causes erosion and slumping of lake edges and stream channels

The Arctic lakes are of vital importance to indigenous people as they are a source of fresh drinking water and also provides important winter transport routes. These lakes not only allows indigenous people access to their traditional hunting and fishing areas, but also to transport goods to remote communities and industries such as mining centers (Vincent et al. 2013). The regime shift would also mean an increase in products extracted from woods such as timber commodities and fuel: “increased areas of tree growth in the Arctic could serve to (…) take supply more wood products and related employment, providing local and global economic benefits” (Hassol et al. 2004).

 

Management options

There are few management options available to maintain the thermokarst lake ecosystems due to the long time scale at which permafrost melts and the natural dynamics within this system e.g. hydraulic fluctuations or site-specific soil conditions. One management option is to continue to reduce the emissions of greenhouse gases while simultaneously investigating further opportunities for carbon offsetting schemes.

For hydroelectric reservoirs, shifting ice conditions will have both positive and negative effects, and may require adaptive changes in operating procedures, with attention to minimize negative impacts associated with ice jams and ice breakup downstream of the spillway (Vincent et al. 2013). Fisheries management plans will also need to be adapted to the changes in migration and productivity of northern fish populations with ongoing climate change (Vincent et al. 2013). These essential resources require the development of integrated freshwater management plans, which include consideration of alternate water sources as traditional supplies change in quantity or quality (Vincent et al. 2011). The Arctic will require increasing vigilance and appropriate water management strategies to avoid and minimize the impacts of changing water impacts in the future (Vincent et al. 2013).

Alternate regimes

The boreal forest in northern Interior Alaska covers an approximate area of 48 million hectares (Mann et al. 2012). Bounded by the Alaska Range to the south, the Seward Peninsula to the west and Brooks Range to the north (Wolken et al. 2011). It is characterized by large areas of gently sloping uplands and flat lowlands isolated by mountain ranges and braided rivers with broad floodplains. During the past 50 years, the mean annual temperature in the interior boreal forest of Alaska has increased by 1.3 ºC (Hartmann and Wendler, 2005) and this has been strongly associated with recent increase in fire frequency and severity (Wolken et al. 2011). The annual area burned in Interior Alaska has doubled during the last decade (Kasischke et al. 2010). The forest has been dominated by coniferous trees (mainly black spruce), which is a group of species that thrive under colder, moist conditions in a deep soil organic layer. However, due to climate warming, Interior Alaska is currently shifting towards a deciduous dominated forest (e.g. aspen and birch) that prefer warmer and drier conditions with a shallow soil organic layer (Johnstone et al. 2010).  

 

 

Low fire frequency coniferous dominated forest

Under cold and wet conditions, coniferous trees, such as the black spruce (Picea mariana), dominate the boreal forest in Interior Alaska. The coniferous trees promote the accumulation of a deep soil organic layer, cold and moist soil temperatures, and slow decomposition rates. Due to the moistness of the soil, fires are not frequent in these forests. However, when they occur, they tend to be severe due to the high flammability of the black spruce. Due to the high water content of the soil organic layer, it will only be entirely consumed by the most severe fires (Johnstone et al. 2010). As long as the entire organic layer has not been consumed, the coniferous trees have a regenerative advantage due to their seed’s ability to sprout in moist organic soils. This means that these forests tend to return to a coniferous dominated state even after a fire (Hollingsworth et al. 2013).

 

High fire frequency deciduous dominated forest

A deciduous dominated forest, mainly consisting of aspen and birch trees in Interior Alaska, prefers and reinforces dry and warm soil conditions. Although these forests have a high organic litter production, the fast decomposition rate and the subsequent fast nutrient turnover, leaves a shallow soil organic layer and a nutrient rich mineral soil that favours deciduous tree regeneration. The warm and dry conditions in the forest lead to a comparatively high fire frequency, but due to the low flammability of the deciduous trees, the fires rarely become very severe (Johnstone et al. 2010). However, because of the shallow pre-fire soil organic layer, the fires tend to expose the mineral soil, which favours the post-fire reproduction of deciduous trees (Hollingsworth et al. 2013).

 

Drivers and causes of the regime shift

The main external direct driver of the regime shift is climate warming, which affects the important feedbacks of the boreal forest ecosystem. Warmer and drier climatic conditions have a direct effect on the most important soil characteristics (i.e. soil temperature, moisture and depth of the soil organic layer), influencing the vegetation cover. Coniferous trees require a cold moist soil with a deep soil organic layer to successfully reproduce. Deciduous trees, on the other hand, are competitively superior under warmer and drier soil conditions with a shallow soil organic layer (Johnstone et al. 2010). Extended summer seasons and the accompanying drier conditions create a suitable environment for an increase in the annual number of natural ignitions by lightning strikes (Kasischke et al. in press). Fires have an immediate effect on the environmental conditions of the forest, affecting factors such as soil moisture and the post-fire forest community composition (Kelly et al. 2012). Once established, the trees support and emphasize the conditions that favour their own reproduction and dominance. Under colder environmental conditions, coniferous trees, such as the black spruce, produce large amounts of organic litter, which, together with the presence of permafrost, maintains a moist soil and slow decomposition rates, contributing to a deep soil organic layer. Under warmer climatic conditions, deciduous trees produce ground layer fuels that decompose at fast rates, maintaining a shallow organic layer (Johnstone et al. 2010). Global socio-economic development indirectly drives climate warming, contributing to the increased atmospheric temperatures through e.g. greenhouse gas emissions (IPCC, 2011). Fire management can have a limited effect on the regime shift through fire suppression practices.

How the regime shift works

Shift from coniferous to deciduous-dominated forest

The coniferous dominated boreal forest of Interior Alaska requires a deep soil organic layer to succeed over other classes of trees. Once the required depth is present, this has a strong reinforcing effect on the coniferous forest. This is a requisite to maintain cold moist soil conditions in which coniferous trees thrive. The cold soil temperature reduces the decomposition rate of organic litter, maintaining a deep soil organic layer (Johnstone et al. 2010).

 

External shocks such as severe and frequent fires or insect outbreaks might cause dramatic shifts in the soil and vegetation by impacting the soil organic layer depth and tree succession (Hollingsworth et al. 2013; McCullough et al. 1998; Wolken et al. 2011). If the severity of such external shocks is sufficient, thresholds in the most important regime characteristics will be crossed (e.g. shallow post-shock soil organic layer) creating windows of opportunity for deciduous species to dominate. Atmospheric temperature influences the frequency and severity of fires, the soil characteristics and the length of the growing season; therefore, climate warming can also cause a change in conditions that would set the grounds for a shift to a new stable state (Mann et al. 2012).

 

The deciduous dominated forest has a competitive advantage in warm and dry conditions with shallow soil organic layers. Again, once the required shallow layer is present, it has a strong reinforcing effect on the deciduous forest. A shallow soil organic layer maintains warm and dry soil conditions that speed up the decomposition of organic litter, maintaining the required depth of soil organic layer (Johnstone et al. 2012).

 

Shift from deciduous-dominated  to coniferous forest 

In order to shift from regime 2 to regime 1, a prolonged climatic cooling would be necessary. A slow change of the environmental conditions towards those that favour a coniferous boreal forest could, if the forest is left undisturbed for a sufficiently long time, lead to late successional coniferous species, like the black spruce, replacing early successional deciduous species (Rupp et al. 2002). Although the spatial scale considered is of extreme importance, sufficiently large external shocks, such as deciduous-specific insect outbreaks, could clear sufficiently large areas to advance the dominance of coniferous trees. In such hypothetical events, coniferous trees could get the opportunity to sprout and start altering the soil conditions to their own favour.

Impacts on ecosystem services and human well-being

The provisioning of wild products such as berries and game is expected to shift some species and decrease others, but it is unclear what the net changes will be (Chapin et al. 2008). The boreal forest is likely to become a net source of carbon to the atmosphere (McGuire et al. 2009; Schuur et al. 2009), while the air quality is expected to decrease periodically due to the increase in fire frequency (Chapin et al. 2008).

Management options

The primary tactic used for reducing impacts of fire and enhance resilience (i.e. maintain structures, feedbacks, functions and processes of regime 1 is fire suppression to reduce the fire severity (Chapin et al. 2008). Decrease in fire severity could prevent the fire from consuming the soil organic layer, which in turn favours the conditions for coniferous tree. Fire suppression should be maintained in Alaska since it minimizes the risk fire imposes to life and property in communities. However, distant fires should be allowed to burn under controlled conditions, since it could have a cooling effect at a regional scale due to the higher albedo of post-fire deciduous species compared to the previous coniferous forest (Chapin et al. 2008).

Reducing air temperature is essential to be able to restore regime 1. Alaska itself accounts for a miniscule proportion of global greenhouse gas emissions and consequently to climate warming (Chapin et al. 2008). Therefore, reducing Alaska’s emissions will not have a major impact on global climate change. Global abatement of emissions is the long-term solution. Regardless of global greenhouse gas emission policies, it is highly likely that recent warming and wildfire frequency will continue for several decades in Alaska due to the multidecadal lag in the climate system (IPCC, 2007). This makes the reduction of emissions an inefficient short-term solution.  Secondary challenges that face efforts to achieve reduced emissions include restricting the economy and changing the focus of developed nations on a continued economic but sustainable growth. Educating global public about the social impacts of climate change in Alaska and other parts of the world (e.g. reduction in ecosystem services) may promote the willingness to reduce emissions (Chapin et al. 2008).

In summary, this means that the regime shift from coniferous to deciduous dominated forest in theory could be reversible through manipulation of e.g. soil conditions or local climate. However, due to projected global socio-economic development and climate change, it is highly unlikely that reversing the regime shift will be possible during the next 100 years.

Key References

  1. Beck, Pieter S.A., Glenn P. Juday, Claire Alix, Valerie A. Barber, Stephen E. Winslow, Emily E. Sousa, Patricia Heiser, James D. Herriges and Scott J. Goetz, 2011. Changes in forest productivity across Alaska consistent with biome shift. Ecology Letters, no. 14: 373-379. Blackwell Publishing Ltd, Hoboken, New Jersey.
  2. Beck, Pieter S.A., Glenn P. Juday, Claire Alix, Valerie A. Barber, Stephen E. Winslow, Emily E. Sousa, Patricia Heiser, James D. Herriges and Scott J. Goetz, 2011. Changes in forest productivity across Alaska consistent with biome shift. Ecology Letters, no. 14: 373-379. Blackwell Publishing Ltd, Hoboken, New Jersey.
  3. Chapin, F. Stuart III, Sarah F. Trainor, Orville Huntington, Amy L. Lovecraft, Erika Zavaleta, David C. Natcher, A. David McGuire, Joanna L. Nelson, Lily Ray, Monika Calef, Nancy Fresco, Henry Huntington, T. Scott Rupp, La’Ona DeWilde and Rosamond L. Naylor, 2008. Increasing wildfire in Alaska’s boreal forests: Pathways to potential solutions of a wicked problem. BioScience, vol. 58, no. 6: 531-540. American Institute of Biological Sciences.
  4. Hartmann, B., and G. Wendler, 2005. The significance of the 1976 Pacific climate shift in the climatology of Alaska. Journal of Climate, vol. 18: 4824–4839.
  5. Hollingsworth, Teresa N., Jill F. Johnstone, Emily L. Bernhardt, F. Stuart Chapin III, 2013: Fire severity filters regeneration traits to shape community assembly in Alaska’s boreal forest. PLoS ONE 8(2): e56033. doi:10.1371/journal.pone.0056033
  6. Huntsinger, E., N. Fried, and D. Robinson. 2007. Alaska economic trends. Alaska Department of Labor and Workforce Development, Juneau, Alaska, USA.
  7. IPCC (Intergovernmental Panel on Climate Change), 2007. Climate Change 2007: The PhysicalScience Basis, Cambridge (MA): Cambridge University Press, (8 May 2008; www.climatescience.gov/Library/ipcc/wgl4ar-review. htm)
  8. IPCC, 2011. Summary for Policymakers. In: Edenhofer, O., R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschoss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds): IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  9. Johnstone, Jill F., F. Stuart Chapin III, Teresa N. Hollingsworth, Michelle C. Mack, Vladimir Romanovsky and Merritt Turetsky, 2010: Fire, climate change, and forest resilience in interior Alaska. Canadian Journal of Forest Research, vol. 40: 1302-1312. NRC Research Press, Ottawa, Ontario.
  10. Kasischke, Eric S., David L. Verbyla, T. Scott Rupp, A. David McGuire, Karen A. Murphy, Randi Jandt, Jennifer L. Barnes, Elizabeth E. Hoy, Paul A. Duffy, Monika Calef, and Merritt R. Turetsky, 2010. Alaska’s changing fire regime: implications for the vulnerability of its boreal forests. Canadian Journal of Forest Research, vol. 40: 1313–1324.
  11. Kasischke, Eric S., T.Scott Rupp and D.L. Verbyla, in press. Fire trends in the Alaskan boreal forest. In: F.S. Chapin III, J. Yarie, K. Van Cleve, L.A. Viereck, M.W. Oswood and D.L. Verbyla (eds.). Alaska’s Changing Boreal Forest. Oxford University Press.
  12. Kelly, R., M. L. Chipman, P. E. Higuera, I. Stefanova, L.B. Brubaker and F. S. Hu, 2013. Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years. Proceedings of the National Academy of Sciences of the United States of America, vol. 110, no. 32: 13055–13060.
  13. Keyser, A. R., J. S. Kimball, R. R. Nemani and S. W. Running, 2000. Simulating the effects of climate change on the carbon balance of North American high-latitude forests. Global Change Biology, vol. 6: 185-195. Blackwell Science Ltd, Hoboken, New Jersey.
  14. Lynch, J. S., J.L. Hollis and F.S. Hu, 2004. Climatic and landscape controls of the boreal forest fire regime: Holocene records from Alaska. Journal of Ecology, vol. 92, no. 3: 477–489.
  15. Mann, Daniel H., T. Scott Rupp, Mark A. Olson and Paul Duffy, 2012. Is Alaska’s boreal forest now crossing a major ecological threshold? Arctic, Antarctic, and Alpine Research, vol. 44, no. 3: 319-331. Institute of Arctic and Alpine Research (INSTAAR), University of Colorado.
  16. McCoy, V. M., and C. R. Burn, 2005: Potential alteration by climate change of the forest-fire regime in the boreal forest of central Yukon Territory. Arctic, vol. 58, no. 3: 276-285. Arctic Institute of North America.
  17. McCullough, D.G., R. A. Werner and D. Neumann, 1998. Fire and insects in northern and boreal forest ecosystems of North America. Annual review of entomology, vol. 43: 107–27.
  18. McGuire, A. David, Leif G. Anderson, Torben R. Christensen, Scott Dallimore, Laodong Guo, Daniel J. Hayes, Martin Heimann, Thomas D. Lorenson, Robie W. Macdonald and Nigel Roulet, 2009: Sensitivity of the carbon cycle in the Arctic to climate change. Ecological Monographs, vol. 79, no. 4: 523-555. Ecological Society of America.
  19. Rupp, T. Scott, A. M. Starfield, F. Stuart Chapin III and Paul Duffy, 2002: Modeling the impact of black spruce on the fire regime of Alaskan boreal forest. Climatic Change, vol. 55: 212-233. Kluwer Academic Publishers.
  20. Schuur, Edward A. G., Jason G. Vogel, Kathryn G. Crummer, Hanna Lee, James O. Sickman and T. E. Osterkamp, 2009: The effect of permafrost thaw on old carbon release and net carbon exchange from tundra. Nature, vol. 459: 556-559. Macmillan Publishers Ltd, London.
  21. Wolken, Jane M., Teresa N. Hollingsworth, T. Scott Rupp, F. Stuart Chapin, III, Sarah F. Trainor, Tara M. Barrett, Patrick F. Sullivan, A. David McGuire, Eugenie S. Euskirchen, Paul E. Hennon, Erik A. Beever, Jeff S. Conn, Lisa K. Crone, David V. D'Amore, Nancy Fresco, Thomas A. Hanley, Knut Kielland, James J. Kruse, Trista Patterson, Edward A. G. Schuur, David L. Verbyla, and John Yarie 2011. Evidence and implications of recent and projected climate change in Alaska's forest ecosystems. Ecosphere 2:art124. http://dx.doi.org/10.1890/ES11-00288.1

Citation

Linda Lindström Lindström, Katja Malmborg, Lara D. Mateos, Juan Carlos Rocha, Garry Peterson. Coniferous to deciduous boreal forest. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-18 12:17:20 GMT.

Marine food webs: community change and trophic level decline

Main Contributors:

Susa Niiranen, Reinette (Oonsie) Biggs, Garry Peterson, Juan Carlos Rocha

Other Contributors:

Henrik Österblom

Summary

A characteristic regime shift in aquatic systems involves an abrupt increase in the dominance of lower trophic level groups within aquatic food webs. This regime shift involves a change from an ecosystem with high numbers of predatory fish to one dominated by pelagic planktivores. The shift is often initiated by high fishing pressure on top-predators followed by a trophic cascade, but can also be brought about by other environmental factors like global warming and upwelling increase. In extreme cases the food web is shortened due to disappearance of top predators and the carbon transfer pathways is dominated by microbial webs instead of the classic trophic chain. The new regime can be enforced and maintained by biological mechanisms including minimum population biomass, competition and dietary relations, or environmental conditions. Despite there being some mechanism that often dissipate the trophic cascade, food web regime shifts do have substantial impact on commercial fisheries, as well as increase the vulnerability of an ecosystem to eutrophication, hypoxia and invasion by non-native species.

Drivers

Key direct drivers

  • Harvest and resource consumption
  • External inputs (eg fertilizers)
  • Global climate change

Land use

  • Fisheries

Impacts

Ecosystem type

  • Marine & coastal

Key Ecosystem Processes

  • Primary production

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries

Cultural services

  • Recreation
  • Aesthetic values

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values
  • Cultural identity

Key Attributes

Typical spatial scale

  • Local/landscape
  • National (country)
  • Sub-continental/regional

Typical time scale

  • Years

Reversibility

  • Hysteretic

Evidence

  • Models
  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Speculative – Regime shift has been proposed, but little evidence as yet

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Links to other regime shifts

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Alternate regimes

A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.  

With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.

Arctic benthos (regime 1)

With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.

Subarctic benthos (regime 2)

After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).

 

Drivers and causes of the regime shift

The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.

The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).

 

How the regime shift works

In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)

Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.

Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions. 

Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.

 

Impacts on ecosystem services and human well-being

Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.

The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being. 

 

Management options

Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.

Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.

 

Alternate regimes

Due to thermokarst processes such as permafrost melting, numerous areas of the Arctic landscape are extremely lake-rich.  The water balance of those lakes may either be maintained due to impermeable permafrost that limits groundwater flow (Marsh 2009) or lost due to seasonal drainage, a natural characteristic of the Arctic system. However, a widespread, unnatural decline in lake abundance is being observed within the Arctic due to decades of rising atmospheric temperatures in the region that has been deteriorating the state of the permafrost (Smith et al. 2005). The lake-rich landscape of the Arctic is therefore slowly transforming into a drained, dry landscape increasingly dominated by vegetation such as shrubs and graminoids (Hinzman 2005).

 

Thermokarst lake ecosystem

 

The thermokarst lake-dominated landscape is formed as a response to some imposed disturbance, such as road construction, wildfire or climatic warming, which causes permafrost to thaw, creating irregular surface topography depressions, called thermokarsts. Those may appear as near-surface massive ice melts, allowing the surface to subside (Hinzman et al. 2005). As permafrost melts over time, those depressions become filled with melted water forming the lakes characteristic of the Arctic region.

 

Terrestrial ecosystem

 

Driven by external and internal factors, Arctic lakes are drying up at an unnatural speed within the region. This shift is accompanied by changes in vegetation, for example, increased shrub- and graminord abundance and a decrease in the cover of mosses and lichens through a broad expanse of the Arctic (Hinzman et al. 2005). The changes in vegetation may then further transform the landscape through sequential effects on foraging mammals and birds, as well as aquatic fauna and insects (Hinzman et al 2005).

 

Drivers and causes of the regime shift

 

Shift from thermokarst lakes to terrestrial ecosystem

The main external driver of this regime shift is climate change, which is caused by elevated greenhouse gas emissions (IPCC 2007). Increased temperatures warm the soil and warms the permafrost. As the permafrost thaws, greenhouse gasses such as CO2 and CH4 that are trapped in the ice are released into the atmosphere, further increasing the effects of climate change (Hinzman 2005; Karlsson et al. 2011). At a local scale, the effects of climate change are incremental. However, as this interaction is occurs over large areas, the effects are aggregated and self-reinforcing, which is significant since permafrost and seasonally frozen grounds store about 25% of the total global soil carbon stock (IPCC 2007).

At a local scale, initial thawing of the permafrost from increased atmospheric temperatures creates a dynamic landscape of depressions (thermokarst) in which lakes are formed (IPCC 2007). As permafrost become more degraded, drainage through outlet channels occurs. Since the temperature of melted water is higher than that of ice, it will exacerbate the melting of permafrost, causing another reinforcing feedback. Once the layer of frozen ground is completely penetrated the lakes can become permanently drained allowing for vegetation to be established (Marsh et al. 2008).

Shift from terrestrial ecosystem to thermokarst lake ecosystem

Heightened air- and soil temperatures melt the ice-rich soils and permafrost on which forests in the Arctic region develop, changing the physical conditions for the forest growing on top of it. When the roots of the trees get flooded, the trees die and ponds and lakes eventually replace the forest (Hinzman 2005). However, it is important to note that the shift from a terrestrial ecosystem to a lake dominated one is not as frequent as the shift from lakes to terrestrial ecosystems. The reason for this are the reinforcing feedbacks as mentioned above.

How the regime shift works

The warming of soil temperatures leads to the melting of continuous permafrost and the subsequent increase in abundance of discontinuous or sporadic permafrost. Melting of permafrost causes depressions in the soil called thermokarsts. Increased precipitation in the form of snowfall may have either an insulating or cooling effect on soil temperatures depending on several variables such a timing, duration, accumulation, and melting processes of seasonal snow cover, density, structure, and thickness of seasonal snow cover etc. (Zhang 2005). Depending upon the local surface energy balance, the thawed ground may refreeze or the permafrost can continue to degrade (Hinzman et al. 2005). On sites where continuous permafrost is thawing the active layer of soil in which melted water can move deepens allowing for the formation of thermokarst lakes (Hinzman et al. 2005). In freezing seasons, if the temperature is low enough and the hydraulic parameters on site suitable, the formation of these lakes can contribute to the re-formation of permafrost soils. Ice-rich permafrost can also prevent surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet. Decreased albedo due to changes in vegetation, the extension of snow-free and ice-free periods on terrestrial and lake surfaces and abundance, and reduction in the area occupied by glaciers and continental ice sheets in high latitudes may act as a positive feedback into atmospheric warming (Hinzman et al. 2005).

On sites where there is an abundance of discontinuous permafrost the active layer also deepens allowing for increased surface area for sub-ground water movement that increases the rate of drainage. Thermokarst lakes and ponds may begin to fill or drain depending upon the direction of the hydraulic gradient beneath the lake (Hinzman et al. 2005). Therefore it is not the depth of the active layer alone that determines the rate of drainage. However, increased drainage of lakes may prevent the reformation of permafrost from lack of available water for freezing in winter months and it can be inferred through advection of heat from decreased albedo. Once these drained lake beds become permanently drained they are likely to be re-vegetated which will change the albedo and may cause the extension of snow-free and ice-free periods and may act as feedback into increasing atmospheric temperature (Hinzman et al. 2005).

The affects of the shifts from lake to terrestrial ecosystems are aggregated at a local, regional and global scale due to the large scale and frequency at which those dynamics occur. The whole regime as described in the causal loop diagram below is a re-enforcing feedback that at its current state is leading to an overall decrease in lake dominated ecosystems and a increase in terrestrial ecosystems (Smith et al. 2005).

Impacts on ecosystem services and human well-being

Even though this regime shift occurs at the local scale, its consequences go beyond that scale because it reshapes regional hydrology (through changes in water balance and surface water- connection and fragmentation) and alters regional supplies of the ecosystem (Karlsson et al. 2011). This regime shift therefore affects provisioning ecosystem services, as it will decrease the availability of freshwater. The above-ground stored freshwater, which is of vital importance to migratory birds, fish, and other wildlife used by indigenous people, will shift to below-ground stored freshwater (Artic Science Journeys Radio Stories 2005). Globally, however, freshwater will become a more abundant ES due to reductions permafrost and subsequently ice sheets and glaciers resulting from the regime’s feedbacks to climate change. Nevertheless, this global increase in the freshwater ES is not a positive one: “Increased freshwater delivery to the Arctic Ocean from reductions in ice sheets and glaciers result in rising sea level (...)” (Hinzman et al. 2005). At a local scale traditional hunting and fishing practices will be impaired (Vincent et al. 2013).  Arctic freshwater systems provide important migratory routes for fish stock. Due to this regime shit, those routes would be greatly altered in connectivity among lakes and river channels, as well as in terms of their physical coupling to the coastal marine ecosystem (Vincent et al. 2012). Globally, fishing practices would suffer as well. Increased discharge of water into the Arctic Ocean due to draining results in increased nutrient and external organic matter inputs to the Arctic Ocean that affect primary processes at the base of the marine food web (Vincent et al. 2012). Climate regulation is heavily affected on a global scale through the processes of this regime shift: as it sets up a positive feedback cycle in which the release of methane and carbon dioxide through the newly exposed soil and melted permafrost feeds into global warming. The shift from lakes to terrestrial ecosystems heightens gully erosion, a type of erosion that occurs when water is channeled across unprotected land and washes away the soil along the drainage lines. As permafrost degrades further, it causes erosion and slumping of lake edges and stream channels

The Arctic lakes are of vital importance to indigenous people as they are a source of fresh drinking water and also provides important winter transport routes. These lakes not only allows indigenous people access to their traditional hunting and fishing areas, but also to transport goods to remote communities and industries such as mining centers (Vincent et al. 2013). The regime shift would also mean an increase in products extracted from woods such as timber commodities and fuel: “increased areas of tree growth in the Arctic could serve to (…) take supply more wood products and related employment, providing local and global economic benefits” (Hassol et al. 2004).

 

Management options

There are few management options available to maintain the thermokarst lake ecosystems due to the long time scale at which permafrost melts and the natural dynamics within this system e.g. hydraulic fluctuations or site-specific soil conditions. One management option is to continue to reduce the emissions of greenhouse gases while simultaneously investigating further opportunities for carbon offsetting schemes.

For hydroelectric reservoirs, shifting ice conditions will have both positive and negative effects, and may require adaptive changes in operating procedures, with attention to minimize negative impacts associated with ice jams and ice breakup downstream of the spillway (Vincent et al. 2013). Fisheries management plans will also need to be adapted to the changes in migration and productivity of northern fish populations with ongoing climate change (Vincent et al. 2013). These essential resources require the development of integrated freshwater management plans, which include consideration of alternate water sources as traditional supplies change in quantity or quality (Vincent et al. 2011). The Arctic will require increasing vigilance and appropriate water management strategies to avoid and minimize the impacts of changing water impacts in the future (Vincent et al. 2013).

Alternate regimes

The boreal forest in northern Interior Alaska covers an approximate area of 48 million hectares (Mann et al. 2012). Bounded by the Alaska Range to the south, the Seward Peninsula to the west and Brooks Range to the north (Wolken et al. 2011). It is characterized by large areas of gently sloping uplands and flat lowlands isolated by mountain ranges and braided rivers with broad floodplains. During the past 50 years, the mean annual temperature in the interior boreal forest of Alaska has increased by 1.3 ºC (Hartmann and Wendler, 2005) and this has been strongly associated with recent increase in fire frequency and severity (Wolken et al. 2011). The annual area burned in Interior Alaska has doubled during the last decade (Kasischke et al. 2010). The forest has been dominated by coniferous trees (mainly black spruce), which is a group of species that thrive under colder, moist conditions in a deep soil organic layer. However, due to climate warming, Interior Alaska is currently shifting towards a deciduous dominated forest (e.g. aspen and birch) that prefer warmer and drier conditions with a shallow soil organic layer (Johnstone et al. 2010).  

 

 

Low fire frequency coniferous dominated forest

Under cold and wet conditions, coniferous trees, such as the black spruce (Picea mariana), dominate the boreal forest in Interior Alaska. The coniferous trees promote the accumulation of a deep soil organic layer, cold and moist soil temperatures, and slow decomposition rates. Due to the moistness of the soil, fires are not frequent in these forests. However, when they occur, they tend to be severe due to the high flammability of the black spruce. Due to the high water content of the soil organic layer, it will only be entirely consumed by the most severe fires (Johnstone et al. 2010). As long as the entire organic layer has not been consumed, the coniferous trees have a regenerative advantage due to their seed’s ability to sprout in moist organic soils. This means that these forests tend to return to a coniferous dominated state even after a fire (Hollingsworth et al. 2013).

 

High fire frequency deciduous dominated forest

A deciduous dominated forest, mainly consisting of aspen and birch trees in Interior Alaska, prefers and reinforces dry and warm soil conditions. Although these forests have a high organic litter production, the fast decomposition rate and the subsequent fast nutrient turnover, leaves a shallow soil organic layer and a nutrient rich mineral soil that favours deciduous tree regeneration. The warm and dry conditions in the forest lead to a comparatively high fire frequency, but due to the low flammability of the deciduous trees, the fires rarely become very severe (Johnstone et al. 2010). However, because of the shallow pre-fire soil organic layer, the fires tend to expose the mineral soil, which favours the post-fire reproduction of deciduous trees (Hollingsworth et al. 2013).

 

Drivers and causes of the regime shift

The main external direct driver of the regime shift is climate warming, which affects the important feedbacks of the boreal forest ecosystem. Warmer and drier climatic conditions have a direct effect on the most important soil characteristics (i.e. soil temperature, moisture and depth of the soil organic layer), influencing the vegetation cover. Coniferous trees require a cold moist soil with a deep soil organic layer to successfully reproduce. Deciduous trees, on the other hand, are competitively superior under warmer and drier soil conditions with a shallow soil organic layer (Johnstone et al. 2010). Extended summer seasons and the accompanying drier conditions create a suitable environment for an increase in the annual number of natural ignitions by lightning strikes (Kasischke et al. in press). Fires have an immediate effect on the environmental conditions of the forest, affecting factors such as soil moisture and the post-fire forest community composition (Kelly et al. 2012). Once established, the trees support and emphasize the conditions that favour their own reproduction and dominance. Under colder environmental conditions, coniferous trees, such as the black spruce, produce large amounts of organic litter, which, together with the presence of permafrost, maintains a moist soil and slow decomposition rates, contributing to a deep soil organic layer. Under warmer climatic conditions, deciduous trees produce ground layer fuels that decompose at fast rates, maintaining a shallow organic layer (Johnstone et al. 2010). Global socio-economic development indirectly drives climate warming, contributing to the increased atmospheric temperatures through e.g. greenhouse gas emissions (IPCC, 2011). Fire management can have a limited effect on the regime shift through fire suppression practices.

How the regime shift works

Shift from coniferous to deciduous-dominated forest

The coniferous dominated boreal forest of Interior Alaska requires a deep soil organic layer to succeed over other classes of trees. Once the required depth is present, this has a strong reinforcing effect on the coniferous forest. This is a requisite to maintain cold moist soil conditions in which coniferous trees thrive. The cold soil temperature reduces the decomposition rate of organic litter, maintaining a deep soil organic layer (Johnstone et al. 2010).

 

External shocks such as severe and frequent fires or insect outbreaks might cause dramatic shifts in the soil and vegetation by impacting the soil organic layer depth and tree succession (Hollingsworth et al. 2013; McCullough et al. 1998; Wolken et al. 2011). If the severity of such external shocks is sufficient, thresholds in the most important regime characteristics will be crossed (e.g. shallow post-shock soil organic layer) creating windows of opportunity for deciduous species to dominate. Atmospheric temperature influences the frequency and severity of fires, the soil characteristics and the length of the growing season; therefore, climate warming can also cause a change in conditions that would set the grounds for a shift to a new stable state (Mann et al. 2012).

 

The deciduous dominated forest has a competitive advantage in warm and dry conditions with shallow soil organic layers. Again, once the required shallow layer is present, it has a strong reinforcing effect on the deciduous forest. A shallow soil organic layer maintains warm and dry soil conditions that speed up the decomposition of organic litter, maintaining the required depth of soil organic layer (Johnstone et al. 2012).

 

Shift from deciduous-dominated  to coniferous forest 

In order to shift from regime 2 to regime 1, a prolonged climatic cooling would be necessary. A slow change of the environmental conditions towards those that favour a coniferous boreal forest could, if the forest is left undisturbed for a sufficiently long time, lead to late successional coniferous species, like the black spruce, replacing early successional deciduous species (Rupp et al. 2002). Although the spatial scale considered is of extreme importance, sufficiently large external shocks, such as deciduous-specific insect outbreaks, could clear sufficiently large areas to advance the dominance of coniferous trees. In such hypothetical events, coniferous trees could get the opportunity to sprout and start altering the soil conditions to their own favour.

Impacts on ecosystem services and human well-being

The provisioning of wild products such as berries and game is expected to shift some species and decrease others, but it is unclear what the net changes will be (Chapin et al. 2008). The boreal forest is likely to become a net source of carbon to the atmosphere (McGuire et al. 2009; Schuur et al. 2009), while the air quality is expected to decrease periodically due to the increase in fire frequency (Chapin et al. 2008).

Management options

The primary tactic used for reducing impacts of fire and enhance resilience (i.e. maintain structures, feedbacks, functions and processes of regime 1 is fire suppression to reduce the fire severity (Chapin et al. 2008). Decrease in fire severity could prevent the fire from consuming the soil organic layer, which in turn favours the conditions for coniferous tree. Fire suppression should be maintained in Alaska since it minimizes the risk fire imposes to life and property in communities. However, distant fires should be allowed to burn under controlled conditions, since it could have a cooling effect at a regional scale due to the higher albedo of post-fire deciduous species compared to the previous coniferous forest (Chapin et al. 2008).

Reducing air temperature is essential to be able to restore regime 1. Alaska itself accounts for a miniscule proportion of global greenhouse gas emissions and consequently to climate warming (Chapin et al. 2008). Therefore, reducing Alaska’s emissions will not have a major impact on global climate change. Global abatement of emissions is the long-term solution. Regardless of global greenhouse gas emission policies, it is highly likely that recent warming and wildfire frequency will continue for several decades in Alaska due to the multidecadal lag in the climate system (IPCC, 2007). This makes the reduction of emissions an inefficient short-term solution.  Secondary challenges that face efforts to achieve reduced emissions include restricting the economy and changing the focus of developed nations on a continued economic but sustainable growth. Educating global public about the social impacts of climate change in Alaska and other parts of the world (e.g. reduction in ecosystem services) may promote the willingness to reduce emissions (Chapin et al. 2008).

In summary, this means that the regime shift from coniferous to deciduous dominated forest in theory could be reversible through manipulation of e.g. soil conditions or local climate. However, due to projected global socio-economic development and climate change, it is highly unlikely that reversing the regime shift will be possible during the next 100 years.

Alternate regimes

This regime shift involves an abrupt reorganization of an aquatic food web due to a decrease in top predators or increase in environmental factors like temperature. Trophic cascades play a central role in these regime shifts. During these cascades groups in adjacent trophic levels often show inverse patterns in their abundance (Carpenter 2003). In other words, every other trophic level displays an increase in biomass, which leads to increased predation pressure and biomass decrease on every other trophic level. Most trophic cascades are described in ecosystems with low species diversity and/or simple food webs and/or small size (Frank et al. 2005). The magnitude of a trophic cascade varies depending on species diversity, regional oceanography, local physical disturbance, habitat complexity and fishery practices (Salomon et al. 2010). Food webs with more trophic levels and biodiversity show less trophic cascades than simpler systems (Salomon et al. 2010); due in part to omnivory and the variation on trophic interaction strength (Bascompte et al. 2005). However some documented examples includes subtidal reefs in New Zealand (Shears and Babcock 2002), Caribbean coral reefs (Bellwood et al. 2004), the Gulf of Maine (Steneck et al. 2004), and the North Atlantic (Kirby et al. 2009).

Predator-dominated food web.

Ecosystems in this regime are characterized by high predatory fish, low planktivorous fish, high zooplankton and low phytoplankton abundance. Such ecosystems can be subject to human influence, such as intensive fishery, but exhibit so called “natural compensation” e.g. via species richness and omnivory (Pace et al. 1999) that maintain the regime. Despite high productivity, such ecosystems are less likely to suffer from eutrophication due to high grazing pressure on phytoplankton. Ecosystem services associated with predator-dominated food webs includes food provision, higher biodiversity, better disease and pest control, as well as cultural services like recreation and aesthetic values (e.g. diving, sport fishing).

Planktivore-dominated food web.

Ecosystems in this regime are characterized by low predatory fish, high planktivorous fish, low zooplankton and high phytoplankton abundance. In extreme cases the actual number of trophic levels (TLs) can decrease. Ecosystems with lower trophic level dominance are more vulnerable to eutrophication due to lowered grazing pressure on phytoplankton, as well as invasion of planktivore organisms like jellyfish. A planktivore-dominated food web can affect the fluxes of carbon, transforming aquatic systems -coastal and lakes- from sinks to sources of green house gases (Bakun et al. 2010; Estes et al. 2011). Other ecosystem services like fisheries and recreation are expected to be significantly reduced.

Drivers and causes of the regime shift

Trophic cascades are usually initiated by fishing pressure that have direct or indirect impacts on the abundance of top predators. The response time to these drivers varies across ecosystems. In the case of coastal areas it has been found that the ecosystem response time to overfishing can vary from decades to centuries where multiple predators exist (Jackson et al. 2001).

Other factors that affect the regime shift are global warming and demand of food and fiber. Global warming have effects on upwelling, water circulatory systems that bring nutrients from the bottom of the sea towards the surface. By altering upwellings, food webs receive either too much or too little nutrients. In both cases the energy transferred towards higher trophic levels diminish as nutrients input variability increase. Demand of food strongly drives fishing, being the main cause of lost of top predators over the world.

How the regime shift works

Predator dominated food webs occur in environments with low disturbance both from fishing and nutrients input. They usually have four or more trophic levels. The regime is maintained by biotic mechanism that comprise predation and competition both intra and inter species. Abiotic factors also play a role maintaining the regime, by allowing an intermediate level of nutrient inputs from the deep ocean necessary for sustaining the food web.

In general, a combination of low top predator productivity due to poor recruitment or somatic growth combined with high exploitation pressure are conditions under which regime shifts in aquatic food webs are likely to occur (Daskalov et al. 2007). Overfishing can unbalance trophic relations, changing the configuration of competition and predation. Weak competitors may become stronger given less fishing pressure on such stocks, or by favoring the reproduction of it’s preys by fishing its competitor species. In addition, changes in nutrients inputs can represent an advantage for lower trophic levels, allowing them to escape the control of its predators.

As result, the ecological function of certain species groups is changed, allowing over abundance of immediate lower level. For example, the decrease of top-predators allows over abundance of meso predators that in turn reduces planktivorous fish and increase zooplankton. Further increase in nutrients from upwellings can lock the food web in a state dominated by planktivore fish as jellyfish. Low nutrients input, on the other hand, would reduce the amount of energy available for the food web and as a result an abrupt reduction of higher trophic levels is expected.

Impacts on ecosystem services and human well-being

Shift from predators to planktivore dominated food webs

Commercial fish stocks that are locked into a regime where they are maintained at low levels of abundance has obvious economic consequences on fisheries industry. However, these fisheries related shifts are not always negative, as for example a decrease in top predators can increase the catches of mid-level prey (e.g. shrimps and clupeids) (Frank et al. 2005). When an ecosystem-wide regime shift occurs it affects not only targeted groups, such as commercial fish (Daskalov et al. 2007) but also impacts other ecosystem functions. Biodiversity as well as ecosystem resilience may be reduced by a shift from an ecosystem with high top predator diversity and dominance to a system with lower trophic level dominance. This means that ecosystem may become more vulnerable to both climatic and anthropogenic change. Such ecosystems may more easily suffer from eutrophication, hypoxia and invasion by non-native species. Eutrophication and hypoxia in turn lead to increased algal blooms and decrease the recreational values of water bodies.

Predators and commercial fish stocks have remained under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the reduction of food webs, which in turn increases the risk of fisheries collapse. Depletion of fish stocks has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Hassan et al. 2005). Fish catches are projected to further decrease in the 21st century affecting protein sources for people, especially in poor regions (Hassan et al. 2005). The estimated current contribution of fisheries to human protein consumption is 29 million tons produced industrially and 24 million tons in small-scale fisheries (Hassan et al. 2005).

Management options

Options for enhancing resilience

In food webs that exhibit strong top-down control, decreasing the fishing pressure on top predators is an obvious strategy (Scheffer et al. 2001; Frank et al. 2005). However, due to the hysteresis effect, return to a predator-dominated food web may often require substantially lower fishing levels than that which induced the regime shift, together with additional measures. Hence, restoration is usually costly, especially to fisherman (Moellmann et al. 2009). As mentioned above, environmental factors can play an important role in maintaining alternative food web regimes. Nutrient loading is one of the key parameters controlling processes such as eutrophication and hypoxia. However, decreasing nutrient loads in marine environments requires large scale actions and often collaboration at international level (e.g. Helsinki Commission Baltic Sea Action Plan in the Baltic Sea). Hence, the time-frame of these actions in addition to time-lag in ecosystem response is often long. Climatic variation cannot be directly controlled by human actions. However, climate impacts can be mediated by employing other management measures that contribute to maintaining healthy marine ecosystems with high biodiversity and resilience, which then can compensate for changes in climate.

Options for reducing resilience to encourage restoration or transformation

In addition, in small closed food webs systems (3 to 4 trophic levels), biomanipulation has been suggested to manage one of the symptoms of degraded marine environments, namely eutrophication. This involves increasing the population of predatory fish such as bass, pike and walleye through stocking or reduced angling quotas. Increased populations of these predators leads to a decrease in the level of zooplanktivores. This in turn allows an increase in the population of planktivores that graze on the algae and zooplanckton, helping to reduce the algal density and phytoplankton respectively(Smith and Schindler 2009). This option, however, has limited reach when it comes to open marine foodwebs, where seasonal migrations and metapopulation dynamics are common.

Key References

  1. Ainley, D. G., and L. K. Blight. 2009. Ecological repercussions of historical fish extraction from the Southern Ocean. Fish And Fisheries 10:13–38.
  2. Allesina, S., and M. Pascual. (n.d.). Googling Food Webs: Can an Eigenvector Measure Species’ Importance for Coextinctions? PLoS Computational Biology.
  3. Bakun, A., D. Field, A. Redondo-Rodriguez, and S. Weeks. 2010. Greenhouse gas, upwelling-favorable winds, and the future of coastal ocean upwelling ecosystems. Global Change Biology 16:1213–1228.
  4. Bascompte, J., C. Melian, and E. Sala. 2005. Interaction strength combinations and the overfishing of a marine food web. Proceedings Of The National Academy Of Sciences Of The United States Of America 102:5443–5447.
  5. Beaugrand, G. 2004. The North Sea regime shift: evidence, causes, mechanisms and consequences. Progress in Oceanography 60:245–262.
  6. Behrenfeld, M. J., R. T. O'Malley, D. A. Siegel, C. R. Mcclain, J. L. Sarmiento, G. C. Feldman, A. J. Milligan, P. G. Falkowski, R. M. Letelier, and E. S. Boss. 2006. Climate-driven trends in contemporary ocean productivity. Nature 444:752–755.
  7. Bellwood, D., T. Hughes, C. Folke, and M. Nyström. 2004. Confronting the coral reef crisis. Nature 429:827–833.
  8. Berkes, F., T. Hughes, R. Steneck, J. Wilson, D. Bellwood, B. Crona, C. Folke, L. Gunderson, H. Leslie, J. Norberg, M. Nystrom, P. Olsson, H. Osterblom, M. Scheffer, and B. Worm. 2006. Ecology - Globalization, roving bandits, and marine resources. Science 311:1557–1558.
  9. Carpenter, S. R. 2003. Regime shifts in lake ecosystems. Ecology Institute.
  10. Daskalov, G. M., A. N. Grishin, S. Rodionov, and V. Mihneva. 2007. Trophic cascades triggered by overfishing reveal possible mechanisms of ecosystem regime shifts. Proceedings Of The National Academy Of Sciences Of The United States Of America 104:10518–10523.
  11. Dunne, J., and R. Williams. 2004. Network structure and robustness of marine food webs. Marine Ecology Progress Series.
  12. Estes, J., J. Terborgh, J. Brashares, and M. Power. 2011. Trophic Downgrading of Planet Earth. Science.
  13. Frank, K. T., B. Petrie, J. S. Choi, and W. C. Leggett. 2005. Trophic Cascades in a Formerly Cod-Dominated Ecosystem. Science 308:1621–1623.
  14. Jackson, J., M. Kirby, W. Berger, K. Bjorndal, L. Botsford, B. Bourque, R. Bradbury, R. Cooke, J. Erlandson, J. Estes, T. Hughes, S. Kidwell, C. Lange, H. Lenihan, J. Pandolfi, C. Peterson, R. Steneck, M. Tegner, and R. Warner. 2001. Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science 293:629–637.
  15. Kirby, R. R., G. Beaugrand, and J. A. Lindley. 2009. Synergistic Effects of Climate and Fishing in a Marine Ecosystem. Ecosystems 12:548–561.
  16. M Hassan, R., R. Scholes, and N. Ash. 2005. Ecosystems and human well-being: current state and trends, Volume 1 1:917.
  17. Moellmann, C., R. Diekmann, B. Muller-Karulis, G. Kornilovs, M. Plikshs, and P. Axe. 2009. Reorganization of a large marine ecosystem due to atmospheric and anthropogenic pressure: a discontinuous regime shift in the Central Baltic Sea. Global Change Biology 15:1377–1393.
  18. Pace, M., J. Cole, S. Carpenter, and J. Kitchell. 1999. Trophic cascades revealed in diverse ecosystems. Trends in Ecology & Evolution 14:483–488.
  19. Pauly, D., V. Christensen, J. Dalsgaard, R. Froese, and F. Torres. 1998. Fishing down marine food webs. Science 279:860–863.
  20. Salomon, A. K., S. K. Gaichas, N. T. Shears, J. E. Smith, E. M. P. Madin, and S. D. Gaines. 2010. Key Features and Context-Dependence of Fishery-Induced Trophic Cascades. Conservation Biology 24:382–394.
  21. Scheffer, M., S. Carpenter, J. Foley, C. Folke, and B. Walker. 2001. Catastrophic shifts in ecosystems. Nature 413:591–596.
  22. Shears, N., and R. Babcock. 2002. Marine reserves demonstrate top-down control of community structure on temperate reefs. Oecologia 132:131–142.
  23. Smith, V. H., and D. W. Schindler. 2009. Eutrophication science: where do we go from here? Trends in Ecology & Evolution 24:201–207.
  24. Steneck, R., J. Vavrinec, and A. Leland. 2004. Accelerating trophic-level dysfunction in kelp forest ecosystems of the western North Atlantic. Ecosystems 7:323–332.

Citation

Susa Niiranen, Reinette (Oonsie) Biggs, Garry Peterson, Juan Carlos Rocha, Henrik Österblom. Marine food webs: community change and trophic level decline. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-30 09:57:23 GMT.
Tuesday, 14 October 2014 13:09

Fisheries collapse

Written by Juan Carlos

Fisheries collapse

Main Contributors:

Garry Peterson, Juan Carlos Rocha

Other Contributors:

Henrik Österblom, Reinette (Oonsie) Biggs

Summary

A fishery collapses when the structure of the marine community (i.e. its species composition) changes radically, trapping the fishery into a regime in which high-valued commercial species cannot recover. These dynamics are often characterized by cascading effects across multiple trophic levels in marine food webs. Both top-down and bottom-up drivers contribute to the collapse of commercial fisheries. Overfishing is the main top-down driver, and is associated with indirect drivers that maintain fishing effort despite variation on fisheries demand, such as number of fishing boats in a fleet and fishing quotas that are insensitive to stock variation, as well as indirect drivers which increase fishing effort, such as demand from new markets, new possibilities to export fish, and technology improvements. The chief bottom-up drivers of collapse are drivers that influence the productivity of the base of marine food web. These include both anthropogenic and natural climate change that can shift the intensity and frequency of upwelling of cool nutrient rich water. Other factors, such as diseases spread, changes in ocean circulation, winds and temperature variation can act as synergistic factors contributing to collapses. The collapse of a commercial fishery can have substantial economic and social impacts.

Drivers

Key direct drivers

  • Harvest and resource consumption
  • External inputs (eg fertilizers)
  • Adoption of new technology
  • Species introduction or removal
  • Disease
  • Global climate change

Land use

  • Fisheries

Impacts

Ecosystem type

  • Marine & coastal

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries

Cultural services

  • Recreation
  • Aesthetic values
  • Knowledge and educational values

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values
  • Cultural identity

Key Attributes

Typical spatial scale

  • Local/landscape
  • National (country)
  • Sub-continental/regional

Typical time scale

  • Years
  • Decades

Reversibility

  • Irreversible (on 100 year time scale)
  • Hysteretic

Evidence

  • Models
  • Paleo-observation
  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Contested – Reasonable evidence both for and against the existence of RS

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Links to other regime shifts

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Alternate regimes

A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.  

With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.

Arctic benthos (regime 1)

With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.

Subarctic benthos (regime 2)

After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).

 

Drivers and causes of the regime shift

The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.

The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).

 

How the regime shift works

In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)

Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.

Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions. 

Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.

 

Impacts on ecosystem services and human well-being

Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.

The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being. 

 

Management options

Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.

Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.

 

Alternate regimes

Due to thermokarst processes such as permafrost melting, numerous areas of the Arctic landscape are extremely lake-rich.  The water balance of those lakes may either be maintained due to impermeable permafrost that limits groundwater flow (Marsh 2009) or lost due to seasonal drainage, a natural characteristic of the Arctic system. However, a widespread, unnatural decline in lake abundance is being observed within the Arctic due to decades of rising atmospheric temperatures in the region that has been deteriorating the state of the permafrost (Smith et al. 2005). The lake-rich landscape of the Arctic is therefore slowly transforming into a drained, dry landscape increasingly dominated by vegetation such as shrubs and graminoids (Hinzman 2005).

 

Thermokarst lake ecosystem

 

The thermokarst lake-dominated landscape is formed as a response to some imposed disturbance, such as road construction, wildfire or climatic warming, which causes permafrost to thaw, creating irregular surface topography depressions, called thermokarsts. Those may appear as near-surface massive ice melts, allowing the surface to subside (Hinzman et al. 2005). As permafrost melts over time, those depressions become filled with melted water forming the lakes characteristic of the Arctic region.

 

Terrestrial ecosystem

 

Driven by external and internal factors, Arctic lakes are drying up at an unnatural speed within the region. This shift is accompanied by changes in vegetation, for example, increased shrub- and graminord abundance and a decrease in the cover of mosses and lichens through a broad expanse of the Arctic (Hinzman et al. 2005). The changes in vegetation may then further transform the landscape through sequential effects on foraging mammals and birds, as well as aquatic fauna and insects (Hinzman et al 2005).

 

Drivers and causes of the regime shift

 

Shift from thermokarst lakes to terrestrial ecosystem

The main external driver of this regime shift is climate change, which is caused by elevated greenhouse gas emissions (IPCC 2007). Increased temperatures warm the soil and warms the permafrost. As the permafrost thaws, greenhouse gasses such as CO2 and CH4 that are trapped in the ice are released into the atmosphere, further increasing the effects of climate change (Hinzman 2005; Karlsson et al. 2011). At a local scale, the effects of climate change are incremental. However, as this interaction is occurs over large areas, the effects are aggregated and self-reinforcing, which is significant since permafrost and seasonally frozen grounds store about 25% of the total global soil carbon stock (IPCC 2007).

At a local scale, initial thawing of the permafrost from increased atmospheric temperatures creates a dynamic landscape of depressions (thermokarst) in which lakes are formed (IPCC 2007). As permafrost become more degraded, drainage through outlet channels occurs. Since the temperature of melted water is higher than that of ice, it will exacerbate the melting of permafrost, causing another reinforcing feedback. Once the layer of frozen ground is completely penetrated the lakes can become permanently drained allowing for vegetation to be established (Marsh et al. 2008).

Shift from terrestrial ecosystem to thermokarst lake ecosystem

Heightened air- and soil temperatures melt the ice-rich soils and permafrost on which forests in the Arctic region develop, changing the physical conditions for the forest growing on top of it. When the roots of the trees get flooded, the trees die and ponds and lakes eventually replace the forest (Hinzman 2005). However, it is important to note that the shift from a terrestrial ecosystem to a lake dominated one is not as frequent as the shift from lakes to terrestrial ecosystems. The reason for this are the reinforcing feedbacks as mentioned above.

How the regime shift works

The warming of soil temperatures leads to the melting of continuous permafrost and the subsequent increase in abundance of discontinuous or sporadic permafrost. Melting of permafrost causes depressions in the soil called thermokarsts. Increased precipitation in the form of snowfall may have either an insulating or cooling effect on soil temperatures depending on several variables such a timing, duration, accumulation, and melting processes of seasonal snow cover, density, structure, and thickness of seasonal snow cover etc. (Zhang 2005). Depending upon the local surface energy balance, the thawed ground may refreeze or the permafrost can continue to degrade (Hinzman et al. 2005). On sites where continuous permafrost is thawing the active layer of soil in which melted water can move deepens allowing for the formation of thermokarst lakes (Hinzman et al. 2005). In freezing seasons, if the temperature is low enough and the hydraulic parameters on site suitable, the formation of these lakes can contribute to the re-formation of permafrost soils. Ice-rich permafrost can also prevent surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet. Decreased albedo due to changes in vegetation, the extension of snow-free and ice-free periods on terrestrial and lake surfaces and abundance, and reduction in the area occupied by glaciers and continental ice sheets in high latitudes may act as a positive feedback into atmospheric warming (Hinzman et al. 2005).

On sites where there is an abundance of discontinuous permafrost the active layer also deepens allowing for increased surface area for sub-ground water movement that increases the rate of drainage. Thermokarst lakes and ponds may begin to fill or drain depending upon the direction of the hydraulic gradient beneath the lake (Hinzman et al. 2005). Therefore it is not the depth of the active layer alone that determines the rate of drainage. However, increased drainage of lakes may prevent the reformation of permafrost from lack of available water for freezing in winter months and it can be inferred through advection of heat from decreased albedo. Once these drained lake beds become permanently drained they are likely to be re-vegetated which will change the albedo and may cause the extension of snow-free and ice-free periods and may act as feedback into increasing atmospheric temperature (Hinzman et al. 2005).

The affects of the shifts from lake to terrestrial ecosystems are aggregated at a local, regional and global scale due to the large scale and frequency at which those dynamics occur. The whole regime as described in the causal loop diagram below is a re-enforcing feedback that at its current state is leading to an overall decrease in lake dominated ecosystems and a increase in terrestrial ecosystems (Smith et al. 2005).

Impacts on ecosystem services and human well-being

Even though this regime shift occurs at the local scale, its consequences go beyond that scale because it reshapes regional hydrology (through changes in water balance and surface water- connection and fragmentation) and alters regional supplies of the ecosystem (Karlsson et al. 2011). This regime shift therefore affects provisioning ecosystem services, as it will decrease the availability of freshwater. The above-ground stored freshwater, which is of vital importance to migratory birds, fish, and other wildlife used by indigenous people, will shift to below-ground stored freshwater (Artic Science Journeys Radio Stories 2005). Globally, however, freshwater will become a more abundant ES due to reductions permafrost and subsequently ice sheets and glaciers resulting from the regime’s feedbacks to climate change. Nevertheless, this global increase in the freshwater ES is not a positive one: “Increased freshwater delivery to the Arctic Ocean from reductions in ice sheets and glaciers result in rising sea level (...)” (Hinzman et al. 2005). At a local scale traditional hunting and fishing practices will be impaired (Vincent et al. 2013).  Arctic freshwater systems provide important migratory routes for fish stock. Due to this regime shit, those routes would be greatly altered in connectivity among lakes and river channels, as well as in terms of their physical coupling to the coastal marine ecosystem (Vincent et al. 2012). Globally, fishing practices would suffer as well. Increased discharge of water into the Arctic Ocean due to draining results in increased nutrient and external organic matter inputs to the Arctic Ocean that affect primary processes at the base of the marine food web (Vincent et al. 2012). Climate regulation is heavily affected on a global scale through the processes of this regime shift: as it sets up a positive feedback cycle in which the release of methane and carbon dioxide through the newly exposed soil and melted permafrost feeds into global warming. The shift from lakes to terrestrial ecosystems heightens gully erosion, a type of erosion that occurs when water is channeled across unprotected land and washes away the soil along the drainage lines. As permafrost degrades further, it causes erosion and slumping of lake edges and stream channels

The Arctic lakes are of vital importance to indigenous people as they are a source of fresh drinking water and also provides important winter transport routes. These lakes not only allows indigenous people access to their traditional hunting and fishing areas, but also to transport goods to remote communities and industries such as mining centers (Vincent et al. 2013). The regime shift would also mean an increase in products extracted from woods such as timber commodities and fuel: “increased areas of tree growth in the Arctic could serve to (…) take supply more wood products and related employment, providing local and global economic benefits” (Hassol et al. 2004).

 

Management options

There are few management options available to maintain the thermokarst lake ecosystems due to the long time scale at which permafrost melts and the natural dynamics within this system e.g. hydraulic fluctuations or site-specific soil conditions. One management option is to continue to reduce the emissions of greenhouse gases while simultaneously investigating further opportunities for carbon offsetting schemes.

For hydroelectric reservoirs, shifting ice conditions will have both positive and negative effects, and may require adaptive changes in operating procedures, with attention to minimize negative impacts associated with ice jams and ice breakup downstream of the spillway (Vincent et al. 2013). Fisheries management plans will also need to be adapted to the changes in migration and productivity of northern fish populations with ongoing climate change (Vincent et al. 2013). These essential resources require the development of integrated freshwater management plans, which include consideration of alternate water sources as traditional supplies change in quantity or quality (Vincent et al. 2011). The Arctic will require increasing vigilance and appropriate water management strategies to avoid and minimize the impacts of changing water impacts in the future (Vincent et al. 2013).

Alternate regimes

The boreal forest in northern Interior Alaska covers an approximate area of 48 million hectares (Mann et al. 2012). Bounded by the Alaska Range to the south, the Seward Peninsula to the west and Brooks Range to the north (Wolken et al. 2011). It is characterized by large areas of gently sloping uplands and flat lowlands isolated by mountain ranges and braided rivers with broad floodplains. During the past 50 years, the mean annual temperature in the interior boreal forest of Alaska has increased by 1.3 ºC (Hartmann and Wendler, 2005) and this has been strongly associated with recent increase in fire frequency and severity (Wolken et al. 2011). The annual area burned in Interior Alaska has doubled during the last decade (Kasischke et al. 2010). The forest has been dominated by coniferous trees (mainly black spruce), which is a group of species that thrive under colder, moist conditions in a deep soil organic layer. However, due to climate warming, Interior Alaska is currently shifting towards a deciduous dominated forest (e.g. aspen and birch) that prefer warmer and drier conditions with a shallow soil organic layer (Johnstone et al. 2010).  

 

 

Low fire frequency coniferous dominated forest

Under cold and wet conditions, coniferous trees, such as the black spruce (Picea mariana), dominate the boreal forest in Interior Alaska. The coniferous trees promote the accumulation of a deep soil organic layer, cold and moist soil temperatures, and slow decomposition rates. Due to the moistness of the soil, fires are not frequent in these forests. However, when they occur, they tend to be severe due to the high flammability of the black spruce. Due to the high water content of the soil organic layer, it will only be entirely consumed by the most severe fires (Johnstone et al. 2010). As long as the entire organic layer has not been consumed, the coniferous trees have a regenerative advantage due to their seed’s ability to sprout in moist organic soils. This means that these forests tend to return to a coniferous dominated state even after a fire (Hollingsworth et al. 2013).

 

High fire frequency deciduous dominated forest

A deciduous dominated forest, mainly consisting of aspen and birch trees in Interior Alaska, prefers and reinforces dry and warm soil conditions. Although these forests have a high organic litter production, the fast decomposition rate and the subsequent fast nutrient turnover, leaves a shallow soil organic layer and a nutrient rich mineral soil that favours deciduous tree regeneration. The warm and dry conditions in the forest lead to a comparatively high fire frequency, but due to the low flammability of the deciduous trees, the fires rarely become very severe (Johnstone et al. 2010). However, because of the shallow pre-fire soil organic layer, the fires tend to expose the mineral soil, which favours the post-fire reproduction of deciduous trees (Hollingsworth et al. 2013).

 

Drivers and causes of the regime shift

The main external direct driver of the regime shift is climate warming, which affects the important feedbacks of the boreal forest ecosystem. Warmer and drier climatic conditions have a direct effect on the most important soil characteristics (i.e. soil temperature, moisture and depth of the soil organic layer), influencing the vegetation cover. Coniferous trees require a cold moist soil with a deep soil organic layer to successfully reproduce. Deciduous trees, on the other hand, are competitively superior under warmer and drier soil conditions with a shallow soil organic layer (Johnstone et al. 2010). Extended summer seasons and the accompanying drier conditions create a suitable environment for an increase in the annual number of natural ignitions by lightning strikes (Kasischke et al. in press). Fires have an immediate effect on the environmental conditions of the forest, affecting factors such as soil moisture and the post-fire forest community composition (Kelly et al. 2012). Once established, the trees support and emphasize the conditions that favour their own reproduction and dominance. Under colder environmental conditions, coniferous trees, such as the black spruce, produce large amounts of organic litter, which, together with the presence of permafrost, maintains a moist soil and slow decomposition rates, contributing to a deep soil organic layer. Under warmer climatic conditions, deciduous trees produce ground layer fuels that decompose at fast rates, maintaining a shallow organic layer (Johnstone et al. 2010). Global socio-economic development indirectly drives climate warming, contributing to the increased atmospheric temperatures through e.g. greenhouse gas emissions (IPCC, 2011). Fire management can have a limited effect on the regime shift through fire suppression practices.

How the regime shift works

Shift from coniferous to deciduous-dominated forest

The coniferous dominated boreal forest of Interior Alaska requires a deep soil organic layer to succeed over other classes of trees. Once the required depth is present, this has a strong reinforcing effect on the coniferous forest. This is a requisite to maintain cold moist soil conditions in which coniferous trees thrive. The cold soil temperature reduces the decomposition rate of organic litter, maintaining a deep soil organic layer (Johnstone et al. 2010).

 

External shocks such as severe and frequent fires or insect outbreaks might cause dramatic shifts in the soil and vegetation by impacting the soil organic layer depth and tree succession (Hollingsworth et al. 2013; McCullough et al. 1998; Wolken et al. 2011). If the severity of such external shocks is sufficient, thresholds in the most important regime characteristics will be crossed (e.g. shallow post-shock soil organic layer) creating windows of opportunity for deciduous species to dominate. Atmospheric temperature influences the frequency and severity of fires, the soil characteristics and the length of the growing season; therefore, climate warming can also cause a change in conditions that would set the grounds for a shift to a new stable state (Mann et al. 2012).

 

The deciduous dominated forest has a competitive advantage in warm and dry conditions with shallow soil organic layers. Again, once the required shallow layer is present, it has a strong reinforcing effect on the deciduous forest. A shallow soil organic layer maintains warm and dry soil conditions that speed up the decomposition of organic litter, maintaining the required depth of soil organic layer (Johnstone et al. 2012).

 

Shift from deciduous-dominated  to coniferous forest 

In order to shift from regime 2 to regime 1, a prolonged climatic cooling would be necessary. A slow change of the environmental conditions towards those that favour a coniferous boreal forest could, if the forest is left undisturbed for a sufficiently long time, lead to late successional coniferous species, like the black spruce, replacing early successional deciduous species (Rupp et al. 2002). Although the spatial scale considered is of extreme importance, sufficiently large external shocks, such as deciduous-specific insect outbreaks, could clear sufficiently large areas to advance the dominance of coniferous trees. In such hypothetical events, coniferous trees could get the opportunity to sprout and start altering the soil conditions to their own favour.

Impacts on ecosystem services and human well-being

The provisioning of wild products such as berries and game is expected to shift some species and decrease others, but it is unclear what the net changes will be (Chapin et al. 2008). The boreal forest is likely to become a net source of carbon to the atmosphere (McGuire et al. 2009; Schuur et al. 2009), while the air quality is expected to decrease periodically due to the increase in fire frequency (Chapin et al. 2008).

Management options

The primary tactic used for reducing impacts of fire and enhance resilience (i.e. maintain structures, feedbacks, functions and processes of regime 1 is fire suppression to reduce the fire severity (Chapin et al. 2008). Decrease in fire severity could prevent the fire from consuming the soil organic layer, which in turn favours the conditions for coniferous tree. Fire suppression should be maintained in Alaska since it minimizes the risk fire imposes to life and property in communities. However, distant fires should be allowed to burn under controlled conditions, since it could have a cooling effect at a regional scale due to the higher albedo of post-fire deciduous species compared to the previous coniferous forest (Chapin et al. 2008).

Reducing air temperature is essential to be able to restore regime 1. Alaska itself accounts for a miniscule proportion of global greenhouse gas emissions and consequently to climate warming (Chapin et al. 2008). Therefore, reducing Alaska’s emissions will not have a major impact on global climate change. Global abatement of emissions is the long-term solution. Regardless of global greenhouse gas emission policies, it is highly likely that recent warming and wildfire frequency will continue for several decades in Alaska due to the multidecadal lag in the climate system (IPCC, 2007). This makes the reduction of emissions an inefficient short-term solution.  Secondary challenges that face efforts to achieve reduced emissions include restricting the economy and changing the focus of developed nations on a continued economic but sustainable growth. Educating global public about the social impacts of climate change in Alaska and other parts of the world (e.g. reduction in ecosystem services) may promote the willingness to reduce emissions (Chapin et al. 2008).

In summary, this means that the regime shift from coniferous to deciduous dominated forest in theory could be reversible through manipulation of e.g. soil conditions or local climate. However, due to projected global socio-economic development and climate change, it is highly unlikely that reversing the regime shift will be possible during the next 100 years.

Alternate regimes

This regime shift involves an abrupt reorganization of an aquatic food web due to a decrease in top predators or increase in environmental factors like temperature. Trophic cascades play a central role in these regime shifts. During these cascades groups in adjacent trophic levels often show inverse patterns in their abundance (Carpenter 2003). In other words, every other trophic level displays an increase in biomass, which leads to increased predation pressure and biomass decrease on every other trophic level. Most trophic cascades are described in ecosystems with low species diversity and/or simple food webs and/or small size (Frank et al. 2005). The magnitude of a trophic cascade varies depending on species diversity, regional oceanography, local physical disturbance, habitat complexity and fishery practices (Salomon et al. 2010). Food webs with more trophic levels and biodiversity show less trophic cascades than simpler systems (Salomon et al. 2010); due in part to omnivory and the variation on trophic interaction strength (Bascompte et al. 2005). However some documented examples includes subtidal reefs in New Zealand (Shears and Babcock 2002), Caribbean coral reefs (Bellwood et al. 2004), the Gulf of Maine (Steneck et al. 2004), and the North Atlantic (Kirby et al. 2009).

Predator-dominated food web.

Ecosystems in this regime are characterized by high predatory fish, low planktivorous fish, high zooplankton and low phytoplankton abundance. Such ecosystems can be subject to human influence, such as intensive fishery, but exhibit so called “natural compensation” e.g. via species richness and omnivory (Pace et al. 1999) that maintain the regime. Despite high productivity, such ecosystems are less likely to suffer from eutrophication due to high grazing pressure on phytoplankton. Ecosystem services associated with predator-dominated food webs includes food provision, higher biodiversity, better disease and pest control, as well as cultural services like recreation and aesthetic values (e.g. diving, sport fishing).

Planktivore-dominated food web.

Ecosystems in this regime are characterized by low predatory fish, high planktivorous fish, low zooplankton and high phytoplankton abundance. In extreme cases the actual number of trophic levels (TLs) can decrease. Ecosystems with lower trophic level dominance are more vulnerable to eutrophication due to lowered grazing pressure on phytoplankton, as well as invasion of planktivore organisms like jellyfish. A planktivore-dominated food web can affect the fluxes of carbon, transforming aquatic systems -coastal and lakes- from sinks to sources of green house gases (Bakun et al. 2010; Estes et al. 2011). Other ecosystem services like fisheries and recreation are expected to be significantly reduced.

Drivers and causes of the regime shift

Trophic cascades are usually initiated by fishing pressure that have direct or indirect impacts on the abundance of top predators. The response time to these drivers varies across ecosystems. In the case of coastal areas it has been found that the ecosystem response time to overfishing can vary from decades to centuries where multiple predators exist (Jackson et al. 2001).

Other factors that affect the regime shift are global warming and demand of food and fiber. Global warming have effects on upwelling, water circulatory systems that bring nutrients from the bottom of the sea towards the surface. By altering upwellings, food webs receive either too much or too little nutrients. In both cases the energy transferred towards higher trophic levels diminish as nutrients input variability increase. Demand of food strongly drives fishing, being the main cause of lost of top predators over the world.

How the regime shift works

Predator dominated food webs occur in environments with low disturbance both from fishing and nutrients input. They usually have four or more trophic levels. The regime is maintained by biotic mechanism that comprise predation and competition both intra and inter species. Abiotic factors also play a role maintaining the regime, by allowing an intermediate level of nutrient inputs from the deep ocean necessary for sustaining the food web.

In general, a combination of low top predator productivity due to poor recruitment or somatic growth combined with high exploitation pressure are conditions under which regime shifts in aquatic food webs are likely to occur (Daskalov et al. 2007). Overfishing can unbalance trophic relations, changing the configuration of competition and predation. Weak competitors may become stronger given less fishing pressure on such stocks, or by favoring the reproduction of it’s preys by fishing its competitor species. In addition, changes in nutrients inputs can represent an advantage for lower trophic levels, allowing them to escape the control of its predators.

As result, the ecological function of certain species groups is changed, allowing over abundance of immediate lower level. For example, the decrease of top-predators allows over abundance of meso predators that in turn reduces planktivorous fish and increase zooplankton. Further increase in nutrients from upwellings can lock the food web in a state dominated by planktivore fish as jellyfish. Low nutrients input, on the other hand, would reduce the amount of energy available for the food web and as a result an abrupt reduction of higher trophic levels is expected.

Impacts on ecosystem services and human well-being

Shift from predators to planktivore dominated food webs

Commercial fish stocks that are locked into a regime where they are maintained at low levels of abundance has obvious economic consequences on fisheries industry. However, these fisheries related shifts are not always negative, as for example a decrease in top predators can increase the catches of mid-level prey (e.g. shrimps and clupeids) (Frank et al. 2005). When an ecosystem-wide regime shift occurs it affects not only targeted groups, such as commercial fish (Daskalov et al. 2007) but also impacts other ecosystem functions. Biodiversity as well as ecosystem resilience may be reduced by a shift from an ecosystem with high top predator diversity and dominance to a system with lower trophic level dominance. This means that ecosystem may become more vulnerable to both climatic and anthropogenic change. Such ecosystems may more easily suffer from eutrophication, hypoxia and invasion by non-native species. Eutrophication and hypoxia in turn lead to increased algal blooms and decrease the recreational values of water bodies.

Predators and commercial fish stocks have remained under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the reduction of food webs, which in turn increases the risk of fisheries collapse. Depletion of fish stocks has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Hassan et al. 2005). Fish catches are projected to further decrease in the 21st century affecting protein sources for people, especially in poor regions (Hassan et al. 2005). The estimated current contribution of fisheries to human protein consumption is 29 million tons produced industrially and 24 million tons in small-scale fisheries (Hassan et al. 2005).

Management options

Options for enhancing resilience

In food webs that exhibit strong top-down control, decreasing the fishing pressure on top predators is an obvious strategy (Scheffer et al. 2001; Frank et al. 2005). However, due to the hysteresis effect, return to a predator-dominated food web may often require substantially lower fishing levels than that which induced the regime shift, together with additional measures. Hence, restoration is usually costly, especially to fisherman (Moellmann et al. 2009). As mentioned above, environmental factors can play an important role in maintaining alternative food web regimes. Nutrient loading is one of the key parameters controlling processes such as eutrophication and hypoxia. However, decreasing nutrient loads in marine environments requires large scale actions and often collaboration at international level (e.g. Helsinki Commission Baltic Sea Action Plan in the Baltic Sea). Hence, the time-frame of these actions in addition to time-lag in ecosystem response is often long. Climatic variation cannot be directly controlled by human actions. However, climate impacts can be mediated by employing other management measures that contribute to maintaining healthy marine ecosystems with high biodiversity and resilience, which then can compensate for changes in climate.

Options for reducing resilience to encourage restoration or transformation

In addition, in small closed food webs systems (3 to 4 trophic levels), biomanipulation has been suggested to manage one of the symptoms of degraded marine environments, namely eutrophication. This involves increasing the population of predatory fish such as bass, pike and walleye through stocking or reduced angling quotas. Increased populations of these predators leads to a decrease in the level of zooplanktivores. This in turn allows an increase in the population of planktivores that graze on the algae and zooplanckton, helping to reduce the algal density and phytoplankton respectively(Smith and Schindler 2009). This option, however, has limited reach when it comes to open marine foodwebs, where seasonal migrations and metapopulation dynamics are common.

Alternate regimes

During the 20th century global fisheries greatly expanded in terms of effort and range. This expansion resulted in the overfishing of many fish stocks, and in some cases triggered fisheries collapses, for example the collapse of the highly productive Newfoundland cod fishery (Worm et al. 2009). While overfishing can severely deplete fish stocks, it will not necessarily trigger a fisheries collapse. A collapse occurs when once fishery pressure has been removed, stocks fail to rebuild (Hutchings 2000, Kirby et al. 2009), suggesting that the system has become locked into an alternate regime. In this case, alternate regimes are usefully conceptualized as alternative food web structures:

High abundance of a commercial fish species.

In this regime a valuable fish species is common and the fishery is highly productive. The main ecosystem services associated with high abundance of commercial fish are the production of food in first place, employment for both artisanal and industrial fishermen; followed by pest and disease regulation by healthy food webs, recreational values as recreational fishing, knowledge and educational values, as well as maintenance of biodiversity.

Low abundance of the commercial fish species.

This regime is characterized by a substantially reduced abundance of the valuable fish species. In cases where the valuable fish species performs key ecological functions, it can lead to trophic cascades that lead to an overabundance of planktivorous fish or primary producers such as seagrass, macroalgae, sponge or phytoplankton (Jackson et al. 2001). With low fish abundance, food production is the most immediate ecosystem services affected. While industrial fishermen often can move and harvest new areas or new species, artisanal fishermen often can lose their livelihoods and traditions.  

Drivers and causes of the regime shift

There are two key direct drivers of fisheries collapse: overfishing as a top-down disturbance, and the change in nutrients input as a bottom-up disturbance.

Overfishing reduces the population size of commercial fish species, reduces their ability to perform their functional role in the ecosystem, can destabilize their population dynamics (Anderson et al. 2008), and ultimately could change the structure of the food web. Nutrient inputs, on the other hand, affect fisheries by increasing the abundance of lower trophic levels e.g. phytoplankton and zooplanktivores such as jellyfish (Daskalov et al. 2007). Such changes in food web configuration can reduce the energy transfer to fish and hence reduce fishing productivity. These drivers can reduce the resilience of a regime making it more vulnerable to other types of regime shift, such as climate driven food web reorganization [see: Climate & Marine Food Webs]. The indirect drivers that explain overfishing and changes in nutrient input interact in synergistic ways, making their identification a challenging process (Ocean Studies Board 2006, Kirby et al. 2009).

Indirect drivers of overfishing include oversupply of fishing boats, inflexible fishing quotas, demand for fish, the development of new fishing technologies, the facilitation of trade (market connectivity), governance failures such as the failure of regulation to halt illegal fishing, and market failures such as subsidies or perverse incentives. Nutrient enrichment of marine ecosystems is also indirectly driven by runoff of fertilizer nutrients from agriculture and sewage from urban areas (Diaz and Rosenberg 2008).

How the regime shift works

A fishery collapses when a high-valued commercial species is substantially depleted and fails to immediately recover when fishing effort is removed (Hutchings 2000). It is usually associated with a substantial and persistent change in the structure of the marine community. Such a change involves changes in the species presence, their relative abundance, size and age.

When the stock of commercial species decreases or even disappears the populations of other similar species may increase. Many commercially fished species are predatory fish, such as tuna, that have a high trophic level. Removing such top predators can cause a trophic cascade to reshuffle a food web (Daskalov et al. 2007, Estes et al. 2011), especially if there are not ecologically similar species present. Such trophic cascades, can increase fish stocks of prey species (Walters and Kitchell 2001), which can in turn lead to either sequential depletion of fish stocks or the development of new fisheries (Essington et al. 2006).

The low abundance of commercial fish species can be maintained by a number of mechanisms. First, when the population of the commercial fish species falls under certain levels their population growth rate can decline from a variety of causes. This is known as depensation in fisheries, or the Allee effect more generally (Liermann and Hilborn 1997). Such an effect can occur if small schools of fish offer poorer protection from predators, smaller populations are less able to hunt or detect food, or mating success rates can decline.

Second, nutrient enrichment can favor the food web shifts towards zooplanktivores, such as jellyfish. By reducing resources for other fish, the rise of such species can suppress fisheries recovery. Fishery pressure on filter-feeding fishes like sardine exacerbates the problem by favoring jellyfish outbreaks (Jackson et al. 2001, Brotz et al. 2012, Condon et al. 2013).

Despite evidence from some well-documented cases, the degree to which fisheries declines are regime shifts remains a controversial issue for a number of reasons (Hilborn 2007, Litzow and Urban 2009). First, time series for most fisheries are only available from 1950, a time when marine ecosystems were already affected by flourishing industrial fishing, and some stocks had already been seriously depleted, especially those of large marine vertebrates like whales, manatees, crocodiles, seals, swordfish, sharks and rays (Kirby et al. 2009). This lack of long term data makes it difficult to identify causation in fisheries time series. Furthermore, marine ecological diversity and food web complexity complicates the detection of changes in fish populations due to fishing, because the signals of regime shift dynamics are small relative to ecological variation (Liermann and Hilborn 1997).

Impacts on ecosystem services and human well-being

Shift from high to low abundance of commercial fish species

Since 1950 the world marine fisheries catch has expanded almost fivefold, and is currently about 80 million tonnes of fish. According to FAO, about 30% of fish stocks are overexploited, 60% are fully exploited and about 10 % are not fully exploited (FAO 2012). Global marine fish catches have plateaued and are not expected to increase. Local fisheries decline has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Pauly et al. 2006). There is not clear assessment of the number of stocks that have collapsed but FAO suggests about 5% of fish stocks are depleted (FAO 2012). Some collapsed stocks have remained at under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the collapse of fisheries.  

Shift from low to high abundance of commercial fish species

An abundant fish population regime will likely better maintain marine biodiversity and will be more resilience to climate variation and climate change. Society will benefit of high abundance regime in terms of high primary productivity that translate in more food, jobs and livelihoods in small coastal communities. It will also better regulate marine hypoxia.

Management options

A review of marine fisheries in the Millennium Ecosystem Assessment offer a synthetic set of managerial options for addressing fisheries collapse (Pauly et al. 2006).

Options for enhancing resilience

There are two types of strategies for enhancing resilience. The first is to stop doing things that are eroding resilience, while the second is to being activities that build resilience. There are a number of fishing practices that could be eliminated because they damage the resilience of fisheries. For example, fishing practices that destroy habitat such as bottom trawling should be avoided. Developing alternative technologies and reduction of fishing effort are suggested. Bycatch, the killing but not consuming of unwanted fish, can also have substantial impacts and can be reduced. Indirectly, fisheries subsidies and inflexible quotas can drive fishing effort to high levels that erode the resilience of a fishery.

Options for reducing resilience of unwanted regime to encourage restoration or transformation

Direct resilience building strategies include the establishment of Marine protected areas (MPAs) (Takashina and Mougi 2014). These areas can play an important role by providing refugia for fish and increasing ecological diversity. Research suggests that they need to cover ecologically significant areas, including different habitats, ensuring connectivity among them. However there remains controversy over the extent to which MPAs shift fishing pressure to less protected sites.

Indirect resilience building efforts include efficient regulation and policing of fishing can reduce illegal fishing, while comprehensive fisheries policies at the international and national level can also build resilience if they result in fisheries policies that reinforce one another. Indirect resilience building strategies include improving the ability of fishers to exit fisheries.

Good management of fishing pressure has been able to restore fisheries stocks. For example, in the US fishing closures and implementation of management measures for bottom fishing have enabled the restoration of George Bank haddock, Atlantic scallops, George Bank yellowtail flounder, Atlantic striped bass, Atlantic Arcadian redfish, Pacific chub mackerel, and Pacific sardine. However, there is no clear strategy that works across many ecosystems or nations (Beddington et al. 2007).

Key References

  1. Ainley, D. G., and L. K. Blight. 2009. Ecological repercussions of historical fish extraction from the Southern Ocean. Fish And Fisheries 10:13–38.
  2. Alheit, J. 2009. Consequences of regime shifts for marine food webs. International Journal Of Earth Sciences 98:261–268.
  3. Anderson, C. N. K., C.-H. Hsieh, S. A. Sandin, R. Hewitt, A. Hollowed, J. Beddington, R. M. May, and G. Sugihara. 2008. Why fishing magnifies fluctuations in fish abundance. Nature 452:835–839.
  4. Bakun, A., D. Field, A. Redondo-Rodriguez, and S. Weeks. 2010. Greenhouse gas, upwelling-favorable winds, and the future of coastal ocean upwelling ecosystems. Global Change Biology 16:1213–1228.
  5. Beddington, J. R., D. J. Agnew, and C. W. Clark. 2007. Current problems in the management of marine fisheries. Science 316:1713–1716.
  6. Behrenfeld, M. J., R. T. O'Malley, D. A. Siegel, C. R. Mcclain, J. L. Sarmiento, G. C. Feldman, A. J. Milligan, P. G. Falkowski, R. M. Letelier, and E. S. Boss. 2006. Climate-driven trends in contemporary ocean productivity. Nature 444:752–755.
  7. Berkes, F. 2008. Sacred ecology. Taylor & Francis
  8. Berkes, F., T. Hughes, R. Steneck, J. Wilson, D. Bellwood, B. Crona, C. Folke, L. Gunderson, H. Leslie, J. Norberg, M. Nystrom, P. Olsson, H. Osterblom, M. Scheffer, and B. Worm. 2006. Ecology - Globalization, roving bandits, and marine resources. Science 311:1557–1558.
  9. Brotz, L., W. W. L. Cheung, K. Kleisner, E. Pakhomov, and D. Pauly. 2012. Increasing jellyfish populations: trends in Large Marine Ecosystems. Hydrobiologia.
  10. Carpenter, S. R. 2003. Regime shifts in lake ecosystems. Ecology Institute.
  11. Clarke, S. 2004. Understanding pressures on fishery resources through trade statistics: a pilot study of four products in the Chinese dried seafood market. Fish And Fisheries 5:53–74.
  12. Condon, R. H., C. M. Duarte, K. A. Pitt, K. L. Robinson, C. H. Lucas, K. R. Sutherland, H. W. Mianzan, M. Bogeberg, J. E. Purcell, M. B. Decker, S.-I. Uye, L. P. Madin, R. D. Brodeur, S. H. D. Haddock, A. Malej, G. D. Parry, E. Eriksen, J. Quiñones, M. Acha, M. Harvey, J. M. Arthur, and W. M. Graham. 2013. Recurrent jellyfish blooms are a consequence of global oscillations. Proceedings of the National Academy of Sciences 110:1000–1005.
  13. Daskalov, G. M., A. N. Grishin, S. Rodionov, and V. Mihneva. 2007. Trophic cascades triggered by overfishing reveal possible mechanisms of ecosystem regime shifts. Proceedings Of The National Academy Of Sciences Of The United States Of America 104:10518–10523.
  14. Diaz, R. J., and R. Rosenberg. 2008. Spreading Dead Zones and Consequences for Marine Ecosystems. Science 321:926–929
  15. Essington, T. E., A. H. Beaudreau, and J. Wiedenmann. 2006. Fishing through marine food webs. Proceedings Of The National Academy Of Sciences Of The United States Of America 103:3171–3175.
  16. Estes, J., J. Terborgh, J. Brashares, and M. Power. 2011. Trophic Downgrading of Planet Earth. Science.
  17. FAO. 2012. The state of world fisheries and aquaculture. UN Food & Agriculture Organization.
  18. Hilborn, R. 2007. Reinterpreting the State of Fisheries and their Management. Ecosystems 10:1362–1369.
  19. Hutchings, J. A. 2000. Collapse and recovery of marine fishes. Nature 406:882–885.
  20. Hutchings, J., and J. Reynolds. 2004. Marine fish population collapses: Consequences for recovery and extinction risk. BioScience 54:297–309.
  21. Jackson, J., M. Kirby, W. Berger, K. Bjorndal, L. Botsford, B. Bourque, R. Bradbury, R. Cooke, J. Erlandson, J. Estes, T. Hughes, S. Kidwell, C. Lange, H. Lenihan, J. Pandolfi, C. Peterson, R. Steneck, M. Tegner, and R. Warner. 2001. Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science 293:629–637.
  22. Jiao, Y. 2009. Regime shift in marine ecosystems and implications for fisheries management, a review. Reviews in Fish Biology and Fisheries 19:177–191.
  23. Kirby, R. R., G. Beaugrand, and J. A. Lindley. 2009. Synergistic Effects of Climate and Fishing in a Marine Ecosystem. Ecosystems 12:548–561.
  24. Lenzen, M., D. Moran, K. Kanemoto, B. Foran, L. Lobefaro, and A. Geschke. 2012. International trade drives biodiversity threats in developing nations. Nature 486:109–112.
  25. Liermann, M., and R. Hilborn. 1997. Depensation in fish stocks: a hierarchic Bayesian meta-analysis. dx.doi.org.
  26. Litzow, M. A., and D. Urban. 2009. Fishing through (and up) Alaskan food webs. Canadian Journal Of Fisheries And Aquatic Sciences 66:201–211.
  27. Moellmann, C., B. Mueller-Karulis, G. Kornilovs, and M. A. St John. 2008. Effects of climate and overfishing on zooplankton dynamics and ecosystem structure: regime shifts, trophic cascade, and feedback coops in a simple ecosystem. Ices Journal Of Marine Science 65:302–310.
  28. Moellmann, C., R. Diekmann, B. Muller-Karulis, G. Kornilovs, M. Plikshs, and P. Axe. 2009. Reorganization of a large marine ecosystem due to atmospheric and anthropogenic pressure: a discontinuous regime shift in the Central Baltic Sea. Global Change Biology 15:1377–1393.
  29. Ocean Studies Board. 2006. Dynamic Changes in Marine Ecosystems: Fishing, Food Webs, and Future Options. National Research Council, Washington D.C.
  30. Pauly, D., J. Alder, A. Bakun, S. Heileman, K. Koch, P. Mace, W. Perrin, K. Stergiou, U. Sumaila, M. Vierros, K. Freire, Y. Sadovy, V. Christensen, K. Kaschner, M. Palomares, P. Tyedmers, C. Wabnitz, R. Watson, and B. Worm. 2006. Chapter 18: Marine Fisheries Systems. Pages 1–35 in R. Hassan, R. Scholes, and N. Ash, editors. Millennium Ecosystem Assessment. Island Press, Washington, D.C.
  31. Scheffer, M., E. H. Nes, M. Holmgren, and T. Hughes. 2008. Pulse-Driven Loss of Top-Down Control: The Critical-Rate Hypothesis. Ecosystems 11:226–237.
  32. Takashina, N., and A. Mougi. 2014. Effects of marine protected areas on overfished fishing stocks with multiple stable states. Journal Of Theoretical Biology 341:64–70.
  33. Takasuka, A., Y. Oozeki, H. Kubota, and S. E. Lluch-Cota. 2008. Contrasting spawning temperature optima: Why are anchovy and sardine regime shifts synchronous across the North Pacific? Progress in Oceanography 77:225–232.
  34. Walters, C., and J. F. Kitchell. 2001. Cultivation/depensation effects on juvenile survival and recruitment: implications for the theory of fishing. dx.doi.org.
  35. Worm, B., R. Hilborn, J. K. Baum, T. A. Branch, J. S. Collie, C. Costello, M. J. Fogarty, E. A. Fulton, J. A. Hutchings, S. Jennings, O. P. Jensen, H. K. Lotze, P. M. Mace, T. R. Mcclanahan, C. Minto, S. R. Palumbi, A. M. Parma, D. Ricard, A. A. Rosenberg, R. Watson, and D. Zeller. 2009. Rebuilding global fisheries. Science 325:578–585.

Citation

Garry Peterson, Juan Carlos Rocha, Henrik Österblom, Reinette (Oonsie) Biggs. Fisheries collapse. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-19 04:44:30 GMT.
Friday, 03 October 2014 16:07

Mangroves transitions

Written by Juan Carlos

Mangroves transitions

Main Contributors:

Juan Carlos Rocha

Other Contributors:

Reinette (Oonsie) Biggs

Summary

Mangroves are ecosystems adapted to the mixture of salt and fresh water in coastal areas. They provide important ecosystem services such as carbon storage, storm protection, ground fields for several marine species, water cleansing, and wood for construction and energy. Despite their importance for both local communities and the global carbon budget, mangroves are at risk of collapsing in tropical areas of the world while they are likely to expand on temperate areas. World’s mangrove cover has been reduced by 30% during the last 50 years mainly driven by aquaculture, deforestation, land cover change towards agricultural fields, shrimp farming, salt extraction or urban development, as well as by infrastructure development changing the water salinity. In the coming century, climate change is expected to add pressure to this ecosystem by increasing sea level rise and changing the distribution of extreme weather events such as storms and droughts with impacts on the balance between salt and fresh water. In temperate areas climate change is expected to raise temperature enough for mangroves development in areas currently dominated by salt marshes. Managerial options include identifying areas where these ecosystems have potential for expansion and migration, reduce human pressure on them and assist them by monitoring drivers and implementing marine protected areas.

Drivers

Key direct drivers

  • Vegetation conversion and habitat fragmentation
  • Harvest and resource consumption
  • External inputs (eg fertilizers)
  • Infrastructure development
  • Environmental shocks (eg floods)
  • Global climate change

Land use

  • Urban
  • Small-scale subsistence crop cultivation
  • Timber production
  • Fisheries
  • Conservation
  • Tourism

Impacts

Ecosystem type

  • Marine & coastal

Key Ecosystem Processes

  • Soil formation
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries
  • Wild animal and plant products
  • Timber
  • Woodfuel

Regulating services

  • Climate regulation
  • Water purification
  • Water regulation
  • Regulation of soil erosion
  • Natural hazard regulation

Cultural services

  • Recreation
  • Aesthetic values

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Years
  • Decades

Reversibility

  • Unknown

Evidence

  • Models
  • Paleo-observation
  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Well established – Wide agreement in the literature that the RS exists

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Links to other regime shifts

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Alternate regimes

A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.  

With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.

Arctic benthos (regime 1)

With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.

Subarctic benthos (regime 2)

After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).

 

Drivers and causes of the regime shift

The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.

The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).

 

How the regime shift works

In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)

Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.

Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions. 

Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.

 

Impacts on ecosystem services and human well-being

Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.

The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being. 

 

Management options

Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.

Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.

 

Alternate regimes

Due to thermokarst processes such as permafrost melting, numerous areas of the Arctic landscape are extremely lake-rich.  The water balance of those lakes may either be maintained due to impermeable permafrost that limits groundwater flow (Marsh 2009) or lost due to seasonal drainage, a natural characteristic of the Arctic system. However, a widespread, unnatural decline in lake abundance is being observed within the Arctic due to decades of rising atmospheric temperatures in the region that has been deteriorating the state of the permafrost (Smith et al. 2005). The lake-rich landscape of the Arctic is therefore slowly transforming into a drained, dry landscape increasingly dominated by vegetation such as shrubs and graminoids (Hinzman 2005).

 

Thermokarst lake ecosystem

 

The thermokarst lake-dominated landscape is formed as a response to some imposed disturbance, such as road construction, wildfire or climatic warming, which causes permafrost to thaw, creating irregular surface topography depressions, called thermokarsts. Those may appear as near-surface massive ice melts, allowing the surface to subside (Hinzman et al. 2005). As permafrost melts over time, those depressions become filled with melted water forming the lakes characteristic of the Arctic region.

 

Terrestrial ecosystem

 

Driven by external and internal factors, Arctic lakes are drying up at an unnatural speed within the region. This shift is accompanied by changes in vegetation, for example, increased shrub- and graminord abundance and a decrease in the cover of mosses and lichens through a broad expanse of the Arctic (Hinzman et al. 2005). The changes in vegetation may then further transform the landscape through sequential effects on foraging mammals and birds, as well as aquatic fauna and insects (Hinzman et al 2005).

 

Drivers and causes of the regime shift

 

Shift from thermokarst lakes to terrestrial ecosystem

The main external driver of this regime shift is climate change, which is caused by elevated greenhouse gas emissions (IPCC 2007). Increased temperatures warm the soil and warms the permafrost. As the permafrost thaws, greenhouse gasses such as CO2 and CH4 that are trapped in the ice are released into the atmosphere, further increasing the effects of climate change (Hinzman 2005; Karlsson et al. 2011). At a local scale, the effects of climate change are incremental. However, as this interaction is occurs over large areas, the effects are aggregated and self-reinforcing, which is significant since permafrost and seasonally frozen grounds store about 25% of the total global soil carbon stock (IPCC 2007).

At a local scale, initial thawing of the permafrost from increased atmospheric temperatures creates a dynamic landscape of depressions (thermokarst) in which lakes are formed (IPCC 2007). As permafrost become more degraded, drainage through outlet channels occurs. Since the temperature of melted water is higher than that of ice, it will exacerbate the melting of permafrost, causing another reinforcing feedback. Once the layer of frozen ground is completely penetrated the lakes can become permanently drained allowing for vegetation to be established (Marsh et al. 2008).

Shift from terrestrial ecosystem to thermokarst lake ecosystem

Heightened air- and soil temperatures melt the ice-rich soils and permafrost on which forests in the Arctic region develop, changing the physical conditions for the forest growing on top of it. When the roots of the trees get flooded, the trees die and ponds and lakes eventually replace the forest (Hinzman 2005). However, it is important to note that the shift from a terrestrial ecosystem to a lake dominated one is not as frequent as the shift from lakes to terrestrial ecosystems. The reason for this are the reinforcing feedbacks as mentioned above.

How the regime shift works

The warming of soil temperatures leads to the melting of continuous permafrost and the subsequent increase in abundance of discontinuous or sporadic permafrost. Melting of permafrost causes depressions in the soil called thermokarsts. Increased precipitation in the form of snowfall may have either an insulating or cooling effect on soil temperatures depending on several variables such a timing, duration, accumulation, and melting processes of seasonal snow cover, density, structure, and thickness of seasonal snow cover etc. (Zhang 2005). Depending upon the local surface energy balance, the thawed ground may refreeze or the permafrost can continue to degrade (Hinzman et al. 2005). On sites where continuous permafrost is thawing the active layer of soil in which melted water can move deepens allowing for the formation of thermokarst lakes (Hinzman et al. 2005). In freezing seasons, if the temperature is low enough and the hydraulic parameters on site suitable, the formation of these lakes can contribute to the re-formation of permafrost soils. Ice-rich permafrost can also prevent surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet. Decreased albedo due to changes in vegetation, the extension of snow-free and ice-free periods on terrestrial and lake surfaces and abundance, and reduction in the area occupied by glaciers and continental ice sheets in high latitudes may act as a positive feedback into atmospheric warming (Hinzman et al. 2005).

On sites where there is an abundance of discontinuous permafrost the active layer also deepens allowing for increased surface area for sub-ground water movement that increases the rate of drainage. Thermokarst lakes and ponds may begin to fill or drain depending upon the direction of the hydraulic gradient beneath the lake (Hinzman et al. 2005). Therefore it is not the depth of the active layer alone that determines the rate of drainage. However, increased drainage of lakes may prevent the reformation of permafrost from lack of available water for freezing in winter months and it can be inferred through advection of heat from decreased albedo. Once these drained lake beds become permanently drained they are likely to be re-vegetated which will change the albedo and may cause the extension of snow-free and ice-free periods and may act as feedback into increasing atmospheric temperature (Hinzman et al. 2005).

The affects of the shifts from lake to terrestrial ecosystems are aggregated at a local, regional and global scale due to the large scale and frequency at which those dynamics occur. The whole regime as described in the causal loop diagram below is a re-enforcing feedback that at its current state is leading to an overall decrease in lake dominated ecosystems and a increase in terrestrial ecosystems (Smith et al. 2005).

Impacts on ecosystem services and human well-being

Even though this regime shift occurs at the local scale, its consequences go beyond that scale because it reshapes regional hydrology (through changes in water balance and surface water- connection and fragmentation) and alters regional supplies of the ecosystem (Karlsson et al. 2011). This regime shift therefore affects provisioning ecosystem services, as it will decrease the availability of freshwater. The above-ground stored freshwater, which is of vital importance to migratory birds, fish, and other wildlife used by indigenous people, will shift to below-ground stored freshwater (Artic Science Journeys Radio Stories 2005). Globally, however, freshwater will become a more abundant ES due to reductions permafrost and subsequently ice sheets and glaciers resulting from the regime’s feedbacks to climate change. Nevertheless, this global increase in the freshwater ES is not a positive one: “Increased freshwater delivery to the Arctic Ocean from reductions in ice sheets and glaciers result in rising sea level (...)” (Hinzman et al. 2005). At a local scale traditional hunting and fishing practices will be impaired (Vincent et al. 2013).  Arctic freshwater systems provide important migratory routes for fish stock. Due to this regime shit, those routes would be greatly altered in connectivity among lakes and river channels, as well as in terms of their physical coupling to the coastal marine ecosystem (Vincent et al. 2012). Globally, fishing practices would suffer as well. Increased discharge of water into the Arctic Ocean due to draining results in increased nutrient and external organic matter inputs to the Arctic Ocean that affect primary processes at the base of the marine food web (Vincent et al. 2012). Climate regulation is heavily affected on a global scale through the processes of this regime shift: as it sets up a positive feedback cycle in which the release of methane and carbon dioxide through the newly exposed soil and melted permafrost feeds into global warming. The shift from lakes to terrestrial ecosystems heightens gully erosion, a type of erosion that occurs when water is channeled across unprotected land and washes away the soil along the drainage lines. As permafrost degrades further, it causes erosion and slumping of lake edges and stream channels

The Arctic lakes are of vital importance to indigenous people as they are a source of fresh drinking water and also provides important winter transport routes. These lakes not only allows indigenous people access to their traditional hunting and fishing areas, but also to transport goods to remote communities and industries such as mining centers (Vincent et al. 2013). The regime shift would also mean an increase in products extracted from woods such as timber commodities and fuel: “increased areas of tree growth in the Arctic could serve to (…) take supply more wood products and related employment, providing local and global economic benefits” (Hassol et al. 2004).

 

Management options

There are few management options available to maintain the thermokarst lake ecosystems due to the long time scale at which permafrost melts and the natural dynamics within this system e.g. hydraulic fluctuations or site-specific soil conditions. One management option is to continue to reduce the emissions of greenhouse gases while simultaneously investigating further opportunities for carbon offsetting schemes.

For hydroelectric reservoirs, shifting ice conditions will have both positive and negative effects, and may require adaptive changes in operating procedures, with attention to minimize negative impacts associated with ice jams and ice breakup downstream of the spillway (Vincent et al. 2013). Fisheries management plans will also need to be adapted to the changes in migration and productivity of northern fish populations with ongoing climate change (Vincent et al. 2013). These essential resources require the development of integrated freshwater management plans, which include consideration of alternate water sources as traditional supplies change in quantity or quality (Vincent et al. 2011). The Arctic will require increasing vigilance and appropriate water management strategies to avoid and minimize the impacts of changing water impacts in the future (Vincent et al. 2013).

Alternate regimes

The boreal forest in northern Interior Alaska covers an approximate area of 48 million hectares (Mann et al. 2012). Bounded by the Alaska Range to the south, the Seward Peninsula to the west and Brooks Range to the north (Wolken et al. 2011). It is characterized by large areas of gently sloping uplands and flat lowlands isolated by mountain ranges and braided rivers with broad floodplains. During the past 50 years, the mean annual temperature in the interior boreal forest of Alaska has increased by 1.3 ºC (Hartmann and Wendler, 2005) and this has been strongly associated with recent increase in fire frequency and severity (Wolken et al. 2011). The annual area burned in Interior Alaska has doubled during the last decade (Kasischke et al. 2010). The forest has been dominated by coniferous trees (mainly black spruce), which is a group of species that thrive under colder, moist conditions in a deep soil organic layer. However, due to climate warming, Interior Alaska is currently shifting towards a deciduous dominated forest (e.g. aspen and birch) that prefer warmer and drier conditions with a shallow soil organic layer (Johnstone et al. 2010).  

 

 

Low fire frequency coniferous dominated forest

Under cold and wet conditions, coniferous trees, such as the black spruce (Picea mariana), dominate the boreal forest in Interior Alaska. The coniferous trees promote the accumulation of a deep soil organic layer, cold and moist soil temperatures, and slow decomposition rates. Due to the moistness of the soil, fires are not frequent in these forests. However, when they occur, they tend to be severe due to the high flammability of the black spruce. Due to the high water content of the soil organic layer, it will only be entirely consumed by the most severe fires (Johnstone et al. 2010). As long as the entire organic layer has not been consumed, the coniferous trees have a regenerative advantage due to their seed’s ability to sprout in moist organic soils. This means that these forests tend to return to a coniferous dominated state even after a fire (Hollingsworth et al. 2013).

 

High fire frequency deciduous dominated forest

A deciduous dominated forest, mainly consisting of aspen and birch trees in Interior Alaska, prefers and reinforces dry and warm soil conditions. Although these forests have a high organic litter production, the fast decomposition rate and the subsequent fast nutrient turnover, leaves a shallow soil organic layer and a nutrient rich mineral soil that favours deciduous tree regeneration. The warm and dry conditions in the forest lead to a comparatively high fire frequency, but due to the low flammability of the deciduous trees, the fires rarely become very severe (Johnstone et al. 2010). However, because of the shallow pre-fire soil organic layer, the fires tend to expose the mineral soil, which favours the post-fire reproduction of deciduous trees (Hollingsworth et al. 2013).

 

Drivers and causes of the regime shift

The main external direct driver of the regime shift is climate warming, which affects the important feedbacks of the boreal forest ecosystem. Warmer and drier climatic conditions have a direct effect on the most important soil characteristics (i.e. soil temperature, moisture and depth of the soil organic layer), influencing the vegetation cover. Coniferous trees require a cold moist soil with a deep soil organic layer to successfully reproduce. Deciduous trees, on the other hand, are competitively superior under warmer and drier soil conditions with a shallow soil organic layer (Johnstone et al. 2010). Extended summer seasons and the accompanying drier conditions create a suitable environment for an increase in the annual number of natural ignitions by lightning strikes (Kasischke et al. in press). Fires have an immediate effect on the environmental conditions of the forest, affecting factors such as soil moisture and the post-fire forest community composition (Kelly et al. 2012). Once established, the trees support and emphasize the conditions that favour their own reproduction and dominance. Under colder environmental conditions, coniferous trees, such as the black spruce, produce large amounts of organic litter, which, together with the presence of permafrost, maintains a moist soil and slow decomposition rates, contributing to a deep soil organic layer. Under warmer climatic conditions, deciduous trees produce ground layer fuels that decompose at fast rates, maintaining a shallow organic layer (Johnstone et al. 2010). Global socio-economic development indirectly drives climate warming, contributing to the increased atmospheric temperatures through e.g. greenhouse gas emissions (IPCC, 2011). Fire management can have a limited effect on the regime shift through fire suppression practices.

How the regime shift works

Shift from coniferous to deciduous-dominated forest

The coniferous dominated boreal forest of Interior Alaska requires a deep soil organic layer to succeed over other classes of trees. Once the required depth is present, this has a strong reinforcing effect on the coniferous forest. This is a requisite to maintain cold moist soil conditions in which coniferous trees thrive. The cold soil temperature reduces the decomposition rate of organic litter, maintaining a deep soil organic layer (Johnstone et al. 2010).

 

External shocks such as severe and frequent fires or insect outbreaks might cause dramatic shifts in the soil and vegetation by impacting the soil organic layer depth and tree succession (Hollingsworth et al. 2013; McCullough et al. 1998; Wolken et al. 2011). If the severity of such external shocks is sufficient, thresholds in the most important regime characteristics will be crossed (e.g. shallow post-shock soil organic layer) creating windows of opportunity for deciduous species to dominate. Atmospheric temperature influences the frequency and severity of fires, the soil characteristics and the length of the growing season; therefore, climate warming can also cause a change in conditions that would set the grounds for a shift to a new stable state (Mann et al. 2012).

 

The deciduous dominated forest has a competitive advantage in warm and dry conditions with shallow soil organic layers. Again, once the required shallow layer is present, it has a strong reinforcing effect on the deciduous forest. A shallow soil organic layer maintains warm and dry soil conditions that speed up the decomposition of organic litter, maintaining the required depth of soil organic layer (Johnstone et al. 2012).

 

Shift from deciduous-dominated  to coniferous forest 

In order to shift from regime 2 to regime 1, a prolonged climatic cooling would be necessary. A slow change of the environmental conditions towards those that favour a coniferous boreal forest could, if the forest is left undisturbed for a sufficiently long time, lead to late successional coniferous species, like the black spruce, replacing early successional deciduous species (Rupp et al. 2002). Although the spatial scale considered is of extreme importance, sufficiently large external shocks, such as deciduous-specific insect outbreaks, could clear sufficiently large areas to advance the dominance of coniferous trees. In such hypothetical events, coniferous trees could get the opportunity to sprout and start altering the soil conditions to their own favour.

Impacts on ecosystem services and human well-being

The provisioning of wild products such as berries and game is expected to shift some species and decrease others, but it is unclear what the net changes will be (Chapin et al. 2008). The boreal forest is likely to become a net source of carbon to the atmosphere (McGuire et al. 2009; Schuur et al. 2009), while the air quality is expected to decrease periodically due to the increase in fire frequency (Chapin et al. 2008).

Management options

The primary tactic used for reducing impacts of fire and enhance resilience (i.e. maintain structures, feedbacks, functions and processes of regime 1 is fire suppression to reduce the fire severity (Chapin et al. 2008). Decrease in fire severity could prevent the fire from consuming the soil organic layer, which in turn favours the conditions for coniferous tree. Fire suppression should be maintained in Alaska since it minimizes the risk fire imposes to life and property in communities. However, distant fires should be allowed to burn under controlled conditions, since it could have a cooling effect at a regional scale due to the higher albedo of post-fire deciduous species compared to the previous coniferous forest (Chapin et al. 2008).

Reducing air temperature is essential to be able to restore regime 1. Alaska itself accounts for a miniscule proportion of global greenhouse gas emissions and consequently to climate warming (Chapin et al. 2008). Therefore, reducing Alaska’s emissions will not have a major impact on global climate change. Global abatement of emissions is the long-term solution. Regardless of global greenhouse gas emission policies, it is highly likely that recent warming and wildfire frequency will continue for several decades in Alaska due to the multidecadal lag in the climate system (IPCC, 2007). This makes the reduction of emissions an inefficient short-term solution.  Secondary challenges that face efforts to achieve reduced emissions include restricting the economy and changing the focus of developed nations on a continued economic but sustainable growth. Educating global public about the social impacts of climate change in Alaska and other parts of the world (e.g. reduction in ecosystem services) may promote the willingness to reduce emissions (Chapin et al. 2008).

In summary, this means that the regime shift from coniferous to deciduous dominated forest in theory could be reversible through manipulation of e.g. soil conditions or local climate. However, due to projected global socio-economic development and climate change, it is highly unlikely that reversing the regime shift will be possible during the next 100 years.

Alternate regimes

This regime shift involves an abrupt reorganization of an aquatic food web due to a decrease in top predators or increase in environmental factors like temperature. Trophic cascades play a central role in these regime shifts. During these cascades groups in adjacent trophic levels often show inverse patterns in their abundance (Carpenter 2003). In other words, every other trophic level displays an increase in biomass, which leads to increased predation pressure and biomass decrease on every other trophic level. Most trophic cascades are described in ecosystems with low species diversity and/or simple food webs and/or small size (Frank et al. 2005). The magnitude of a trophic cascade varies depending on species diversity, regional oceanography, local physical disturbance, habitat complexity and fishery practices (Salomon et al. 2010). Food webs with more trophic levels and biodiversity show less trophic cascades than simpler systems (Salomon et al. 2010); due in part to omnivory and the variation on trophic interaction strength (Bascompte et al. 2005). However some documented examples includes subtidal reefs in New Zealand (Shears and Babcock 2002), Caribbean coral reefs (Bellwood et al. 2004), the Gulf of Maine (Steneck et al. 2004), and the North Atlantic (Kirby et al. 2009).

Predator-dominated food web.

Ecosystems in this regime are characterized by high predatory fish, low planktivorous fish, high zooplankton and low phytoplankton abundance. Such ecosystems can be subject to human influence, such as intensive fishery, but exhibit so called “natural compensation” e.g. via species richness and omnivory (Pace et al. 1999) that maintain the regime. Despite high productivity, such ecosystems are less likely to suffer from eutrophication due to high grazing pressure on phytoplankton. Ecosystem services associated with predator-dominated food webs includes food provision, higher biodiversity, better disease and pest control, as well as cultural services like recreation and aesthetic values (e.g. diving, sport fishing).

Planktivore-dominated food web.

Ecosystems in this regime are characterized by low predatory fish, high planktivorous fish, low zooplankton and high phytoplankton abundance. In extreme cases the actual number of trophic levels (TLs) can decrease. Ecosystems with lower trophic level dominance are more vulnerable to eutrophication due to lowered grazing pressure on phytoplankton, as well as invasion of planktivore organisms like jellyfish. A planktivore-dominated food web can affect the fluxes of carbon, transforming aquatic systems -coastal and lakes- from sinks to sources of green house gases (Bakun et al. 2010; Estes et al. 2011). Other ecosystem services like fisheries and recreation are expected to be significantly reduced.

Drivers and causes of the regime shift

Trophic cascades are usually initiated by fishing pressure that have direct or indirect impacts on the abundance of top predators. The response time to these drivers varies across ecosystems. In the case of coastal areas it has been found that the ecosystem response time to overfishing can vary from decades to centuries where multiple predators exist (Jackson et al. 2001).

Other factors that affect the regime shift are global warming and demand of food and fiber. Global warming have effects on upwelling, water circulatory systems that bring nutrients from the bottom of the sea towards the surface. By altering upwellings, food webs receive either too much or too little nutrients. In both cases the energy transferred towards higher trophic levels diminish as nutrients input variability increase. Demand of food strongly drives fishing, being the main cause of lost of top predators over the world.

How the regime shift works

Predator dominated food webs occur in environments with low disturbance both from fishing and nutrients input. They usually have four or more trophic levels. The regime is maintained by biotic mechanism that comprise predation and competition both intra and inter species. Abiotic factors also play a role maintaining the regime, by allowing an intermediate level of nutrient inputs from the deep ocean necessary for sustaining the food web.

In general, a combination of low top predator productivity due to poor recruitment or somatic growth combined with high exploitation pressure are conditions under which regime shifts in aquatic food webs are likely to occur (Daskalov et al. 2007). Overfishing can unbalance trophic relations, changing the configuration of competition and predation. Weak competitors may become stronger given less fishing pressure on such stocks, or by favoring the reproduction of it’s preys by fishing its competitor species. In addition, changes in nutrients inputs can represent an advantage for lower trophic levels, allowing them to escape the control of its predators.

As result, the ecological function of certain species groups is changed, allowing over abundance of immediate lower level. For example, the decrease of top-predators allows over abundance of meso predators that in turn reduces planktivorous fish and increase zooplankton. Further increase in nutrients from upwellings can lock the food web in a state dominated by planktivore fish as jellyfish. Low nutrients input, on the other hand, would reduce the amount of energy available for the food web and as a result an abrupt reduction of higher trophic levels is expected.

Impacts on ecosystem services and human well-being

Shift from predators to planktivore dominated food webs

Commercial fish stocks that are locked into a regime where they are maintained at low levels of abundance has obvious economic consequences on fisheries industry. However, these fisheries related shifts are not always negative, as for example a decrease in top predators can increase the catches of mid-level prey (e.g. shrimps and clupeids) (Frank et al. 2005). When an ecosystem-wide regime shift occurs it affects not only targeted groups, such as commercial fish (Daskalov et al. 2007) but also impacts other ecosystem functions. Biodiversity as well as ecosystem resilience may be reduced by a shift from an ecosystem with high top predator diversity and dominance to a system with lower trophic level dominance. This means that ecosystem may become more vulnerable to both climatic and anthropogenic change. Such ecosystems may more easily suffer from eutrophication, hypoxia and invasion by non-native species. Eutrophication and hypoxia in turn lead to increased algal blooms and decrease the recreational values of water bodies.

Predators and commercial fish stocks have remained under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the reduction of food webs, which in turn increases the risk of fisheries collapse. Depletion of fish stocks has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Hassan et al. 2005). Fish catches are projected to further decrease in the 21st century affecting protein sources for people, especially in poor regions (Hassan et al. 2005). The estimated current contribution of fisheries to human protein consumption is 29 million tons produced industrially and 24 million tons in small-scale fisheries (Hassan et al. 2005).

Management options

Options for enhancing resilience

In food webs that exhibit strong top-down control, decreasing the fishing pressure on top predators is an obvious strategy (Scheffer et al. 2001; Frank et al. 2005). However, due to the hysteresis effect, return to a predator-dominated food web may often require substantially lower fishing levels than that which induced the regime shift, together with additional measures. Hence, restoration is usually costly, especially to fisherman (Moellmann et al. 2009). As mentioned above, environmental factors can play an important role in maintaining alternative food web regimes. Nutrient loading is one of the key parameters controlling processes such as eutrophication and hypoxia. However, decreasing nutrient loads in marine environments requires large scale actions and often collaboration at international level (e.g. Helsinki Commission Baltic Sea Action Plan in the Baltic Sea). Hence, the time-frame of these actions in addition to time-lag in ecosystem response is often long. Climatic variation cannot be directly controlled by human actions. However, climate impacts can be mediated by employing other management measures that contribute to maintaining healthy marine ecosystems with high biodiversity and resilience, which then can compensate for changes in climate.

Options for reducing resilience to encourage restoration or transformation

In addition, in small closed food webs systems (3 to 4 trophic levels), biomanipulation has been suggested to manage one of the symptoms of degraded marine environments, namely eutrophication. This involves increasing the population of predatory fish such as bass, pike and walleye through stocking or reduced angling quotas. Increased populations of these predators leads to a decrease in the level of zooplanktivores. This in turn allows an increase in the population of planktivores that graze on the algae and zooplanckton, helping to reduce the algal density and phytoplankton respectively(Smith and Schindler 2009). This option, however, has limited reach when it comes to open marine foodwebs, where seasonal migrations and metapopulation dynamics are common.

Alternate regimes

During the 20th century global fisheries greatly expanded in terms of effort and range. This expansion resulted in the overfishing of many fish stocks, and in some cases triggered fisheries collapses, for example the collapse of the highly productive Newfoundland cod fishery (Worm et al. 2009). While overfishing can severely deplete fish stocks, it will not necessarily trigger a fisheries collapse. A collapse occurs when once fishery pressure has been removed, stocks fail to rebuild (Hutchings 2000, Kirby et al. 2009), suggesting that the system has become locked into an alternate regime. In this case, alternate regimes are usefully conceptualized as alternative food web structures:

High abundance of a commercial fish species.

In this regime a valuable fish species is common and the fishery is highly productive. The main ecosystem services associated with high abundance of commercial fish are the production of food in first place, employment for both artisanal and industrial fishermen; followed by pest and disease regulation by healthy food webs, recreational values as recreational fishing, knowledge and educational values, as well as maintenance of biodiversity.

Low abundance of the commercial fish species.

This regime is characterized by a substantially reduced abundance of the valuable fish species. In cases where the valuable fish species performs key ecological functions, it can lead to trophic cascades that lead to an overabundance of planktivorous fish or primary producers such as seagrass, macroalgae, sponge or phytoplankton (Jackson et al. 2001). With low fish abundance, food production is the most immediate ecosystem services affected. While industrial fishermen often can move and harvest new areas or new species, artisanal fishermen often can lose their livelihoods and traditions.  

Drivers and causes of the regime shift

There are two key direct drivers of fisheries collapse: overfishing as a top-down disturbance, and the change in nutrients input as a bottom-up disturbance.

Overfishing reduces the population size of commercial fish species, reduces their ability to perform their functional role in the ecosystem, can destabilize their population dynamics (Anderson et al. 2008), and ultimately could change the structure of the food web. Nutrient inputs, on the other hand, affect fisheries by increasing the abundance of lower trophic levels e.g. phytoplankton and zooplanktivores such as jellyfish (Daskalov et al. 2007). Such changes in food web configuration can reduce the energy transfer to fish and hence reduce fishing productivity. These drivers can reduce the resilience of a regime making it more vulnerable to other types of regime shift, such as climate driven food web reorganization [see: Climate & Marine Food Webs]. The indirect drivers that explain overfishing and changes in nutrient input interact in synergistic ways, making their identification a challenging process (Ocean Studies Board 2006, Kirby et al. 2009).

Indirect drivers of overfishing include oversupply of fishing boats, inflexible fishing quotas, demand for fish, the development of new fishing technologies, the facilitation of trade (market connectivity), governance failures such as the failure of regulation to halt illegal fishing, and market failures such as subsidies or perverse incentives. Nutrient enrichment of marine ecosystems is also indirectly driven by runoff of fertilizer nutrients from agriculture and sewage from urban areas (Diaz and Rosenberg 2008).

How the regime shift works

A fishery collapses when a high-valued commercial species is substantially depleted and fails to immediately recover when fishing effort is removed (Hutchings 2000). It is usually associated with a substantial and persistent change in the structure of the marine community. Such a change involves changes in the species presence, their relative abundance, size and age.

When the stock of commercial species decreases or even disappears the populations of other similar species may increase. Many commercially fished species are predatory fish, such as tuna, that have a high trophic level. Removing such top predators can cause a trophic cascade to reshuffle a food web (Daskalov et al. 2007, Estes et al. 2011), especially if there are not ecologically similar species present. Such trophic cascades, can increase fish stocks of prey species (Walters and Kitchell 2001), which can in turn lead to either sequential depletion of fish stocks or the development of new fisheries (Essington et al. 2006).

The low abundance of commercial fish species can be maintained by a number of mechanisms. First, when the population of the commercial fish species falls under certain levels their population growth rate can decline from a variety of causes. This is known as depensation in fisheries, or the Allee effect more generally (Liermann and Hilborn 1997). Such an effect can occur if small schools of fish offer poorer protection from predators, smaller populations are less able to hunt or detect food, or mating success rates can decline.

Second, nutrient enrichment can favor the food web shifts towards zooplanktivores, such as jellyfish. By reducing resources for other fish, the rise of such species can suppress fisheries recovery. Fishery pressure on filter-feeding fishes like sardine exacerbates the problem by favoring jellyfish outbreaks (Jackson et al. 2001, Brotz et al. 2012, Condon et al. 2013).

Despite evidence from some well-documented cases, the degree to which fisheries declines are regime shifts remains a controversial issue for a number of reasons (Hilborn 2007, Litzow and Urban 2009). First, time series for most fisheries are only available from 1950, a time when marine ecosystems were already affected by flourishing industrial fishing, and some stocks had already been seriously depleted, especially those of large marine vertebrates like whales, manatees, crocodiles, seals, swordfish, sharks and rays (Kirby et al. 2009). This lack of long term data makes it difficult to identify causation in fisheries time series. Furthermore, marine ecological diversity and food web complexity complicates the detection of changes in fish populations due to fishing, because the signals of regime shift dynamics are small relative to ecological variation (Liermann and Hilborn 1997).

Impacts on ecosystem services and human well-being

Shift from high to low abundance of commercial fish species

Since 1950 the world marine fisheries catch has expanded almost fivefold, and is currently about 80 million tonnes of fish. According to FAO, about 30% of fish stocks are overexploited, 60% are fully exploited and about 10 % are not fully exploited (FAO 2012). Global marine fish catches have plateaued and are not expected to increase. Local fisheries decline has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Pauly et al. 2006). There is not clear assessment of the number of stocks that have collapsed but FAO suggests about 5% of fish stocks are depleted (FAO 2012). Some collapsed stocks have remained at under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the collapse of fisheries.  

Shift from low to high abundance of commercial fish species

An abundant fish population regime will likely better maintain marine biodiversity and will be more resilience to climate variation and climate change. Society will benefit of high abundance regime in terms of high primary productivity that translate in more food, jobs and livelihoods in small coastal communities. It will also better regulate marine hypoxia.

Management options

A review of marine fisheries in the Millennium Ecosystem Assessment offer a synthetic set of managerial options for addressing fisheries collapse (Pauly et al. 2006).

Options for enhancing resilience

There are two types of strategies for enhancing resilience. The first is to stop doing things that are eroding resilience, while the second is to being activities that build resilience. There are a number of fishing practices that could be eliminated because they damage the resilience of fisheries. For example, fishing practices that destroy habitat such as bottom trawling should be avoided. Developing alternative technologies and reduction of fishing effort are suggested. Bycatch, the killing but not consuming of unwanted fish, can also have substantial impacts and can be reduced. Indirectly, fisheries subsidies and inflexible quotas can drive fishing effort to high levels that erode the resilience of a fishery.

Options for reducing resilience of unwanted regime to encourage restoration or transformation

Direct resilience building strategies include the establishment of Marine protected areas (MPAs) (Takashina and Mougi 2014). These areas can play an important role by providing refugia for fish and increasing ecological diversity. Research suggests that they need to cover ecologically significant areas, including different habitats, ensuring connectivity among them. However there remains controversy over the extent to which MPAs shift fishing pressure to less protected sites.

Indirect resilience building efforts include efficient regulation and policing of fishing can reduce illegal fishing, while comprehensive fisheries policies at the international and national level can also build resilience if they result in fisheries policies that reinforce one another. Indirect resilience building strategies include improving the ability of fishers to exit fisheries.

Good management of fishing pressure has been able to restore fisheries stocks. For example, in the US fishing closures and implementation of management measures for bottom fishing have enabled the restoration of George Bank haddock, Atlantic scallops, George Bank yellowtail flounder, Atlantic striped bass, Atlantic Arcadian redfish, Pacific chub mackerel, and Pacific sardine. However, there is no clear strategy that works across many ecosystems or nations (Beddington et al. 2007).

Alternate regimes

Mangroves are ecosystems adapted to the mixture of fresh and salt water common in intertidal zones of marine coastal environments. Mangroves cover between 1.7-1.8 x 105 km2 globally. They develop within 30ºN and 38ºS and grow optimally between 15ºC and 25ºC (Lovelock 2008). Outside this geographical and thermal range mangroves show reduced leaf formation and above 38ºC or below freezing temperatures they do not photosynthesize (Mcleod and Salm 2006; Cavanaugh et al. 2014). Mangrove ecosystems are often used by local communities who depend on fishing and timber harvesting.

Mangrove forest

Mangrove forest is characterized by an ensemble of species with aerial roots adapted to survive in environments where water salinity and tides are highly variable. Mangroves grow in tropical and subtropical waters, where the optimal temperature range is between 15-24ºC. Different species occur under different salinity gradients and some trees reach 40m high in areas with high precipitation. Mangroves usually establish in soil enriched by the sediment discharge of rivers (Mcleod and Salm 2006). Mangroves are ecologically important because they provide habitat for many marine species at some stages of their lifespan, increasing both marine and terrestrial biodiversity.

Salt marshes, rocky tidal, shrimp farms.

Mangroves can become dominated by salt marshes or other configuration of coastal wetlands depending on the substrate, sediment deposition and the topography (Mcleod and Salm 2006). The alternative regime once a mangrove has collapsed is not straightforward. It strongly depends on the land use, topological characteristics, latitude and the history of the system disturbance (Cavanaugh et al. 2014). For example, many mangroves around the world have been converted into ponds for shrimp farming, salt extraction, or simply dry out to establish agriculture, cattle ranching, infrastructure development or human settlements.

Drivers and causes of the regime shift

The main drivers of mangrove collapse have typically been land use/cover change during the last 50 years. One third of world’s mangroves have been lost due to overexploitation of forest resources (deforestation) or conversion into agriculture, salt extraction, or ponds for shrimp farming (Cavanaugh et al. 2014). Other threats to mangroves include the development of infrastructure (roads, dams), the diversion of fresh water for irrigation and the development of urban areas. These impacts are typically stronger in developing countries where it is expected that mangroves will decline 25% by 2025 (Mcleod and Salm 2006), while recent studies show that mangrove areas are declining 1-2% yearly (Duke et al. 2007; Alongi 200; Cavanaugh et al. 2014).

Current climate change is also expected to affect mangroves via the increase in frequency and severity of storms, floods and droughts (climate extreme events), the increase of temperature, sea level rise, and ocean acidification. Frequent extreme events that potentially decrease the flow of fresh water will increase salinity, putting stress on some species and reducing their growing rates and seedling survival (Mcleod and Salm 2006). Precipitation increase, on the other hand, could favor mangrove over salt marshes by decreasing salinity and giving a competitive advantage to mangroves (Cavanaugh et al. 2014). However, strong storms and flooding events could prevent mangroves from respiring when their aerial roots are underwater. Although temperature is expected to increase 2-6ºC before 2100, its effect on mangroves is contested. On the one hand, a 6ºC increase would heterogeneously affect mangroves around the world, although it is likely to induce thermal stress, it won’t be strong enough to surpass the temperature tolerance of these ecosystems. An increase in temperature will increase the melting rate of ice and expand the volume of world’s oceans increasing the level of the sea. On the other hand, it has also been observed that warming on the coldest time of the year can favor mangroves over taking salt marshes in temperate areas of the planet, with a suggested threshold of -4ºC for Florida (Cavanaugh et al. 2014).

Sea level rise is perhaps the most important threat to mangroves. While the expected sea level rise projection range from 0.09 to 0.88 m (or 0.9 to 8.8 mm per year) several studies report that mangrove ability to migrate upwards range from 1.2 to 4.5mm per year (Mcleod and Salm 2006). Although faster migration has been observed in Holocene stratigraphic records for Florida, migration also depends of local topological conditions. Increase in atmospheric carbon dioxide could increase mangroves growth but also ocean acidification. While the increase in growth is not expected to be strong enough to overcome the pressure of sea level rise, acidification is expected to have an indirect effect on mangroves by reducing coral reefs accretion, increasing in turn the erosive effects of waves on mangrove soils (Mcleod and Salm 2006).

How the regime shift works

Shift from mangroves to salt marshes, rocky tidal or shrimp ponds.

By having aerial roots mangroves adapt to very specific conditions where salt and fresh water meets in coastal zones. Mangroves build up carbon rich soils also known as peat, which favors further mangrove growth.

Loss of mangrove area decreases peat production. Peat is lost by wave erosion and reduces the likelihood of recovery of mangrove forest. Mangrove area loss is mainly driven by conversion to agriculture, urban settlements, overexploitation of wood, shrimp farming, salt extraction, or infrastructure development (e.g. roads, dams, water divergence to irrigation) (Cavanaugh et al. 2014). Human perturbations are maintained by social feedbacks such as demand for food and market access, population growth and the cumulative benefit of human settlements (higher provision of services). In contrast, communities with poor services access such as electric energy or gas for cooking strongly rely on mangrove wood as source of energy and construction material (Mcleod and Salm 2006).

If sea level rise occurs faster than the ability of mangroves to build up peat or migrate to higher areas, mangrove forests will disappear in major areas of the world (Alongi 2008). Both the impact of sea level rise and mangrove’s ability to adapt depend on local conditions and are highly uncertain. For example, mangroves in carbonate substrates (such as coralline islands) and microtidal systems (short tidal change from high to low) are less likely to adapt and migrate as climate change continues. Mangroves in rich soil substrates and macrotidal systems are more likely to migrate if salinity balance and sedimentation do not overcome their tolerance range (Mcleod and Salm 2006).  

 

Shift from salt marshes to mangroves  

Mangroves have also been observed to migrate pole wards colonizing habitats previously dominated by salt marshes (Cavanaugh et al. 2014). The shift occurs when the minimum temperature in winter increase for a longer period of time, giving mangroves an advantage to over compete salt marshes. It has been observed in areas such as Florida, Louisiana or Australia. Although there is uncertainty regarding the mechanism, the threshold is thought to be related with the number of days that minimum temperatures are colder than -4ºC, at least for Florida case (Cavanaugh et al. 2014). The authors of the Florida study also report that it’s unlikely that local scale drivers such as nutrients inputs, sedimentation or hydrology have had a determinant effect on the shift. However, sea level rise has contributed to increasing mangrove area (since it has happen slowly enough) and other common driver of the shift can reduce mangrove accretion such as coastal development, aquaculture and timber production (Cavanaugh et al. 2014).

Impacts on ecosystem services and human well-being

Mangroves provide a variety of ecosystem services. Mangrove ecosystems provide habitat for economically important marine species for fishing (e.g. shrimp or lobster among other crustaceans and mollusc), and they provide habitats for important groups of terrestrial species such as reptiles, monkeys, and birds. They are therefore important for maintaining both marine and terrestrial biodiversity. Mangroves also provide fuelwood, charcoal, and wood for construction. As a forest it provides protection against storms, floods, river born siltation and also traps water pollutants and sediments, reducing the erosive action of waves, particularly protecting adjacent ecosystems such as coral reefs, and sea grass beds (Duke et al. 2007). Mangroves have also been shown to play a key role in the global carbon budget, capturing up to 15% of global carbon and exporting up to 11% of particular terrestrial carbon to the oceans (Alongi 2014). In fact, recent studies support the hypothesis that mangroves are able to capture and store carbon in a greater extend that terrestrial ecosystems (Lovelock 2008), nearly identical to those of tropical forest (Alongi 2014). Therefore, losing mangrove area implies the loss of carbon storage.

While in the short term local communities could benefit by converting mangroves into shrimp farms, agriculture fields, or using its wood; in the long term they mean the loss of important services such as coastline protection, biodiversity important for tourism and fishing, water cleansing and carbon storage (Ewel et al. 1998; Cavanaugh et al. 2014). In fact, their services has been valued over $1.6 trillion per year (Costanza et al. 1997).

Management options

McLeod et al. (2006) propose a series of tools to monitor and plan for mangroves adaptation to climate change. First they propose to assess mangrove vulnerability based on the local conditions. Management options for mangrove forests are highly context dependent. As the main threat to mangroves is climate change through sea level rise, mangrove ecosystems will likely migrate upwards and northwards (Cavanaugh et al. 2014). However, such migration is constrained by local conditions such as substrate types, infrastructure development, sediment input and changes in salinity.

Second, they propose to apply risk-spreading principles (Duke et al. 2007) and identify areas that are likely to be sources and sinks of the migratory process (Mcleod and Salm 2006).  By identifying the role of each mangrove patch, it is easier to prioritize where green belts are needed to increase connectivity, which patches should be protected and which patches are more likely to respond to restoration efforts. They emphasize the importance of establishing a baseline and monitoring program with local communities in order to assess the main drivers locally. Developing partnerships with local stakeholders and developing alternative livelihoods for mangrove dependent communities are key managerial efforts (Mcleod and Salm 2006). The resilience of mangroves to future climate change scenarios also depend on how successful management efforts are at reducing the influence of other drivers: deforestation, overexploitation, infrastructure development, divergence of water for irrigation, and urbanization. Management of such local forcing will increase resilience to regional forcing such as increasing storm events or ocean acidification.

Third, it has been suggested that policy and market-based are critical to foster conservation efforts, reduce mangrove encroachment and maintain carbon storage through funding mechanisms such as REDD or other payment for ecosystem services schemes (Hutchison et al. 2013). Effective governance structures and education has also been suggested as managerial strategies (Duke et al. 2007).

Key References

  1. Alongi, D. M. 2008. Mangrove forests: Resilience, protection from tsunamis, and responses to global climate change. Estuarine, Coastal and Shelf Science 76:1–13.
  2. Alongi, D. M. 2014. Carbon Cycling and Storage in Mangrove Forests. Annual review of marine science 6:195–219.
  3. Cavanaugh, K. C., J. R. Kellner, A. J. Forde, D. S. Gruner, J. D. Parker, W. Rodriguez, and I. C. Feller. 2014. Poleward expansion of mangroves is a threshold response to decreased frequency of extreme cold events. Proceedings of the National Academy of Sciences 111:723–727.
  4. Costanza, R., R. dArge, R. deGroot, S. Farber, M. Grasso, B. Hannon, K. Limburg, S. Naeem, R. ONeill, J. Paruelo, R. Raskin, P. Sutton, and M. vandenBelt. 1997. The value of the world's ecosystem services and natural capital. Nature 387:253–260.
  5. Duke, N. C., Meynecke, J. O., Dittmann, S. & Ellison, A. M. A world without mangroves? Science (2007).
  6. Ewel, K. C., R. R. Twilley, and J. E. Ong. 1998. Different kinds of mangrove forests provide different goods and services. Global Ecology and Biogeography Letters.
  7. Hutchison, J., A. Manica, R. Swetnam, A. Balmford, and M. Spalding. 2013. Predicting global patterns in mangrove forest biomass. Conservation Letters
  8. Lovelock, C. E. 2008. Soil Respiration and Belowground Carbon Allocation in Mangrove Forests. Ecosystems 11:342–354
  9. Mcleod, E., and R. V Salm. 2006. Managing Mangroves for Resilience to Climate Change. Page 64. IUCN, Gland, Switzerland.
  10. Millennium Ecosystem Assessment. 2005. Ecosystems and human well-being: synthesis‎:137.
  11. Restrepo, J. D., and A. Kettner. 2012. Human induced discharge diversion in a tropical delta and its environmental implications: The Patía River, Colombia. Journal of Hydrology 424-425:124–142.

Citation

Juan Carlos Rocha, Reinette (Oonsie) Biggs. Mangroves transitions. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-30 08:39:15 GMT.
Thursday, 18 September 2014 07:21

Common pool resource harvesting

Written by Steven Lade

Common pool resource harvesting

Main Contributors:

Steven Lade, Maja Schlüter

Other Contributors:

Simon Levin, Alessandro Tavoni

Summary

A community derives ecosystem services by extracting from a renewable resource such as water, fish, or forest. If a harvester is overharvesting (a defector), he/she is ostracised by the community, for example by obstructing their access to necessary machinery or to market, which encourages the defector to co-operate by harvesting at a lower, socially optimal level. The alternative regimes are (1) high co-operation and resource levels and (2) overharvesting. Key drivers include resource inflow, the effectiveness of the ostracism norm, and the cost of harvesting. Key impacts include degradation in the state of the resource and harvester payoff and wellbeing. Evidence is currently confined to modelling studies.

Drivers

Key direct drivers

  • Harvest and resource consumption
  • Environmental shocks (eg floods)

Land use

  • Urban
  • Small-scale subsistence crop cultivation
  • Large-scale commercial crop cultivation
  • Intensive livestock production (eg feedlots)
  • Extensive livestock production (rangelands)
  • Timber production
  • Fisheries
  • Mining
  • Conservation
  • Tourism

Impacts

Ecosystem type

  • Marine & coastal
  • Freshwater lakes & rivers
  • Temperate & boreal forests
  • Tropical forests
  • Moist savannas & woodlands
  • Drylands & deserts
  • Mediterranean shrubs (eg Fynbos)
  • Grasslands
  • Tundra
  • Polar
  • Agro-ecosystems

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Food crops
  • Livestock
  • Fisheries
  • Wild animal and plant products
  • Timber
  • Woodfuel
  • Fuel and fiber crops
  • Hydropower

Human Well-being

  • Livelihoods and economic activity

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Weeks
  • Months
  • Years
  • Decades

Reversibility

  • Hysteretic

Evidence

  • Models

Confidence: Existence of RS

  • Speculative – Regime shift has been proposed, but little evidence as yet

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Alternate regimes

Steppe

The steppe vegetation coverage is dominated by grasses growing in dry soils (Eroglu et al. 2012). In this regime, herbivores increase nutrient availability by accelerating nutrient cycling. This increases primary production. During the Pleistocene epoch, arctic steppes sustained large grazing herbivores such as mammoths, bison and yaks (Zimov 2005). Today, grasses still sustain reindeer/caribou population. Currently, the steppe communities of Yakutia, Siberia are confined to the terraces above floodplains and south facing slopes of the river valleys (Yurtzev 1982).

Tundra

The tundra vegetation coverage is dominated by mosses and shrub such as dwarf birches and willows growing in waterlogged soils. The ecosystem has lower primary production and nutrient cycling thus the growth rate is slow. Moss growth is limited by moisture so the moist and nutrient low soil promote and sustain moss growth.

Drivers and causes of the regime shift

Herbivory has caused large-scale vegetation changes across the Arctic (Speed et al. 2009; Bråthen et al. 2007; van der Wal 2005; Srivastava & Jefferies 1996). Herbivores trample the vegetation and deposit faeces to nutrient limited soils which promotes grass growth sustaining herbivore population. In the absence of herbivores grass litter increases causing soil moisture to increase which reduces nutrient availability promoting the growth of unpalatable mosses.

Climate can act as a driver by changing air temperatures which is associated with changes in soil moisture. Soil moisture is key component in sustaining moss or grass coverage. Moss growth is more limited by water than by nutrients. However, nutrients are more available in drier soils which promote grass growth. Currently, climate is changing, in the long term whether steppe, tundra or another regime will dominate Arctic ecosystems in uncertain.

How the regime shift works

Shift from Steppe to Tundra
Grass-dominated steppe is maintained by trampling and nutrient inputs from herbivore faeces. The enriched soils promote the growth of grass coverage (Shaver et al. 1986). Grasses have a strong influence on soil moisture as they dry out the soil through high transpiration (Zimov 2005). Dry soil conditions are associated with increased nutrient availability intensifying grass growth (Nadelhoffer et al. 1991).

In the absence of herbivores, moss coverage increases creating water-logged soils (Zimov 1995). Water-logged soils prevent decomposition of organic matter, so nutrient availability decreases, which limits the growth of grasses and, hence, promotes moss growth instead (Zimov 1995). As moss growth increases, herbivore density is further reduced because moss have lower nutritional value.

Moss-dominated tundra is susceptible to trampling, a disturbance caused by large herbivores; therefore, moss coverage increases with low herbivore density (van der Wal 2001). Mosses causes an increase in soil moisture. The combination of high soil moisture creates the conditions for water-logged soils establishing optimal conditions for moss growth (Zimov 1995).

 

Shift from Tundra to Steppe

Climate is the main driver that could shift tundra back to steppe; this is because large grazers don't exist any more and arctic ecosystems are vulnerable to climate change. Rising of air temperatures would deepen the thawing layer of permafrost and increase evaporation. Therefore, frozen nutrients from soil would become available, which would promote grass growth; in addition to that, higher temperatures will enhance organic matter decomposition and hence nutrient availability. Then, grasses will increase evaporation. Nevertheless, moist soil conditions might be resilient since permafrost melting will make frozen water available. If there are no draining conditions, moisture will provide a suitable environment for mosses that will insulate soil temperatures and then buffer permafrost melting.


Rising of temperatures will create dynamics on arctic ecosystems that are difficult to predict. However, recent evidence shows that tundra could turn into different types of vegetation (e.g. shrub land, forest, lake, grassland) depending on local conditions (Karlsson et al. 2011). Steppe-grasses will prevail on drier areas, but more probably as patches within a mosaic since large herbivores that maintained steppe conditions during the Pleistocene, are not present.

Impacts on ecosystem services and human well-being

Shift from Steppe to tundra

Shifting from steppe to tundra had a great impact on provision of food. The thermal insulation of the tundra prevents permafrost melting, which prevents destabilization and collapse of infrastructure (Schaefer 2012). Increasing air temperature due to climate change deepens the active layer of permafrost. However, soils covered by moss, permafrost is less susceptible to degradation (Ivanova 2003), this ecosystem service is essential since permafrost contains almost twice as much carbon as the atmosphere today and its effects would be irreversible at human time scale (Schaefer et al. 2012).

The wellbeing of nomadic indigenous peoples such as Nenets, Enets, Sami, Nganasans and Selkups, are connected to Arctic that provide opportunities for grazing and food sources, as well as ecosystems that co-produce cultural ecosystem services i.e. production and conservation of indigenous knowledge, practices and believes, as well as production of scientific knowledge (Huntington 2013). Tourism is also a cultural ecosystem service that will be affected.

 

Tundra to steppe

Arctic soils covered by mosses, permafrost is less susceptible to degradation (Ivanova 2003). Transition from tundra to steppe implies loss of this buffering layer. Consequence, the following ecosystem services will be impacted: Water availability. Local and regional freshwater dynamics will change leading to a decline in the number of lakes and wetlands (Wrona et al. 2016; Schaefer 2012). Increased abundance of grassland will affect livestock production, steppe might benefit both reindeer hunters and herders. Protection from infrastructure destabilization; pipelines, railways and power lines across the arctic are built on solid ground provided by the permafrost that if degraded their stability might be lost. This might cause economic costs and serious ecological damages (Schaefer, 2012). Fire regulation; as moss tundra shifts to a drier regime, possibilities of wild fires will increase, which contributes to thermal erosion of permafrost (Schaefer, 2012). Prevention of climate change; permafrost degradation turns permafrost's carbon stock into a carbon source, which would accelerate climate change (Schuur et al. 2015). Loss of this ecosystem service impacts the global level, since permafrost contains twice as much carbon as the atmosphere and effects of release is irreversible at human time scale (Schaefer, 2012).

Cultural ecosystem services will also be affected; conservation of indigenous knowledge, practices and believes related to tundra would be lost, affecting the well being of t he Nenets, Enets, Sami, Nganasans and Selkups (Huntington 2013).

Management options

To prevent the steppe from shifting into tundra, greenhouse gas emissions must be reduced in order to avoid the effects of climate change. The planetary boundary value for atmospheric CO2 concentration is 350 ppm above pre-industrial level (Rockstrom et al. 2009). This was further supported by the COP21 agreement, a treaty signed to limit temperature increase to 1.5oC compared to pre-industrial levels. These efforts are not enough to prevent the feedback loop, eliminating emission is the only solution. In the Artic, land management strategies can address feedback loop on the local level. In a study, by Post and Pederson (2008), a 5-year experimental investigation of Arctic plant community response to warming, showed that warming under continuous grazing pressure from muskoxen did not differ from plant composition without warming suggest that management of large herbivores may be an important aspect for mitigating ecosystem response to future climate change

Attempts are being made today to restore the Pleistocene steppe ecosystem on a limited spatial extent in Siberia. Sergey Zimov's Pleistonce Park, 160 km2, attempts to reestablish the steppe ecosystem by reintroducing megafauna to Northern Siberia (Republic of Yakutia). Zimov (2005) and colleagues tried to test if the 'key-herbivore´ hypothesis can be verified, as today's Holocene climate should be optimal for the Pleistocene steppe vegetation. The megafauna consisting of reindeer, moose, Yakutian horses, musk oxen and bison would influence the vegetation and soil composition, by trampling on grassland and returning nutrients to the soil through their manure (Zimov 2005). The grass root systems stabilize the soil and trampling reduces the albedo, exposing ground to colder temperatures, both would prevent permafrost from melting.

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on  freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants as perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Alternate regimes

Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).

Polar regime

The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).

Temperate regime

Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).

Drivers and causes of the regime shift

Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.

Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).

The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).

How the regime shift works

The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.

The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).

The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.

 



Impacts on ecosystem services and human well-being

Shift from polar regime to temperate regime

Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).


Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).

Management options

From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.

It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.

Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.

Alternate regimes

A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.  

With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.

Arctic benthos (regime 1)

With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.

Subarctic benthos (regime 2)

After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).

 

Drivers and causes of the regime shift

The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.

The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).

 

How the regime shift works

In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)

Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.

Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions. 

Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.

 

Impacts on ecosystem services and human well-being

Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.

The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being. 

 

Management options

Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.

Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.

 

Alternate regimes

Due to thermokarst processes such as permafrost melting, numerous areas of the Arctic landscape are extremely lake-rich.  The water balance of those lakes may either be maintained due to impermeable permafrost that limits groundwater flow (Marsh 2009) or lost due to seasonal drainage, a natural characteristic of the Arctic system. However, a widespread, unnatural decline in lake abundance is being observed within the Arctic due to decades of rising atmospheric temperatures in the region that has been deteriorating the state of the permafrost (Smith et al. 2005). The lake-rich landscape of the Arctic is therefore slowly transforming into a drained, dry landscape increasingly dominated by vegetation such as shrubs and graminoids (Hinzman 2005).

 

Thermokarst lake ecosystem

 

The thermokarst lake-dominated landscape is formed as a response to some imposed disturbance, such as road construction, wildfire or climatic warming, which causes permafrost to thaw, creating irregular surface topography depressions, called thermokarsts. Those may appear as near-surface massive ice melts, allowing the surface to subside (Hinzman et al. 2005). As permafrost melts over time, those depressions become filled with melted water forming the lakes characteristic of the Arctic region.

 

Terrestrial ecosystem

 

Driven by external and internal factors, Arctic lakes are drying up at an unnatural speed within the region. This shift is accompanied by changes in vegetation, for example, increased shrub- and graminord abundance and a decrease in the cover of mosses and lichens through a broad expanse of the Arctic (Hinzman et al. 2005). The changes in vegetation may then further transform the landscape through sequential effects on foraging mammals and birds, as well as aquatic fauna and insects (Hinzman et al 2005).

 

Drivers and causes of the regime shift

 

Shift from thermokarst lakes to terrestrial ecosystem

The main external driver of this regime shift is climate change, which is caused by elevated greenhouse gas emissions (IPCC 2007). Increased temperatures warm the soil and warms the permafrost. As the permafrost thaws, greenhouse gasses such as CO2 and CH4 that are trapped in the ice are released into the atmosphere, further increasing the effects of climate change (Hinzman 2005; Karlsson et al. 2011). At a local scale, the effects of climate change are incremental. However, as this interaction is occurs over large areas, the effects are aggregated and self-reinforcing, which is significant since permafrost and seasonally frozen grounds store about 25% of the total global soil carbon stock (IPCC 2007).

At a local scale, initial thawing of the permafrost from increased atmospheric temperatures creates a dynamic landscape of depressions (thermokarst) in which lakes are formed (IPCC 2007). As permafrost become more degraded, drainage through outlet channels occurs. Since the temperature of melted water is higher than that of ice, it will exacerbate the melting of permafrost, causing another reinforcing feedback. Once the layer of frozen ground is completely penetrated the lakes can become permanently drained allowing for vegetation to be established (Marsh et al. 2008).

Shift from terrestrial ecosystem to thermokarst lake ecosystem

Heightened air- and soil temperatures melt the ice-rich soils and permafrost on which forests in the Arctic region develop, changing the physical conditions for the forest growing on top of it. When the roots of the trees get flooded, the trees die and ponds and lakes eventually replace the forest (Hinzman 2005). However, it is important to note that the shift from a terrestrial ecosystem to a lake dominated one is not as frequent as the shift from lakes to terrestrial ecosystems. The reason for this are the reinforcing feedbacks as mentioned above.

How the regime shift works

The warming of soil temperatures leads to the melting of continuous permafrost and the subsequent increase in abundance of discontinuous or sporadic permafrost. Melting of permafrost causes depressions in the soil called thermokarsts. Increased precipitation in the form of snowfall may have either an insulating or cooling effect on soil temperatures depending on several variables such a timing, duration, accumulation, and melting processes of seasonal snow cover, density, structure, and thickness of seasonal snow cover etc. (Zhang 2005). Depending upon the local surface energy balance, the thawed ground may refreeze or the permafrost can continue to degrade (Hinzman et al. 2005). On sites where continuous permafrost is thawing the active layer of soil in which melted water can move deepens allowing for the formation of thermokarst lakes (Hinzman et al. 2005). In freezing seasons, if the temperature is low enough and the hydraulic parameters on site suitable, the formation of these lakes can contribute to the re-formation of permafrost soils. Ice-rich permafrost can also prevent surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet. Decreased albedo due to changes in vegetation, the extension of snow-free and ice-free periods on terrestrial and lake surfaces and abundance, and reduction in the area occupied by glaciers and continental ice sheets in high latitudes may act as a positive feedback into atmospheric warming (Hinzman et al. 2005).

On sites where there is an abundance of discontinuous permafrost the active layer also deepens allowing for increased surface area for sub-ground water movement that increases the rate of drainage. Thermokarst lakes and ponds may begin to fill or drain depending upon the direction of the hydraulic gradient beneath the lake (Hinzman et al. 2005). Therefore it is not the depth of the active layer alone that determines the rate of drainage. However, increased drainage of lakes may prevent the reformation of permafrost from lack of available water for freezing in winter months and it can be inferred through advection of heat from decreased albedo. Once these drained lake beds become permanently drained they are likely to be re-vegetated which will change the albedo and may cause the extension of snow-free and ice-free periods and may act as feedback into increasing atmospheric temperature (Hinzman et al. 2005).

The affects of the shifts from lake to terrestrial ecosystems are aggregated at a local, regional and global scale due to the large scale and frequency at which those dynamics occur. The whole regime as described in the causal loop diagram below is a re-enforcing feedback that at its current state is leading to an overall decrease in lake dominated ecosystems and a increase in terrestrial ecosystems (Smith et al. 2005).

Impacts on ecosystem services and human well-being

Even though this regime shift occurs at the local scale, its consequences go beyond that scale because it reshapes regional hydrology (through changes in water balance and surface water- connection and fragmentation) and alters regional supplies of the ecosystem (Karlsson et al. 2011). This regime shift therefore affects provisioning ecosystem services, as it will decrease the availability of freshwater. The above-ground stored freshwater, which is of vital importance to migratory birds, fish, and other wildlife used by indigenous people, will shift to below-ground stored freshwater (Artic Science Journeys Radio Stories 2005). Globally, however, freshwater will become a more abundant ES due to reductions permafrost and subsequently ice sheets and glaciers resulting from the regime’s feedbacks to climate change. Nevertheless, this global increase in the freshwater ES is not a positive one: “Increased freshwater delivery to the Arctic Ocean from reductions in ice sheets and glaciers result in rising sea level (...)” (Hinzman et al. 2005). At a local scale traditional hunting and fishing practices will be impaired (Vincent et al. 2013).  Arctic freshwater systems provide important migratory routes for fish stock. Due to this regime shit, those routes would be greatly altered in connectivity among lakes and river channels, as well as in terms of their physical coupling to the coastal marine ecosystem (Vincent et al. 2012). Globally, fishing practices would suffer as well. Increased discharge of water into the Arctic Ocean due to draining results in increased nutrient and external organic matter inputs to the Arctic Ocean that affect primary processes at the base of the marine food web (Vincent et al. 2012). Climate regulation is heavily affected on a global scale through the processes of this regime shift: as it sets up a positive feedback cycle in which the release of methane and carbon dioxide through the newly exposed soil and melted permafrost feeds into global warming. The shift from lakes to terrestrial ecosystems heightens gully erosion, a type of erosion that occurs when water is channeled across unprotected land and washes away the soil along the drainage lines. As permafrost degrades further, it causes erosion and slumping of lake edges and stream channels

The Arctic lakes are of vital importance to indigenous people as they are a source of fresh drinking water and also provides important winter transport routes. These lakes not only allows indigenous people access to their traditional hunting and fishing areas, but also to transport goods to remote communities and industries such as mining centers (Vincent et al. 2013). The regime shift would also mean an increase in products extracted from woods such as timber commodities and fuel: “increased areas of tree growth in the Arctic could serve to (…) take supply more wood products and related employment, providing local and global economic benefits” (Hassol et al. 2004).

 

Management options

There are few management options available to maintain the thermokarst lake ecosystems due to the long time scale at which permafrost melts and the natural dynamics within this system e.g. hydraulic fluctuations or site-specific soil conditions. One management option is to continue to reduce the emissions of greenhouse gases while simultaneously investigating further opportunities for carbon offsetting schemes.

For hydroelectric reservoirs, shifting ice conditions will have both positive and negative effects, and may require adaptive changes in operating procedures, with attention to minimize negative impacts associated with ice jams and ice breakup downstream of the spillway (Vincent et al. 2013). Fisheries management plans will also need to be adapted to the changes in migration and productivity of northern fish populations with ongoing climate change (Vincent et al. 2013). These essential resources require the development of integrated freshwater management plans, which include consideration of alternate water sources as traditional supplies change in quantity or quality (Vincent et al. 2011). The Arctic will require increasing vigilance and appropriate water management strategies to avoid and minimize the impacts of changing water impacts in the future (Vincent et al. 2013).

Alternate regimes

The boreal forest in northern Interior Alaska covers an approximate area of 48 million hectares (Mann et al. 2012). Bounded by the Alaska Range to the south, the Seward Peninsula to the west and Brooks Range to the north (Wolken et al. 2011). It is characterized by large areas of gently sloping uplands and flat lowlands isolated by mountain ranges and braided rivers with broad floodplains. During the past 50 years, the mean annual temperature in the interior boreal forest of Alaska has increased by 1.3 ºC (Hartmann and Wendler, 2005) and this has been strongly associated with recent increase in fire frequency and severity (Wolken et al. 2011). The annual area burned in Interior Alaska has doubled during the last decade (Kasischke et al. 2010). The forest has been dominated by coniferous trees (mainly black spruce), which is a group of species that thrive under colder, moist conditions in a deep soil organic layer. However, due to climate warming, Interior Alaska is currently shifting towards a deciduous dominated forest (e.g. aspen and birch) that prefer warmer and drier conditions with a shallow soil organic layer (Johnstone et al. 2010).  

 

 

Low fire frequency coniferous dominated forest

Under cold and wet conditions, coniferous trees, such as the black spruce (Picea mariana), dominate the boreal forest in Interior Alaska. The coniferous trees promote the accumulation of a deep soil organic layer, cold and moist soil temperatures, and slow decomposition rates. Due to the moistness of the soil, fires are not frequent in these forests. However, when they occur, they tend to be severe due to the high flammability of the black spruce. Due to the high water content of the soil organic layer, it will only be entirely consumed by the most severe fires (Johnstone et al. 2010). As long as the entire organic layer has not been consumed, the coniferous trees have a regenerative advantage due to their seed’s ability to sprout in moist organic soils. This means that these forests tend to return to a coniferous dominated state even after a fire (Hollingsworth et al. 2013).

 

High fire frequency deciduous dominated forest

A deciduous dominated forest, mainly consisting of aspen and birch trees in Interior Alaska, prefers and reinforces dry and warm soil conditions. Although these forests have a high organic litter production, the fast decomposition rate and the subsequent fast nutrient turnover, leaves a shallow soil organic layer and a nutrient rich mineral soil that favours deciduous tree regeneration. The warm and dry conditions in the forest lead to a comparatively high fire frequency, but due to the low flammability of the deciduous trees, the fires rarely become very severe (Johnstone et al. 2010). However, because of the shallow pre-fire soil organic layer, the fires tend to expose the mineral soil, which favours the post-fire reproduction of deciduous trees (Hollingsworth et al. 2013).

 

Drivers and causes of the regime shift

The main external direct driver of the regime shift is climate warming, which affects the important feedbacks of the boreal forest ecosystem. Warmer and drier climatic conditions have a direct effect on the most important soil characteristics (i.e. soil temperature, moisture and depth of the soil organic layer), influencing the vegetation cover. Coniferous trees require a cold moist soil with a deep soil organic layer to successfully reproduce. Deciduous trees, on the other hand, are competitively superior under warmer and drier soil conditions with a shallow soil organic layer (Johnstone et al. 2010). Extended summer seasons and the accompanying drier conditions create a suitable environment for an increase in the annual number of natural ignitions by lightning strikes (Kasischke et al. in press). Fires have an immediate effect on the environmental conditions of the forest, affecting factors such as soil moisture and the post-fire forest community composition (Kelly et al. 2012). Once established, the trees support and emphasize the conditions that favour their own reproduction and dominance. Under colder environmental conditions, coniferous trees, such as the black spruce, produce large amounts of organic litter, which, together with the presence of permafrost, maintains a moist soil and slow decomposition rates, contributing to a deep soil organic layer. Under warmer climatic conditions, deciduous trees produce ground layer fuels that decompose at fast rates, maintaining a shallow organic layer (Johnstone et al. 2010). Global socio-economic development indirectly drives climate warming, contributing to the increased atmospheric temperatures through e.g. greenhouse gas emissions (IPCC, 2011). Fire management can have a limited effect on the regime shift through fire suppression practices.

How the regime shift works

Shift from coniferous to deciduous-dominated forest

The coniferous dominated boreal forest of Interior Alaska requires a deep soil organic layer to succeed over other classes of trees. Once the required depth is present, this has a strong reinforcing effect on the coniferous forest. This is a requisite to maintain cold moist soil conditions in which coniferous trees thrive. The cold soil temperature reduces the decomposition rate of organic litter, maintaining a deep soil organic layer (Johnstone et al. 2010).

 

External shocks such as severe and frequent fires or insect outbreaks might cause dramatic shifts in the soil and vegetation by impacting the soil organic layer depth and tree succession (Hollingsworth et al. 2013; McCullough et al. 1998; Wolken et al. 2011). If the severity of such external shocks is sufficient, thresholds in the most important regime characteristics will be crossed (e.g. shallow post-shock soil organic layer) creating windows of opportunity for deciduous species to dominate. Atmospheric temperature influences the frequency and severity of fires, the soil characteristics and the length of the growing season; therefore, climate warming can also cause a change in conditions that would set the grounds for a shift to a new stable state (Mann et al. 2012).

 

The deciduous dominated forest has a competitive advantage in warm and dry conditions with shallow soil organic layers. Again, once the required shallow layer is present, it has a strong reinforcing effect on the deciduous forest. A shallow soil organic layer maintains warm and dry soil conditions that speed up the decomposition of organic litter, maintaining the required depth of soil organic layer (Johnstone et al. 2012).

 

Shift from deciduous-dominated  to coniferous forest 

In order to shift from regime 2 to regime 1, a prolonged climatic cooling would be necessary. A slow change of the environmental conditions towards those that favour a coniferous boreal forest could, if the forest is left undisturbed for a sufficiently long time, lead to late successional coniferous species, like the black spruce, replacing early successional deciduous species (Rupp et al. 2002). Although the spatial scale considered is of extreme importance, sufficiently large external shocks, such as deciduous-specific insect outbreaks, could clear sufficiently large areas to advance the dominance of coniferous trees. In such hypothetical events, coniferous trees could get the opportunity to sprout and start altering the soil conditions to their own favour.

Impacts on ecosystem services and human well-being

The provisioning of wild products such as berries and game is expected to shift some species and decrease others, but it is unclear what the net changes will be (Chapin et al. 2008). The boreal forest is likely to become a net source of carbon to the atmosphere (McGuire et al. 2009; Schuur et al. 2009), while the air quality is expected to decrease periodically due to the increase in fire frequency (Chapin et al. 2008).

Management options

The primary tactic used for reducing impacts of fire and enhance resilience (i.e. maintain structures, feedbacks, functions and processes of regime 1 is fire suppression to reduce the fire severity (Chapin et al. 2008). Decrease in fire severity could prevent the fire from consuming the soil organic layer, which in turn favours the conditions for coniferous tree. Fire suppression should be maintained in Alaska since it minimizes the risk fire imposes to life and property in communities. However, distant fires should be allowed to burn under controlled conditions, since it could have a cooling effect at a regional scale due to the higher albedo of post-fire deciduous species compared to the previous coniferous forest (Chapin et al. 2008).

Reducing air temperature is essential to be able to restore regime 1. Alaska itself accounts for a miniscule proportion of global greenhouse gas emissions and consequently to climate warming (Chapin et al. 2008). Therefore, reducing Alaska’s emissions will not have a major impact on global climate change. Global abatement of emissions is the long-term solution. Regardless of global greenhouse gas emission policies, it is highly likely that recent warming and wildfire frequency will continue for several decades in Alaska due to the multidecadal lag in the climate system (IPCC, 2007). This makes the reduction of emissions an inefficient short-term solution.  Secondary challenges that face efforts to achieve reduced emissions include restricting the economy and changing the focus of developed nations on a continued economic but sustainable growth. Educating global public about the social impacts of climate change in Alaska and other parts of the world (e.g. reduction in ecosystem services) may promote the willingness to reduce emissions (Chapin et al. 2008).

In summary, this means that the regime shift from coniferous to deciduous dominated forest in theory could be reversible through manipulation of e.g. soil conditions or local climate. However, due to projected global socio-economic development and climate change, it is highly unlikely that reversing the regime shift will be possible during the next 100 years.

Alternate regimes

This regime shift involves an abrupt reorganization of an aquatic food web due to a decrease in top predators or increase in environmental factors like temperature. Trophic cascades play a central role in these regime shifts. During these cascades groups in adjacent trophic levels often show inverse patterns in their abundance (Carpenter 2003). In other words, every other trophic level displays an increase in biomass, which leads to increased predation pressure and biomass decrease on every other trophic level. Most trophic cascades are described in ecosystems with low species diversity and/or simple food webs and/or small size (Frank et al. 2005). The magnitude of a trophic cascade varies depending on species diversity, regional oceanography, local physical disturbance, habitat complexity and fishery practices (Salomon et al. 2010). Food webs with more trophic levels and biodiversity show less trophic cascades than simpler systems (Salomon et al. 2010); due in part to omnivory and the variation on trophic interaction strength (Bascompte et al. 2005). However some documented examples includes subtidal reefs in New Zealand (Shears and Babcock 2002), Caribbean coral reefs (Bellwood et al. 2004), the Gulf of Maine (Steneck et al. 2004), and the North Atlantic (Kirby et al. 2009).

Predator-dominated food web.

Ecosystems in this regime are characterized by high predatory fish, low planktivorous fish, high zooplankton and low phytoplankton abundance. Such ecosystems can be subject to human influence, such as intensive fishery, but exhibit so called “natural compensation” e.g. via species richness and omnivory (Pace et al. 1999) that maintain the regime. Despite high productivity, such ecosystems are less likely to suffer from eutrophication due to high grazing pressure on phytoplankton. Ecosystem services associated with predator-dominated food webs includes food provision, higher biodiversity, better disease and pest control, as well as cultural services like recreation and aesthetic values (e.g. diving, sport fishing).

Planktivore-dominated food web.

Ecosystems in this regime are characterized by low predatory fish, high planktivorous fish, low zooplankton and high phytoplankton abundance. In extreme cases the actual number of trophic levels (TLs) can decrease. Ecosystems with lower trophic level dominance are more vulnerable to eutrophication due to lowered grazing pressure on phytoplankton, as well as invasion of planktivore organisms like jellyfish. A planktivore-dominated food web can affect the fluxes of carbon, transforming aquatic systems -coastal and lakes- from sinks to sources of green house gases (Bakun et al. 2010; Estes et al. 2011). Other ecosystem services like fisheries and recreation are expected to be significantly reduced.

Drivers and causes of the regime shift

Trophic cascades are usually initiated by fishing pressure that have direct or indirect impacts on the abundance of top predators. The response time to these drivers varies across ecosystems. In the case of coastal areas it has been found that the ecosystem response time to overfishing can vary from decades to centuries where multiple predators exist (Jackson et al. 2001).

Other factors that affect the regime shift are global warming and demand of food and fiber. Global warming have effects on upwelling, water circulatory systems that bring nutrients from the bottom of the sea towards the surface. By altering upwellings, food webs receive either too much or too little nutrients. In both cases the energy transferred towards higher trophic levels diminish as nutrients input variability increase. Demand of food strongly drives fishing, being the main cause of lost of top predators over the world.

How the regime shift works

Predator dominated food webs occur in environments with low disturbance both from fishing and nutrients input. They usually have four or more trophic levels. The regime is maintained by biotic mechanism that comprise predation and competition both intra and inter species. Abiotic factors also play a role maintaining the regime, by allowing an intermediate level of nutrient inputs from the deep ocean necessary for sustaining the food web.

In general, a combination of low top predator productivity due to poor recruitment or somatic growth combined with high exploitation pressure are conditions under which regime shifts in aquatic food webs are likely to occur (Daskalov et al. 2007). Overfishing can unbalance trophic relations, changing the configuration of competition and predation. Weak competitors may become stronger given less fishing pressure on such stocks, or by favoring the reproduction of it’s preys by fishing its competitor species. In addition, changes in nutrients inputs can represent an advantage for lower trophic levels, allowing them to escape the control of its predators.

As result, the ecological function of certain species groups is changed, allowing over abundance of immediate lower level. For example, the decrease of top-predators allows over abundance of meso predators that in turn reduces planktivorous fish and increase zooplankton. Further increase in nutrients from upwellings can lock the food web in a state dominated by planktivore fish as jellyfish. Low nutrients input, on the other hand, would reduce the amount of energy available for the food web and as a result an abrupt reduction of higher trophic levels is expected.

Impacts on ecosystem services and human well-being

Shift from predators to planktivore dominated food webs

Commercial fish stocks that are locked into a regime where they are maintained at low levels of abundance has obvious economic consequences on fisheries industry. However, these fisheries related shifts are not always negative, as for example a decrease in top predators can increase the catches of mid-level prey (e.g. shrimps and clupeids) (Frank et al. 2005). When an ecosystem-wide regime shift occurs it affects not only targeted groups, such as commercial fish (Daskalov et al. 2007) but also impacts other ecosystem functions. Biodiversity as well as ecosystem resilience may be reduced by a shift from an ecosystem with high top predator diversity and dominance to a system with lower trophic level dominance. This means that ecosystem may become more vulnerable to both climatic and anthropogenic change. Such ecosystems may more easily suffer from eutrophication, hypoxia and invasion by non-native species. Eutrophication and hypoxia in turn lead to increased algal blooms and decrease the recreational values of water bodies.

Predators and commercial fish stocks have remained under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the reduction of food webs, which in turn increases the risk of fisheries collapse. Depletion of fish stocks has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Hassan et al. 2005). Fish catches are projected to further decrease in the 21st century affecting protein sources for people, especially in poor regions (Hassan et al. 2005). The estimated current contribution of fisheries to human protein consumption is 29 million tons produced industrially and 24 million tons in small-scale fisheries (Hassan et al. 2005).

Management options

Options for enhancing resilience

In food webs that exhibit strong top-down control, decreasing the fishing pressure on top predators is an obvious strategy (Scheffer et al. 2001; Frank et al. 2005). However, due to the hysteresis effect, return to a predator-dominated food web may often require substantially lower fishing levels than that which induced the regime shift, together with additional measures. Hence, restoration is usually costly, especially to fisherman (Moellmann et al. 2009). As mentioned above, environmental factors can play an important role in maintaining alternative food web regimes. Nutrient loading is one of the key parameters controlling processes such as eutrophication and hypoxia. However, decreasing nutrient loads in marine environments requires large scale actions and often collaboration at international level (e.g. Helsinki Commission Baltic Sea Action Plan in the Baltic Sea). Hence, the time-frame of these actions in addition to time-lag in ecosystem response is often long. Climatic variation cannot be directly controlled by human actions. However, climate impacts can be mediated by employing other management measures that contribute to maintaining healthy marine ecosystems with high biodiversity and resilience, which then can compensate for changes in climate.

Options for reducing resilience to encourage restoration or transformation

In addition, in small closed food webs systems (3 to 4 trophic levels), biomanipulation has been suggested to manage one of the symptoms of degraded marine environments, namely eutrophication. This involves increasing the population of predatory fish such as bass, pike and walleye through stocking or reduced angling quotas. Increased populations of these predators leads to a decrease in the level of zooplanktivores. This in turn allows an increase in the population of planktivores that graze on the algae and zooplanckton, helping to reduce the algal density and phytoplankton respectively(Smith and Schindler 2009). This option, however, has limited reach when it comes to open marine foodwebs, where seasonal migrations and metapopulation dynamics are common.

Alternate regimes

During the 20th century global fisheries greatly expanded in terms of effort and range. This expansion resulted in the overfishing of many fish stocks, and in some cases triggered fisheries collapses, for example the collapse of the highly productive Newfoundland cod fishery (Worm et al. 2009). While overfishing can severely deplete fish stocks, it will not necessarily trigger a fisheries collapse. A collapse occurs when once fishery pressure has been removed, stocks fail to rebuild (Hutchings 2000, Kirby et al. 2009), suggesting that the system has become locked into an alternate regime. In this case, alternate regimes are usefully conceptualized as alternative food web structures:

High abundance of a commercial fish species.

In this regime a valuable fish species is common and the fishery is highly productive. The main ecosystem services associated with high abundance of commercial fish are the production of food in first place, employment for both artisanal and industrial fishermen; followed by pest and disease regulation by healthy food webs, recreational values as recreational fishing, knowledge and educational values, as well as maintenance of biodiversity.

Low abundance of the commercial fish species.

This regime is characterized by a substantially reduced abundance of the valuable fish species. In cases where the valuable fish species performs key ecological functions, it can lead to trophic cascades that lead to an overabundance of planktivorous fish or primary producers such as seagrass, macroalgae, sponge or phytoplankton (Jackson et al. 2001). With low fish abundance, food production is the most immediate ecosystem services affected. While industrial fishermen often can move and harvest new areas or new species, artisanal fishermen often can lose their livelihoods and traditions.  

Drivers and causes of the regime shift

There are two key direct drivers of fisheries collapse: overfishing as a top-down disturbance, and the change in nutrients input as a bottom-up disturbance.

Overfishing reduces the population size of commercial fish species, reduces their ability to perform their functional role in the ecosystem, can destabilize their population dynamics (Anderson et al. 2008), and ultimately could change the structure of the food web. Nutrient inputs, on the other hand, affect fisheries by increasing the abundance of lower trophic levels e.g. phytoplankton and zooplanktivores such as jellyfish (Daskalov et al. 2007). Such changes in food web configuration can reduce the energy transfer to fish and hence reduce fishing productivity. These drivers can reduce the resilience of a regime making it more vulnerable to other types of regime shift, such as climate driven food web reorganization [see: Climate & Marine Food Webs]. The indirect drivers that explain overfishing and changes in nutrient input interact in synergistic ways, making their identification a challenging process (Ocean Studies Board 2006, Kirby et al. 2009).

Indirect drivers of overfishing include oversupply of fishing boats, inflexible fishing quotas, demand for fish, the development of new fishing technologies, the facilitation of trade (market connectivity), governance failures such as the failure of regulation to halt illegal fishing, and market failures such as subsidies or perverse incentives. Nutrient enrichment of marine ecosystems is also indirectly driven by runoff of fertilizer nutrients from agriculture and sewage from urban areas (Diaz and Rosenberg 2008).

How the regime shift works

A fishery collapses when a high-valued commercial species is substantially depleted and fails to immediately recover when fishing effort is removed (Hutchings 2000). It is usually associated with a substantial and persistent change in the structure of the marine community. Such a change involves changes in the species presence, their relative abundance, size and age.

When the stock of commercial species decreases or even disappears the populations of other similar species may increase. Many commercially fished species are predatory fish, such as tuna, that have a high trophic level. Removing such top predators can cause a trophic cascade to reshuffle a food web (Daskalov et al. 2007, Estes et al. 2011), especially if there are not ecologically similar species present. Such trophic cascades, can increase fish stocks of prey species (Walters and Kitchell 2001), which can in turn lead to either sequential depletion of fish stocks or the development of new fisheries (Essington et al. 2006).

The low abundance of commercial fish species can be maintained by a number of mechanisms. First, when the population of the commercial fish species falls under certain levels their population growth rate can decline from a variety of causes. This is known as depensation in fisheries, or the Allee effect more generally (Liermann and Hilborn 1997). Such an effect can occur if small schools of fish offer poorer protection from predators, smaller populations are less able to hunt or detect food, or mating success rates can decline.

Second, nutrient enrichment can favor the food web shifts towards zooplanktivores, such as jellyfish. By reducing resources for other fish, the rise of such species can suppress fisheries recovery. Fishery pressure on filter-feeding fishes like sardine exacerbates the problem by favoring jellyfish outbreaks (Jackson et al. 2001, Brotz et al. 2012, Condon et al. 2013).

Despite evidence from some well-documented cases, the degree to which fisheries declines are regime shifts remains a controversial issue for a number of reasons (Hilborn 2007, Litzow and Urban 2009). First, time series for most fisheries are only available from 1950, a time when marine ecosystems were already affected by flourishing industrial fishing, and some stocks had already been seriously depleted, especially those of large marine vertebrates like whales, manatees, crocodiles, seals, swordfish, sharks and rays (Kirby et al. 2009). This lack of long term data makes it difficult to identify causation in fisheries time series. Furthermore, marine ecological diversity and food web complexity complicates the detection of changes in fish populations due to fishing, because the signals of regime shift dynamics are small relative to ecological variation (Liermann and Hilborn 1997).

Impacts on ecosystem services and human well-being

Shift from high to low abundance of commercial fish species

Since 1950 the world marine fisheries catch has expanded almost fivefold, and is currently about 80 million tonnes of fish. According to FAO, about 30% of fish stocks are overexploited, 60% are fully exploited and about 10 % are not fully exploited (FAO 2012). Global marine fish catches have plateaued and are not expected to increase. Local fisheries decline has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Pauly et al. 2006). There is not clear assessment of the number of stocks that have collapsed but FAO suggests about 5% of fish stocks are depleted (FAO 2012). Some collapsed stocks have remained at under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the collapse of fisheries.  

Shift from low to high abundance of commercial fish species

An abundant fish population regime will likely better maintain marine biodiversity and will be more resilience to climate variation and climate change. Society will benefit of high abundance regime in terms of high primary productivity that translate in more food, jobs and livelihoods in small coastal communities. It will also better regulate marine hypoxia.

Management options

A review of marine fisheries in the Millennium Ecosystem Assessment offer a synthetic set of managerial options for addressing fisheries collapse (Pauly et al. 2006).

Options for enhancing resilience

There are two types of strategies for enhancing resilience. The first is to stop doing things that are eroding resilience, while the second is to being activities that build resilience. There are a number of fishing practices that could be eliminated because they damage the resilience of fisheries. For example, fishing practices that destroy habitat such as bottom trawling should be avoided. Developing alternative technologies and reduction of fishing effort are suggested. Bycatch, the killing but not consuming of unwanted fish, can also have substantial impacts and can be reduced. Indirectly, fisheries subsidies and inflexible quotas can drive fishing effort to high levels that erode the resilience of a fishery.

Options for reducing resilience of unwanted regime to encourage restoration or transformation

Direct resilience building strategies include the establishment of Marine protected areas (MPAs) (Takashina and Mougi 2014). These areas can play an important role by providing refugia for fish and increasing ecological diversity. Research suggests that they need to cover ecologically significant areas, including different habitats, ensuring connectivity among them. However there remains controversy over the extent to which MPAs shift fishing pressure to less protected sites.

Indirect resilience building efforts include efficient regulation and policing of fishing can reduce illegal fishing, while comprehensive fisheries policies at the international and national level can also build resilience if they result in fisheries policies that reinforce one another. Indirect resilience building strategies include improving the ability of fishers to exit fisheries.

Good management of fishing pressure has been able to restore fisheries stocks. For example, in the US fishing closures and implementation of management measures for bottom fishing have enabled the restoration of George Bank haddock, Atlantic scallops, George Bank yellowtail flounder, Atlantic striped bass, Atlantic Arcadian redfish, Pacific chub mackerel, and Pacific sardine. However, there is no clear strategy that works across many ecosystems or nations (Beddington et al. 2007).

Alternate regimes

Mangroves are ecosystems adapted to the mixture of fresh and salt water common in intertidal zones of marine coastal environments. Mangroves cover between 1.7-1.8 x 105 km2 globally. They develop within 30ºN and 38ºS and grow optimally between 15ºC and 25ºC (Lovelock 2008). Outside this geographical and thermal range mangroves show reduced leaf formation and above 38ºC or below freezing temperatures they do not photosynthesize (Mcleod and Salm 2006; Cavanaugh et al. 2014). Mangrove ecosystems are often used by local communities who depend on fishing and timber harvesting.

Mangrove forest

Mangrove forest is characterized by an ensemble of species with aerial roots adapted to survive in environments where water salinity and tides are highly variable. Mangroves grow in tropical and subtropical waters, where the optimal temperature range is between 15-24ºC. Different species occur under different salinity gradients and some trees reach 40m high in areas with high precipitation. Mangroves usually establish in soil enriched by the sediment discharge of rivers (Mcleod and Salm 2006). Mangroves are ecologically important because they provide habitat for many marine species at some stages of their lifespan, increasing both marine and terrestrial biodiversity.

Salt marshes, rocky tidal, shrimp farms.

Mangroves can become dominated by salt marshes or other configuration of coastal wetlands depending on the substrate, sediment deposition and the topography (Mcleod and Salm 2006). The alternative regime once a mangrove has collapsed is not straightforward. It strongly depends on the land use, topological characteristics, latitude and the history of the system disturbance (Cavanaugh et al. 2014). For example, many mangroves around the world have been converted into ponds for shrimp farming, salt extraction, or simply dry out to establish agriculture, cattle ranching, infrastructure development or human settlements.

Drivers and causes of the regime shift

The main drivers of mangrove collapse have typically been land use/cover change during the last 50 years. One third of world’s mangroves have been lost due to overexploitation of forest resources (deforestation) or conversion into agriculture, salt extraction, or ponds for shrimp farming (Cavanaugh et al. 2014). Other threats to mangroves include the development of infrastructure (roads, dams), the diversion of fresh water for irrigation and the development of urban areas. These impacts are typically stronger in developing countries where it is expected that mangroves will decline 25% by 2025 (Mcleod and Salm 2006), while recent studies show that mangrove areas are declining 1-2% yearly (Duke et al. 2007; Alongi 200; Cavanaugh et al. 2014).

Current climate change is also expected to affect mangroves via the increase in frequency and severity of storms, floods and droughts (climate extreme events), the increase of temperature, sea level rise, and ocean acidification. Frequent extreme events that potentially decrease the flow of fresh water will increase salinity, putting stress on some species and reducing their growing rates and seedling survival (Mcleod and Salm 2006). Precipitation increase, on the other hand, could favor mangrove over salt marshes by decreasing salinity and giving a competitive advantage to mangroves (Cavanaugh et al. 2014). However, strong storms and flooding events could prevent mangroves from respiring when their aerial roots are underwater. Although temperature is expected to increase 2-6ºC before 2100, its effect on mangroves is contested. On the one hand, a 6ºC increase would heterogeneously affect mangroves around the world, although it is likely to induce thermal stress, it won’t be strong enough to surpass the temperature tolerance of these ecosystems. An increase in temperature will increase the melting rate of ice and expand the volume of world’s oceans increasing the level of the sea. On the other hand, it has also been observed that warming on the coldest time of the year can favor mangroves over taking salt marshes in temperate areas of the planet, with a suggested threshold of -4ºC for Florida (Cavanaugh et al. 2014).

Sea level rise is perhaps the most important threat to mangroves. While the expected sea level rise projection range from 0.09 to 0.88 m (or 0.9 to 8.8 mm per year) several studies report that mangrove ability to migrate upwards range from 1.2 to 4.5mm per year (Mcleod and Salm 2006). Although faster migration has been observed in Holocene stratigraphic records for Florida, migration also depends of local topological conditions. Increase in atmospheric carbon dioxide could increase mangroves growth but also ocean acidification. While the increase in growth is not expected to be strong enough to overcome the pressure of sea level rise, acidification is expected to have an indirect effect on mangroves by reducing coral reefs accretion, increasing in turn the erosive effects of waves on mangrove soils (Mcleod and Salm 2006).

How the regime shift works

Shift from mangroves to salt marshes, rocky tidal or shrimp ponds.

By having aerial roots mangroves adapt to very specific conditions where salt and fresh water meets in coastal zones. Mangroves build up carbon rich soils also known as peat, which favors further mangrove growth.

Loss of mangrove area decreases peat production. Peat is lost by wave erosion and reduces the likelihood of recovery of mangrove forest. Mangrove area loss is mainly driven by conversion to agriculture, urban settlements, overexploitation of wood, shrimp farming, salt extraction, or infrastructure development (e.g. roads, dams, water divergence to irrigation) (Cavanaugh et al. 2014). Human perturbations are maintained by social feedbacks such as demand for food and market access, population growth and the cumulative benefit of human settlements (higher provision of services). In contrast, communities with poor services access such as electric energy or gas for cooking strongly rely on mangrove wood as source of energy and construction material (Mcleod and Salm 2006).

If sea level rise occurs faster than the ability of mangroves to build up peat or migrate to higher areas, mangrove forests will disappear in major areas of the world (Alongi 2008). Both the impact of sea level rise and mangrove’s ability to adapt depend on local conditions and are highly uncertain. For example, mangroves in carbonate substrates (such as coralline islands) and microtidal systems (short tidal change from high to low) are less likely to adapt and migrate as climate change continues. Mangroves in rich soil substrates and macrotidal systems are more likely to migrate if salinity balance and sedimentation do not overcome their tolerance range (Mcleod and Salm 2006).  

 

Shift from salt marshes to mangroves  

Mangroves have also been observed to migrate pole wards colonizing habitats previously dominated by salt marshes (Cavanaugh et al. 2014). The shift occurs when the minimum temperature in winter increase for a longer period of time, giving mangroves an advantage to over compete salt marshes. It has been observed in areas such as Florida, Louisiana or Australia. Although there is uncertainty regarding the mechanism, the threshold is thought to be related with the number of days that minimum temperatures are colder than -4ºC, at least for Florida case (Cavanaugh et al. 2014). The authors of the Florida study also report that it’s unlikely that local scale drivers such as nutrients inputs, sedimentation or hydrology have had a determinant effect on the shift. However, sea level rise has contributed to increasing mangrove area (since it has happen slowly enough) and other common driver of the shift can reduce mangrove accretion such as coastal development, aquaculture and timber production (Cavanaugh et al. 2014).

Impacts on ecosystem services and human well-being

Mangroves provide a variety of ecosystem services. Mangrove ecosystems provide habitat for economically important marine species for fishing (e.g. shrimp or lobster among other crustaceans and mollusc), and they provide habitats for important groups of terrestrial species such as reptiles, monkeys, and birds. They are therefore important for maintaining both marine and terrestrial biodiversity. Mangroves also provide fuelwood, charcoal, and wood for construction. As a forest it provides protection against storms, floods, river born siltation and also traps water pollutants and sediments, reducing the erosive action of waves, particularly protecting adjacent ecosystems such as coral reefs, and sea grass beds (Duke et al. 2007). Mangroves have also been shown to play a key role in the global carbon budget, capturing up to 15% of global carbon and exporting up to 11% of particular terrestrial carbon to the oceans (Alongi 2014). In fact, recent studies support the hypothesis that mangroves are able to capture and store carbon in a greater extend that terrestrial ecosystems (Lovelock 2008), nearly identical to those of tropical forest (Alongi 2014). Therefore, losing mangrove area implies the loss of carbon storage.

While in the short term local communities could benefit by converting mangroves into shrimp farms, agriculture fields, or using its wood; in the long term they mean the loss of important services such as coastline protection, biodiversity important for tourism and fishing, water cleansing and carbon storage (Ewel et al. 1998; Cavanaugh et al. 2014). In fact, their services has been valued over $1.6 trillion per year (Costanza et al. 1997).

Management options

McLeod et al. (2006) propose a series of tools to monitor and plan for mangroves adaptation to climate change. First they propose to assess mangrove vulnerability based on the local conditions. Management options for mangrove forests are highly context dependent. As the main threat to mangroves is climate change through sea level rise, mangrove ecosystems will likely migrate upwards and northwards (Cavanaugh et al. 2014). However, such migration is constrained by local conditions such as substrate types, infrastructure development, sediment input and changes in salinity.

Second, they propose to apply risk-spreading principles (Duke et al. 2007) and identify areas that are likely to be sources and sinks of the migratory process (Mcleod and Salm 2006).  By identifying the role of each mangrove patch, it is easier to prioritize where green belts are needed to increase connectivity, which patches should be protected and which patches are more likely to respond to restoration efforts. They emphasize the importance of establishing a baseline and monitoring program with local communities in order to assess the main drivers locally. Developing partnerships with local stakeholders and developing alternative livelihoods for mangrove dependent communities are key managerial efforts (Mcleod and Salm 2006). The resilience of mangroves to future climate change scenarios also depend on how successful management efforts are at reducing the influence of other drivers: deforestation, overexploitation, infrastructure development, divergence of water for irrigation, and urbanization. Management of such local forcing will increase resilience to regional forcing such as increasing storm events or ocean acidification.

Third, it has been suggested that policy and market-based are critical to foster conservation efforts, reduce mangrove encroachment and maintain carbon storage through funding mechanisms such as REDD or other payment for ecosystem services schemes (Hutchison et al. 2013). Effective governance structures and education has also been suggested as managerial strategies (Duke et al. 2007).

Alternate regimes

Co-operation and sustainable resource levels

Harvesters extract from the resource at rates that are socially optimal, ensuring that resource levels stay at their most productive level and the community-average payoff is high. Social capital exists within the community, ensuring any defectors are ostracised.

Over-harvesting

Harvesters exert high efforts in extracting the resource, leading to depletion of the resource and low payoffs for the community. Social capital is absent and defectors are not ostracised.

Drivers and causes of the regime shift

Shift from ‘Co-operation and sustainable resource levels’ to ‘Over-harvesting’

The regime shift occurs when ostracism ceases to be an effective mechanism for encouraging defectors to co-operate, because the benefits of overharvesting begin to outweigh the disadvantages of ostracism. A number of factors could drive this shift. Increasing resource level, for example due to increased inflow, can lead to ineffective ostracism because at higher resource levels defection becomes more attractive by providing higher gains from resource over-extraction. Increasing defector payoff compared to ostracism, for example due to decreased costs or decreased ostracism strength, could also lead to defection becoming increasingly attractive.

Shift from ‘Over-harvesting’ to ‘Co-operation and sustainable resource levels’

Reversal of any of the above trends can cause a shift from over-harvesting to co-operation: decrease in resource levels, for example due to decreased inflow; increased costs; or increased ostracism strength.

How the regime shift works

Co-operation and sustainable resource levels occur when there is sufficient social capital to ostracise defectors and thereby make defection less attractive than co-operation. The key negative feedback that keeps the regime stable is the following: if the fraction of cooperators increases the social capital and hence strength of ostracism increases which reduces the utility of defectors, making defection less attractive. At the same time, however, resource productivity increases which increases the utility of defectors, so stability ultimately depends on the respective strength of the two feedback loops.

The over-harvesting regime exists when there are only few cooperators and hence there is insufficient social capital to ostracise defectors. The feedback that maintains full defection is as follows: any agent that 'co-operates' (i.e. harvests at a lower level) will have a payoff substantially less than the defectors. In the absence of social capital to encourage co-operation through ostracism, the co-operator will switch back to defection. The key threshold for a collapse in co-operation is when the costs of ostracism are lower than the benefits of overharvesting, i.e. ostracism ceases to be an effective mechanism to discourage defection. A variety of drivers can cause this shift, as discussed below.

To trigger a transition from full defection back to co-operation, co-operation must become more attractive than defection. This could be achieved by increasing the costs of harvesting, or by decreasing the level of the resource to low levels so that the benefits of overharvesting are minimal. However, once the co-operation strategy is lost in the community, it may be very difficult for it to re-emerge.

Impacts on ecosystem services and human well-being

The ability of the natural system to provide resources for harvesting is lost with the regime shift from co-operation to over-harvesting (gained for over-harvesting to co-operation). Depending on context, loss of other ecosystem services many accompany the decline in resources. The income that members of the harvesting community obtain by harvesting is severely decreased by this regime shift. Income may even no longer exceed the costs of harvesting and the community may need to find other means of survival.

Management options

Options for preventing regime shift to over-harvesting

Management actions that stop the drivers discussed above reaching their thresholds may help to prevent regime shifts. Activities that strengthen social norms and trust in the community and thus enhance cooperation and decrease the incentive to defect and overharvest for the individual benefit (hence increasing the strength of the ostracism).

Options for restoration of co-operation:

Introducing measures that build trust in the community and disincentivise free riding. 

Key References

  1. Lade SJ, Tavoni A, Levin SA & Schlüter M. 2013. Regime shifts in a social-ecological system. Journal of Theoretical Ecology 6:359-372.
  2. Tavoni A, Schlüter M, Levin S (2012), The survival of the conformist: Social pressure and renewable resource management, Journal of Theoretical Biology 299:152-161

Citation

Steven Lade, Maja Schlüter, Simon Levin, Alessandro Tavoni. Common pool resource harvesting. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2014-10-10 10:37:50 GMT.
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