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Regime Shifts
Regime Shifts

Regime Shifts (31)

Wednesday, 21 March 2012 13:37

Sprawling vs Compact City

Written by Anneli

Sprawling vs Compact City

Main Contributors:

Beom-Sik Yoo, Maja Berggren, Anneli Sundin

Other Contributors:

Reinette (Oonsie) Biggs, Juan Carlos Rocha

Summary

Urban sprawl, i.e. expansion of cities into low density, single use development, is a growing problem across the world leading to loss of ecosystem services, air pollution, class segregation and increased energy use. It is mainly driven by population growth, housing preferences, demand for social security and aesthetic preferences. The key maintainers of sprawl are road infrastructure designed with automobile use in mind and government's intentional and unintentional support for city expansion. Many cities are beginning to realize the negative impacts of urban sprawl, and governments are working towards shifting from sprawling of cities to development towards a more compact structure. This is made by e.g. investments in public transportation and new compact and mixed used areas where automobiles are not necessary. 

Drivers

Key direct drivers

  • Infrastructure development
  • Environmental shocks (eg floods)

Land use

  • Urban

Impacts

Ecosystem type

  • Planetary

Key Ecosystem Processes

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

Biodiversity

  • Biodiversity

Provisioning services

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

Regulating services

  • Air quality regulation
  • Climate regulation
  • Water purification
  • Water regulation
  • Regulation of soil erosion
  • Pollination
  • Natural hazard 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

  • Local/landscape

Typical time scale

  • Years
  • Decades

Reversibility

  • Irreversible (on 100 year time scale)
  • Hysteretic

Evidence

  • Models
  • Contemporary observations

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

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Key References

  1. Arbury, J., 2005. From Urban Sprawl to Compact City – An Analysis of urban growth management in Auckland, thesis, University of Auckland, Available at: http://portal.jarbury.net/thesis.pdf.
  2. Bairoch, P. (1991) Cities and economic development: from the dawn of history to the present. University of Chicago Press.
  3. Breheny, M., 2001. Densities and Sustainable Cities: the UK experience. In Echenique M. and Saint A. (eds.) Cities for the New Millennium, London.
  4. Brown, D.G. and Robinson, D.T. (2006). Effects of Heterogeneity in Residential Preferences on an Agent-Based Model of Urban Sprawl. Ecology and Society.
  5. Carpenter, S. R., and K. L. Cottingham. 1997. Resilience and restoration of lakes. Conservation Ecology [online]1(1): 2. Available from the Internet. URL: http://www.consecol.org/vol1/iss1/art2/
  6. Dye, C., 2008. Health and Urban Living. Science 8 February 2008: Vol. 319 no. 5864 pp. 766-769.
  7. EEA, 2006. Urban sprawl in Europe: The ignored challenge. EEA-report, no 10, 2006, European Environment Agency (EEA), Copenhagen, Denmark.
  8. Ewing, R. H., 2008. Characteristics, Causes, and Effects of Sprawl: A Literature Review. Pages 519-535 in Marzluff, J. M., Shulenberger, E., Endlicher, W., Albert, M., Bradley, G., Ryan, C., Simon, U., and ZumBrunnen, C., editors. Urban Ecology. Springer US. Available at: http://www.springerlink.com/content/v8522178lm8g7370/fulltext.pdf.
  9. Hendriksen, B. and de Boer, Y., 2011. CDP Cities 2011: Global report on C40 cities. Carbon Disclosure Project (CDP).
  10. Scheffer, M., Carpenter, S., Foley, J. A., Folke, C., Walker, B., 2001. Catastrophic shifts in ecosystems. Nature 413:591–596.
  11. Scoffham, E. and Vale, B., 1996. ‘How compact is sustainable – how sustainable is compact?’ in Jenks, Burton and Williams (eds.) The Compact City: a sustainable urban form? E & FN Spoon, London.
  12. Williams, K., Burton, E., and Jenks, M., 1996. ‘Achieving the Compact City through Intensification: an acceptable option?’ in Jenks, M., Burton, E. and Williams, K. (eds.) The Compact City: a sustainable urban form? E & FN Spoon, London.

Citation

Beom-Sik Yoo, Maja Berggren, Anneli Sundin, Reinette (Oonsie) Biggs, Juan Carlos Rocha. Sprawling vs Compact City. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-06 09:18:59 GMT.
Friday, 16 March 2012 10:00

River Channel Position

Written by Reinette (Oonsie) Biggs

River Channel Position

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

In freshwater lake and river systems, a river channel position regime shift occurs when the main channel of a river abruptly changes its course to a new river channel. Meandering and braided rivers are especially vulnerable to such shifts. The actual shift of the channel usually follows a large flood event, but other factors make the system susceptible to the shift. Most commonly, sediment buildup blocks the riverflow due to changes in current and riverbed gradient. In other cases, a cutoff occurs at the meandering neck in rivers with high channel sinuosity. Human activities such as land clearance and artificial channel widening can also make the river system vulnerable to a sudden course change. A shift in river channel position has large impacts on the ecology, economy and society, especially through impacts on water availability which is important for agriculture and transportation. On a 100 year time-scale the shift is irreversible. Only enormous engineering efforts can prevent a river from switching to a new channel, or restore a former river course. However, such efforts are very complex and costly. 

Drivers

Key direct drivers

  • Infrastructure development
  • Soil erosion & land degradation
  • 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)
  • Fisheries

Impacts

Ecosystem type

  • Freshwater lakes & rivers

Key Ecosystem Processes

  • Soil formation
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Food crops
  • Fisheries

Regulating services

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

Cultural services

  • Recreation
  • Aesthetic values
  • Spiritual and religious

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity
  • Security of housing & infrastructure
  • Social conflict

Key Attributes

Typical spatial scale

  • Local/landscape
  • Sub-continental/regional

Typical time scale

  • Weeks
  • Months
  • Years

Reversibility

  • Irreversible (on 100 year time scale)

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

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Key References

  1. Biedenharn DS, Thorne CR, Watson CC. 2000. Recent morphological evolution of the Lower Mississippi River. Geomorphology 34, 227-249.
  2. Coomes OT, Abizaid C, Lapointe M. 2009. Human Modification of a Large Meandering Amazonian River: Genesis, Ecological and Economic Consequences of The Masisea Cutoff on the Central Ucayali, Peru. Ambio 38, 130-134.
  3. Dent CL, Cumming GS, Carpenter SR. 2002. Multiple states in river and lake ecosystems. Phil. Trans. R. Soc. Lond. 357, 635-645.
  4. Gordon L, Peterson GD, Bennett E. 2008. Agricultural modifications of hydrological flows create ecological surprises. TREE 23, No. 4.
  5. Hooke JM. 2003. River meander behaviour and instability: a framework for analysis. Transactions of the Institute of British Geographers 28, 238–253.
  6. Hooke JM. 2004. Cutoffs galore!: occurrence and causes of multiple cutoffs on a meandering river. Geomorphology 61, 225-238.
  7. Knox JC. 2006. Floodplain sedimentation in the Upper Mississippi Valley: Natural versus human accelerated. Geomorphology 79, 286-310.
  8. Shields JR, Simon A, Steffen LJ. 2000. Reservoir effects on downstream river channel migration. Environmental Conservation 27, 54-66.
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Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. River Channel Position. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-13 14:21:21 GMT.
Tuesday, 06 March 2012 16:30

Salt Marsh to Tidal Flat

Written by Steven

Salt Marsh to Tidal Flat

Main Contributors:

Steven Alexander

Other Contributors:

Reinette (Oonsie) Biggs

Summary

The shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services such as pollution filtration, storm protection, and fisheries enhancement. This regime shift is primarily driven by the rate of sea level rise and the rate of sediment delivery. Transitions to consumer control either through the overharvesting of predators or the introduction of invasive/ exotic species can also contribute to this regime shift. It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms. However, thresholds exists in the rate of sea level rise (RSLR) and the rate of sediment delivery, where upon the mechanisms that effectively control the platform elevations are no longer able to keep up with sea level rise. Effective management options largely depend on the regional variables of the system. These options range from the reintroduction of top predators and removal of invasive/ exotic species to coordinated dam releases to provide necessary sediment pulses. 

Drivers

Key direct drivers

  • Vegetation conversion and habitat fragmentation
  • Infrastructure development
  • Species introduction or removal
  • Global climate change

Land use

  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Marine & coastal

Key Ecosystem Processes

  • Soil formation
  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries
  • Fuel and fiber crops

Regulating services

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

Cultural services

  • Recreation
  • Aesthetic values
  • Knowledge and educational values

Human Well-being

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

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Years
  • Decades

Reversibility

  • 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

  • Well established – Wide agreement on the underlying mechanism

Alternate regimes

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Key References

  1. Alongi, D. 1998. Coastal Ecosystem Processes. New York, New York: CRC Press.
  2. Bertness, M. & Silliman, B. 2008. Consumer control of salt marshes driven by human disturbance. Conservation Biology 22(3), 618 – 623.
  3. Defina, A., Carniello, L., Fagherazzi, S., & D’Alpaos, L. 2007. Self-organization of shallow basins in tidal flats and salt marshes. Journal of Geophysical Research 12, F03001.
  4. Fagherazzi, S., Carniello, L., D’Alpaos, L., & Defina, A. 2006. Critical bifurcation of shallow microtidal landforms in tidal flats and salt marshes. Proceedings of the National Academy of Sciences 103(22), 8337 - 8341.
  5. Gedan, K., Silliman, B., & Bertness, M. 2009. Centuries of human-driven change in salt marsh ecosystems. Annual Review of Marine Science 1, 117 -141.
  6. Kirwan, M. & Murray, A. 2007. A coupled geomorphic and ecological model of tidal marsh evolution. Proceedings of the National Academy of Sciences 104(15), 6118 – 6122.
  7. Kirwan, M., Guntenspergen, Gl, D’Alpaos, A., Morris, J., Mudd, S., & Temmerman, S. 2010. Limits on the adaptability of coastal marshes to rising sea level. Geophysical Research Letters 37, L23401.
  8. Kirwan, M., Murray, A., Donnelly, J., & Corbett, D. 2011. Rapid wetland expansion during European settlement and its implication for marsh survival under modern sediment delivery rates. Geology 39(5), 507-510.
  9. Marani, M., D’Alpaos, A., Lanzoni, S., Carniello, L., & Rinaldo, A. 2007. Biologically-controlled multiple equilibria of tidal landforms and the fate of the Venice lagoon. Geophysical Research Letters 34, L11402.
  10. Marani, M., D’Alpaos, A., Lanzoni, S., Carniello, L., & Rinaldo, A. 2010. The importance of being coupled: Stable states and catastrophic shifts in tidal biomorphodynamics. Journal of Geophysical Research 115, F04004.
  11. Mariotti, G., Fagherazzi, S., Wiberg, P., McGlathery, K., Carniello, L., & Defina, A. 2010. Influence of storm surges and sea level on shallow tidal basin erosive processes. Journal of Geophysical Research 115, C11012.
  12. McKee, K. & Patrick, W. 1988. The relationship of Smooth Cordgrass (Spartina alterniflora) to tidal datums: A review. Estuaries 11(3), 143-151.
  13. Morris, J., Sundareshwar, P., Nietch, C., Kjerfve, B., & Cahoon, D. 2002. Responses of coastal wetlands to rising sea level. Ecology 83(10), 2869 – 2877.
  14. Mudd, S., Howell, S., & Morris, J. 2009. Impact of dynamic feedbacks between sedimentation, sea-level rise, and biomass production on near-surface stratigraphy and caron accumulation. Estuarine, Coastal and Shelf Science 82, 377 – 389.
  15. Murray, A., Knaapen, M., Tal, M., & Kirwan, M. 2008. Biomorphodynamics: Physical-biological feedbacks that shape landscapes. Water Resources Research 44, W11301.
  16. Pasternack, G., Brush, G., & Hilgartner, W. 2001. Impact of historic land-use change on sediment delivery to a Chesapeake Bay subestuarine delta. Earth Surface Processes and Landforms 26, 409-427.
  17. UNEP. 2006. Marine and coastal ecosystems and human well-being: A synthesis report based on the findings of the Millennium Ecosystem Assessment. UNEP.
  18. van de Koppel J, van der Wal D, Bakker JP, & Herman PM. 2005. Self-organization and vegetation collapse in salt marsh ecosystems. American Naturalist 165, 1-12.

Citation

Steven Alexander, Reinette (Oonsie) Biggs. Salt Marsh to Tidal Flat. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-04-10 09:28:24 GMT.
Thursday, 08 September 2011 15:39

Submerged to Floating Plants

Written by Henning

Submerged to Floating Plants

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

The shift from submerged to floating plants in aquatic ecosystems such as ponds, canals, ditches or tropical lakes generates a loss of ecosystem services such as freshwater, fisheries and biodiversity. This regime shift is primarily driven by nutrient enrichment in the water body, as well as invasion by exotic species. Other drivers are turbidity, changes of the water depth and fluctuations in the water-level. The main mechanism that maintains floating plant dominance is the decrease of in situ light due to an increase of shading by floating plant biomass in higher strata which leads to dark and anoxic conditions under the leaf surface, leaving little opportunity for plant or animal life. Harvesting of floating plants is a management strategy that can shift the floating plant dominated regime back to a submerged plant dominated system. 

Drivers

Key direct drivers

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

Land use

  • Large-scale commercial crop cultivation
  • Intensive livestock production (eg feedlots)
  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Freshwater lakes & rivers

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Fisheries

Regulating services

  • Water purification
  • Water regulation

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

Typical time scale

  • Weeks
  • Months

Reversibility

  • Hysteretic
  • Readily reversible

Evidence

  • Models
  • Contemporary observations
  • Experiments

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

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Alternate regimes

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

 

Drivers and causes of the regime shift

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

How the regime shift works

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Impacts on ecosystem services and human well-being

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

Management options

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

Key References

  1. Coops H, Doef RW. 1996. Submerged vegetation development in two shallow, eutrophic lakes. Hydrobiologia 340, 115-120.
  2. Janse JH, Van Puijenbroeck PJTM. 1998. Effects of eutrophication in drainage ditches. Environmental Pollution 102, 547-552
  3. Oliver JD. 1993. A review of the biology of Giant Salvinia (Salvinia molesta Mitchell). Journal of Aquatic Plant Management 31, 227-231
  4. Scheffer M, Szabó S, Gragnani A, van Nes EH, Rinaldi S, Kautsky N, Norberg J, Roijackers RMM, Franken RJM. 2003. Floating plant dominance as a stable state. PNAS 100, Issue 7, 4040-4045.

Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. Submerged to Floating Plants. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-06 10:26:14 GMT.
Monday, 28 February 2011 22:10

Tundra to Boreal forest

Written by Rolands

Tundra to Boreal forest

Main Contributors:

Juan Carlos Rocha, Rolands Sadauskis

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

The main driver behind the shift from tundra to boreal forest is the increasingly warm climate due to high concentrations of carbon in the atmosphere, allowing pioneer shrubs associated with the boreal forest regime to increase significantly. The actual shift to boreal forest with spruce and pine as the dominant species is unlikely to occur this century due to time lags involved with species migration. Shrub expansion in the Arctic tundra is the first phase of this regime shift, which is reinforced by carbon release due to permafrost degradation, which in turn increases climate warming and microbial activity enhancing shrub growth. Sufficient numbers of herbivores can limit shrub expansion and potentially maintain the shrub state on a long term basis.

Drivers

Key direct drivers

  • Global climate change

Land use

  • Extensive livestock production (rangelands)
  • Conservation
  • Tourism

Impacts

Ecosystem type

  • Tundra

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Livestock
  • Wild animal and plant products
  • Timber

Regulating services

  • Climate regulation

Cultural services

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

Human Well-being

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

Key Attributes

Typical spatial scale

  • Local/landscape
  • Sub-continental/regional

Typical time scale

  • Decades
  • Centuries

Reversibility

  • Irreversible (on 100 year time scale)
  • Unknown

Evidence

  • Models
  • Paleo-observation
  • 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

Links to other regime shifts

Alternate regimes

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Alternate regimes

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

 

Drivers and causes of the regime shift

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

How the regime shift works

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Impacts on ecosystem services and human well-being

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

Management options

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

Alternate regimes

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

Drivers and causes of the regime shift

The main external driver of this regime shift is CO2 emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

How the regime shift works

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO2 emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Impacts on ecosystem services and human well-being

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

 

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

Management options

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO2 levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Key References

  1. Bonan GB, Pollard D, and Thompson SL. 1992. Effects of Boreal Forest Vegetation on Global Climate. Nature 359,716-18
  2. CAVM Team: Circumpolar Arctic Vegetation. Arctic Portal. http://web.arcticportal.org/uploads/OY/RT/OYRTN1ieqI5IVfPL2U3SqA/side1_031016.pdf. Last visited: 1 Apr. 2010
  3. Chapin III FS. et al. 2005. Role of land-surface changes in Arctic summer warming. Science 310,657-660.
  4. Frelich LE, and Reich PB. 1995. Spatial patterns and succession in a Minnesota southern boreal forest. Ecological Monographs 65(3),325-346.
  5. Hinzman LD, et al. 2005. Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions. Climatic Change 72(3).
  6. IPCC 2007. Climate Change 2007: The Physical Science Basis. Cambridge University Press, New York.
  7. Lloyd AH. 2003. Effects of Permafrost Degradation on Woody Vegetation at Arctic Treeline on the Seward Peninsula, Alaska. Permafrost Periglac. Process. 14,93–101.
  8. Myers-Smith I. 2007. Shrub line advance in alpine tundra of the Kluane Region: mechanisms of expansion and ecosystem impacts. Arctic 60(4),447-451.
  9. Olofsson J, Oksanen L, Callaghan T, Hulme PE, Oksanen T, Suominen O. 2009. Herbivores inhibit climate-driven shrub expansion on the tundra. Global Change Biology 15(11),2681 – 2693.
  10. Rockström J. et al. 2009. Planetary Boundaries: Exploring the Safe Operating Space for Humanity. Ecology and Society 14(2),32.
  11. Sturm M, Schimel J, Michaelson G, Welker JM, Oberbauer SF, Liston GE, Fahnestock J, and Romanovsky VE. 2005. Winter biological processes could help convert arctic tundra to shrubland. Bioscience 55(1),17-26.
  12. UCMP: The tundra biome. University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/exhibits/biomes/tundra.php Last visited: 26 Jan. 2010.
  13. Welker JM, Fahnestock JT, 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.
  14. Zimov SA, Schuur EAG, Chapin III SF. 2006. Permafrost and the Global Carbon Budget. Science 312(16),1612-1613.

Citation

Juan Carlos Rocha, Rolands Sadauskis, Reinette (Oonsie) Biggs, Garry Peterson. Tundra to Boreal forest. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-07 09:45:51 GMT.
Monday, 28 February 2011 14:52

Forest to Savannas

Written by Juan Carlos

Forest to Savannas

Main Contributors:

Juan Carlos Rocha

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

Forest to savannas is a regime shift typical of tropical areas where forests, an ecosystem dominated by trees changes to a savanna dominated by a mixture of grasslands and shrublands. Several feedbacks play an important role in this regime shift including albedo effects, evapotranspiration and cloud formation, fragmentation and fire-prone area expansion, change in ocean circulation and self organizing vegetation patterns. However, these feedbacks are not always strong enough to produce alternative regimes. In some areas shifts are expected to occur under stochastic events like ENSO droughts or unlikely events like Earth orbit change.

Drivers

Key direct drivers

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

Land use

  • Small-scale subsistence crop cultivation
  • Extensive livestock production (rangelands)
  • Timber production

Impacts

Ecosystem type

  • Tropical forests
  • Grasslands

Key Ecosystem Processes

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

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Food crops
  • Livestock
  • Timber
  • Woodfuel
  • Wild animal and plant foods

Regulating services

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

  • Decades

Reversibility

  • 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

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

Alternate regimes

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Alternate regimes

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

 

Drivers and causes of the regime shift

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

How the regime shift works

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Impacts on ecosystem services and human well-being

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

Management options

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

Alternate regimes

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

Drivers and causes of the regime shift

The main external driver of this regime shift is CO2 emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

How the regime shift works

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO2 emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Impacts on ecosystem services and human well-being

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

 

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

Management options

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO2 levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Alternate regimes

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Drivers and causes of the regime shift

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

How the regime shift works

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Impacts on ecosystem services and human well-being

 

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Management options

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Key References

  1. Bonan, G. 2008. Forests and climate change: Forcings, feedbacks, and the climate benefits of forests. Science 320:1444-1449.
  2. Da Silva, R., D. Werth, and R. Avissar. 2008. Regional impacts of future land-cover changes on the amazon basin wet-season climate. J Climate 21:1153-1170.
  3. Dekker, S. C., H. J. de Boer, V. Brovkin, K. Fraedrich, M. J. Wassen, and M. Rietkerk. 2010. Biogeophysical feedbacks trigger shifts in the modelled vegetation-atmosphere system at multiple scales. BIOGEOSCIENCES 7:1237-1245.
  4. Dekker, S. C., M. Rietkerk, and M. F. P. Bierkens. 2007. Coupling microscale vegetation-soil water and macroscale vegetation-precipitation feedbacks in semiarid ecosystems. Global Change Biol 13:671-678.
  5. Falkenmark, M. and J. Rockström. 2008. Building resilience to drought in desertification-prone savannas in Sub-Saharan Africa: The water perspective. Nat. Resour. Forum 32:93-102.
  6. Foley, J., R. DeFries, G. Asner, C. Barford, G. Bonan, S. Carpenter, F. Chapin, M. Coe, G. Daily, and H. Gibbs. 2005. Global consequences of land use. Science 309:570-574.
  7. Geist, H. and E. Lambin. 2002. Proximate causes and underlying driving forces of tropical deforestation. BioScience 52:143-150.
  8. Hutyra, L., J. Munger, C. Nobre, S. Saleska, S. Vieira, and S. Wofsy. 2005. Climatic variability and vegetation vulnerability in Amazonia. Geophys Res Lett 32:L24712.
  9. Laurance, W. and G. Williamson. 2001. Positive feedbacks among forest fragmentation, drought, and climate change in the Amazon. Conservation biology 15:1529-1535.
  10. Los, S. O., G. P. Weedon, P. R. J. North, J. D. Kaduk, C. M. Taylor, and P. M. Cox. 2006. An observation-based estimate of the strength of rainfall-vegetation interactions in the Sahel. Geophys Res Lett 33:L16402.
  11. Millennium Ecosystem Assessment. 2005. Ecosystems and human well-being: synthesis?.137.
  12. Nobre, P., M. Malagutti, D. F. Urbano, R. A. F. De Almeida, and E. Giarolla. 2009. Amazon Deforestation and Climate Change in a Coupled Model Simulation. J Climate 22:5686.
  13. Oyama, M. and C. Nobre. 2003. A new climate-vegetation equilibrium state for tropical South America. Geophys Res Lett 30:2199.
  14. Oyama, M. and C. Nobre. 2004. Climatic consequences of a large-scale desertification in northeast Brazil: A GCM simulation study. J Climate 17:3203-3213.
  15. Pinto, E., Y. Shin, S. A. Cowling, and C. D. Jones. 2009. Past, present and future vegetation-cloud feedbacks in the Amazon Basin. Clim Dynam 32:741-751.
  16. Reynolds, J., D. Smith, E. Lambin, T. Ii, B L, M. Mortimore, S. Batterbury, T. Downing, H. Dowlatabadi, R. Fernandez, J. Herrick, E. Huber-Sannwald, H. Jiang, R. Leemans, T. Lynam, F. Maestre, M. Ayarza, and B. Walker. 2007. Global Desertification: Building a Science for Dryland Development. Science 316:847.
  17. Rietkerk, M., S. Dekker, P. de Ruiter, and J. van de Koppel. 2004. Self-organized patchiness and catastrophic shifts in ecosystems. Science 305:1926-1929.
  18. Saleska, S., K. Didan, A. Huete, and H. da Rocha. 2007. Amazon forests green-up during 2005 drought. Science 318:612-612.
  19. Scheffer, M. 2009. Critical transitions in nature and society.
  20. Sternberg, L. 2001. Savanna-forest hysteresis in the tropics. Global Ecology and Biogeography:369-378.

Citation

Juan Carlos Rocha, Reinette (Oonsie) Biggs, Garry Peterson. Forest to Savannas. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-05-11 14:22:36 GMT.
Monday, 28 February 2011 10:47

Kelp Transitions

Written by Juan Carlos

Kelp Transitions

Main Contributors:

Juan Carlos Rocha

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

Kelp forests are marine coastal ecosystems located in shallow areas where large macroalgae ecologically engineer the environment to produce a coastal marine environment substantially different from the same area without kelp.  Kelp forests can undergo a regime shift to turf-forming algae or urchin barrens. This regime shift leads to loss of habitat and ecological complexity. Shifts to turf algae are related to nutrient input, while shifts to urchin barrens are related to trophic-level changes. The consequent loss of habitat complexity may affect commercially important fisheries. Managerial options include restoring biodiversity and installing wastewater treatment plants in coastal zones.

Drivers

Key direct drivers

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

Land use

  • Large-scale commercial crop cultivation
  • Fisheries
  • Conservation
  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Marine & coastal

Key Ecosystem Processes

  • Primary production

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries
  • Wild animal and plant products
  • Other crops (eg cotton)

Regulating services

  • 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

  • Months
  • Years

Reversibility

  • Hysteretic
  • Readily reversible

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

  • Well established – Wide agreement on the underlying mechanism

Links to other regime shifts

Alternate regimes

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Alternate regimes

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

 

Drivers and causes of the regime shift

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

How the regime shift works

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Impacts on ecosystem services and human well-being

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

Management options

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

Alternate regimes

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

Drivers and causes of the regime shift

The main external driver of this regime shift is CO2 emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

How the regime shift works

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO2 emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Impacts on ecosystem services and human well-being

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

 

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

Management options

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO2 levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Alternate regimes

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Drivers and causes of the regime shift

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

How the regime shift works

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Impacts on ecosystem services and human well-being

 

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Management options

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Alternate regimes

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 


Drivers and causes of the regime shift

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

How the regime shift works

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Impacts on ecosystem services and human well-being

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

 

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Management options

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

 

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Key References

  1. Bakun, A., et al. (2010) Greenhouse gas, upwelling-favorable winds, and the future of coastal ocean upwelling ecosystems. Global Change Biol 16, 1213-1228
  2. Estes, J., et al. (2011) Trophic Downgrading of Planet Earth. Science
  3. Gorman, D. and S. Connell. 2009. Recovering subtidal forests in human-dominated landscapes. J Appl Ecol 46:1258-1265.
  4. Gorman, D., B. D. Russell, and S. D. Connell. 2009. Land-to-sea connectivity: linking human-derived terrestrial subsidies to subtidal habitat change on open rocky coasts. Ecological Applications 19:1114-1126.
  5. Konar, B. and J. Estes. 2003. The stability of boundary regions between kelp beds and deforested areas. Ecology 84:174-185.
  6. Lauzon-Guay, J.-S., R. Scheibling, and M. Barbeau. 2009. Modelling phase shifts in a rocky subtidal ecosystem. Mar Ecol-Prog Ser 375:25-39.
  7. Ling, S., C. Johnson, S. Frusher, and K. Ridgway. 2009. Overfishing reduces resilience of kelp beds to climate-driven catastrophic phase shift. P Natl Acad Sci Usa 106:22341-22345.
  8. Pauly, D., V. Christensen, J. Dalsgaard, R. Froese, and F. Torres. 1998. Fishing down marine food webs. Science 279:860-863.
  9. Scheffer, M. 2009. Critical transitions in nature and society.
  10. Smith, V.H., and Schindler, D.W. (2009) Eutrophication science: where do we go from here? Trends Ecol. Evol. 24, 201-207
  11. 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.
  12. Steneck, R., M. Graham, B. Bourque, D. Corbett, J. Erlandson, J. Estes, and M. Tegner. 2002. Kelp forest ecosystems: biodiversity, stability, resilience and future. Environ. Conserv. 29:436-459.

Citation

Juan Carlos Rocha, Reinette (Oonsie) Biggs, Garry Peterson. Kelp Transitions. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-25 08:10:16 GMT.
Monday, 28 February 2011 10:33

Bivalves Collapse

Written by Juan Carlos

Bivalves Collapse

Main Contributors:

Christine Hammond, Juan Carlos Rocha

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

Bivalves form reefs that filter water removing sediments and nutrients maintaining clear water.  Bivalve reefs also produce spatial structure that provides habitat to other aquatic species. A low abundance regime can be induced by harvesting. Low abundances of bivalves do not provide water filtering, leading to murkier water, which can impede bivalve population growth. 

Drivers

Key direct drivers

  • Harvest and resource consumption
  • External inputs (eg fertilizers)
  • Adoption of new technology
  • Disease
  • Environmental shocks (eg floods)

Land use

  • Urban
  • Large-scale commercial crop cultivation
  • Intensive livestock production (eg feedlots)
  • Fisheries
  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Marine & coastal
  • Freshwater lakes & rivers

Key Ecosystem Processes

  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Fisheries

Regulating services

  • Water purification

Cultural services

  • Aesthetic values

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Months
  • Years

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

  • Well established – Wide agreement on the underlying mechanism

Links to other regime shifts

Alternate regimes

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Alternate regimes

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

 

Drivers and causes of the regime shift

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

How the regime shift works

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Impacts on ecosystem services and human well-being

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

Management options

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

Alternate regimes

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

Drivers and causes of the regime shift

The main external driver of this regime shift is CO2 emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

How the regime shift works

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO2 emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Impacts on ecosystem services and human well-being

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

 

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

Management options

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO2 levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Alternate regimes

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Drivers and causes of the regime shift

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

How the regime shift works

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Impacts on ecosystem services and human well-being

 

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Management options

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Alternate regimes

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 


Drivers and causes of the regime shift

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

How the regime shift works

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Impacts on ecosystem services and human well-being

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

 

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Management options

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

 

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Alternate regimes

Bivalve mollusks play an important role in aquatic ecosystems by filtering and sequestering nutrients. When bivalve abundance changes, it can have substantial ecosystem impacts, creating two different, self-reinforcing regimes.

Bivalve mollusk reef regime

When aquatic ecosystems have high bivalve mollusk density, they form reefs that include many bivalves and are shallow, nearing the water's surface. The filtering activity of the bivalves results in clear water with high levels of dissolved oxygen. Plankton populations are limited in these conditions. The bivalves themselves operate as reinforcing feedbacks in that their abundance ensures sufficient filtration to maintain this clear water regime. The clear water in turn reinforces bivalve abundance by maintaining hydrodynamics that are conducive to bivalve health (Scheffer 2009). Large, shallow bivalve mollusk reefs encourage biodiversity by providing habitat and filtering nutrients.

Isolated, low density bivalve mollusk regime

When aquatic ecosystems have low bivalve mollusk density, the reefs are small and deep. Water is turbid, with low levels of dissolved oxygen. Plankton and filamentous algae flourish under these conditions. Poor reef size and water filtration limit biodiversity. The low bivalve abundance state is reinforced as bivalve health and fecundity is weakened by turbid water conditions. The turbid water conditions are in turn reinforced by low bivalve abundance(Weijerman et al. 2005, Powell et al. 2008).

The regime of low bivalve abundance is expected to reduce fish populations, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fisheries, human health, and recreation. 

Drivers and causes of the regime shift

Mechanized anthropogenic over-harvesting of bivalve mollusks is the main driver of the shift from high to low abundance (Thrush and Dayton 2002).  High inputs of nutrients from agricultural or urban sources can act as a slow driver that weakens bivalve health by creating plankton blooms that the bivalves are unable to filter.  Plankton blooms can further weaken the health of bivalves by producing organic matter, whose decay, reduces oxygen availability.  A disturbance that can trigger a regime shift is bivalve disease (Powell et al. 2008).  When populations are weakened by eutrophication and anoxia, they become physiologically susceptible to diseases that can rapidly devastate remaining populations (Leniham 1999). 

How the regime shift works

Bivalves are a class of mollusks containing over 30,000 species which include scallops, clams, oysters, and mussels. Bivalve mollusks, like corals, are often referred to as "ecosystem engineers" because they engineer the physical and hydrodynamic structures of the ecosystems they occupy in a way that produces conditions that support their own population growth. Bivalves live on the sediment surface, where they form reefs that provide increased surface area and habitat for a variety of species. They filter nutrients from the pelagic zone and transfer it to the benthic zone, maintaining the system in a clear water state. If bivalves are not able to maintain clear water, due to declines in bivalve populations or increases in low water quality, the ecosystem can shift to a turbid water state. The main drivers of this shift is direct anthropogenic over-harvesting of the bivalves, followed by a gradual increase of nutrients discharge from agricultural activities and coastal urban settlements.  Once mollusk abundance is reduced, plankton and filamentous algae populations can substantially increase. As result, oxygen and light available in the water column is reduced, while nutrients levels accumulate due to the lack of filtration.  These processes produce conditions which favour algae over bivalves.  If nutrient inputs are substantial, eutrophication and areas of hypoxia (low oxygen) are likely to appear in areas dominated by plankton instead of mollusks. 

Impacts on ecosystem services and human well-being

Shift from high to low bivalve mollusk abundance regime

The loss of abundant bivalve mollusk reefs threatens diverse ecosystem services. The most direct impact is the loss of valuable shellfisheries. A secondary, but potentially substantial impact is the loss of the filtering service provided by bivalves.  In urbanized estuaries, it can necessitate the installation of costly synthetic water filtration technology (Gren et al. 2009).  In other areas it can lead to declines in water quality that shift populations and reduce recreation opportunities, and potentially the value of waterfront property.

The loss of bivalve habitat structure leads to lower species abundances, and to declines in species richness (Airoldi et al. 2008). Both structural and functional biodiversity is threatened by loss of bivalve abundance, with far reaching effects on marine ecosystems and human ability to exploit such systems (Worm et al. 2006).  The regime of low bivalve abundance is expected to reduce fisheries, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fishing productivity and human health.

The loss of bivalve nutrient filtration can lead to other regime shifts, such as eutrophication and hypoxia, which can produce unproductive and undesirable ecosystems (see Hypoxia and Eutrophication regime shifts). 

Management options

Options for enhancing resilience

Estuaries are the world's most degraded marine ecosystems, receiving land-based pollution from crop cultivation and urbanization, and suffering from anthropogenic over-harvesting (Lotze et al. 2006).  The resulting problems are complex, impacting bivalve health and fecundity as well as many other species, with far reaching implications in both biological and social domains.

Experience has shown that management focused on one species or problem tends to be ineffective.  Newer research stresses the need to address natural resources as part of complex social-ecological systems, often with long histories of human exploitation (Jackson et al. 2001).   Rather than addressing problems on a species-by-species basis, multi -species management has shown to have synergistic effects.  For example, in Chesapeake bay, fisheries management looks at oyster, blue crab, striped bass and shad together (Boesch 2004).

Taking this approach further, Ecosystem-based fishery management efforts focus on recognizing interactions between multiple species and environmental stressors, such as low dissolved oxygen levels. Success is measured by the degree to which management efforts include ecosystem-based approaches, rather than by an assessment of fishing stocks (Lotze et al. 2006). 

 

Options for reducing resilience to encourage restoration or transformation

The main option exercised for preventing or reversing a regime shift regarding bivalve abundance is to import bivalve mollusks from another region for aquaculture (Van de Koppel 2008) .  Another practice commonly used is to hang bivalves from the surface or build artificial reefs to elevate bivalves in order to avoid the hypoxic conditions in deeper water (Carlsson et al. 2009). 

Key References

  1. Airoldi, L., Balata, B. Beck, M.W. 2008 The Grey Zone: Relationships between habitat loss and marine diversity. Journal of experimental marine biology and ecology: 366 pp. 8-15
  2. Boesch, D.F. 2004. Scientific requirements for ecosystem-based management in the restoration of Chesapeake Bay and Coastal Louisiana. Ecological Engineering. 26 (1) pp 6-26
  3. Burns KA, Smith JL. 1981. Biological monitoring of ambient water quality: the case for using bivalves as sentinel organisms for monitoring petroleum pollution in coastal waters. Estuarine, Coastal and Shelf Science 30(4), 433–443. doi:10.1016/S0302-3524(81)80039-4.
  4. Carlsson, M.S., Holmer, M., Petersen, J.K. 2009 Seasonal and spatial variations of benthic impacts of mussel longline farming in a eutrophic Danish Fjord, Limfjorden. Journal of Shellfish Research. 28 (4) pp 791-801
  5. Gren, I., Lindahl, O., Lindqvist, M. 2009 Values of Mussel farming for combating eutrophication: An application to the Baltic Sea. Ecological Engineering. In Press: doi:10.1016/j.ecoleng.2008.12.033
  6. Jackson, J; Kirby, M; Berger, W; Bjorndal, K; Botsford, L; Bourque, B; Bradbury, R; Cooke, R; Erlandson, J; Estes, J; Hughes, T; Kidwell, S; Lange, C; Lenihan, H; Pandofi, J; Peterson, C; Steneck, R; Tegner, M; and Warner, R. 2001. Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science. 293 pp. 639-637
  7. Leniham, H.S., Micheli, F., SHelton, S.W., Peterson, C.H. 1999 The influence of multiple environmental stressors on susceptibility to parasites: An experimental determination with oysters. Limnology and Oceanography: 44 (3) pp. 910-924
  8. Loo, L.O. ,and R.Rosenber., 1989. Bivalve suspension-feeding dynamics and benthic-pelagic coupling in an eutrophicated marine bay. J. Exp. Mar. Biol. Ecol. 130: pp. 253-276
  9. Lotze H. K., et Lenihan, H.S., Bourque, B., Bradbury, R.H., Cooke, R.G., Kay, M.C., Kidwell, S.M., Kirby, M.X., Peterson, C.H., Jackson, J.B.C. 2006 Depletion, degradation and recovery potential of estuaries and coastal seas. Science 312 pp. 1806–1809.
  10. Norroko, A., Hewitt, J., Thrush, S., Funnell, G. 2006 Conditional outcomes of facilitation by a habitat-modifying subtidal bivalve. Ecology 87(1) pp. 226-234
  11. Powell, EN, Ashton-Alcox, K.A, Kraeuter JN, Ford SE, Bushek D. Long-term trends in oyster population dynamics in Delaware Bay: Regime shifts and response to disease. J Shellfish Res (2008) vol. 27 (4) pp. 729-755
  12. Scheffer, M. 2009. Critical Transitions in Nature and Society. Princeton Studies in Complexity pp. 207-208
  13. Thrush SF, JE Hewitt, S Parkes, AM Lohrer, C Pilditch, SA Woodin, DS Wethey, M Chiantore, V Asnaghi, S De Juan, C Kraan, I Rodil, C Savage, aC Van Colen 2014. Experimenting with ecosystem interaction networks in search of threshold potentials in real-world marine ecosystems. Ecology 95:1451–1457.
  14. Thrush, S.F. and Paul K. Dayton. 2002 Disturbance to marine benthic habitats by trawling and dredging: Implications for marine biodiversity. Annu. Rev. Ecol. Syst. 33 pp. 449-473
  15. Van de Koppel, J., Gascoigne, J.C., Theraulaz, G., Rietkerk, M., Mooij W.M., & Herman, P.M.J. (2008) Experimental evidence for spatial self-organization and its emergent effects in mussel beds. Science 322
  16. Weijerman, M., Lindeboom, H., Zuur, A. Regime shifts in marine ecosystems of the North Sea and Wadden Sea 2005 Mar Ecol Prog Ser 298 pp. 21-39
  17. Worm, B., Barbier, E., Beaumont, N., Duffy, E., Folke, C., Halpern, B.S., Jackson, J.B.C., Lotze, H.k., Micheli, F., Palumbi, S.R., Sala, E., Selkoe, K.A., Stachowicz, J.J., Watson, R. 2006 Impacts of biodiversity loss on ecosystem services. Science: 314 pp. 787-790

Citation

Christine Hammond, Juan Carlos Rocha, Reinette (Oonsie) Biggs, Garry Peterson. Bivalves Collapse. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-22 06:54:22 GMT.
Monday, 28 February 2011 10:23

Bush Encroachment

Written by Juan Carlos

Bush Encroachment

Main Contributors:

Bob Scholes, Reinette (Oonsie) Biggs, Juan Carlos Rocha, Linda Luvuno

Other Contributors:

Garry Peterson

Summary

Woody encroachment occurs when a grassy landscape with a relatively low cover of woody plants rapidly and apparently irreversibly increases in tree or shrub cover. Encroachment typically occurs when savanna landscapes formerly under wild herbivores or nomadic pastoralism are converted to commercial cattle ranching, involving fencing, water provision for livestock, a fixed (sometimes high) stocking rate, and intentional or unintentional grass fire suppression. Encroachment reduces the grass productivity and can make access by cattle difficult, with substantial negative economic impacts on ranchers. Woody encroachment is usually very difficult and costly to reverse. Managerial recommendations therefore focus on avoidance through moderate grazing and fires of sufficient intensity and frequency to prevent the recruitment of young trees. 

Drivers

Key direct drivers

  • Harvest and resource consumption
  • Species introduction or removal
  • Environmental shocks (eg floods)

Land use

  • Extensive livestock production (rangelands)
  • Conservation
  • Tourism

Impacts

Ecosystem type

  • Grasslands

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Livestock
  • Wild animal and plant products
  • Woodfuel

Regulating services

  • Climate regulation

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Years
  • Decades

Reversibility

  • 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

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Alternate regimes

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

 

Drivers and causes of the regime shift

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

How the regime shift works

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Impacts on ecosystem services and human well-being

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

Management options

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

Alternate regimes

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

Drivers and causes of the regime shift

The main external driver of this regime shift is CO2 emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

How the regime shift works

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO2 emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Impacts on ecosystem services and human well-being

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

 

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

Management options

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO2 levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Alternate regimes

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Drivers and causes of the regime shift

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

How the regime shift works

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Impacts on ecosystem services and human well-being

 

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Management options

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Alternate regimes

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 


Drivers and causes of the regime shift

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

How the regime shift works

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Impacts on ecosystem services and human well-being

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

 

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Management options

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

 

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Alternate regimes

Bivalve mollusks play an important role in aquatic ecosystems by filtering and sequestering nutrients. When bivalve abundance changes, it can have substantial ecosystem impacts, creating two different, self-reinforcing regimes.

Bivalve mollusk reef regime

When aquatic ecosystems have high bivalve mollusk density, they form reefs that include many bivalves and are shallow, nearing the water's surface. The filtering activity of the bivalves results in clear water with high levels of dissolved oxygen. Plankton populations are limited in these conditions. The bivalves themselves operate as reinforcing feedbacks in that their abundance ensures sufficient filtration to maintain this clear water regime. The clear water in turn reinforces bivalve abundance by maintaining hydrodynamics that are conducive to bivalve health (Scheffer 2009). Large, shallow bivalve mollusk reefs encourage biodiversity by providing habitat and filtering nutrients.

Isolated, low density bivalve mollusk regime

When aquatic ecosystems have low bivalve mollusk density, the reefs are small and deep. Water is turbid, with low levels of dissolved oxygen. Plankton and filamentous algae flourish under these conditions. Poor reef size and water filtration limit biodiversity. The low bivalve abundance state is reinforced as bivalve health and fecundity is weakened by turbid water conditions. The turbid water conditions are in turn reinforced by low bivalve abundance(Weijerman et al. 2005, Powell et al. 2008).

The regime of low bivalve abundance is expected to reduce fish populations, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fisheries, human health, and recreation. 

Drivers and causes of the regime shift

Mechanized anthropogenic over-harvesting of bivalve mollusks is the main driver of the shift from high to low abundance (Thrush and Dayton 2002).  High inputs of nutrients from agricultural or urban sources can act as a slow driver that weakens bivalve health by creating plankton blooms that the bivalves are unable to filter.  Plankton blooms can further weaken the health of bivalves by producing organic matter, whose decay, reduces oxygen availability.  A disturbance that can trigger a regime shift is bivalve disease (Powell et al. 2008).  When populations are weakened by eutrophication and anoxia, they become physiologically susceptible to diseases that can rapidly devastate remaining populations (Leniham 1999). 

How the regime shift works

Bivalves are a class of mollusks containing over 30,000 species which include scallops, clams, oysters, and mussels. Bivalve mollusks, like corals, are often referred to as "ecosystem engineers" because they engineer the physical and hydrodynamic structures of the ecosystems they occupy in a way that produces conditions that support their own population growth. Bivalves live on the sediment surface, where they form reefs that provide increased surface area and habitat for a variety of species. They filter nutrients from the pelagic zone and transfer it to the benthic zone, maintaining the system in a clear water state. If bivalves are not able to maintain clear water, due to declines in bivalve populations or increases in low water quality, the ecosystem can shift to a turbid water state. The main drivers of this shift is direct anthropogenic over-harvesting of the bivalves, followed by a gradual increase of nutrients discharge from agricultural activities and coastal urban settlements.  Once mollusk abundance is reduced, plankton and filamentous algae populations can substantially increase. As result, oxygen and light available in the water column is reduced, while nutrients levels accumulate due to the lack of filtration.  These processes produce conditions which favour algae over bivalves.  If nutrient inputs are substantial, eutrophication and areas of hypoxia (low oxygen) are likely to appear in areas dominated by plankton instead of mollusks. 

Impacts on ecosystem services and human well-being

Shift from high to low bivalve mollusk abundance regime

The loss of abundant bivalve mollusk reefs threatens diverse ecosystem services. The most direct impact is the loss of valuable shellfisheries. A secondary, but potentially substantial impact is the loss of the filtering service provided by bivalves.  In urbanized estuaries, it can necessitate the installation of costly synthetic water filtration technology (Gren et al. 2009).  In other areas it can lead to declines in water quality that shift populations and reduce recreation opportunities, and potentially the value of waterfront property.

The loss of bivalve habitat structure leads to lower species abundances, and to declines in species richness (Airoldi et al. 2008). Both structural and functional biodiversity is threatened by loss of bivalve abundance, with far reaching effects on marine ecosystems and human ability to exploit such systems (Worm et al. 2006).  The regime of low bivalve abundance is expected to reduce fisheries, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fishing productivity and human health.

The loss of bivalve nutrient filtration can lead to other regime shifts, such as eutrophication and hypoxia, which can produce unproductive and undesirable ecosystems (see Hypoxia and Eutrophication regime shifts). 

Management options

Options for enhancing resilience

Estuaries are the world's most degraded marine ecosystems, receiving land-based pollution from crop cultivation and urbanization, and suffering from anthropogenic over-harvesting (Lotze et al. 2006).  The resulting problems are complex, impacting bivalve health and fecundity as well as many other species, with far reaching implications in both biological and social domains.

Experience has shown that management focused on one species or problem tends to be ineffective.  Newer research stresses the need to address natural resources as part of complex social-ecological systems, often with long histories of human exploitation (Jackson et al. 2001).   Rather than addressing problems on a species-by-species basis, multi -species management has shown to have synergistic effects.  For example, in Chesapeake bay, fisheries management looks at oyster, blue crab, striped bass and shad together (Boesch 2004).

Taking this approach further, Ecosystem-based fishery management efforts focus on recognizing interactions between multiple species and environmental stressors, such as low dissolved oxygen levels. Success is measured by the degree to which management efforts include ecosystem-based approaches, rather than by an assessment of fishing stocks (Lotze et al. 2006). 

 

Options for reducing resilience to encourage restoration or transformation

The main option exercised for preventing or reversing a regime shift regarding bivalve abundance is to import bivalve mollusks from another region for aquaculture (Van de Koppel 2008) .  Another practice commonly used is to hang bivalves from the surface or build artificial reefs to elevate bivalves in order to avoid the hypoxic conditions in deeper water (Carlsson et al. 2009). 

Alternate regimes

Savannas are systems that consist of a mixture of woody vegetation (trees or shrubs) and grasses. Savannas, dry forest and shrublands cover 40% of the world's land area, host up to 42% of the world's human population (Reynolds et al. 2007, Falkenmark and Rockström 2008), and together with drylands sustain 50% of the world's livestock (Millennium Ecosystem Assessment 2005).

At small scales (up to about 10 km2) savanna systems, especially those used for extensive cattle ranching, may stabilize in two different self-reinforcing regimes (Walker 1993, Scheffer et al. 2001, Scholes 2003):  

Open, grassy savanna regime

In this regime the landscape has a productive grass layer with few mature trees. The canopy in savannas never closes, and the floor layer is dominated by grass, especially C4 species. Most young trees are unable to establish because, while often numerous, seedlings are constantly knocked back to ground level by herbivory and fire. There is enough grass after grazing to support a fire with flame-length taller than the young saplings sufficiently often to keep them in a 'fire trap' (Dublin et al. 1990, Roques et al. 2001b, a). Open savanna systems are suitable for ranching, and are maintained by fire dynamics and grazing.

Closed, woody savanna regime

In this regime the landscape is dominated by woody shrubs or trees. Once established, woody vegetation is stable because adult trees are seldom killed by herbivory or fire. These alternate regimes can occur at a range of spatial scales. Sometimes larger areas (e.g. an entire cattle ranch) may shift from a grass-dominated to a persistent woody-dominated state (Dublin et al. 1990, Walker 1993). In other cases, the alternate regimes are expressed as a mosaic of small patches of trees or bush interspersed with patches of grass, where the respective patches are highly persistent over time (Rietkerk et al. 2004).   

Drivers and causes of the regime shift

Bush encroachment refers to a shift from a grassy system to a persistently woody system. It typically occurs in areas used for free-range cattle ranching, and is usually caused by a combination of grazing and fire management practices. Bush encroachment involves a change in the outcome of the competitive interaction between woody vegetation (shrubs and trees) and herbaceous vegetation (grasses and herbs), mediated by nutrients, grazing, fire, rainfall variability and use of either the woody or grassy components by humans (Anderies et al. 2002, Janssen et al. 2004, Wiegand et al. 2006). The encroachment typically occurs in episodes rather than continuously, and involves a particular set of encroaching species rather than the entire woody community.

Bush encroachment typically occurs in areas used for commercial cattle ranching (as opposed to subsistence, communal, or nomadic) and may follow episodes of sustained severe overgrazing, though not necessarily so. It may also occur under other land uses (Wiegand et al. 2006). It tends to be an episodic phenomenon, where the tree cohorts can often be linked to issues in the ranching enterprise – such as drought-induced debt or downturns in the cattle price cycle (Scholes 2003, Wiegand et al. 2006). 

How the regime shift works

There are several different hypotheses regarding the mechanism by which bush encroachment occurs. Different mechanisms (or combinations of mechanisms) may be important in different places. One proposed mechanism is based on changes in fire regime: in the sustained presence of high numbers of grazers (typically cattle) accumulation of grass fuel is reduced, leading to period without intense fire long enough for woody plants to grow beyond the fire-susceptible stage, which in turn suppresses grass production and fires, further enhancing the establishment of woody vegetation (Higgins et al. 2000, Staver et al. 2009, Staver et al. 2011). A related hypothesis notes the elimination of browsers (especially very large browsers such as elephant and giraffe, but also the more-numerous small browsers) from the system when cattle are introduced (Dublin et al. 1990). Similarly, alien species, such as Prosopis in South Africa or Acacia nilotica in Australia, both deliberately introduced, can play an important role in bush encroachment by affecting fire regimes (Poynton 1990). Another hypothesis focuses on changes in water availability based on the rooting depths of plants: grasses are thought to be more shallowly-rooted than trees, so if grass cover is reduced by overgrazing, this is more water available for trees, which promotes their growth and establishment, further suppressing grass growth (Noy-Meir 1982). Refinements of these hypotheses emphasize combinations of events, such as a multiyear drought or fireless period providing a "window" for the establishment of trees (Wiegand et al. 2006).

Yet other hypotheses focus on the role that increases in global CO2 levels may play in the observed proliferation of woody plants in many, widely-separated areas of the world during the 20th century. The underlying mechanism is still debated, but several possibilities have been proposed: i) that rising CO2 levels favour C3 (woody plant) photosynthesis relative to C4 (tropical grass) photosynthesis  ii) elevated CO2 may reduce transpiration of grasses, leading to greater water percolation and therefore favoring deeper rooted woody species, iii) faster growth of woody plants due to CO2 enrichment, and therefore faster escape of seedlings from susceptibility to fire, iv) investments in carbon-based defense compounds such as tannins, which are the main defense compounds in many encroaching trees but not in grasses (Midgley and Bond 2001, Wiegand et al. 2006). It is striking that encroachment is almost unheard of on communal land, and is not universal on commercial farms. A suggested explanation for this is that communal lands use the trees for firewood and run goats along with cattle, inhibiting the establishment of trees (Scholes 2003). Bush encroachment has been documented in East and Southern Africa (but not West Africa), South America (Uruguay/Argentina and Chile), North America (Texas, New Mexico) and Australia, but not in India, also a savanna environment. Furthermore, it did not happen simultaneously in those places, but 30-50 years after the widespread establishment of sedentary grazing management, what has been referred to as 'commercial' ranching above. This tends to disfavor the rising CO2 hypothesis, although rising CO2 may predispose the shift (Midgley and Bond 2001).  

Impacts on ecosystem services and human well-being

Shift from grassy to woody savanna

Woody encroachment brings a relatively rapid change, over a decade or two, from a highly productive grass layer to a sparse and unproductive grass component. Since cattle are grass-eaters, this change substantially reduces cattle productivity (Anderies et al. 2002, Scholes 2003), with major impacts on cattle ranchers. On the other hand, encroachment increases the supply of tree-based ecosystem services, such as wood for fuel, charcoal-making and building material. This is somewhat dependent on the species involved. The increase in woody cover could potentially also have macro and micro-climatic effects through impacts on albedo and CO2 uptake, in addition to the decrease in methane emissions from cattle.

Difficulties in mustering the cattle in dense bush are a contributing factor. Therefore, wood encroachment leads to economic losses for cattle ranchers in what is frequently an economically marginal area for other agricultural uses such as croplands. 

 

Management options

Options for enhancing resilience

There is some agreement among researchers and extension workers that encroachment can be avoided by stocking lightly and burning frequently to prevent the establishment of trees and maintain grass crowns - the productive part of the grass that is less affected by fires (Roques et al. 2001b, Janssen et al. 2004). However, this is seldom reflected in management practice. 

Options for reducing resilience to encourage restoration or transformation

Bush encroachment is expensive to reverse, since rapid results rely on arboricides or repeated mechanical or manual clearing. A common method involves the manual removal of woody vegetation, with repeated follow-up control and the use of fire to enhance the establishment and competitive advantage of grasses (Scholes 1985, 2003). Attempts to reverse bush encroachment often have poor results, either due to the rapid resprouting of the trees or the conversion of the grass layer to less desirable species in the process. 

There are anecdotal reports of widespread mortality of near-dominant encroaching species after several decades, possibly related to disease, prolonged drought or simply old age, which provides windows for grass establishment and fuel load for intense fires.

Key References

  1. Anderies, J.M.; Janssen, M.A; ans Walker, B. 2002. Grazing management, resilience, and the dynamics of a fire-driven rangeland system. Ecosystems 5 (1): 23-44.
  2. Cavelier, J., T. Aide, C. Santos, A. Eusse, and J. Dupuy. 1998. The savannization of moist forests in the Sierra Nevada de Santa Marta, Colombia. Journal of Biogeography:901-912.
  3. Dublin, H. T., Sinclair, A. R. and McGlade, J. (1990). Elephants and fire as causes of multiple stable states in the Serengeti-Mara woodlands. Journal of Animal Ecology 59, 1147-1164.
  4. Higgins S. I., W. J. Bond, and W. S. W. Trollope. 2000. Fire, resprouting and variability: a recipe for grass-tree coexistence in savanna. Journal of Ecology, 88:213-229.
  5. Janssen, M.A; Anderies, J.M; and Walker, B. 2004. Robust strategies for managing rangelands with multiple stable attractors. Journal of Environmental Economics and Management
  6. Midgley, J. J. and Bond, W. J. (2001). A synthesis of the demography of African acacias. Journal of Tropical Ecology 17, 871-886.
  7. Noy-Meir, I. (1982). Stability of plant-herbivore models and possible applications to savanna. In: Ecology of Tropical Savannas (Huntley, B. J. and Walker, B. H. ed.), pp.591-609. Berlin: Springer.
  8. Poynton, R.J. 1990. The genus Prosopis in South Africa. S. Afr. For. J. 152: 62–66.
  9. Rietkerk, M., Dekker, S. C., de Ruiter, P. C. and van de Koppel, J. (2004). Self-organized patchiness and catastrophic shifts in ecosystems. Science 305, 1926-1929.
  10. Roques, K.G; O’Connor, T.G; Watkinson, A.R. 2001. Dynamics of shrub encroachment in an African savanna: relative influences of fire, herbivory, rainfall and density dependence. J Appl Ecol 38 (2): 268-280
  11. Scheffer, M., Carpenter, S. R., Foley, J. A., Folke, C. and Walker, B. H. (2001). Catastrophic shifts in ecosystems. Nature 413, 591-596.
  12. Scholes R. J. 2003. Convex Relationships in Ecosystems Containing Mixtures of Trees and Grass. Environmental and Resource Economics, 26:559-574.
  13. Scholes R.J. 1985. A Guide to Bush Clearing in the Eastern Transvaal Lowveld. Occasional Report of the Resource Ecology Group, University of the Witwatersrand. 50 pp.
  14. Scholes, RJ & S. Archer. 1997. Tree-grass interactions in savannas. Annual Review of Ecology and Systematics 28, 517-44.
  15. Staver A. C., W. J. Bond, W. D. Stock, S. J. van Rensburg, and M. S. Waldram. 2009. Browsing and fire interact to suppress tree density in an African savanna. Ecological Applications, 19:1909-1919.
  16. van de Koppel, J. and Rietkerk, M. 2000. Herbivore regulation and irreversible vegetation change in semi-arid grazing systems. Oikos 90 (2): 253-260
  17. Walker, B.H. 1993. Rangeland ecology: understanding and managing change. Ambio 22: 2-3.
  18. Wiegand, K; Saitz, D; and Ward, D. 2006. A patch-dynamics approach to savanna dynamics and woody plant encroachment - Insights from an arid savanna. Perspect Plant Ecol 7 (4): 229-242

Citation

Bob Scholes, Reinette (Oonsie) Biggs, Juan Carlos Rocha, Linda Luvuno, Garry Peterson. Bush Encroachment. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-02-13 10:45:38 GMT.
Monday, 28 February 2011 10:05

Hypoxia

Written by Juan Carlos

Hypoxia

Main Contributors:

Juan Carlos Rocha

Other Contributors:

Garry Peterson, Rutger Rosenberg, Reinette (Oonsie) Biggs

Summary

The critical variable in the hypoxia regime shift is dissolved oxygen in the water (DO). Different self-reinforcing regimes can be identified as normoxia, hypoxia and anoxia. Hypoxia is typically associated with eutrophication, and related to excess nutrient inputs from fertilizers or untreated sewage. As a result, hypoxic environments are also know as dead zones, areas where fish and crustaceans are not able to live. Anoxia occurs when hypoxia is exacerbated by releasing hydrogen sulfide, then changing water acidity (pH). Management options include the reduction of nutrient inputs (nitrogen and phosphorous), i.e. by closing the nutrient cycle in agricultural systems and through waste-water treatment. 

Drivers

Key direct drivers

  • External inputs (eg fertilizers)
  • Global climate change

Land use

  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Freshwater lakes & rivers

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries
  • Wild animal and plant products

Regulating services

  • Water purification

Cultural services

  • Recreation

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity

Key Attributes

Typical spatial scale

  • Local/landscape
  • Sub-continental/regional

Typical time scale

  • Months
  • Years
  • Decades

Reversibility

  • Hysteretic
  • Readily reversible

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

Links to other regime shifts

Alternate regimes

When discussing the two different regimes we will focus on the archetypes for compact city and sprawled city as large cities and megacities that still attract people. Thus, rather than a proper regime shift, development of cities is often path dependent according to one of the two trajectories towards either a sprawling city or a compact city. Our focus is on urban areas in developed countries, therefore we do not take e.g. poor city slums into consideration.   

Compact city

A compact city is more than just a city that is densely populated. "Dwelling density, the advancement of mixed-use development, and reaffirmed focus on the nature and quality of development have been identified as important aspects in the creation of the compact city" (Arbury 2005).Three elements are consistently found in many literatures that describe a compact city - mix-used development, greater focus on public transportation and quality urban design (Breheny 2001, Arbury 2005). Studies have included the promotion of urban regeneration, the revitalisation of town centres, and restraint on development in rural areas (Breheny 2001) and pedestrian friendly with large pavements (Arbury 2005) as part of the definition. 

Sprawling city

Urban sprawl can be defined as "unplanned, uncontrolled, and uncoordinated single use development that does not provide for a functional mix of uses and/or is not functionally related to surrounding land uses and which variously appears as low-density ...development" (Arbury 2005). The European Environment Agency (EEA) has described sprawl as the physical pattern of low-density expansion of large urban areas, under market conditions, mainly into the surrounding agricultural areas. Low-density, single-use and automobile dependent type of development has been the key attributes with lack of planning and control being the key enablers for urban sprawl.  

Unfortunately, there is not one single measurement for compact city or sprawling city. It requires a set of indicators that incorporate economic, social and environmental attributes of city development. Furthermore, defining what is desirable is quite subjective to preferences held by residents of different cities, which implies what may be desirable for one city might be undesirable for others. 

Drivers and causes of the regime shift

Shift from Compact City to Sprawling City

The main drivers and causes contributing to sprawl are urban population growth and the demand for housing. City expansion impels economic development, through the production of jobs, creativity, technology and the accumulation of knowledge and economic markets, which in many ways can lead to better welfare (Bairoch 1991). This induces the demand to live more spaciously, own a private lot and drive a car. Moreover, there are many people wishing to live closer to nature, hence people preferring aesthetic quality of the landscape seems to be contributing to sprawl development (Brown 2006). At the same time living in the inner city becomes more and more expensive and undesirable.

In Europe, many cities developed long before cars existed, and therefore have a more compact structure. Cities in North America, Australia and New Zealand, on the other hand, are planned for automobile use as a central part of the city system. Infrastructure, such as construction of roads, works as a feedback in the system and facilitates transportation. Increased private automobile ownership and low prices of fuel raises the automobile use even further. Another cause is the sudden rush of people to a city due to natural disasters (famines, flooding etc.) leading to uncontrolled sprawled or slum areas.  

 

How the regime shift works

A compact city has a diversity of functions congregated at the same place. It is designed with a mixed-use construction, pedestrian friendly roads and alternative means of transport. Instead of a "laissez faire" trend in urban planning, there is usually conscious control of city expansion with urban intensification, investments in public transportation and urban quality design (Arbury 2005). The functions of a compact city are self-reinforcing since its structure promotes public transportation, biking and walking before automobile use, with housing, work and shopping within reasonable distances. However, city expansion without conscious planning usually results in the city sprawling, also in originally compact cities, which explains why many old European cities have a compact core but a sprawled periphery.

Thus, almost all cities have elements of both sprawl and compact development, and once an area is built, it is hard to change its structure. Therefore, there are no precise thresholds and there is no regime shift in its proper sense, but rather a path dependency towards either sprawl or compact development. This pattern is described by Scheffer and Carpenter:"For conditions in which alternative equilibriums exist, the initial state (i.e. place in the landscape) determines the equilibrium to which the system will settle"(2003).

The regime of a sprawling city refers to low-density development of urban areas with a single use structure. This means the functions of housing, service and work opportunities are separated into different areas in the city. Infrastructure of roads and the automobile use make transport between these areas possible, hence maintaining the system. In countries such as USA, Australia and New Zealand, many cities were constructed after the invention of cars, rendering less dense city centers suited for automobile use. This construction automatically implies a bigger risk of sprawling compared to older European cities. Population growth and demand for housing are the main factors that drive this system. 

Impacts on ecosystem services and human well-being

Shift from Compact City to Sprawling City 

Sprawl leads to loss of land, which could have a negative impact on all provisioning ecosystem services depending on what type of land that is removed. Examples of such provisioning services are food crops, livestock, fisheries, wild animals, plant products, timber, fuel, fibre crops and woodfuel. This also leads to loss of air quality regulation from trees. Furthermore, there is biodiversity loss affecting other regulating services such as pollination, carbon sequestration, pest control and disease control. Cities in general and the expansion of the cities in particular result in increased emissions of greenhouse gases. Cities emit 70% of the world's greenhouse gases, which have vast consequences for the climate (CDP Cities 2011).

Since sprawled areas consist of private owned lots, commercial centres and roads, there is a lack of public space (Arbury 2005). Thus, people who own a private lot gain from sprawl whereas others loose. The suburban residents get the advantages of living in a house, away from urban stress and insecurities, but still have access to work opportunities and activities in the city (Arbury 2005). Also pollution and car accidents have a bigger impact on inner city dwellers since cars gather in the centre (Ewing 2008). Segregation is another problem that increases with sprawl, leading to poorer health among people who are less well off (Arbury 2005). Shifting from a compact to sprawling city affects cultural services such as recreation areas, aesthetic values, knowledge, educational values and spiritual and religious values. The compact city model encourages people to use public transportation and to walk and bike more, which have a positive impact on human health. However, city life is often considered stressful and can contribute to mental strains. 

 

 

Management options

Governments have an important role to play to ensure that feedback mechanisms enable sprawling cities to develop more effectively. An important first step is to formulate and enforce a city plan with wide public support (EEA 2006). Bold steps can be taken to abolish subsidies and other governmental support on outskirt road construction and "new town" developments. Full cost of providing public utilities, i.e. electricity, water and sanitation supply and waste treatment can be incurred to residents in newly developed areas (EEA 2006). Full cost of externalities from private automobile use can also be incurred through tax on vehicle ownership, fuel price and other schemes such as city tolls. Public finance from these resources along with savings from less road construction can be invested to promote inner-city mobility, such as investment on public transportation and bicycle and pedestrian roads. Without city planning, there will always be economic, social and environmental driving forces motivating people to move to the outskirts, as long as there is population growth. However, as long as the government does not allow feedback mechanisms of sprawl to be initiated, such as government support for road construction and large-scale outskirt development, or become too strong, this will be enough to prevent sprawl. This will enhance the resilience of the city to stay compact.  

However, in order to restore a sprawled city back to a compact city, a comprehensive and balanced approach that fundamentally reverse all drivers and feedbacks are necessary (EEA 2006). Another possible approach will be to establish new feedbacks that lead to compact city structure. Foremost, inner city building structures should be transformed into mixed-use structure so that affordable inner-city housing becomes more accessible. City planners should be on the lookout for "windows of opportunity", i.e. brownfield projects, to turn unattractive urban areas into mixed used housing or urban green areas. Governments should work actively to prevent inner-city crime and improve environmental conditions in order to make living in the city more safe and attractive (Ewing 2008).  

 

Alternate regimes

Freshwater river systems experience river channel position regime shifts when the main channel of a river abruptly changes its course to a new river channel. Meandering rivers are especially vulnerable to channel changes because of their high sinuosity which supports meander cutoffs. Other river types with a lower sinuosity can also experience river channel position shifts, for example braided rivers which have many small channels. Changes in river channel position can be understood as regime shifts when one considers the timescale at which they occur and the irreversibility of the shift. Large channel shifts (shifting the river position tens of kilometers) occur only approximately once a millennium. Within this timeframe humans settle along the river and establish complex social and economic structures which depend on the river. When a large, abrupt shift in the channel position occurs it results in huge disruption and the need for large-scale reorganization of the economy and societies that depend on the river. Once the river channel has shifted it will very rarely return to its former position, but instead become stabilized in its new position. In this context the alternate regimes are:

Old channel course

In this regime the river flows along a path which it has followed for many decades. People have therefore adapted to and have based their activities on this position of the river channel. For example, the floodplain area is often used by farmers to cultivate crops. Cities are located in the floodplain area, often protected by levees in case of a flood event. The mentality of the local inhabitants is that a channel shift should not be allowed to happen because their well-being is closely linked to the river in its current position. Therefore, large defense infrastructure is sometimes a characteristic of this regime (see also Tisza River case study).

 

New channel course

In this regime the river has switched its main course to a new path, often tens of kilometers from its previous course. Typically, riverflow is more rapid, and the length of the river is shorter. In the case of a meander cutoff, a so-called oxbow lake is formed. This happens when the meandering necks connect with each other and the abandoned part of the river after the cutoff is disconnected from the riverflow. The size of a cutoff and the resulting oxbow lake varies and depends on the size of the river. 

Drivers and causes of the regime shift

The main direct driver of the regime shift is strong floods, associated with large rainfall events. Such events cause the river to shift its course because they have enough power to break through a natural river levee or dyke, and to breach defense infrastructure such as a spillway that tries to control the riverflow (see Mississippi Case Study).

Human activities such as artificial channel widening, removal of debris, changes in the river course or cutting of vegetation play an important role in making a river more susceptible to channel shifts. The reason is that people may change the actual channel to improve transport or create a shortcut to decrease travel time and costs. Often, these activities might start very small by (e.g. removal of debris) and then slowly grow (e.g. by digging of a channel to make a shortcut) until a critical threshold has been reached and the river suddenly changes its course. In the case of the Ucayali River in Peru, an inconspicuous ditch of approximately one meter width was slowly but systematically widened which eventually led to a 71 kilometer cutoff after a strong flood event (see Ucayali River case study) with large negative impacts for the people living in the region. The destruction of dykes or levees can also cause a river channel to shift its position. For instance, a dyke at the Yellow River was destroyed to check the advance of the Jin army in China in 1128, leading to a shift in the river's course (see Yellow River Case Study). 

How the regime shift works

Floods are usually the direct cause of a shift in river channel position. However, the interplay of many different drivers is responsible for making a river vulnerable to changes in its course. The most common process is that over time sediment buildup gradually blocks the riverflow. A river always tries to take the shortest path and the steepest gradient due to gravity. When sediment is deposited on the riverbed, the gradient declines and slows down the current. This in turn leads to more deposition of sediment because of lower discharge until the river is blocked and spills to the side. This is a natural process, occurring approximately once a millennium in large rivers such as the Mississippi River (see Mississippi River case study).

In other cases, it is common that cutoffs occur at the meandering neck in rivers with high channel sinuosity. Erosion processes and scouring lead to channel incision and erosion of the river walls until the river suddenly breaks through the meandering neck, forming a cutoff. The river is then straightened and a so-called oxbow lake forms. This process is especially pronounced in rivers with a high sediment loading, for example the Amazon River in South America. Cutoffs vary in size depending on the size of the river and can occur alone or consecutively within a short period of time. Such shifts can be linked to the theory of self-organized criticality (SOC). According to SOC, cutoffs occur when the sinuosity of the river meander increases to a critical threshold at which the cutoff occurs.

A crevasse splay can also be responsible for a river channel position shift. It occurs when a natural levee, which was formed by sediment loading along the floodplain, suddenly breaks. This process is known as avulsion. This type of river channel position shift is common in river deltas (so-called delta switching) such as the Mississippi River delta or the Yellow River delta. Over a long period of time, a river delta can shift hundreds of kilometers due to repeated shifts in its main river channel and tributaries. Channel cutoffs in river deltas can also lead to the formation of so-called delta lobes through sedimentation which can form superlobes which in turn can cause further channel shifts. 

Impacts on ecosystem services and human well-being

Large rivers are of vital importance for the ecology, the economy and the society. They provide for example freshwater for inhabitants living in the region and are an important economically, for example as transportation routes, for inland navigation, the supply of fresh water, food and nutrition through small-scale and large-scale crop cultivation, fisheries, water regulation and regulation of soil erosion.When a river channel shifts course, local industries, cities and agriculture along the old channel course are therefore severly affected. Economic activities along the old channel will usually decline, and in some cases seaports have to be moved. However, impacts might also be positive, for example a decrease in flood events and flood levels along the old channel course. This may lead to new economic opportunities such as changes in subsistence and cash crops that generate a higher income. However, they may also experience greater threats of flooding. The impacts might also consist of an increase in flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These negative impacts might in turn lead to migration. 

Large rivers may be very important for regional and global trade. Therefore, a significant change in the position of the main channel usually has significant, mostly negative, impacts on human well-being.

 

Management options

Typically, river channel position regime shifts are irreversible. Without human action the river will almost never return to its original course. Managing river channel position regime shifts is very difficult because of the sudden and nonlinear nature of such shifts.

Options for enhancing resilience

In some situations attempts are made to retard or prevent a channel shift by controlling the riverflow (volume, current) through engineering works such as levees, spillways and weirs. This is usually done to protect housing, infrastructure and floodplain agriculture. However, efforts to modify the riverflow or implement defense infrastructure are usually extremely costly and requires enormous engineering efforts. Moreover, it is possible that the point where the shift is predicted to occur moves upstream or downstream, in which case the defense infrastructure becomes useless. The most iconic example of extensive engineering control structures to prevent a channel shift is the Mississippi River. The main channel is threatened by capture by the Atchafalaya River, and if this were to occur, New Orleans and Baton Rouge would suffer enormous negative economic consequences (see Mississippi River case study).

Options for reducing resilience to encourage restoration or transformation

It is possible to reduce the resilience of the system to encourage it to shift to a new channel position or return to a previous position, for example by removing levees. However, this is usually extremely costly and risky. 

Alternate regimes

Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:

 

Salt Marsh

This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).

 

Tidal Flat/ Subtidal Flat

This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged. 

Drivers and causes of the regime shift

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).

Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.

Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008). 

How the regime shift works

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.

An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.

However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).

Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.

The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor [1] at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor [2]. This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.

Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007). [1] Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction. [2] Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat. 

Impacts on ecosystem services and human well-being

Shift from Salt Marsh to Tidal Flat/ Subtidal Flat

Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection). 

Management options

Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).

Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined. 

Alternate regimes

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

 

Drivers and causes of the regime shift

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

How the regime shift works

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Impacts on ecosystem services and human well-being

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

Management options

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

Alternate regimes

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

Drivers and causes of the regime shift

The main external driver of this regime shift is CO2 emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

How the regime shift works

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO2 emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Impacts on ecosystem services and human well-being

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

 

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

Management options

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO2 levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Alternate regimes

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Drivers and causes of the regime shift

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

How the regime shift works

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Impacts on ecosystem services and human well-being

 

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Management options

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Alternate regimes

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 


Drivers and causes of the regime shift

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

How the regime shift works

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Impacts on ecosystem services and human well-being

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

 

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Management options

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

 

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Alternate regimes

Bivalve mollusks play an important role in aquatic ecosystems by filtering and sequestering nutrients. When bivalve abundance changes, it can have substantial ecosystem impacts, creating two different, self-reinforcing regimes.

Bivalve mollusk reef regime

When aquatic ecosystems have high bivalve mollusk density, they form reefs that include many bivalves and are shallow, nearing the water's surface. The filtering activity of the bivalves results in clear water with high levels of dissolved oxygen. Plankton populations are limited in these conditions. The bivalves themselves operate as reinforcing feedbacks in that their abundance ensures sufficient filtration to maintain this clear water regime. The clear water in turn reinforces bivalve abundance by maintaining hydrodynamics that are conducive to bivalve health (Scheffer 2009). Large, shallow bivalve mollusk reefs encourage biodiversity by providing habitat and filtering nutrients.

Isolated, low density bivalve mollusk regime

When aquatic ecosystems have low bivalve mollusk density, the reefs are small and deep. Water is turbid, with low levels of dissolved oxygen. Plankton and filamentous algae flourish under these conditions. Poor reef size and water filtration limit biodiversity. The low bivalve abundance state is reinforced as bivalve health and fecundity is weakened by turbid water conditions. The turbid water conditions are in turn reinforced by low bivalve abundance(Weijerman et al. 2005, Powell et al. 2008).

The regime of low bivalve abundance is expected to reduce fish populations, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fisheries, human health, and recreation. 

Drivers and causes of the regime shift

Mechanized anthropogenic over-harvesting of bivalve mollusks is the main driver of the shift from high to low abundance (Thrush and Dayton 2002).  High inputs of nutrients from agricultural or urban sources can act as a slow driver that weakens bivalve health by creating plankton blooms that the bivalves are unable to filter.  Plankton blooms can further weaken the health of bivalves by producing organic matter, whose decay, reduces oxygen availability.  A disturbance that can trigger a regime shift is bivalve disease (Powell et al. 2008).  When populations are weakened by eutrophication and anoxia, they become physiologically susceptible to diseases that can rapidly devastate remaining populations (Leniham 1999). 

How the regime shift works

Bivalves are a class of mollusks containing over 30,000 species which include scallops, clams, oysters, and mussels. Bivalve mollusks, like corals, are often referred to as "ecosystem engineers" because they engineer the physical and hydrodynamic structures of the ecosystems they occupy in a way that produces conditions that support their own population growth. Bivalves live on the sediment surface, where they form reefs that provide increased surface area and habitat for a variety of species. They filter nutrients from the pelagic zone and transfer it to the benthic zone, maintaining the system in a clear water state. If bivalves are not able to maintain clear water, due to declines in bivalve populations or increases in low water quality, the ecosystem can shift to a turbid water state. The main drivers of this shift is direct anthropogenic over-harvesting of the bivalves, followed by a gradual increase of nutrients discharge from agricultural activities and coastal urban settlements.  Once mollusk abundance is reduced, plankton and filamentous algae populations can substantially increase. As result, oxygen and light available in the water column is reduced, while nutrients levels accumulate due to the lack of filtration.  These processes produce conditions which favour algae over bivalves.  If nutrient inputs are substantial, eutrophication and areas of hypoxia (low oxygen) are likely to appear in areas dominated by plankton instead of mollusks. 

Impacts on ecosystem services and human well-being

Shift from high to low bivalve mollusk abundance regime

The loss of abundant bivalve mollusk reefs threatens diverse ecosystem services. The most direct impact is the loss of valuable shellfisheries. A secondary, but potentially substantial impact is the loss of the filtering service provided by bivalves.  In urbanized estuaries, it can necessitate the installation of costly synthetic water filtration technology (Gren et al. 2009).  In other areas it can lead to declines in water quality that shift populations and reduce recreation opportunities, and potentially the value of waterfront property.

The loss of bivalve habitat structure leads to lower species abundances, and to declines in species richness (Airoldi et al. 2008). Both structural and functional biodiversity is threatened by loss of bivalve abundance, with far reaching effects on marine ecosystems and human ability to exploit such systems (Worm et al. 2006).  The regime of low bivalve abundance is expected to reduce fisheries, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fishing productivity and human health.

The loss of bivalve nutrient filtration can lead to other regime shifts, such as eutrophication and hypoxia, which can produce unproductive and undesirable ecosystems (see Hypoxia and Eutrophication regime shifts). 

Management options

Options for enhancing resilience

Estuaries are the world's most degraded marine ecosystems, receiving land-based pollution from crop cultivation and urbanization, and suffering from anthropogenic over-harvesting (Lotze et al. 2006).  The resulting problems are complex, impacting bivalve health and fecundity as well as many other species, with far reaching implications in both biological and social domains.

Experience has shown that management focused on one species or problem tends to be ineffective.  Newer research stresses the need to address natural resources as part of complex social-ecological systems, often with long histories of human exploitation (Jackson et al. 2001).   Rather than addressing problems on a species-by-species basis, multi -species management has shown to have synergistic effects.  For example, in Chesapeake bay, fisheries management looks at oyster, blue crab, striped bass and shad together (Boesch 2004).

Taking this approach further, Ecosystem-based fishery management efforts focus on recognizing interactions between multiple species and environmental stressors, such as low dissolved oxygen levels. Success is measured by the degree to which management efforts include ecosystem-based approaches, rather than by an assessment of fishing stocks (Lotze et al. 2006). 

 

Options for reducing resilience to encourage restoration or transformation

The main option exercised for preventing or reversing a regime shift regarding bivalve abundance is to import bivalve mollusks from another region for aquaculture (Van de Koppel 2008) .  Another practice commonly used is to hang bivalves from the surface or build artificial reefs to elevate bivalves in order to avoid the hypoxic conditions in deeper water (Carlsson et al. 2009). 

Alternate regimes

Savannas are systems that consist of a mixture of woody vegetation (trees or shrubs) and grasses. Savannas, dry forest and shrublands cover 40% of the world's land area, host up to 42% of the world's human population (Reynolds et al. 2007, Falkenmark and Rockström 2008), and together with drylands sustain 50% of the world's livestock (Millennium Ecosystem Assessment 2005).

At small scales (up to about 10 km2) savanna systems, especially those used for extensive cattle ranching, may stabilize in two different self-reinforcing regimes (Walker 1993, Scheffer et al. 2001, Scholes 2003):  

Open, grassy savanna regime

In this regime the landscape has a productive grass layer with few mature trees. The canopy in savannas never closes, and the floor layer is dominated by grass, especially C4 species. Most young trees are unable to establish because, while often numerous, seedlings are constantly knocked back to ground level by herbivory and fire. There is enough grass after grazing to support a fire with flame-length taller than the young saplings sufficiently often to keep them in a 'fire trap' (Dublin et al. 1990, Roques et al. 2001b, a). Open savanna systems are suitable for ranching, and are maintained by fire dynamics and grazing.

Closed, woody savanna regime

In this regime the landscape is dominated by woody shrubs or trees. Once established, woody vegetation is stable because adult trees are seldom killed by herbivory or fire. These alternate regimes can occur at a range of spatial scales. Sometimes larger areas (e.g. an entire cattle ranch) may shift from a grass-dominated to a persistent woody-dominated state (Dublin et al. 1990, Walker 1993). In other cases, the alternate regimes are expressed as a mosaic of small patches of trees or bush interspersed with patches of grass, where the respective patches are highly persistent over time (Rietkerk et al. 2004).   

Drivers and causes of the regime shift

Bush encroachment refers to a shift from a grassy system to a persistently woody system. It typically occurs in areas used for free-range cattle ranching, and is usually caused by a combination of grazing and fire management practices. Bush encroachment involves a change in the outcome of the competitive interaction between woody vegetation (shrubs and trees) and herbaceous vegetation (grasses and herbs), mediated by nutrients, grazing, fire, rainfall variability and use of either the woody or grassy components by humans (Anderies et al. 2002, Janssen et al. 2004, Wiegand et al. 2006). The encroachment typically occurs in episodes rather than continuously, and involves a particular set of encroaching species rather than the entire woody community.

Bush encroachment typically occurs in areas used for commercial cattle ranching (as opposed to subsistence, communal, or nomadic) and may follow episodes of sustained severe overgrazing, though not necessarily so. It may also occur under other land uses (Wiegand et al. 2006). It tends to be an episodic phenomenon, where the tree cohorts can often be linked to issues in the ranching enterprise – such as drought-induced debt or downturns in the cattle price cycle (Scholes 2003, Wiegand et al. 2006). 

How the regime shift works

There are several different hypotheses regarding the mechanism by which bush encroachment occurs. Different mechanisms (or combinations of mechanisms) may be important in different places. One proposed mechanism is based on changes in fire regime: in the sustained presence of high numbers of grazers (typically cattle) accumulation of grass fuel is reduced, leading to period without intense fire long enough for woody plants to grow beyond the fire-susceptible stage, which in turn suppresses grass production and fires, further enhancing the establishment of woody vegetation (Higgins et al. 2000, Staver et al. 2009, Staver et al. 2011). A related hypothesis notes the elimination of browsers (especially very large browsers such as elephant and giraffe, but also the more-numerous small browsers) from the system when cattle are introduced (Dublin et al. 1990). Similarly, alien species, such as Prosopis in South Africa or Acacia nilotica in Australia, both deliberately introduced, can play an important role in bush encroachment by affecting fire regimes (Poynton 1990). Another hypothesis focuses on changes in water availability based on the rooting depths of plants: grasses are thought to be more shallowly-rooted than trees, so if grass cover is reduced by overgrazing, this is more water available for trees, which promotes their growth and establishment, further suppressing grass growth (Noy-Meir 1982). Refinements of these hypotheses emphasize combinations of events, such as a multiyear drought or fireless period providing a "window" for the establishment of trees (Wiegand et al. 2006).

Yet other hypotheses focus on the role that increases in global CO2 levels may play in the observed proliferation of woody plants in many, widely-separated areas of the world during the 20th century. The underlying mechanism is still debated, but several possibilities have been proposed: i) that rising CO2 levels favour C3 (woody plant) photosynthesis relative to C4 (tropical grass) photosynthesis  ii) elevated CO2 may reduce transpiration of grasses, leading to greater water percolation and therefore favoring deeper rooted woody species, iii) faster growth of woody plants due to CO2 enrichment, and therefore faster escape of seedlings from susceptibility to fire, iv) investments in carbon-based defense compounds such as tannins, which are the main defense compounds in many encroaching trees but not in grasses (Midgley and Bond 2001, Wiegand et al. 2006). It is striking that encroachment is almost unheard of on communal land, and is not universal on commercial farms. A suggested explanation for this is that communal lands use the trees for firewood and run goats along with cattle, inhibiting the establishment of trees (Scholes 2003). Bush encroachment has been documented in East and Southern Africa (but not West Africa), South America (Uruguay/Argentina and Chile), North America (Texas, New Mexico) and Australia, but not in India, also a savanna environment. Furthermore, it did not happen simultaneously in those places, but 30-50 years after the widespread establishment of sedentary grazing management, what has been referred to as 'commercial' ranching above. This tends to disfavor the rising CO2 hypothesis, although rising CO2 may predispose the shift (Midgley and Bond 2001).  

Impacts on ecosystem services and human well-being

Shift from grassy to woody savanna

Woody encroachment brings a relatively rapid change, over a decade or two, from a highly productive grass layer to a sparse and unproductive grass component. Since cattle are grass-eaters, this change substantially reduces cattle productivity (Anderies et al. 2002, Scholes 2003), with major impacts on cattle ranchers. On the other hand, encroachment increases the supply of tree-based ecosystem services, such as wood for fuel, charcoal-making and building material. This is somewhat dependent on the species involved. The increase in woody cover could potentially also have macro and micro-climatic effects through impacts on albedo and CO2 uptake, in addition to the decrease in methane emissions from cattle.

Difficulties in mustering the cattle in dense bush are a contributing factor. Therefore, wood encroachment leads to economic losses for cattle ranchers in what is frequently an economically marginal area for other agricultural uses such as croplands. 

 

Management options

Options for enhancing resilience

There is some agreement among researchers and extension workers that encroachment can be avoided by stocking lightly and burning frequently to prevent the establishment of trees and maintain grass crowns - the productive part of the grass that is less affected by fires (Roques et al. 2001b, Janssen et al. 2004). However, this is seldom reflected in management practice. 

Options for reducing resilience to encourage restoration or transformation

Bush encroachment is expensive to reverse, since rapid results rely on arboricides or repeated mechanical or manual clearing. A common method involves the manual removal of woody vegetation, with repeated follow-up control and the use of fire to enhance the establishment and competitive advantage of grasses (Scholes 1985, 2003). Attempts to reverse bush encroachment often have poor results, either due to the rapid resprouting of the trees or the conversion of the grass layer to less desirable species in the process. 

There are anecdotal reports of widespread mortality of near-dominant encroaching species after several decades, possibly related to disease, prolonged drought or simply old age, which provides windows for grass establishment and fuel load for intense fires.

Alternate regimes

The hypoxia regime shift involves a radical change in oxygen concentration in aquatic ecosystems such as rivers, lakes and marine ecosystems. The severity and persistence of hypoxic conditions varies. Episodic oxygen depletion represents 17% of known hypoxia cases, and occurs infrequently with several years sometimes elapsing between events. Episodic oxygen depletion is the first signal that a system has reached a critical point of eutrophication, which in combination with physical processes that stratify the water column, tips the system into hypoxic conditions (Diaz and Rosenberg 2008). Seasonal hypoxia tends to occur periodically during the summer, after algal spring blooms have sunk to the bottom and are being decomposed. It lasts from days to weeks, represents half of the known dead zones, and typically abates in the autumn (Diaz and Rosenberg 2008). Boom-and-bust cycles of animal populations are frequent. Persistent hypoxia occurs when hypoxia becomes persistent due to the build-up of organic matter in the sediments over time, particularly in systems prone to persistent stratification. Persistent hypoxia accounts for 8% of reported hypoxia cases. Anoxia occurs when DO levels fall below 0.2 ml per litre. The accumulation of organic matter is exacerbated, and poisonous hydrogen sulphide (H2S) is released due to microbial metabolism.

Normoxia: This regime is characterized by normal levels of dissolved oxygen, typically 5 to 8 ml per liter. Most aquatic organisms are adapted to survive under these conditions. Particularly important are the benthic organisms. Benthos are a community of organisms which live in sedimentary environments at the bottom of water bodies. They constitute an important key functional group (scavengers and detritivores) in the food web responsible for the decay of dead matter. Macrobenthos facilitate bioturbation processes, in other words, they displace and mix particles facilitating chemical exchange between sediments and the water column. This allows the sediments to "capture" phosphorous and nitrogen from the water column. By removing these nutrients from the water column, bioturbation therefore reduces algal growth and helps ensure that the water remains oxygenated and suitable for the survival of benthos - creating a reinforcing feedback that maintains normoxia conditions.

Hypoxia: The hypoxic regime is reached when dissolved oxygen level falls below 2 ml per liter (Diaz and Rosenberg 2008). Hypoxia is associated with so-called "dead zones". These are areas where dissolved oxygen levels are so low that most life is not able to persist and only very few specialized microorganisms survive. In the hypoxic state, accumulation of organic matter produced by eutrophic processes favors the growth of microbes who, by decomposing the organic matter, consume the oxygen in the water column.

Anoxia: Anoxia occurs when hypoxia is exacerbated to DO levels under 0.2 ml per liter (Diaz and Rosenberg 2008). Under such conditions, benthic animals suffer from mass mortality and decay processes are carried out by microbial metabolism. As A result, hydrogen sulfide is released changing water acidity (pH) and then further favoring bacteria habitat. 

Drivers and causes of the regime shift

Hypoxia is driven by increasing nutrient input, both natural and anthropogenic. Natural sources come from the bottom of the ocean transported by upwellings, water currents that flow vertically driven by the interaction of winds and the temperature gradient between the ocean surface and bottom. Anthropogenic sources are related mainly with the use of fertilizers in agriculture. However, growing urban settlements in coastal areas also increase the water storm runoff and sewage.

The leakage of nutrients from agriculture is further exacerbated by rainfall variability and deforestation, since vegetation offers resistance to erosion and trap nutrients and moisture in the soil. Agriculture is driven in turn by increasing demand of food and fibers as well as population growth. Another important factor leading to hypoxic conditions is the stratification of the water column, which reduce the exchange of nutrients between the surface and the bottom. Water stratification is usually driven by anomalous increase in sea surface temperature. 

How the regime shift works

The fast variable in the hypoxia regime shift is DO, and can also include faunal composition. The slow variables are the accumulation of nutrients, mainly phosphorous and nitrogen in the water column. Large areas of the sea are oxygen minimum zones due to natural conditions. Hypoxic-prone areas generally have a semi-enclosed hydrogeomorphology and water-column stratification that restrict water exchange and reoxygenation of the deeper water layers (Diaz and Rosenberg 2008). Another natural condition that facilitates hypoxia and eutrophication is coastal upwelling, that lead to nutrient enrichment along continental margins.

The normoxia regime is usually  maintained by feedbacks that control the nutrients content in the water column and keep oxygen concentrations high. Zooplankton for example plays an important role controlling micro algae numbers, hence limiting their ability to use nutrients available and reduce oxygen through their metabolism. Macrophytes or macro algae also control micro algae by consuming the same resources. Under such conditions nutrients are kept on inorganic forms unavailable for micro algae to use.

Under the hypoxia regime the feedbacks above are weakened, allowing micro algae to escape predation and competition controls; and reducing the level of DO in water. As DO decreases, mass mortality of different organism increases the availability of organic matter in the water column, decreasing DO and promoting more micro algae growth. When DO reaches critical levels under 0.5mL per liter, hydrogen sulfide is released further decreasing pH in water and reducing biodiversity. Such extreme regime is called anoxia. 

Impacts on ecosystem services and human well-being

Shift from normoxia to hypoxia and anoxia

Normoxia is associated with the provision of food for humans and wildife in aquatic systems as well as regulating services such as water purification and pest regulation. This regime shift into hypoxia, often characterised as a dead zone, has been reported in more than 400 systems affecting more than 245,000 square kilometers and including important fisheries such as the Baltic Sea, Kattegat, Black Sea, Gulf of Mexico, and East China Sea (Diaz and Rosenberg 2008). A major impact on ecosystems is a change in the flux of matter and energy through trophic levels. Consequently, fisheries and hence ecosystem services such as food production are affected. For example, Diaz & Rosenberg (2008) report that biomass in the Baltic Sea has been reduced by approximately 264,000 metric tons of carbon due to hypoxic conditions. Assuming that ~40% of benthic energy passes up the food chain, 106,000 metric tons of carbon of food energy for fisheries has been lost(Diaz and Rosenberg 2008). This implies a reduction in yields and consequent impact on employment in fisheries communities. Another example of such effects is the lobster fishery collapse in Norway (Diaz and Rosenberg 2008).

Besides its effect on fisheries and employment, hypoxia also affect the health and other cultural services of the people living near by dead zones. Decaying matter after mass mortality events create odors and risk of diseases. Recreation, aesthetic and touristic values are lost.

Shift from Hypoxia to Normoxia

When hypoxia is not severe, the system is likely to return to normoxia regime. It implies the recovery of fisheries when species hasn't been extinct locally. However, food webs not always fully recover, and usually top predators are lost. With the recovery of the system, fishing, recreational and touristic activities recover too, although not necessary to the same configuration than before the regime shift. 

Management options

Options for enhancing resilience

Since one of the main drivers of hypoxia is eutrophication, Diaz & Rosenberg (2008) recommend managing the input of fertilizers on agricultural land. They recognize the necessity of developing new methods to close the nutrient cycle on farms, in order to avoid the drainage of nutrients to water sources. Nutrient reductions can also be achieved relatively cost-efficiently by improving waste-water treatment system in regions where this is applicable (e.g. Baltic Sea).

Options for reducing resilience to encourage restoration or transformation

Only 4% of the reported cases of hypoxia have shown improvement, principally due to the reduction of organic and nutrient loading, stratification strength, and freshwater runoff (Diaz and Rosenberg 2008).

Managers could take advantage of windows of opportunity provided by these variables. For example, in the Baltic Sea the stratification of the water column is determined by input of saltwater from the North Sea (Conley et al. 2009). A policy of nutrient load reduction would therefore be more effective in years when saltwater input is low and it is a particularly rainy year. While nutrient loading is largely periodic and rain dependent, stratification and freshwater runoff rely more on physical processes and climate variability. Finally, it has been suggested that an appropriate goal is the reduction of nutrient loads to the level of the mid-1900s (Diaz and Rosenberg 2008). 

Key References

  1. Conley, D; .Björck, S; Bonsdorff, E; Cartensen, J; Destouni, G; Gustafsson, B.G; Hietanene, S; Kortekaas, M; Kuosa, H; Meier, H.E.M; Mueller-Karulis, B; Nordberg, K; Norkko, A; Nuernberg, G; Pitkanen, H; Rabalais, N.N; Rosenberg, R; Savchuk, O.P; Slomp, C.P; Voss, M; Wulff, F; Zillen, L. 2009. Hypoxia-Related Processes in the Baltic Sea. Environ Sci Technol 43(10); 3412-3420
  2. Díaz, Robert and Rosenberg, Rutger. 2008. Spreading Dead Zones and Consequences for Marine Ecosystems. Science 321: 926-29

Citation

Juan Carlos Rocha, Garry Peterson, Rutger Rosenberg, Reinette (Oonsie) Biggs. Hypoxia. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-23 10:38:03 GMT.
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