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Mangroves transitions

Feedback mechanisms

Mangrove forest

  • Soil build up (local, well-established R1): As mangroves grow, their roots and leaves build up an organic rich soil matter also known as peat (Mcleod and V Salm 2006). As the peat soils increase, mangroves reduce the deepness of the water level, favoring appropriate water mixing between fresh and saline water, recreating in turn optimal conditions for mangroves to grow (Lovelock 2008).
  • Temperature feedback (regional, proposed R2): Mangroves and in general wetlands, are thought to be key stocks of carbon storage on their peat soils (Alongi 2014, Cavanaugh et al. 2014). However, recent studies show that their storage capacity is affected by temperature (Lovelock 2008). As mangrove forest is maintained, more carbon is captured in the peat soils, reducing the likelihood of temperature increase via atmospheric CO2. This feedback is however only proposed and is not expected to be strong enough to avoid mangrove transitions since it does not account for other sources of CO2 in the atmosphere. Temperature also play an important role in temperate areas where warming reduces the number of days with freezing temperatures (about -4ºC for latitudes and species of Florida). Such subtle increase on the lower end of the temperature range has favor mangroves to overcome saltmarshes in coastal North America and Australia (Cavanaugh et al. 2014).
  • Temperature feedback (regional, proposed R2): As temperature increase mangroves are push outside their comfort zone, their canopy production is reduced while activity of microbial communities is enhanced in the soils (Lovelock 2008). Increasing temperature is also expected to effect sea level rise, increasing the deepness of mangroves and salinity. Both factors reduce mangrove’s niche. Release of the peat carbon storage reinforces climate warming.
  • Competition mechanism (local, well-established R3): Mangroves are in competition with other species or configuration of the ecosystem that favor other species. One possible alternative regime is such of salt marshes, as indicated in the causal loop diagram. Thus, the more salt marshes, the less available resources for mangroves to grow. However, other configurations are also possible which include rocky shores, lagoons, sandy beaches, among others. The possible alternative regimes highly depend on the history of the system and local conditions (Mcleod and V Salm 2006).
  • Shrimp farming (local, well-established B1): The demand of shrimp production and development policies favor the establishment of shrimp farm industries both at local and regional scales (Mcleod and V Salm 2006). The farming requires the deforestation of mangrove forest and creating ponds for shrimps. This balancing feedback constraint the expansion of mangrove.

Salt marshes, rocky tidal, shrimp farms

  • Temperature feedback (regional, proposed R2): As temperature increase mangroves are push outside their comfort zone, their canopy production is reduced while activity of microbial communities is enhanced in the soils (Lovelock 2008). Increasing temperature is also expected to effect sea level rise, increasing the deepness of mangroves and salinity. Both factors reduce mangrove’s niche. Release of the peat carbon storage reinforces climate warming.
  • Competition mechanism (local, well-established R3): Mangroves are in competition with other species or configuration of the ecosystem that favor other species. One possible alternative regime is such of salt marshes, as indicated in the causal loop diagram. Thus, the more salt marshes, the less available resources for mangroves to grow. However, other configurations are also possible which include rocky shores, lagoons, sandy beaches, among others. The possible alternative regimes highly depend on the history of the system and local conditions (Mcleod and V Salm 2006).
  • Shrimp farming (local, well-established B1): The demand of shrimp production and development policies favor the establishment of shrimp farm industries both at local and regional scales (Mcleod and V Salm 2006). The farming requires the deforestation of mangrove forest and creating ponds for shrimps. This balancing feedback constraint the expansion of mangrove.

 

Drivers

Collapse of mangroves

Important shocks (eg droughts, floods) that contribute to the regime shift include:

  • Floods (local, well-established): Floods are pulse dynamics that bring large quantities of fresh water and sediments to the mangrove area, it favors peat soil formation. However, long exposure to fresh water could stress mangrove trees (Mcleod and V Salm 2006).
  • Droughts (local, well-established): In contrast, droughts are expected to reduce fresh water income and increase salinity, creating stress for mangroves (Mcleod and V Salm 2006, Lovelock 2008).

The main external direct drivers that contribute to the shift include:

  • Deforestation (local, well-established): Deforestation of mangrove is one of the main drivers of mangrove area loss. Mangroves are deforested mainly for their wood either for construction, wood fuel, aquaculture or agriculture (Mcleod and V Salm 2006). Deforestation of areas adjacent to mangrove forest, for example in the same watershed, can also influence mangrove development since it increases erosion and sedimentation (Restrepo and Kettner 2012).
  • Aquaculture (local, well-established): Shrimp aquaculture is by far the most important driver of mangrove area loss in the last 50 years, in fact, it accounts for 20 to 50% of area loss worldwide(Duke et al. 2007).
  • Infrastructure development (local, well-established): Infrastructure development includes the construction of dams, shipping channels, dikes, seawalls, roads, water channels, and urban facilities such as aqueduct and sewage. The disturb mangroves when affecting the mixture of fresh and salted water, or when affecting the sediments and nutrient inflow(Mcleod and V Salm 2006, Duke et al. 2007).
  • Temperature (regional, contested): Increase in temperature due to climate change is expected to affect mangroves in different ways. Although some scientists have demonstrated that temperature stress could reduce leaf production and hence mangrove growth above 25ºC or below 15ºC, and above 35ºC root structures and seedlings are affected, crossing such temperature thresholds in current mangrove areas are not likely to be a major threat under current climate change scenarios(Mcleod and V Salm 2006, Lovelock 2008). On the other hand, increase on the lower limit of temperature range gives mangrove a competitive advantage over saltmarshes, it has been observed expansion polewards of mangrove areas (Cavanaugh et al. 2014).
  • Sea level rise (regional, well-established): Higher sea levels means deeper mangrove substratum. Since mangroves use aerial roots and depend on the mixture of fresh and salt water, changes in depth will substantially affect mangroves. If sea level rise is faster than mangrove ability to up-migrate both in latitude or altitude, mangrove habitat will be decimated. This threat, however, is heterogeneous in space and strongly depend on local conditions such as topology, substratum type and tides (Mcleod and V Salm 2006). It has been proposed that temperature increase will make water less dense and melt ice reservoirs of water with a imminent effect on sea level rise (Mcleod and V Salm 2006).
  • Erosion (regional, well-established): Coastal erosion threatens the formation of peat soil on which mangroves depend (Mcleod and V Salm 2006). Coastal erosion is due to wave action and it could become stronger due to loss of natural wave barriers such as coral reefs.

The main external indirect drivers that contribute to the shift include:

  • Agriculture (regional, well-established): Agriculture is a driver that affects mangroves in two ways. First, when expanding the agricultural frontier over mangrove areas, the forest is lost the soil is usually dried through channels (Mcleod and V Salm 2006). Second, agricultural activities at the watershed level have strong effects on nutrients inputs and erosion, which have secondary effects on mangrove development (Restrepo and Kettner 2012).
  • Sea surface temperature (regional, well-established): Higher SST makes water less dense, increasing sea level rise (Mcleod and V Salm 2006).
  • Ocean acidification (global, well-established): The acidification of oceans is expected to affect accretion of coral reefs, decreasing in turn the protecting shore service that they provide to coastal ecosystems such as mangroves (Mcleod and V Salm 2006).
  • Irrigation (regional, well-established): Diversion of water for agriculture means less fresh water input to mangrove forests, increasing in turn salinization and creating stress for species less adapted to such conditions (Mcleod and V Salm 2006).
  • Fragmentation (regional, well-established): Many mangroves depend on landscape connectivity either through meta-population dynamics (the flow of individuals among different populations) or through the services provided by other key ecosystems such as coral reefs and sea grass beds (Mcleod and V Salm 2006).
  • Urbanization (regional, well-established): Many urban settlements are in coastal regions, some of them former mangrove habitat (Mcleod and V Salm 2006). Over half of world’s mangrove area are located within 25km or urban settlements inhabited by 100.000 or more people (Millennium Ecosystem Assessment 2005, Mcleod and V Salm 2006). As settlements become denser, the demand for wood fuel, fish and other pressures on mangroves such as deposition of waste or sewage grow.

Slow internal system changes that contribute to the regime shift include:

  • Peat soils (local, well-established): Mangroves both build and depend of rich organic matter soils for their development. However, peat soils building is a very slow process (~3.5 mm per year), and its conservation depends upon on a higher rate of sedimentation than sea level rise. Loss of peat could be triggered by strong wave action leading to erosion (Mcleod and V Salm 2006).
  • Water deepness (regional, well-established): Mangrove survival depends upon a greater rate of sedimentation and peat building than sea level rise. If the mangrove substratum is too deep, it means that roots wont have an aerial section exposed for respiration and that water would be have higher salinity. Both factors induce stress in mangroves, inhibiting growth.

 

Summary of Drivers

# Driver (Name) Type (Direct, Indirect, Internal, Shock) Scale (local, regional, global) Uncertainty (speculative, proposed, well-established)
1 Deforestation Direct Local Well-established
2 Agriculture Indirect Local Well-established
3 Aquaculture Direct Local Well-established
4 Infrastructure development Direct Regional Well-established
5 Urbanisation Indirect Local Well-established
6 Floods Shock Regional Proposed
7 Droughts Shock Regional Proposed
8 Temperature Direct Regional Proposed
9 Sea surface temperature Indirect Regional Well-established
10 Sea level rise Direct Local Well-established
11 Ocean acidification Indirect Regional Proposed
12 Erosion Direct Local Proposed
13 Irrigation infrastructure Indirect Regional Well-established
14 Fragmentation Indirect Regional Well-established
15 Hurricanes / storms Shock Local Well-established

Key thresholds

Shift from mangrove forest to other estuarine system

  • Temperature below 15ºC or above 25ºC –  reduces leaf production due to temperature stress. Temperatures higher than -4ºC in the more temperate areas of the globe for the coldest period of the year has been observed to favor mangrove take over salt marshes areas.

Leverage points

It has been suggested that a leverage point to preserve mangrove areas could be using conservation schemes such as REDD, in order to protect the carbon storage service that this coastal ecosystem provides. Education and capacitation of local communities to develop livelihoods less dependent on mangroves commodities are also important fronts of action.

Summary of Ecosystem Service impacts on different User Groups

References (if available)
Provisioning Services
Freshwater +/-
Food Crops +
Feed, Fuel and Fibre Crops 0
Livestock 0
Fisheries -
Wild Food & Products -
Timber -
Woodfuel -
Hydropower 0
Regulating Services
Air Quality Regulation 0
Climate Regulation -
Water Purification -
Soil Erosion Regulation -
Pest & Disease Regulation 0
Pollination 0
Protection against Natural Hazards -
Cultural Services
Recreation -
Aesthetic Values -
Cognitive & Educational 0
Spiritual & Inspirational 0

Uncertainties and unresolved issues

It is uncertain whether temperature will affect the tolerance range of mangroves for leaf production, roots structures and seedlings. According with current projections of temperature rise, this is not likely to happen (Mcleod and V Salm 2006). Temperature however, under the same scenarios, will affect sea level rise strong enough as to induce mangroves migration if peat soil building is fast enough.