• Mangroves transitions
    • Summary
    • Categorical attributes
    • Detail information
    • Regime shift Analysis
      • References

Mangroves transitions

Main contributors: Juan Carlos Rocha

Other contributors: Reinette (Oonsie) Biggs

Last update: 2014-10-03

Summary

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

Categorical attributes

Impacts

Ecosystem type:’

  • Marine & coastal

Key ecosystem processes:

  • Soil formation
  • Water cycling

Biodiversity:

  • Biodiversity

Provisioning services:

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

Regulating services:

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

Cultural services:

  • Recreation
  • Aesthetic values

Human well-being:

  • Food and nutrition
  • Livelihoods and economic activity
  • Security of housing & infrastructure
  • Cultural
  • Aesthetic and recreational values

Links to other regime shifts:

  • Coral Transitions
  • Seagrass transitions
  • Hypoxia
  • Fisheries collapse
  • Marine eutrophication

Drivers

Key drivers:

  • Vegetation conversion and habitat fragmentation
  • Harvest and resource consumption
  • External inputs (e.g. fertilizers
  • pest control
  • irrigation)
  • Infrastructure development (e.g. roads
  • pipelines)
  • Soil erosion & land degradation
  • Environmental shocks (e.g. fire
  • floods
  • droughts)
  • Global climate change

Land use:

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

Key attributes

Spatial scale:

  • Local/landscape (e.g. lake
  • catchment
  • community)

Time scale:

  • Years
  • Decades

Reversibility:

  • Unknown

Evidence:

  • Models
  • Paleo-observation
  • Contemporary observations
  • Experiments

Confidence: existence of the regime shift

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

Confidence: mechanisms underlying the regime shift

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

Detail information

Alternative regimes

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

Mangrove forest

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

Salt marshes, rocky tidal, shrimp farms.

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

Drivers and causes of the regime shift

The main drivers of mangrove collapse have typically been land use / cover change during the last 50 years. One third of world’s mangroves have been lost due to overexploitation of forest resources (deforestation) or conversion into agriculture, salt extraction, or ponds for shrimp farming (Cavanaugh et al. 2014). Other threats to mangroves include the development of infrastructure (roads, dams), the diversion of fresh water for irrigation and the development of urban areas. These impacts are typically stronger in developing countries where it is expected that mangroves will decline by 25% by 2025 (Mcleod and V Salm 2006), while recent studies show that mangroves area are declining 1-2% yearly(Duke et al. 2007, Alongi 2008, Cavanaugh et al. 2014).

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

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

Impacts on ecosystem services and human well-being

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

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

Management options

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

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

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

Regime shift Analysis

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.

Citation

Acknowledge this review as:

Juan Carlos Rocha, Reinette (Oonsie) Biggs. Mangroves transitions. In: Regime Shift Database, www.regimeshifts.org. Last revised: 2014-10-03

References

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This work is licensed under CC BY-NC-SA 4.0. It is an initiative lead by the Stockholm Resilience Centre. The website was developed by Juan Rocha and build with Rmarkdown.