Mangrove restoration

Mangrove restoration is the regeneration of mangrove forest ecosystems in areas where they have previously existed. The practice of mangrove restoration is grounded in the discipline of restoration ecology, which aims to “[assist] the recovery of resilience and adaptive capacity of ecosystems that have been degraded, damaged, or destroyed”.[1] Since environmental impacts are an ongoing threat, to successfully restore an ecosystem implies not merely to recreate its former condition, but to strengthen its capacity to adapt to change over time.

Environmental context

Mangrove forests, along with the animal species they shelter, represent globally significant sources of biodiversity and provide humanity with valuable ecosystem services. They are used by mammals, reptiles and migratory birds as feeding and breeding grounds, and provide crucial habitats for fish and crustacean species of commercial importance. The roots of the mangrove physically buffer shorelines from the erosive impacts of ocean waves and storms. Additionally, they protect riparian zones by absorbing floodwaters and slowing down the flow of sediment-loaded river water. This allows sediments to drop to the bottom where they are held in place, thus containing potentially toxic waste products and improving the quality of water and sanitation in coastal communities.

To the human communities who rely on them, mangrove forests represent local sources of sustainable income from the harvest of fish and timber, as well as non-timber forest products such as medicinal plants, palm leaves and honey. On a global scale, they have been shown to sequester carbon in quantities comparable to higher-canopy terrestrial rainforests, which means that they may play a role in climate change mitigation,[2] in addition to physically protecting coastlines from the projected sea-level rise associated with climate change.[3] However, there are limits to the capacity of mangroves to adapt to climate change. It is projected that a 1-meter rise in sea level could inundate and destroy mangrove forests in many regions around the globe,[4] which would leave coastal communities vulnerable to the risks of flooding, shoreline erosion, saline intrusion and increased storm activity.[5]

Mangrove loss and degradation

The issue of restoration is critical today since mangrove forests are being lost very quickly – at an even faster rate than tropical rainforests inland.[6] A recent estimate puts the total mangrove area worldwide in 2005 at 152,000 km2 – down from 188,000 km2 in 1980.[7] In other words some 36,000 km2, or nearly 20% of the world’s mangroves, were lost over a period of twenty-five years. Other estimates of loss may differ due to having been drawn from a smaller pool of data. The Millennium Ecosystem Assessment estimates the total loss worldwide at 35% between 1980 and 2000, but this result was drawn from data on only slightly more than half of the total mangrove area.[3] Much of this lost mangrove area was destroyed to make room for industry, housing and tourism development; for aquaculture, primarily shrimp farms; and for agriculture, such as rice paddies, livestock pasture and salt production.[7] Other drivers of mangrove forest destruction include activities that divert their sources of freshwater, such as groundwater withdrawals, the building of dams, and the building of roads and drainage canals across tidal flats.

Mangrove restoration

Mangroves are sensitive ecosystems, changing dynamically in response to storms, sediment blockage, and fluctuations in sea level [8] and present a “moving target” for restoration efforts. Different restoration approaches face this challenge in different ways. The most common method simply consists in planting single-species stands of mangroves in areas thought to be suitable, without consideration of whether or not they supported mangroves in the past.[9] This approach usually fails over the long term because the underlying soil and hydrological requirements of the mangroves are not being met. More informed methods aim to bring a damaged mangrove area back into its preexisting condition, taking into account not only ecosystem factors but also social, cultural and political perspectives.[8] These approaches begin with the understanding that a damaged mangrove area may be able to repair itself through the natural processes of secondary succession, without being physically planted, provided that its tidal and freshwater hydrology is functioning normally and there is an adequate supply of seedlings.[10] Taking this into account, it becomes crucial to the success of a restoration project to evaluate what the hydrology of a disturbed mangrove site should look like under normal conditions, and the ways in which it has been modified. One example of this approach is the Ecological Mangrove Restoration method [10] which recommends the following steps, to be undertaken using healthy mangroves of the surrounding area as a reference:

  1. Assess the ecology, especially reproduction and distribution patterns, of the mangrove species at the disturbed site;
  2. Map the topographical elevations and hydrological patterns that determine how seedlings should establish themselves at the site;
  3. Assess the changes made to the site that currently prevent the site from recovering by itself;
  4. Design a restoration plan that begins by restoring the normal range of elevations and tidal hydrology at the site; and
  5. Monitor the site to determine if the restoration has been successful in light of the original objectives.

The actual planting of seedlings is a last resort, since it fails in many cases;[10] it should be considered only if natural recruitment of seedlings fails to reach the restoration objective.

Additional considerations

An important but often overlooked aspect of mangrove restoration efforts is the role that the local communities play as stakeholders in the process and the outcome. Since they may directly feel the effects of restoration projects, they should be involved in the process as much as feasibly possible, from decision-making to maintenance over the long term. Their involvement and local knowledge, as well as collaboration with other stakeholders such as sponsors and governing agencies, is crucial to the success of restoration projects.

Limitations

In some areas, restoration may be prohibitively difficult due to the degradation of the soil that regularly follows the clear-cutting of mangrove forests. Common effects include advanced erosion of the soil, loss of nutrients, high levels of salinity, and/or buildup of toxins.[8] However, even without this extent of degradation, the soil may become unable to host plant life at all due to the loss of the live mangrove roots, which exuded oxygen and carbohydrate into the soil and maintained its quality. Using foresight early in the restoration process to carefully select sites that are likely to succeed as self-maintaining ecosystems, as well as ensuring that proper management is built into the conservation effort, can prevent the waste of time and energy that often accompanies restoration projects. The long-lasting aftereffects of mangrove degradation underscore the importance of eliminating its causes, since once sites are cleared, it is difficult for them to recover without a scientific intervention.

References

  1. ↑ "FSM 2000 – National Forest Resource Management, Chapter 2020 – Ecological Restoration and Resilience". Retrieved 9 April 2012.
  2. ↑ Spalding, Mark; Kainuma, Mami; Collins, Lorna (2010). World Atlas of Mangroves. London, UK: Washington, DC: Earthscan.
  3. 1 2 "Millennium Ecosystem Assessment. Ecosystems and Human Well-Being: Wetlands and Water Synthesis." (PDF). Washington, DC: World Resources Institute. 2005. Retrieved 10 July 2012.
  4. ↑ "Working Group II: Impacts, Adaptation and Vulnerability. 19.3.3.5, "Mangrove Ecosystems"". IPCC Fourth Assessment Report: Climate Change 2001. Retrieved 24 June 2012.
  5. ↑ Field, C.D. (1995). "Impact of expected climate change on mangroves". Hydrobiologia. 295 (1-3): 75–81. doi:10.1007/BF00029113.
  6. ↑ Duke, N.C.; Meynecke, J.-O., Dittmann, S., Ellison, A.M., Anger, K., Berger, U., Cannicci, S., Diele, K., Ewel, K.C., Field, C.D., Koedam, N., Lee, S.Y., Marchand, C., Nordhaus, I. & Dahdouh-Guebas, F. (2007). "A world without mangroves?". Science. 317 (5834): 41–42. doi:10.1126/science.317.5834.41b. Cite uses deprecated parameter |coauthors= (help)
  7. 1 2 The world's mangroves 1980-2005. Food and Agriculture Organization. 2007. ISBN 9789251058565.
  8. 1 2 3 Field, C.D. (1998). "Rehabilitation of Mangrove Ecosystems: An Overview". Marine Pollution Bulletin. 37 (8-12): 383–392. doi:10.1016/s0025-326x(99)00106-x.
  9. ↑ "Ecological Mangrove Restoration in Thailand". Wetlands International. 2012.
  10. 1 2 3 Lewis, Roy R. (2005). "Ecological engineering for successful management and restoration of mangrove forests". Ecological Engineering. 24: 403–418. doi:10.1016/j.ecoleng.2004.10.003.

Sources

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