Present state and future of the world’s mangrove forests

Study Number:

23

Author:

D. M. Alongi

Abstract:

Mangroves, the only woody halophytes living at the confluence of land and sea, have been heavily used traditionally for food, timber, fuel and medicine, and presently occupy about 181 000 km2 of tropical and subtropical coastline. Over the past 50 years, approximately one-third of the world’s mangrove forests have been lost, but most data show very variable loss rates and there is considerable margin of error in most estimates. Mangroves are a valuable ecological and economic resource, being important nursery grounds and breeding sites for birds, fish, crustaceans, shellfish, reptiles and mammals; a renewable source of wood; accumulation sites for sediment, contaminants, carbon and nutrients; and offer protection against coastal erosion. The destruction of mangroves is usually positively related to human population density. Major reasons for destruction are urban coastal development, aquaculture, mining and overexploitation for timber, fish, crustaceans and shellfish. Over the next 25 years, unrestricted clear felling, aquaculture, and overexploitation of fisheries will be the greatest threats, with lesser problems being alteration of hydrology, pollution and global warming (climate change). Loss of biodiversity is, and will continue to be, a severe problem as even pristine mangroves are species-poor compared with other tropical ecosystems (extinction). The future is not entirely bleak. The number of rehabilitation and restoration projects is increasing worldwide with some countries showing increases in mangrove area. The intensity of coastal aquaculture appears to have leveled off in some parts of the world. Some commercial projects and economic models indicate that mangroves can be used as a sustainable resource, especially for wood. The brightest note is that the rate of population growth is projected to slow during the next 50 years, with a gradual decline thereafter to the end of the century. Mangrove forests will continue to be exploited at current rates to 2025, unless they are seen as a valuable resource to be managed on a sustainable basis. After 2025, the future of mangroves will depend on technological and ecological advances in multi-species silviculture, genetics, and forestry modeling, but the greatest hope for their future is for a reduction in human population growth.

Main Results and Conclusions:

• Environmental factors that jeopardize mangrove health include: “Mangroves become more susceptible to diseases and pests when stressed by changes in salinity, tidal inundation, sedimentation and soil physicochemistry, the introduction of pollutants such as oils, herbicides, metals, sewage and acids, and damage from storms and cyclones”(333).
• The ecosystem services provided by mangroves are extensive: foods, tannins and resins, medicines and other bioproducts, furniture, fencing, poles (timber), artisanal and commercial fishing, charcoal, cage aquaculture, ecotourism, recreation, and education (334).
• Threats to mangrove habitat are discussed in great detail:
  • Deforestation: “Deforestation remains the single greatest threat to the survival of mangroves. Although reforestation programmes will continue and are likely to increase in future, the loss of biodiversity, especially from old-growth forests, is unlikely to be regained until at least several decades, and perhaps permanently lost if species experience local extinction due to excessive fragmentation of habitats”(340).
  • Aquaculture: “The loss of mangroves for pond aquaculture is currently one of the largest threats to mangrove forests worldwide. The list of direct and indirect problems caused by pond aquaculture is long and includes: immediate loss of mangroves to construct ponds; blockage of tidal creeks; alteration of natural tidal flows; alteration of the groundwater table; increase in sedimentation rates and turbidity in natural waters; release of toxic wastes; overexploitation of wild seed stocks; development of acid sulphate soils; reduced water quality; introduction of excess nutrients; and alteration of natural food chains”(334).
  • Coastal development: “Mangrove losses are positively related to human population density and growth; the fewer people` who live at or near a forest, the less destruction and exploitation there will be” (346).
• The author makes the unusual prediction that overexploitation is temporary and that there is hope for restoration and protection in the future, if current practices change. However, the extinction of specific mangrove species remains a threat. “Given the apparent link between the exploitation of mangroves and human population density, this implies that overexploitation will continue until 2050, but decline thereafter. Coupled with technological improvements in aquaculture, restoration ecology and genetics, hopefully the worst direct exploitation will be over by 2025. The biggest problem in future is the loss of biodiversity”(346).