17/07/2008 5:55 pmMangrove forests are one of the most productive ecosystems on the planet. They are found on the coastlines of tropical and subtropical countries between the latitudes of 32 degrees north and 38 degrees south; approximately 40% of these areas are located in Asia. The world’s largest mangrove forest is located in the Bay of Bengal and covers 10,000 km2 of the coast. These areas have numerous benefits: they constitute food reserves, offer a very wide variety of raw materials, provide coastal protection against storms, filter pollutants, thus limiting the amount that flows into the ocean... Enduring harsh treatment from human activity—aquaculture, salt-marshes, and overuse of leading resources—these vivariums are endangered. According to the FAO, the areas covered by mangroves have decreased from 19.8 million hectares in 1980 to less than 15 million in 2000. In other words, 5 million hectares were destroyed in a period of 20 years. However, the rate of deforestation has slowed from 1.7% per year, between 1980 and 1990, to 1% per year, between 1990 and 2000. The products generated each year by mangroves are valued at 186 million dollars. If mangrove forests were sustainably managed, they would be more productive and guarantee continual income for coastal populations.
Mangroves are odd coastal plant formations—rich in silt, salt, and humidity—which develop in intertidal areas (under tidal influence). The mangrove—symbolic—dominates the mangrove ecosystem. There are few other plant species in these tidal balance areas: these environments are anoxic, the ground is unstable, and the salinity variable.
Lining the coasts of a little less than a hundred countries, the largest areas are located in Indonesia (34,931 km2), followed by Brazil (10,124 km2), Nigeria (9,977 km2), Australia (9,553 km2 in 1997), India (6,700 km2), Malaysia (6,424 km2), and Bangladesh (6,225 km2).
Mangroves protect the coast
Mangroves play a role in protecting the coastline against erosion caused by wind, waves, and currents, reducing the impact of storms and hurricanes. The regions where mangroves were sacrificed or damaged by human activity (construction, tourism, and agriculture) were among the most affected by the tsunami that hit the coasts of Thailand in December 2004 (IUCN report on tourist zones of Phuket, Phang Nga, and Krabi). The dense tree root system keeps the sediment carried from the soil above from pouring into the ocean all at once, which stabilizes the banks, protects the corals from choking, reduces turbidity, and filters and traps pollutants. At the interface between the ocean and earth, mangroves play a fundamental role in reducing monsoon flooding.
A tropical system on a razor-edge
Long perceived as foul-smelling, unwelcoming, and not very productive marshes, mangrove forests were cleared to establish communities and farm land. Since 1980, these areas have reduced by 47% in South America (20,303 km2 in 1992), 20% in North and Central America (21,029 km2 in 1994), 15% in Asia (66,617 km2 in 1991) and in Oceana (15,780 km2 in 1995), and 8% in Africa (33,901 km2 in 1993).
The importance of mangroves for coastal economies
Mangrove ecosystems serve as nutrient reservoirs and a habitat for a large number of fish, crabs, shrimp, and mollusks, including various commercial species. They also shelter animal species, mammals, reptiles, amphibians, and birds, some of which are endangered. They are a major source of protein for thousands of coastal communities, particularly in developing countries.
The wood from mangroves is known for being rotproof and insect resistant, making it an outstanding construction material. It is also used as a combustible.
Mangroves provide additional products and services: tannin extraction, fibers for the textile industry (rayon), medicinal plants, bark used as a condiment, charcoal, fodder, straw, honey, etc.
Many factors threaten the existence of mangroves
Cleared mangrove plots are used to develop shrimp breeding areas; in order to avoid disease, these aquaculture basins are moved every two years, leading to more areas being cleared. Shrimp aquaculture is responsible for less than 5% of the disappearance of mangroves.
Pesticides and chemical fertilizers transported by streams of water that feed the mangroves disturb the biological balance of the ecosystems, which are also being threatened by the rise of the sea level.
Dam construction and the rerouting of rivers lead to higher salinity levels in these environments, their drying up and, in the long-term, to the death of mangroves.
If the deforestation of areas above mangroves is too extensive, the soil erodes and sediment accumulates around the mangroves, which then lose their filtering abilities.