Temps de lecture :4 minutes
The tropical regions of South America comprise 415 million hectares of wetland, followed by Europe (258 million), North America (242 million), Asia (204 million), Africa (121 to 125 million) and Oceania (36 million). One of the best known examples of wetland destruction is that of the Aral Sea (Kazakhstan, Ouzbekistan) which has lost 75% of its surface area since the 1960s, as the water supply was redirected for irrigation. The Mesopotamia marshland, between the Tigris and Euphrates, in southern Iraq, met the same fate, notably for political reasons, going from around 20,000 km2 in the 1950s to 400 km2 today. In Africa, climate change, the demand for water for irrigation and poor management decisions in the basin reduced the size of Lake Chad, which is shared by Cameroon, Nigeria, Niger and Chad, by 90% in 40 years. For 20 million people, essentially the fishermen and farmers and their families who depend directly on the lake, malnutrition and illness have increased.
Wetlands and their value
The greatest population densities are found either near coasts or interior wetlands: more than half of the largest cities are located less than 50 kilometers from the sea and the average population density is 2.6 times greater near the coast than in land-locked regions. Many of the populations living near wetlands are greatly dependent on these particular wetland ecosystems and often derive their principle source of protein there. For example, in Cambodia, 60 to 80% of protein consumed comes from fish caught in the Tonlé Sap. Underground networks associated with the wetlands supply water to 1.5 to 3 billion people. Marshland and swampland are remarkable systems for removing pollutants: some are capable of reducing nitrate concentration by more than 80%. Peat bogs, by slowly capturing and releasing carbon, have a regulating effect on the climate. They store 25 to 30% of terrestrial carbon in their soil and vegetation. Wetlands also play an essential role in flood regulation; their destruction has increased the risk of flooding for the two billion people living in floodplains.
Wetlands and their destruction
The largest threats to wetlands are posed by population growth and economic development. Drainage, building construction , land conversion, overexploitation, pollution and the introduction of invasive species all take their toll. The greatest pressure is drainage; since 1985, between 56% and 65% of wetlands have been drained for farming in Europe and in North America, 27% in Asia, 6% in South America and 2% in Africa. Farming practices, which often involve the application of massive amounts of fertilizer and pesticides, lead to pollution, which can extend hundreds of kilometers from the source. An excessive amount of nutrients, nitrogen and phosphate cause eutrophication.
Invasive species introduced accidentally or voluntarily can cause extinction of native species. Two-thirds of species introduced in tropical regions and more than half of those introduced in temperate regions were established during colonization. In San Francisco Bay, at least 210 invasive species are found; between 1961 and 1995, one new species was discovered there every fourteen weeks.
Diverting the watercourse upstream from deltas and estuaries leads to degradation, causing them to dry out and diminishes the contribution of sediment rich in organic material.. In addition, millions of people lose their principle means of subsistence. The Mediterranean Basin, Florida, Australia, South Asia and the Caribbean are the regions most affected by these threats.
According to the work of the Intergovernmental Panel on Climate Change, climate change can seriously affect both terrestrial and intertidal wetlands, notably by modifying their geographic distribution. In addition the discoloration and mortality of coral reefs is caused.
Wetlands and the endangered species
In the last twenty years, 25% of mangrove swamps have disappeared. And almost 40% of some estuaries have been destroyed. The state of interior wetlands can be assessed by the condition of the animal populations dependent on them. Aquatic birds are paying a high price: out of 1,138 species monitored 41% are in decline; and out of 964 species semi-exclusively dependent on wetlands, 21% are extinct or on the path to extinction. Concerning mammals, 37% of wetland species are on the Red List of the World Conservation Union, including the manatee, the freshwater dolphin, the porpoise, the sea lion, the walrus, and the seal. Approximately 20% of freshwater fish, out of the 10,000 species described and listed, are either very threatened, in danger or have disappeared in the last few decades. Furthermore, with regard to amphibians, a third of the species are threatened, more than half of which live in freshwater; the decline is evidenced by the loss of 43% of their colonies, which means that the proportion of threatened species is rising rapidly. Out of 200 species of freshwater turtles, more than half are in danger of extinction. In Asia, this threat has increased to 75%, with 18 species in a critical situation or extinction. For example of the 23 species of crocodile, 4 of them are almost extinct, 3 in danger, and 3 others are vulnerable.