How much of the earth's surface is presently composed of wetlands is not known exactly. Mangroves cover some 240,000 km 2 of coastal area, and an estimated 600,000km 2 of coral reefs remain worldwide. Wetlands are among the world's most productive environments. They are cradles of biological diversity, providing the water and primary productivity upon which countless species of plants and animals depend for survival. They support high concentrations of birds, mammals, reptiles, amphibians, fish and invertebrate species.
Wetlands are also important storehouses of plant genetic material.
Rice, for example, which is a common wetland plant, is the staple diet of more than half of humanity. More and more economists and other scientists are working in the field of the valuation of ecosystem services. This is a difficult task, still full of uncertainties, but there is no other choice than to progress in this direction. These functions, values and attributes can only be maintained if the ecological processes of wetlands are allowed to continue functioning. Unfortunately, and in spite of important progress made in recent decades, wetlands continue to be among the world's most threatened ecosystems, owing mainly to ongoing drainage, conversion, pollution, and over-exploitation of their resources.