An estimated 45,000 described species rely on freshwater habitats. With the inclusion of undescribed species, this number could rise to over one million. Species richness in relation to area of habitat is extremely high in many freshwater groups. Freshwater fishes comprise 40% of all fishes and freshwater molluscs about 25% of all molluscs. An estimated 12,500 fish, 5,700 amphibian and 4,000-5,000 mollusc species depend on freshwater habitats. Other major groups dependant upon freshwater include, reptiles, insects, plants, and mammals.
Past Red List assessments of freshwater taxa have rarely been conducted on a comprehensive basis so it is difficult to provide a reliable estimate for their global levels of threat. However, we have now completed several comprehensive regional assessments and the initial results are very worrying. For example, 54% of Madagascar’s native freshwater fish species are threatened. In Eastern Africa 54% of freshwater crabs and 16% of the molluscs are threatened, as are 56% of freshwater fishes endemic to the Mediterranean basin. The SSC's Global Amphibian Assessment also reports high levels of threat with 32% of amphibians assessed as globally threatened and a further 23% as “data deficient”. The freshwater mussels are perhaps the best known of freshwater molluscs and their status is well known for North American populations of which 35% of the 297 taxa are listed as Extinct, Endangered, or are candidates for listing as Endangered.
The major threats to freshwater biodiversity are habitat loss, introduction of alien species, pollution, and overharvesting. There are many examples to illustrate the severity of the situation:
- In the United States, 98% of an estimated 5.2 million kilometres of streams are sufficiently degraded to be unworthy of federal designation as wild or scenic rivers.
- Industrial agriculture around the Aral Sea in the last 30 years has resulted in an approximate halving of the lake's surface area and depth, and a tripling of its salinity.
- Only two of Japan's 30,000 rivers are neither dammed nor modified in some way.
- In Lake Victoria, the introduction of a single alien species, the Nile Perch (Lates niloticus) has led to the possible extinction of an estimated 200 species of endemic fishes.
- Nearly all inland fisheries, with the possible exception of North America and parts of Europe are considered to be overexploited.
was published in 2004.
