The leading causes of aquatic species decline and ecosystem degradation are physical alteration, habitat degradation and destruction, water withdrawal, overexploitation, pollution and the introduction of non-native species.
It is rare that a species or habitat is endangered by a single threat and to determine the root cause is very difficult, if not impossible to determine, because of the complexity of the systems involved.
While only 12 percent of species find their habitat within coastal or freshwater ecosystems, nearly all terrestrial species depend on water for their survival. In addition, in Europe, 25 percent of birds and 11 percent of mammals use freshwater wetlands in order to breed and feed.
Studies regarding global aquatic biodiversity only began in earnest in the early 1990s and there are still relatively few global assessments that have taken place.
More freshwater ecosystem species are threatened with extinction than either terrestrial or marine species. In one study, it was found that, on average, about 50 percent of freshwater species populations fell between 1970 and 2000.
According to the IUCN, there are over 3,000 freshwater species that are listed as threatened, over 1,000 are fish and nearly 1,900 are amphibian.
For larger freshwater species, four out of the five river dolphins and two out of the tree species of manatees and 40 freshwater turtles are threatened.
Source: UNESCO Water Portal, June 2007