Posted on June, 05 2003

The WWF Living Planet Index suggests that populations of freshwater species have fallen by a half on average worldwide since 1970. This compares with a fall of 30 per cent for marine species and 10 per cent for forest species over the same period of time.

The biodiversity of freshwater ecosystems, of all the major habitat types on the earth, has undergone the most catastrophic decline in recent years.Ten thousand out of the 25,000 known species of fish live in freshwater, 40 per cent of the world total, yet freshwater makes up only about 2.5 per cent of the world’s water, or less than 0.01 per cent if ice caps and underground waters are excluded. In terms of their size relative to the earth’s surface, freshwater ecosystems — wetlands, rivers and lakes — account for a disproportionately large fraction of global biodiversity.

The largest river system in the world, by a long way, is the Amazon and its tributaries. The Amazon Basin covers nearly 6 million km2. Its sheer size, and its position along the equator, make it the earth’s most biodiverse freshwater ecosystem.

The largest, deepest, and oldest freshwater lake in the world is Russia's Lake Baikal. Unsurprizingly perhaps, it is also very rich in freshwater species. Out of its total of 1,825 animal species, 982 of them are endemic — species that are found nowhere else in the world. This reflects the age of the lake, which has given time for so many unique species to evolve, as much as its size. Most of the world’s lakes are young, formed in the last ice age, and so comparatively poor in species.All around the world, freshwater fish species are threatened with extinction.

In the 20 countries which have been comprehensively assessed for the IUCN Red List, about 20 per cent of the freshwater fish species are threatened with extinction. It is estimated that among freshwater turtle species, around 35 per cent are threatened.

Proportionately fewer freshwater birds and mammals are threatened, probably because it is easier for them to move from one wetland, lake, or river basin to another, but this is not the case for those species of freshwater mammals that are wholly aquatic.

Of the five species of river dolphin, for example, four are threatened (the Amazon, Ganges, Indus, and Yangtze), as are all three species of manatee (Amazonian, Caribbean, and West African).The most critically endangered river dolphin is the Yangtze river dolphin (Lipotes vexillifer), known in China simply as the baiji.

Chinese government scientists are attempting to relocate the last remaining baiji in an attempt to save the species. Pollution, fishing and traffic on the river have decimated the dolphin population which was estimated to be about 6,000 in the 1950s. If caught, the dolphins will be released in the specially-created Tian'erzhou nature reserve in Hubei province. The reserve is centred around an isolated 21km stretch of the river which became cut off when the Yangtze changed course.The rate of extinction of freshwater fish species in the last century far exceeded background extinction rates. Ninety-one species were listed as having gone extinct in the hundred years prior to 1996, including 50 of Lake Victoria's cichlid fishes. A further 11 species are recorded as extinct in the wild, but still survive in captivity.

Ecological degradation of freshwater ecosystems over the last 100 years has largely come about as a result of four kinds of human activities.

First is the simple withdrawal of water from a water body for human use, and the pollution of the water body by returning water after it has been used, whether in farms, by industry, or in the home. It is estimated that humans use over half of the accessible freshwater runoff worldwide. Withdrawal of most of the available surface water has led to the destruction of the Aral Sea in Central Asia in recent decades.

The second major cause of biodiversity loss is the direct alteration of freshwater habitats, by dam construction for example, or drainage of wetlands and periodically inundated floodplains for farming. Dam construction has had the major influence on the aquatic environment in many river catchment areas. Fish species which migrate up rivers to spawn and return back down river to the sea find that their passage is barred. The Colorado River in the US has been so severely disrupted by dams that its waters no longer reach the sea. Consequently all the fish species in its lower reaches have died out or survive only in isolated pockets. The ecological decline is not restricted to the Colorado River itself, but also in the Gulf of California where the input of freshwater from the Colorado has been severely curtailed.

The freshwater snail fauna of the Mobile Bay drainage basin in the US was once the richest in the world. The construction of 33 major hydro-electric dams and associated infrastructure in the catchment area has resulted in the extinction of 38 out of 118 species that were present at the beginning of the 20th century. One of the rivers flowing into the Bay, the Coosa, which was the most biodiverse of the entire drainage basin, has only 30 species surviving out of an original 80. Silt accumulation behind dams and the submergence of shallows has destroyed most of the snails’ habitat.

The third pressure on freshwater ecosystems comes from direct exploitation, usually over-fishing. However, it is not only fishes that are harvested, but other freshwater species such as crustaceans and molluscs that are caught commercially, and some species such as crocodiles and caimans that are hunted for their meat or skins.

The fourth cause of the decline in freshwater biodiversity is the deliberate or inadvertent introduction of non-native species which are either predators, parasites, or competitors of the native species. Many biologists now believe that it is introduced species that are the greatest cause of decline in freshwater biodiversity.

Like Lake Baikal, the lakes of the African Rift Valley are ancient and support a great variety of species. Lake Malawi has the highest species richness of any lake, due to its phenomenal variety of cichlid fishes. Out of 640 species of fish, over 600 are found nowhere other than Lake Malawi. Many of the endemic freshwater fish species of the Rift Valley have become very rare or extinct in recent decades, following the introduction of the Nile perch (Lates niloticus).

This species was introduced Lake Victoria as a food source, but turned out to be a voracious predator of the endemic cichlids. Before the introduction of the Nile perch around 1970, Lake Victoria supported around 300 species of haplochromine cichlid fishes. Many of these species had not even been scientifically described but were known to experts by their common names only. Over half of these species are now extinct or too rare to be caught and studied.Exotic plant species can also be a huge problem.

The free-floating South American water hyacinth (Eichhornia crassipes) has become a major pest wherever it has been introduced around the world, especially in Africa and Asia. It grows fast and spreads rapidly, blocking water channels, clogging hydro-electric installations, impeding boat traffic, and preventing fishing.Apart from these four causes, regional and global changes such as acid rain and global warming are also thought to be adversely affecting freshwater ecosystems.

Although there are clear links between acid rain and a loss of diversity in rivers and streams, these widespread phenomena have not yet been implicated in a species extinction or major decline.*

Johnathan Loh is author of WWF's Living Planet Report 2002. This article appeared in the Our Planet magazine in June 2003.