Jonathan Loh and Lisa Hadeed detail the rapid loss of species in freshwaters, the worst affected habitat on Earth

Biodiversity has declined more catastrophically in freshwater ecosystems in recent years than in any other of the Earth’s major habitat types. The WWF Living Planet Index suggests that populations of freshwater species have fallen by a half on average worldwide since 1970 – compared with a drop of 30 per cent for marine species and 10 per cent for those of the forests.

Ten thousand of the 25,000 known species of fish – 40 per cent of the world total – live in freshwater, yet it makes up only about 2.5 per cent of the world’s water – and less than 0.01 per cent if ice caps and underground waters are excluded. Freshwater ecosystems – wetlands, rivers and lakes – also account for a disproportionately large fraction of global biodiversity in terms of their size relative to the Earth’s surface.

Much the largest river system in the world – covering nearly 6 million square kilometres – is the Amazon and its tributaries. Its size, and its position along the equator, make it the Earth’s most biodiverse freshwater ecosystem. The world’s largest, deepest and oldest freshwater lake, Lake Baikal, is also very rich in freshwater species. Over half its animal species – 982 out of a total of 1,825 – are endemic, found nowhere else in the world. This reflects the age of the lake as much as its size, since time itself has allowed so many unique species to evolve. By contrast, most of the world’s lakes are young, formed in the last ice age, and so are comparatively poor in species.

Worldwide threat
All around the world, freshwater fish species are threatened with extinction: about 20 per cent of them in the 20 countries which have been comprehensively assessed for the IUCN-The World Conservation Union Red List. Around 35 per cent of freshwater turtle species are similarly threatened. Freshwater birds and mammals are less at risk – probably because it is easier for them to move from one wetland, lake or river basin to another – but wholly aquatic mammals are less fortunate. Four of the five species of river dolphin (the Amazon, Ganges, Indus and Yangtze) are threatened, for example, as are all three species of manatee (Amazonian, Caribbean and West African).

The Yangtze river dolphin Lipotes vexillifer – known in China as the baiji – is the most critically endangered. Chinese Government scientists are attempting to relocate the last 100 remaining baiji to try to save the species: pollution, fishing and river traffic have brought its population down from an estimated 6,000 in the 1950s. If caught, the dolphins will be released in the specially created Tian’erzhou nature reserve in Hubei province, centred around an isolated 21 kilometre stretch of the river, which became cut off when the Yangtze changed course.

The rate of extinction of freshwater fish species far exceeded natural background rates in the last century. Ninety-one species are listed as having become extinct in the 100 years up to 1996, including 50 Lake Victoria cichlid fishes. A further 11 species are recorded as extinct in the wild, but still survive in captivity.

Multiple causes
Ecological degradation of freshwater ecosystems over the last 100 years has largely resulted from four kinds of human activities. The first is the withdrawal of water for human use – whether in farms, by industry or in the home – and the pollution caused by returning the water afterwards. People are estimated to use over half of the accessible freshwater runoff worldwide. Withdrawing most of the available surface water from its tributary rivers has led to the destruction of the Aral Sea in Central Asia.

The second major cause of biodiversity loss is the direct alteration of freshwater habitats, such as by building dams, or draining wetlands and periodically inundated floodplains for farming. Dam building has had the biggest influence on the aquatic environment in many river catchment areas. Fish which migrate up rivers to spawn before returning to the sea find that their passage is barred. The Colorado River in the United States has been so severely disrupted by dams that its waters barely reach the sea: as a result, all the fish species in its lower reaches have died out or survive only in isolated pockets. The lack of freshwater flowing out from the river has also caused ecological decline in the Gulf of Mexico.

Building 33 major hydroelectric dams and associated infrastructure in the catchment area of the Mobile Bay drainage basin in the United States has caused the extinction of 38 out of 118 species of what was the richest freshwater snail fauna in the world. Most of the snails’ habitat has been destroyed by silt accumulation behind dams and the submergence of shallows.

Direct exploitation
The third pressure on freshwater ecosystems comes from direct exploitation, usually overfishing. Not just fish, but other freshwater species – such as crustaceans and molluscs – are caught commercially, while some – like crocodiles and caimans – are hunted for their meat or skins.

The fourth is the deliberate or inadvertent introduction of alien species which are either predators, parasites or competitors of native ones. Many biologists now believe that this is the greatest of all causes 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; over 600 of its 640 fish species are endemic. 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. Introduced to Lake Victoria around 1970 for harvesting for food, it turned out to be a voracious predator of the endemic cichlids. Before its introduction the lake supported around 300 species of haplochromine cichlid fishes, many never scientifically described and known to experts only by their common names. Now over half are extinct or too rare to be caught and studied.

Alien plants
Exotic plant species can also present huge problems. 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 hydroelectric installations, impeding boat traffic and preventing fishing.

Regional and global changes such as acid rain and global warming are also thought to be adversely affecting freshwaters. 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 the extinction or major decline of a species

Jonathan Loh is Editor of WWF’s Living Planet Report; Lisa Hadeed is WWF International’s Communications Manager.

PHOTOGRAPH: Maria Cecilia Goin/UNEP

This issue:
Contents | Editorial K. Toepfer | World Environment Day | Water is life | The water century | Taking it at the flood | Renewing the commitment | Waterless cities | Keeping pollution at bay | People | At a glance | Changing agenda | Nor any drop to drink | Bridging troubled waters | Books & products | Getting there | Sinking fast | Waste not | Water – the poor’s priority | Atomic power

Complementary articles in other issues:
Issue on Water, 1996
Issue on Freshwater, 1998

AAAS Atlas of Population and Environment:
Freshwater wetlands
Mangroves and estuaries