Figure 1: The world’s water cycle: global precipitation, evaporation, evapotranspiration and run-off

Source: UNEP 2002a adapted from Shiklomanov 1999

Water is a key component of all ecosystems. These provide critical goods and services to people, including materials, food and other organic products, water storage and purification, biogeochemical cycling and waste removal. Most global freshwater is bound up as ice in the Polar regions. Only a small, but varying proportion is active at any one time within the global water cycle (Figure 1).

The world’s ecosystems are under pressure from numerous human activities and developments, including urbanization, industrialization and food production. Such activities require freshwater, and to meet demands, the water cycle is inevitably disrupted. It is also influenced through land use changes that directly affect water quantity, quality and water flows. Ecosystems can be damaged, functions lost and vulnerable plants and animals endangered. Much of the degradation of freshwater ecosystems results from overexploitation, habitat destruction, pollution and the introduction of non-native species.
At the earth’s surface, freshwater provides the habitat for large numbers of organisms. Species richness in relation to habitat extent is extremely high in many freshwater groups. For example, 40 per cent of the 25 000 known fish species are freshwater forms. Given the distribution of water on the earth’s surface, this is equivalent to one fish species for every 15 km³ of freshwater, compared with one species for every 100 000 km³ of sea water. The species richness increases strongly toward the equator.

In addition to the negative impacts on aquatic species caused by disrupting the quantity and quality of surface water sources, encroachment of non-native species is also having a major impact on aquatic ecosystems around the world, reducing or eliminating native species in many cases (Heywood and Gardner 1995). Studies of the introduction of non-native fish in Europe, North America, Australia, and New Zealand revealed that 77 per cent of the species introduced led to a drastic reduction or elimination of native fish species (Ross 1991).

Similarly, the introduction in the 1970s of Nile perch and Nile tilapia to Lake Victoria, the world’s largest tropical lake, has fundamentally changed the fish and associated biological communities of the lake. Approximately half the 350 species of cichlids have died out due to the introduction of these two exotic fish species which fed on, and out-competed, the resident populations. Although a new fishery has now been developed based on Nile perch, which currently generates about US$400 million in export income, few within the local community are benefiting, as they have not made the transition to this industry (UNDP and others 2000). The unintended introduction of zebra mussels into the Great Lakes of North America has almost completely displaced native mussel species. This organism has already cost more than US$1 000 million merely to control (Great Lakes Water Quality Board 2001). In African wetlands, countries spend billions of dollars every year to control alien species, such as the water hyacinth, with little success (IUCN 2003).