Both Old World and New World cultures have been centered on freshwater habitats--Babylon between the Tigris and Euphrates Rivers, Egypt on the Nile, Rome on the Tiber, the Aztec capital built on man-made islands in Lake Tenochtitlan, Paris on the Seine, Kinshasa on the Zaire River. The world's rivers, lakes, streams, and wetlands provide most of the world's water for drinking, agriculture, sanitation, and industry, as well as huge quantities of fish and shellfish.

Freshwaters are also home to a tremendous diversity of fish, amphibians, aquatic plants, invertebrates, and microorganisms. The Amazon River alone contains an estimated 3000 species of fish -- only 25 percent less than the total number of mammals worldwide. And freshwater biodiversity is among the most poorly known on Earth. Scientists believe that Thailand may have as many as 1000 species of freshwater fish, but only some 475 have actually been recorded.

Freshwater biodiversity is seriously threatened today -- a telling indicator of the status of the world's freshwater ecosystems. All native fishes in the valley of Mexico are extinct. A recent survey in Malaysia found fewer than half of the 266 fish species previously known from the country. On the island of Singapore, 18 out of 53 species of freshwater fish collected in 1934 could not be located in exhaustive searches only 30 years later. In the southeastern United States, 40 to 50 percent of freshwater snail species are now extinct or endangered due to the impoundment and channelization of rivers. Even on a continental scale, species loss can be very high. In North America, one-third of the native freshwater fish species are extinct or endangered to some degree.

Biodiversity in freshwater systems is distributed in a fundamentally different pattern from that in marine or terrestrial systems. Organisms on land or in the sea live in media that are more or less continuous over extensive regions, and species adjust their ranges to some degree as climate or ecological conditions change. But freshwater habitats are relatively discontinuous, and many freshwater species do not disperse easily across the land barriers that separate river drainages into discrete units. This has three important consequences:

• freshwater species must survive climatic and ecological changes in place;

• freshwater biodiversity is usually highly localized, and even small lake or stream systems often harbor unique, locally evolved forms of life; and

• freshwater species diversity is high even in regions where the number of species at any given site is low, since species differ between one site and the next.

Freshwater lakes are classical examples of "habitat islands" (in this case, bodies of water surrounded by expanses of land). Like islands in general, the larger, more ancient lakes tend to have high levels of endemism, and in the rift lakes of Africa or Lake Baikal of Central Asia, species diversity can be spectacular. With hundreds of species each -- 90 percent of them in some cases found nowhere else -- the East African lakes harbor some of the world's greatest concentrations of locally endemic species.

Unfortunately, lakes are like islands in another way too: they suffer high rates of extinction when habitat modification begins or when exotic species are introduced. The introduction of non-native species -- regrettably still often sanctioned or promoted by governments -- is associated with the depletion of biodiversity and the collapse of major fisheries in such lakes as Lake Chapala of Mexico, Lake Gatun of Panama, and the Great Lakes of North America.

Other factors contributing to the decline of freshwater ecosystems and their native biota are chemical and thermal pollution, over-harvesting, and habitat modifications (such as dam construction). These factors have affected biodiversity to different degrees in both industrialized and developing regions. In Europe and North America, pollution, acidification, and the physical modification of streams have had the greatest impact. In much of South America and Africa, over-harvesting and introduction of non-native species are relatively more important as agents of biodiversity loss.

Programs to protect freshwater biodiversity in industrialized countries have lagged far behind the programs for saving terrestrial biota. Many protected areas include lakes or small portions of watersheds, but rivers and streams are often too linear to incorporate adequately into protected areas. Moreover, rivers and streams frequently pass through more than one political jurisdiction or may themselves constitute political boundaries. (The Danube crosses or borders upon seven European nations.) Consequently, effective management of riverine biodiversity is often a casualty of politics.

The primary method of protecting freshwater biodiversity has been to designate particular species as threatened or endangered making them subject to national recovery programs or international protection. Unfortunately, this approach is failing. In the United States, for example, no aquatic species has ever graduated from the government's endangered species list, but 10 species of fish have been removed due to extinction.

Sources:

• Usher, A.D. 1991. Inhabitants of the Mool River. The Nation, March 31. Bangkok, Thailand.

• Diamond, J.M. 1989. The present, past and future of human-caused extinctions. Phil. Trans. R. Soc. Lond. B 325:469-477.

• Miller, R.R., J.D. Williams, and J.E. Williams. 1989. Extinctions of North American fishes during the past century. Fisheries 14(6):22-38.

• Reid, W.V., and K.R. Miller. 1989. Keeping Options Alive: The Scientific Basis for Conserving Biodiversity. World Resources Institute, Washington, DC, U.S.A.

• Williams, J.E., J.E. Johnson, D.A. Hendrickson, S. Contreras-Balderas, J.D. Williams, M. Navarro-Mendoza, D.E. McCallister, and J.E. Deacon. 1989. Fishes of North America: endangered, threatened or of special concern. Fisheries 14(6):2-20.

• Prance, G.T. 1987. The Amazon: Paradise Lost? Pp. 63-106 In: L. Kaufman and K. Malory (eds.), The Last Extinction. The MIT Press, Cambridge, Mass. U.S.A.

• The World Conservation Union (IUCN). 1983. The IUCN Invertebrate Red Data Book. IUCN, Gland, Switzerland.

• Mohsin, A.K.M., M.A. Ambak. 1983. Freshwater Fishes of Peninsular Malaysia. University Pertanian Malaysia Press, Kuala Lumpur, Malaysia.