Biodiversity in freshwater systems is distributed in a fundamentally different pattern from that in marine systems. Organisms in the sea live in media that is 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:

a) freshwater species must survive climatic and ecological changes in place; b) freshwater biodiversity is usually highly localized, and even small lake or stream systems often harbor unique, locally evolved forms of life; and c) 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.