Lake Malawi

Of the 25,000 living species of fish, about 60 percent are marine—a surprisingly low proportion given the extent of the world’s oceans compared to its fresh waters. This is equivalent to one species for every 24,000 cubic miles (100,000 km3) of sea water, compared to one species for every 31 ⁄ 2 cubic miles (15 km3) of freshwater. The diversity of freshwater fish relative to the size of their habitat is due to the ease with which groups of fish can become separated and geographically isolated in rivers and pools, compared with the sea. Geography and species formation There is no better example of diversification and species formation than Lake Malawi in Africa’s Great Rift Valley. Created about two million years ago by a geological fault, the 365 mile- (584 km-) long lake is today home to over 1,600 species of cichlids—more than occur in all of the rest of Africa. It is thought that all these cichlids developed from just one or two ancestral species, which entered the lake at the time it was formed. The early lake cichlids adapted to the entire range of different habitats that they encountered in the lake. Some became predators; others plant-eaters. Some became restricted to the shoreline; others occupied the depths of the lake. Some grew to large sizes; others diminished. They also diversified in their breeding habits, some species scattering their eggs, others displaying a long period of parental care. By dividing up the biological “niches” available in the lake, the cichlids were able to explode in number, without directly competing with one another for limited resources.