ABSTRACT
Species adapt through processes of natural selection to the conditions- biological and non-biological- which exist in the ecosystem. Several theories stress present-day environmental gradients and niches as the critical factors causing large biodiversity in the tropics. Biological diversity at any moment in time for any geographical location is the balance of immigration and extinction. In successions on new land surfaces (volcanic islands, salt marshes), new species arrive and colonize; initially the rate of immigration exceeds the rate of extinction, but as more species compete for space the rate of extinction increases. The destruction of the world's vegetation is recognized as one of the most serious of human impacts. An immediate result is the extinction of plants and animals and the loss of habitat. It is estimated that between 1990 and 2015 between 2 per cent and 10 per cent of the flora of tropical forests will become extinct.
