Species richness

Abstract One of the most famous features of tropical rain forests is their extreme species richness which has impressed scientists ever since the earliest explorations (Chapter 1). Some species are common, but most are rare: as has been described above (p. 32), in the richest rain forests every second tree on a hectare or so is a different species. Even disregarding the trees, some rain forests are richer in species than any other vegetation on earth; and the richest communities outside the humid tropics have about the same total number of species on small plots as there are trees of 0.1 m in diameter and greater in many rain forests. The way such very large numbers of species have evolved and are packed together has been the driving force for endless speculation, constrained to varying degrees by observation, and commonly involving massive extrapolation. It is one of the most basic and fascinating aspects of these forests and one that energizes much of tropical biology.