Tropical rainforests: how is diversity maintained?

General considerations

Tropical rainforests are among the most diverse ecosystems on land. Amazonian rainforests, for example, can have more than 280 species of trees in one hectare (with a diameter at breast height of at least 10 cm), and a 0.52 km2 plot in Borneo had 1175 species (with a diameter at breast height of at least 1 cm) (references in Wright 2002). Diversity further down in the hierarchy is considerably greater, and it is much more difficult to assess: many if not most species have not yet even been described. For these reasons, most detailed long-term ecological studies that have been conducted in rainforests are concerned with the larger species, i.e., trees. Wright (2002) reviews much of the work done on mechanisms that permit coexistence of so many species in tropical rainforests. Connell and various co-workers have made long-term studies of rainforests and equally diverse marine systems, i.e., coral reefs in eastern Australia, extending over more than 30 years. I discuss their work on tropical and subtropical rainforests in northern and southeastern Queensland in some detail, because it tests clearly formulated hypotheses using long-term extensive and intensive data sets from habitats little disturbed by man (Connell 1978, 1979; Connell et al. 1984; Connell and Green 2000).

The main problem Connell addresses is the degree to which equilibrium or nonequilibrium conditions contribute to the great diversity in tropical rainforests.