Tropical forests are very rich in species, but not uniformly so. The number and kinds of species found in any given area depend heavily upon its history: the immigration, evolution and survival of species under past and current conditions. Rainfall, light, soil fertility, and the abiotic (nonbiological) environment are also determining features. Each species has a range of conditions under which it can survive and reproduce. Some can function only within a very narrow range of conditions; others have a broader tolerance. For plants, rainfall is usually the most critical variable; species richness of trees in the Neotropics can increase as much as six times as rainfall increases from one meter to four meters per year (Wright, 1996). Soil fertility is especially important for understory herbs and shrubs, and even for trees. In Borneo, for instance, the diversity of the large hardwood canopy trees (dipterocarps) is greatest when the soil is of intermediate fertility (Wright, 1996).
Little is known of the interactions between species abundance and various aspects of ecosystem functioning in tropical rainforests. However, soil nutrient levels are greater in areas where there is a greater diversity of plant species. Nutrient retention is also greater, as is the maintenance of soil processes favorable for plant growth. Soils under monoculture suffer more nutrient depletion in comparison with forested areas; in some cases this depletion is so severe as to lead to plant death (Silver, et al., 1996). A certain degree of diversity seems essential to maintain soil-plant interactions at a level which can support plant life. There are many pathways of nutrient flow in natural tropical forests, and these must be maintained for the forest to survive. Synergistic interactions among species are also thought to affect resource consumption and, thereby, ecosystem productivity. For example, Cardinale, Palmer and Collins (2002) found that increasing the diversity of aquatic arthropods boosted their feeding success (“facilitation”). A biodiverse ecosystem may also be able to resist the invasion of exotic species more readily than a less diverse one. Kennedy, et al. (2002), demonstrated this principle in experimental grassland plots. In these experiments, there were fewer invading plants and these invaders were more limited in size in biodiverse grass plots than in less diverse ones.