Seed Predation and the Coexistence of Tree Species in Tropical Forests

Abstract

The hypothesis that seed or seedling predation by host-specific herbivores can explain the coexistence of the large number of tree species in tropical forests, and produce a low density and uniform dispersion of adults of these tree species, is examined. On theoretical grounds, it is shown that spacing of conspecific adults can account for only a very small fraction of the observed tree species richness in tropical forests unless interadult distances are quite large. Moreover, in growing or declining populations, seed predation will not result in large-scale uniformity in adult dispersion patterns throughout the population. It results in uniformity in equilibrium populations only if the same spacing rule applies to all adult trees. However, adult trees differ enormously in seed production from year to year, and from young (small) to old (large) adults. High variance in seed production from tree to tree and year to year leads to heterogeneity in the intensity of seed predation and in the spacing distance between adults. This explains why the adults of most tropical tree species are clumped or randomly dispersed, and not uniformly spaced, in spite of heavy seed predation in many species. Because tree species are not everywhere as dense as whould be predicted from observed nearest neighbor distances, it is concluded that other factors than seed predation limit the abundance of tropical tree species and prevent single-species dominance.