A unified theory of biogeography and relative species abundance and its application to tropical rain forests and coral reefs

Abstract. Theories of island biogeography and of relative species abundance are of central importance in biogeography and community ecology, yet these two bodies of theory heretofore have been largely unconnected. Incorporating speciation into the theory of island biogeography unexpectedly results in unification of these two theories. The unified theory predicts the existence of a fundamental biodiversity number θ that controls not only species richness, but also relative species abundance in the source area metacommunity at equilibrium between speciation and extinction. With additional parameters for island size and migration rate, the theory also predicts relative species abundance on islands or local regions of continuous landscapes. Application of the theory to the biogeography and biodiversity of communities of tropical trees and reef-building corals are discussed. One important result is that only relatively modest migration rates are sufficient to dynamically couple the regional metacommunity and stabilize community structure on large spatiotemporal scales. Thus, regional, long-term compositional stasis in tropical rain forests and coral reefs can arise just as easily from the stabilizing effect of large numbers as from niche-assembly rules that limit species membership in communities. Because of the higher intrinsic vagility of corals, the theory predicts greater regional similarity in coral reef communities than in tropical tree communities.