Intermediate dispersal hypothesis of species diversity: New insights

Abstract

The origin and patterns of species diversity are fundamental themes in ecology and evolutionary biology. Insular systems play an important role in biogeography, and the species richness within an insular system has classically been considered as determined by the balance between the rate of speciation plus net migration and the rate of species extinction. A recent wave of studies integrating comprehensive phenotypic, phylogenetic, and environmental data is accumulating additional macroevolutionary insights at unprecedented scales. In this review, I summarize and discuss the hypothesis that intermediate dispersal ability leads to clades with high species diversity by recurrent speciation events (referred to as the intermediate dispersal hypothesis, IDH). Although some recent empirical and theoretical studies have supported the IDH, further integration of other ecological and evolutionary concepts spanning different timescales is needed to resolve long-standing debates about the non-linear relationship between diversification and organismal dispersal ability. This paper presents a framework for future studies that intend to test the IDH; I organize the factors that should be taken into account, including the indices for quantifying dispersal ability and species diversification, and methods of taxon sampling. The IDH requires more attention and could be used to unveil the diversity of extant species and how dispersal ability affects rates of speciation and extinction.