Equilibrium Models of Evolutionary Species Diversity and the Number of Empty Niches

Evidence from ecological studies indicates that there may be many unoccupied niches in modern ecosystems. To quantify this "emptiness" of adaptive space, a mathematical model of diversification is derived which makes explicit the relationship between maximum species capacity and realized diversity. The proportion of niches empty at equilibrium is a function only of the intrinsic rates of species origination and extinction. Estimates of these rates for eight marine invertebrate groups suggest that the mean proportion of empty niches is somewhere in the range of 12% to 54%. Evolution in such an open adaptive space should be characterized by unremitting taxonomic turnover and continuous faunal change (but only occasional adaptive improvements), and should permit the rapid establishment of new morphospecies. These expectations are qualitatively borne out by the fossil record.