Ecological Limits on Clade Diversification in Higher Taxa

Species richness varies dramatically among groups of organisms, yet the causes of this variation remain poorly understood. Variation in species-level diversification rates may partially explain differential species richness among clades, but older clades should also be more diverse, because they will have had more time to accumulate species. Surprisingly, studies that have investigated this question have reached dramatically different conclusions: several claim to find no such age-diversity relationship, whereas a recent and more inclusive study reported that clade age and not diversification rate explains the variation in species richness among animal taxa. Here I address the relationship between clade age and species richness using a model-based approach that controls for variation in diversification rates among clades. I find that species richness is effectively independent of clade age in four of five data sets. Even extreme among-clade variation in diversification rates cannot account for the absence of a positive age-diversity relationship in angiosperms, birds, and teleost fishes. I consider two alternative explanations for these results and find that a clade volatility model positing correlated speciation-extinction dynamics does not underlie these patterns. Rather, ecological limits on clade growth, such as geographic area, appear to mediate temporal declines in diversification within higher taxa.