than the significance) of the clumping is high in most realms apart from the Nearctic (Fig. 14.3). It also appears to be stronger in ecoregions with high disparity (Spearman’s ρ = 0.316) and with relatively old diversity (Spearman’s ρ = 0.195). These correlations should not be taken as evidence of a functional syndrome unless confirmed at more local scales: Some of the signal probably derives from differences among, rather than within, major biogeographic realms. The prevalence of clumping of risk implies that, faced with approximately equal pressures, species differ in their ability to persist because of lineage-specific characteristics. This finding invites a search for biological correlates of extinction risk.
Perhaps the most obvious proposed risk factor for extinction is large body size. The end-Pleistocene mass extinction of mammals removed mostly large species (Barnosky, Chapter 12, this volume), and declining mammals are an order of magnitude heavier, on average, than are non-threatened species (Cardillo et al., 2005). There are several possible reasons: Large-bodied species are more tempting targets than small ones for hunters; they are, on average, less abundant; and they take longer to reach sexual maturity, have smaller litters of larger offspring, and have larger individual home ranges. Narrow ecological tolerances are also a plausible risk factor—habitat specialists may be more at risk than generalists from