Explaining latitudinal diversity gradients.

DIFFERENCES IN MEASURED RATES OF SPECIation and extinction among living species are commonly used to explain latitudinal diversity gradients. The logic is straightforward: the higher the net diversification rate (the difference between the speciation and extinction rate), the greater the number of species. In this vein, J. T. Weir and D. Schluter conclude from their recent study of birds and mammals that decreased extinction rates rather than increased speciation rates account for the higher net diversification rates posited to drive latitudinal diversity gradients (“The latitudinal gradient in recent speciation and extinction rates of birds and mammals,” Reports, 16 March, p. 1574). However, speciation and extinction rates may have little bearing on the cause of latitudinal diversity gradients. Although we have only a poor understanding of what controls species numbers, it is clear that resource availability is of critical importance, and in particular, that the harshness (and reduced area) of the poles limits population numbers. That is, the polar carrying capacity—the number of species that can be supported—is less than that of equatorial regions, and thus we should expect a latitudinal biodiversity gradient. Critically, rates of speciation and extinction do not control carrying capacities, but only reflect rates of species turnover, and the rate at which biotas reapproach their carrying capacities in response to perturbation, or as their carrying capacities shift in response to changing biotic and abiotic influences. Under this framework, Weir and Schluter’s exciting discovery that origination and extinction rates for birds and mammals increase with latitude may have little bearing on the reason that there are latitudinal diversity gradients. Instead, their data suggest that the frequency of extinction and replacement for these animals has been higher at high latitudes than at lower latitudes. This observation implies that ecological disturbance is more frequent, or has greater impact, at higher latitudes, consistent with the greater effects of the Plio-Pleistocene glacial cycles at high latitudes, as Weir and Schluter note. If we are to explain latitudinal diversity gradients, we need to focus more on the determinants of carrying capacities, and less on the rates at which species turn over, or on how quickly species numbers adjust to