A HEMISPHERIC ASSESSMENT OF SCALE DEPENDENCE IN LATITUDINAL GRADIENTS OF SPECIES RICHNESS

Considerable controversy surrounds the importance of historical, evolutionary, and ecological factors affecting continental patterns in species richness. Although the importance of area and latitude are both well documented, few attempts have been made to integrate their effects in a single model. Most studies have been conducted by superimposing grids on equal‐area projection maps and counting the number of species occurring within grid cells (i.e., quadrats). Unfortunately, different grid‐based studies use different quadrat sizes, making comparisons tenuous. We developed a hierarchical model to evaluate the degree to which area (based on a nested series of quadrats of five sizes, 1000–25000 km 2 ) affects the latitudinal gradient in species richness. The model allows the relationship between latitude and area to be nonlinear and, in its simple form, evaluates how well species richness can be predicted by the additive influences of latitude and area. The complex model evaluates whether an area × latitude interaction accounts for significant additional variation in species richness above that in the simple model (i.e., assesses the scale dependence of the latitudinal gradient). For bats and marsupials, the simple model included only latitudinal effects and accounted for over half of the variation in species richness of each of the two taxa. The interactive effect was nonsignificant for each taxon, accounting for <0.1% of additional variation in species richness in each case. If other taxa or land masses produce similar relationships, then the form of the latitudinal gradient is relatively invariant with respect to area at 1000–25000 km 2 scales, and comparisons among studies at this spatial scale are straightforward.