Geographical Ecology at the Community Level: Perspectives on the Diversity of New World Bats

The ubiquity of the latitudinal gradient of species richness is well documented at coarse scales of resolution, but the extent to which the pattern is recapitulated at the level of local communities for any aspect of biodiversity (i.e., richness, evenness, or diversity) is unclear. We examined how attributes of New World bat diversity vary with each other and with latitude at two scales of resolution, local communities and regional species pools. We calculated 14 indices of diversity (species richness [three], evenness [four], dominance [three], and diversity [four]) from species abundance distributions for 32 intensively sampled local sites between 42.25° N and 24.12° S latitude. The species richness of each corresponding regional pool was estimated from published range maps. In general, the gradient of local species richness was less steep than the corresponding gradient of regional species richness; beta diversity in the tropics is greater than that for temperate communities. All aspects of diversity at the local scale did not vary with latitude in the same manner. The latitudinal gradient in species diversity of local communities was primarily a consequence of the corresponding gradient in species richness. Local richness increased and became more variable with decreasing latitude. In contrast, species evenness did not vary in a systematic fashion with latitude. Although the absolute number of rare species in communities increased faster with latitude than did that of common species, both abundance classes proportionately increased with latitude in equivalent ways throughout the New World. In general, latitudinal variation at the community level was detected in diversity indices that were insensitive to the abundance of species. The dramatic increase in species richness at broad scales of resolution toward the tropics (gamma diversity) was as much a consequence of increased richness at the local level (alpha diversity) as it was a consequence of the latitudinal increase in species turnover among communities (beta diversity). Future theoretical research should examine the correlates of latitude that enhance differentiation among communities at low latitudes (i.e., those which enjoy high productivities). Conservation strategies based on assessments of diversity at coarse levels of resolution (gamma diversity) should be implemented with caution because beta diversity inflates regional estimates of diversity.