Spatial patterns in the diversity of mammal species: A test of the geographic area hypothesis in South America

Abstract

The geographic distributions of 577 South American mammalian species are analysed (i) to depict the latitudinal patterns of variation in mammal species richness, and (ii) to test the geographic area hypothesis in the southern hemisphere at two scales of analysis. This hypothesis predicts that variation in the geographic area of biomes (rather than their particular environmental conditions) is the best predictor of mammal species richness. The areal effect is expected to be stronger after tropical species are excluded from the analysis, because the spillover of these species into extra-tropical latitudes can mask the species-area relationship. A latitudinal gradient in mammal species richness is confirmed in South America, although the pattern is stronger in bats than in non-volant quadrupeds. Values of species richness estimated from equal area samples (‘species density’) reveal that species richness of different mammal taxa peaks at different latitudes. The species density pattern in the mammal species assemblage taken as a whole offers support to the geographic area hypothesis after excluding tropical species from the analyses. The number of principally extra-tropical (PE-T) mammal species per unit area depends on the biome area. In contrast, bats and rodents taken separately do not support the geographic area hypothesis at this cell-scaled analysis. At the scale of biomes, the variation in the geographic area of different macrohabitats is the best predictor of mammal species richness. The species richness-area relationship holds even after controlling for the effect of latitude and differences in mean net primary productivity of the different macrohabitats. There is a difference between bats and rodents, to the extent that a strong species richness-area relationship is observed for rodents at the biome-scaled analysis. Excluding tropical species does not enhance the species richness-area relationship at this scale, thus suggesting that the ability to capture the impact of the spillover effect of tropical species into extra-tropical latitudes might depend on the scale of habitat analysis.