The Regulation of Mammalian Faunal Similarity Among the Continents

Cluster and regression analyses of the distribution of 969 terrestrial mammal genera from throughout the world reveal that the degree of similarity between continents is controlled by existing geographic patterns. Mammalian faunas form three distinct, geographically coherent groups: Western Hemisphere, Eurasia-Africa and Australia-New Guinea. Faunal similarity, as measured by the Jaccard Index, is a decreasing function of (in decreasing order of importance) the overland distance between regions, the degree of longitudinal separation, and the difference in area. These variables appear to measure the progressive inhibition of dispersal with increasing distance, the extent of the oceans between the Eastern and Western Hemispheres, and the effect of area on differences in diversity between regions. As much as 70% of the variation in faunal similarity can be accounted for by present-day geographic conditions. Historical effects appear to be minor and regional in their extent. Prior continental configurations play only a small role in regulating the distribution of living mammals.