Gradients of species density and turnover in marsupials: A hemispheric perspective

Although marsupials represent a taxonomically and ecologically diverse group of mammals (Infraclass Metatheria), little is known concerning their large-scale areographic patterns. Distributional data for North and South America were analyzed by statistical techniques to ascertain the degree to which latitude, biome richness, and area account for patterns of species density. In general, mid-latitude accounted for more variation among bands than did other descriptors, but the form of the relation between species density and latitude differed between North and South America. Marsupials in North America exhibited more simple relations (quadratic) between species density and latitude than did their South American counterparts (cubic). Nonetheless, both models are consistent with a subtropical maximum. Turnover of species between quadrats within a band did not contribute to latitudinal gradients in species density in a significant fashion in either continent. Such complex results may reflect the dynamic interaction of contemporary ecological forces (habitat selection, competition, physiological constraints) superimposed on a pervasive historical legacy (i.e., extensive radiation in South America, subsequent reduction in diversity after placental invasion during the late Pliocene, re-invasion of Central America from the south).