BIOGEOGRAPHY OF EXTANT NORTH AMERICAN MAMMALS: SPATIAL TURNOVER IN RELATION TO CLIMATIC AND PHYSIOGRAPHIC VARIABLES

This study examined spatial turnover of extant North American mammals geographically and in relation to climatic and physiographic variables that were shown to have strong quantitative relationships to richness and ecological structure. The spatial approach to turnover could allow us to predict its magnitude, location, and rates, with possible paleontological and conservation applications. A grid of 150 mi² quadrats was superimposed over North America, and species presence/absence per quadrat was assigned based on range maps. We identified regions with concentrations of range boundaries. The frequency of boundaries per quadrat was compared to climatic variables describing aspects of temperature, moisture, and topography. The Jaccard index, which quantifies similarity of species composition per pair of adjacent quadrats, was compared to the absolute value of the difference in each environmental variable for the quadrat pairs. Calculations were repeated with body size and trophic categories. The most obvious geographic pattern in the range boundary data is the concentrations of boundaries on the edges of the Great Plains. Pairwise and multiple regressions of Jaccard indices for species and ecological categories on differences in environmental variables do not indicate strong quantitative relationships between turnover and the environmental variables. Some variables (actual evapotranspiration, elevation, maximum temperature) explain more of the variation in Jaccard indices than others. Jaccard indices are generally high and spatial patterns of turnover may be more obvious over longer distances. The environmental variables seem to explain patterns of species and ecological richness better than turnover in composition at the spatial scale of adjacent quadrats.