Assemblage structure and quantitative habitat relations of small mammals along an ecological gradient in the Colorado Desert of southern California

Ecological gradients have intrigued ecologists for many years. In southern California the Deep Canyon Transect spans a range of habitats and elevations from Lower Sonoran Desert sand dunes and creosote scrub to Upper Transition coniferous forest, where relict species typical of the Sierra Nevada are found. I sampled a 1050 m elevational range in this transect to evaluate the ecological distributions of small mammals and to better characterize community structure. Results complement and substantially extend a previous study of this fauna, and provide insights into the habitat associations of species in this complex fauna. Assemblage structure changed greatly between summer and winter, largely due to reduced presence of pocket mice Chaetodipus in winter. Additionally, the distribution of abundance and species richness was different than reported earlier, suggesting that patterns across this gradient may be temporally variable, and strongly influenced by local dynamics. Most taxa exhibited significantly nonrandom use of a large number of habitat variables but this was not a simple consequence of the elevational gradient. A mid-elevation bulge in species richness was indicated, but likely is not a consequence of mass effects since a number of animals captured in intermediate regions were reproductively active.