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 elevation s from Lower Sonoran Desert sand dunes and creosote scrub to Upper Transiti on coniferous forest, where relict species typical of the Sierra Nevada are found. I sampled a 1050 m elevational range in this transect to evaluate t he ecological distributions of small mammals and to better characterize com munity structure. Results complement and substantially extend a previous st udy of this fauna, and provide insights into the habitat associations of sp ecies in this complex fauna. Assemblage structure changed greatly between s ummer and winter, largely due to reduced presence of pocket mice Chaetodipu s in winter. Additionally. the distribution of abundance and species richne ss was different than reported earlier, suggesting that patterns across thi s gradient may be temporally variable, and strongly influenced by local dyn amics. Most taxa exhibited significantly nonrandom use of a large number of habitat variables but this was nor a simple consequence of the elevational gradient. A mid-elevation bulge in species richness was indicated, bur lik ely is nor a consequence of mass effects since a number of animals captured in intermediate regions were reproductively active.