Elevational distributions of kangaroo rats (Genus dipodomys): Long-term trends at a Mojave Desert site

Distributions of related species along environmental gradients provide ecol ogists with insights into factors that limit distributions of species. We a pply this approach to two species of kangaroo rats. A survey in 1983 showed that Dipodomys merriami is replaced by D. panamintinus at the tops of elev ational gradients in parts of the Mojave Desert where both species occur. O ver the 17 y since the initial survey we have conducted six additional cens uses along one such gradient. In years of high population densities D.,merr iami is more abundant at low and D. panamintinus at high elevations along t his gradient. Following periods of drought, however, when population densit ies are reduced overall, D. merriami expands upward along the gradient, whe reas D. panamintinus becomes restricted to even higher elevations. This pat tern suggests that D. merriami normally is restricted to lower elevations b y competition from the larger D. panamintinus, and experiences competitive release when the latter is at low density. The distribution of D. panaminti nus, in contrast, responds to changes with elevation in primary productivit y or in physical factors, rather than to competition from D. merriami Becau se of relatively large body mass, individuals of D. panamintinus may be res tricted to higher elevations, particularly in drought years, because primar y productivity at these elevations suffices to support their metabolic requ irements. This interpretation of limits to distribution has implications fo r conservation of these and similar species especially under impending clim ate change.