Distributions of related species along environmental gradients provide ecologists with insights into factors that limit distributions of species. We apply 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 elevational gradients in parts of the Mojave Desert where both species occur. Over the 17 y since the initial survey we have conducted six additional censuses along one such gradient. In years of high population densities D. merriami is more abundant at low and D. panamintinus at high elevations along this gradient. Following periods of drought, however, when population densities are reduced overall, D. merriami expands upward along the gradient, whereas D. panamintinus becomes restricted to even higher elevations. This pattern suggests that D. merriami normally is restricted to lower elevations by competition from the larger D. panamintinus, and experiences competitive release when the latter is at low density. The distribution of D. panamintinus, in contrast, responds to changes with elevation in primary productivity or in physical factors, rather than to competition from D. merriami. Because of relatively large body mass, individuals of D. panamintinus may be restricted to higher elevations, particularly in drought years, because primary productivity at these elevations suffices to support their metabolic requirements. This interpretation of limits to distribution has implications for conservation of these and similar species especially under impending climate change.
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