Perturbation analysis of competition and overlap in habitat utilization between Dipodomys ordii and Dipodomys merriami

Abstract

The populations of two coexisting species of Dipodomys (Heteromyidae, Rodentia) were manipulated on 10, large, unenclosed, trapping grids. These manipulations revealed that, although many kangaroo rats are established residents in an area, a large number are transient individuals who quickly occupy vacated habitats. On plots from which residents had been removed, transients settled at rates of up to 5% of carrying capacity per day. These immigrants were invariably of the same species that was removed, indicating a strong element of intraspecific competition with little or no evidence of competition between the species.Trapping records suggest that these species avoid competition through habitat selection. Dipodomys ordii prefer a grassier habitat, and D. merriami a habitat dominated by creosote bush. Apparent overlap in their utilization of habitats, based on sites of capture, predicts competition coefficients to be higher than those permitted by the theory of limiting similarity and much higher than those actually shown by the perturbation experiments.This study demonstrates the dangers of estimating alpha without experimentation. This is especially true in cases where habitat selection may be important, since organisms may travel in habitats without collecting resources therein. Our results are discussed in light of a theory which examines the optimal (rather than tolerable) amount of overlap in habitat utilization between two potential competitors in a mixed habitat. This theory predicts that the pressure of natural selection should eliminate the interspecific competition entirely.However, the conclusion that the interspecific competitive alpha is zero does not lead to the conclusion that interspecific competition is unimportant in the system. Instead, if our interpretation is correct, such competition has molded the system, and were there not a continual threat of interspecific competition, the habitat specializations would soon disappear.

Quantification of Competition among Coexisting Heteromyids in the Southwest

“…The competitive nature of the interaction was suggested by increase in the number of Perognathus in response to removal of Dipodomys. This result is contrary to that found by Schroder and Rosenzweig (1975) who found no density response between two species of Dipodomys of very similar size. In addition, the quick increase of the number of Perognathus to the removal of Dipodomys differs from the findings of Munger and Brown (1981) were it took 8 months for the small rodents to increase in number in response to the removal of Dipodomys.…”

“…Schroder and Rosenzweig (1975) removed rodents from large study plots with the surprising result that little competition exists between two coexisting species of similar size. They concluded that competition had been important in the past, but is not presently affecting interspecific interactions.…”

“…Schroder and Rosenzweig (1975) found it difficult to lower a species' density because after removing most individuals from an experimental grid other individuals of that same species took their places, often within two weeks. This might indicate an intraspecific mechanism for adjusting local population density that worked very quickly.…”

Explaining patterns in abundances of organisms on boulders: the failure of 'natural experiments'

“…Dayton (1973) showed that patterns consistent with 2 aspects of competition theory actually had quite different causes. Schroder & Rosenzweig (1975) tested previous ideas that 'habitat specializat i o n~, food habits, and species diversity of desert rodents . .…”

Management of amphibians, reptiles, and small mammals in North America: Proceedings of the symposium

“…Hence, there appears to be a limit to how far the community can compensate for the absence of certain species. Among the possible explanations for this might be that assemblages of desert rodents have been associating together for a sufficient time to have lost the flexibility to respond to situations where one or more of the species are absent (Schroder and Rosenzweig 1975). Another is that quadrupedal species may have a limited ability to avoid predators in open microhabitats and this limits the degree to which they can compensate when the bipeds are removed.…”