Factors Influencing Species Composition and Population Sizes in a Ground Beetle Community (Carabidae): Predation by Rodents

Abstract

In this study, we examined the hypothesis that rodent predation significantly alters the ground beetle community in a shrub-steppe ecosystem in southwestern Wyoming, USA. Four resident rodent species (Peromyscus maniculatus, Onychomys leucogaster, Perognathus parvus and Spermophilus armatus) were shown to feed readily on ground beetles in captivity. We tested the impact of these rodent predators on field populations of ground beetles by excluding rodents from ten 15 x 15 m plots for 2 yr (1984-86). We found that total carbid abundances were 111% higher on rodent exclosure plots relative to control plots. Seven of eight carabid species (Amara farcta, A. quenseli, Calosoma luxatum, Carabus taedatus, Cyminidis cribricollis, Harpalus fraternus and H. innocuus) exhibited greater population sizes in rodent-free plots than in control plots, while only one species (Philophuga viridis) was less abundant in rodent exclosures. Rodent-free plots also contained significantly greater mean numbers of carabid species (per plot) than control plots. Total densities of insectivorous and omnivorous rodents in the study area during 1986 averaged about 10 rodents ha-1, which was slightly larger than in previous years but far smaller than densities during rodent cycle peaks. Hence, we assert that typical densities of rodents can significantly reduce the abundances of ground beetles in our shrub-steppe ecosystem, and may contribute to temporary, localized extinctions of rarer carabid species.