Long-term Experimental Study of a Chihuahuan Desert Rodent Community: 13 Years of Competition'

d4hstract. An experimental study of competition between kangaroo rats (Dipodornys spp.) and other sympatric desert rodents using exclosures with "semipermeable" fences has been continuously maintained at a site in the northern Chihuahuan Desert since 1977. A new set of experimental manipulations begun in 1988 at the same site repeated this study. As reported previously for this community, exclusion of three species of Dipodotnys from both original and new experimental plots resulted in greater abundances of five species of small granivorous rodents (Chaetodipus penicillatus, Perognath~ts .flavus, Perornyscus eremicus, P. rnaniculatus, Reithrodorztomys megalotis) on these plots relative to controls. In contrast, there were no significant treatment effects on the abundances of insectivorous grasshopper mice (0n~)chorn~)s spp.). The long time lag before the response by small granivores to Dipodotnys removal observed in the original experiment was not repeated in the experiment begun in 1988. Long-term (10 yr) exclusion of kangaroo rats from experimental plots has resulted in changes in vegetative cover, particularly increased grassiness. on these plots relative to controls. We used the repetition of the Dipodotnys exclusion experiment in 1988 to evaluate the importance of this potential indirect effect of kangaroo rats on other rodents in this community. By examining differences in rodent capture numbers on the original and new sets of Dipodomys exclusion plots, we could identify four species (C. penicillat~ts, Perognathus jlavl~s, Perornyscus eremicus, P. marzicl~latus) whose responses to kangaroo rat removal reflected direct competition from kangaroo rats, one species (R. megalotis) whose response reflected both direct and indirect (via vegetation changes and habitat selection) effects, and two species (Sigmodon hispidus, S. .fi~lviventer) whose responses reflected only vegetation-mediated effects. The continuous presence of competition between small granivores and kangaroo rats over the 13-yr study despite large. species-specific fluctuations in abundances suggests that competition is pervasive within this community.