Experiments on Interspecific Competition between Two Species of Pocket Gophers

Abstract

Banded individuals of two species of pocket gophers (Thomomys bottae and T. talpoides) were introduced into three study areas in south-central Colorado in the summers of 1959 and 1960. One year after each set of introductions the pocket gophers remaining in the areas were trapped. The patterns of introduction are described and the trapping results are presented and discussed. The two species were able to live and reproduce side by side, but in each experiment one species perpetuated itself better than did the other. The two major factors influencing the outcomes of the experiments seemed to be: The difference between the motility of the two species as it affected immigration, emigration and reproduction at low population densities; and, compared to T. bottae, the greater environmental tolerance shown by T. talpolides. INTRODUCTION The species of pocket gophers (Geomyidae) are almost entirely allopatric or parapatric in distribution. The similarity in morphology and mode of life among the species of pocket gophers seemingly prevents sympatric distributions and suggests that intense interspecific competition may exist where the ranges of two species meet. Among the authors who have remarked on allopatry in pocket gophers are Bailey (1926, 1931), Davis (1939), Hall (1946), and Kennerly (1958, 1959). The present study contributes to an understanding of interspecific competition in mammals by presenting observations on experimentally established colonies of the valley pocket gopher, Thomomys bottae, and the northern pocket gopher, T. talpoides. Acknowledgments.-This study was supported by a National Science Foundation Grant G-9807. For critically reading the manuscript we would like to thank Ronald A. Ryder and Donald F. Hervey, Colorado State University. For help with field and laboratory work we wish to thank Roger E. Baker, George D. Bear, Keith E. Evans, LeMoyne B. Marlatt, Gary T. Myers, and Ronald L. Perry. MATERIALS AND METHODS Field experiments were conducted approximately 6 miles north of Cotopaxi, Fremont Co., Colorado, at about 7,500 ft elevation, in a section of canyon where the ranges of T. bottae and T. talpoides meet. Field work began in April, 1959, and terminated in August, 1962. The terrain of the study area is rocky and precipitous, and the slopes are dotted with pinion pines (Pinus edulU's) and juniper (Juniperus scopulorum). Pocket gophers are largely restricted to the deeper soils of the canyon bottoms or alluvial slopes, where blue grama (Bouteloua gracilis) and sleepygrass (Stipa robusta) are usually the dominant plants. Vegetative compositions for plants occurring on the study areas are listed in Table I. Vegetative composition for any * Technical journal article S.S. 917, Colorado Agricultural Experiment Station.