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Spatial variation in keystone effects: small mammal diversity associated with black-tailed prairie dog colonies


J. F. Cully, Jr (, United States Geological Survey, Kansas Cooperative Fish and Wildlife Research Unit, and Div. of Biology, 204 Leasure Hall, Kansas State Univ., Manhattan, KS 66506, USA. – S. K. Collinge, Dept of Ecology and Evolutionary Biology, and Environmental Studies Program, Univ. of Colorado-Boulder, 334 UCB, Boulder, CO 80309-0334, USA. – R. E. VanNimwegen and B. Thiagarajan, Div. of Biology, 204 Leasure Hall, Kansas State Univ., Manhattan, KS 66506, USA. – C. Ray, W. C. Johnson and D. B. Conlin, Dept of Ecology and Evolutionary Biology, Univ. of Colorado-Boulder, 334 UCB, Boulder, CO 80309-0334, USA. – B. E. Holmes, Colorado Division of Wildlife, P.O. Box 1181, Meeker, CO 81641, USA.


Species with extensive geographic ranges may interact with different species assemblages at distant locations, with the result that the nature of the interactions may vary spatially. Black-tailed prairie dogs Cynomys ludovicianus occur from Canada to Mexico in grasslands of the western Great Plains of North America. Black-tailed prairie dogs alter vegetation and dig extensive burrow systems that alter grassland habitats for plants and other animal species. These alterations of habitat justify the descriptor “ecological engineer,” and the resulting changes in species composition have earned them status as a keystone species. We examined the impact of black-tailed prairie dogs on small mammal assemblages by trapping at on- and off-colony locations at eight study areas across the species' geographic range. We posed 2 nested hypotheses: 1) prairie dogs function as a keystone species for other rodent species; and 2) the keystone role varies spatially. Assuming that it does, we asked what are the sources of the variation?

Black-tailed prairie dogs consistently functioned as a keystone species in that there were strong statistically significant differences in community composition on versus off prairie dog colonies across the species range in prairie grassland. Small mammal species composition varied along both latitudinal and longitudinal gradients, and species richness varied from 4 to 11. Assemblages closer together were more similar; such correlations approximately doubled when including only on- or off-colony grids. Black-tailed prairie dogs had a significant effect on associated rodent assemblages that varied regionally, dependent upon the composition of the local rodent species pool. Over the range of the black-tailed prairie dog, on-colony rodent richness and evenness were less variable, and species composition was more consistent than off-colony assemblages.