Competition among Small Mammals in Experimentally Perturbed Areas of the Shortgrass Prairie

Abstract

Vegetation structure and species composition of vegetation differed on shortgrass prairie areas maintained as controls and manipulated through the application of water, nitrogen, and both water and nitrogen. Different small mammal communities formed on these 4 habitat types. Microtus ochrogaster and Reithrodontomys megalotis, rare on most habitat types of shortgrass prairie ecosystem, invaded the nitrogen / water treatment in relatively high densities. The native mammal species of the shortgrass prairie (Peromyscus maniculatus, Onychomys leucogaster, and Spermophilus tridecemlineatus) completely or partly avoided the nitrogen / water treatment. A removal experiment was conducted to test the prediction that continuing competition between the 2 invader species and the native species restricted the distributions of the latter. In this experiment, both M. ochrogaster and R. megalotis were removed from 1 plot of the nitrogen / water treatment. The second plot of the same treatment served as control for the removal experiment. Peromyscus maniculatus increased in density on the removal plot relative to the nonremoval plot. Onychomys leucogaster did not invade the removal plot; S. tridecemlineatus hibernated during most of the experiment, and its response could not be studied. The experimental results provide direct evidence that competition from M. ochrogaster affects the distribution and abundance of P. maniculatus. The results also suggest that O. leucogaster avoided the nitrogen / water treatment because of different habitat preference. Overlap indices, measured over the food and habitat dimensions of the niche, agree well with competition coefficients directly measured during the perturbation. Even though the overlap between M. ochrogaster and P. maniculatus is relatively small, the 2 species are strong competitors.