Competition Between Microtus pennsylvanicus and Microtus ochrogaster

The closely related species, Microtus pennsylvanicus and M. ochrogaster, coexist in many grassland areas in southern Indiana. Three predictions were examined to determine if natural populations of these species are in competition. No evidence of detrimental interactions could be described from snap-trapping studies or from detailed live- trapping work. Clear competition occurred only in fenced areas which supported atypically high densities and suffered from overgrazing. Microtus pennsylvanicus and M. ochrogaster seem to coexist in southern Indiana grasslands with little interspecific competition. competition. Habitat segregation between these two Microtus species is com- monly reported in the literature {e.g., Lewin, 1968; DeCoursey, 1957; Zimmerman, 1965). We have not found very distinct habitat segrega- tion in our studies of M. pennsylvanicus and M. ochrogaster in south- ern Indiana (Keller and Krebs, 1970). Over half of the 15 areas studied contained both species. In this study I have attempted to determine whether there is evidence of competitive interactions be- tween M. pennsylvanicus and M. ochrogaster in southern Indiana grasslands which contain mixed populations. I began by assuming that interspecific competition for food or space occurs between the two species. What predictions can be made for this situation? Discussion will be restricted to habitats which support pop- ulations of both species. Competition theory would suggest three pre- dictions if Microtus pennsylvanicus has a detrimental effect on M. ochrogaster: {1 ) the density of M. ochrogaster should be inversely related to the density of M. pennsylvanicus; (2) if M. pennsylvanicus is experimentally removed from a habitat suitable for both species, M. ochrogaster should increase in numbers and reach a higher than average density; (3) if periodic fluctuations occur in both species, M. ochrogaster should respond to low phases of M. pennsylvanicus by increasing so