Winter Coexistence of Clethrionomys gapperi and Microtus pennsylvanicus in a Grassland Habitat

In October 1969 numerous red-backed voles (Clethrionomys gapperi) immigrated into a field normally occupied mainly by meadow voles (Microtus pennsylvanicus), established home ranges, and survived well until February 1970, when they began to disappear. Both species were at peak populations during the winter of 1969-70, in the Pinawa, Manitoba, area. The times of immigration and emigration corresponded with the end and onset of reproductive activity in the resident Microtus. It is suggested that Clethrionomys are normally excluded from grassland habitats by reproductively active and aggressive Microtus. In winter M. pennsylvanicus is less aggressive and the species can coexist. Winter colonization, allowed by the breakdown of habitat separation due to aggressive behavior, may explain the ubiquitous presence of forest animals in the isolated groves which are common in the aspen parkland. INTRODUCTION The mechanisms which preserve habitat separation between meadow voles (Microtus) and red-backed voles (Clethrionomys) have been the subject of several recent studies (Clough, 1964; Cameron, 1964; Morris, 1969; Grant, 1969, 1970a, 1970b), but are still incompletely understood (Clough, 1964; Morris, 1969). This paper describes a temporary breakdown of habitat separation, and discusses the implications of this event to an understanding of the mechanisms which normally preserve habitat separation.