Competitive Exclusion between Microtus and Clethrionomys in the Aspen Parkland of Saskatchewan

Habitat preferences of Microtus pennsylvanicus and Clethrionomys gapperi were noted in the aspen parkland near Saskatoon, Saskatchewan (52°10’N, 106°41’W). An almost complete habitat separation was observed between the two species. In summer, in an area where both species occurred in numbers, resident Clethrionomys were confined to an aspen stand, whereas resident Microtus were in adjacent grassland. Where Clethrionomys were absent, resident Microtus favored habitat normally used by Clethrionomys. Late autumn trapping suggests that in winter the two species coexist in the same aspen stand where there is a protective cover of snow. The data suggest competitive exclusion of Microtus by Clethrionomys from aspen habitat during the summer. The possibility of a beachhead phenomenon and the exclusion of Clethrionomys by Microtus from certain habitats is considered.