Spatial Interactions and Microhabitat Selections of Two Locally Sympatric Voles, Microtus Montanus and Microtus Pennsylvanicus

Abstract

Microtus pennsylvanicus and Microtus montanus are sympatric in south central Montana, and the two occur in intermixed populations. This study attempted to assess the relative importance of interspecific interactions and habitat preference on movements and habitat selection of the two species. Radioisotope telemetry in 20—m x 20—m field enclosures showed that M. montanus had larger home ranges in the presence of M. pennsylvanicus than in single species treatment. Spacing between individuals was closer for M. montanus when alone as compared to mixed treatments. There was more overlap in M. montanus home ranges in single species treatments than in mixed treatments. Microtus montanus selected different vegetation when alone than in the presence of M. pennsylvanicus. The vegetation selected by each species was different from that selected by the other in all treatments. Enclosure experiments indicated only slight differences in emigration rates, survival rates, or weight changes for either species between single and mixed species treatments. Trap lines in natural areas outside of the enclosures showed that the two species are sympatric throughout the year but have different habitat preferences. Both interspecific interactions and divergent habitat preferences are important in maintaining separation between M. pennsylvanicus and M. montanus but the importance of each changes along habitat gradients. As habitats become uniformly optimal for M. pennsylvanicus, they are able to exclude M. montanus from an area.