Vertical Stratification of Activity of Two Peromyscus Species: An Experimental Analysis

The importance of competitive interactions in maintaining the vertical microhabitat segregation of two ecologically similar Peromyscus species was evaluated by reciprocal species—removal experiments during a year of peak density. The white—footed mouse, Peromyscus leucopus noveboracensis, and cloudland deer mouse, Peromyscus maniculatus nubiterrae, are syntopic in southwest Virginia and are commonly trapped aboveground in mixed deciduous forest. Patterns of arboreal habitat use and levels of arboreal activity were determined for both species before the removal experiments. During the summer and fall of 1981, six live—trapping grids, equipped with both ground and aboveground traps, were randomly allocated as control, leucopus removal, or maniculatus removal grids. On all grids, maniculatus used arboreal habitat more extensively than leucopus. Overall, 48% of 1357 maniculatus captures, but only 24% of 2270 leucopus captures, were in aboveground traps. Before removals, both species occupied structurally similar arboreal habitat, and following removals were detected no shifts in arboreal habitat use that would indicate competitive release by the unmanipulated species. Removals also had no effect on the degree of individual arboreal activity shown by the remaining species. Both before and after removals, maniculatus consistently showed more arboreal activity than leucopus for all treatments. While maniculatus significantly increased the number of arboreal sites occupied in the absence of leucopus, and encroached substantially on arboreal habitat formerly used by its congener, this expanded arboreal habitat use occurred in the absence of any numerical increase or change in activity levels. We attributed it to greater arboreal orientation of maniculatus and opportunistic use of arboreal habitat in areas of former range overlap with leucopus. The experiments suggest that differential use of arboreal habitat by these species results from differences in their behavioral responses to vertical habitat, and is independent of dynamic competitive interactions.