Experimental studies of competitive interaction in a two-species system. I. Microtus and Clethrionomys species in enclosures

A 17-week experiment was performed in 1966 to determine under what conditions Microtus pennsylvanicus and Clethrionomys gapperi leave their usual habitat and enter another, and the significance of interspecific interaction. Three 1-ac enclosures were used, each containing equisized blocks of deciduous woodland and grassland. Into one enclosure three pairs of the woodland species Clethrionomys gapperi only were introduced. Into another, four pairs of the grassland species Microtus pennsylvanicus only were introduced. Into the remaining enclosure three pairs of Clethrionomys gapperi and four pairs of Microtus Pennsylvanicus were introduced. All these were placed in the appropriate habitats. Subsequent movements of the animals were determined by live-trapping with Longworth traps arranged in a grid pattern and used on one to three consecutive nights and (or) days per week. Both species reproduced. Because of poor recruitment, the population densities of Clethrionomys gapperi changed very little. Microtus pennsylvanicus reproduced with greater success, and the final densities were about three times greater than the starting densities.But for one individual, Microtus pennsylvanicus stayed entirely within the grassland habitat. From the beginning, and throughout the experiment, Clethrionomys gapperi individuals moved into the grassland habitat. Recruits entered the grassland proportionately more frequently than did the adults, but the adults entered the grassland more frequently in the second half of the experiment than in the first half. One adult female and her presumed offspring were trapped only in the grassland. Several data implicate density effects as contributors to the movement of Clethrionomys gapperi into the grassland habitat. It is proposed that the carrying capacity of the woodland habitat was exceeded by the initial number of animals introduced, and that the stimulus or stimuli to move into the grassland arose from social interaction above a threshold level.Many more Clethrionomys gapperi were trapped in the grassland (I) without Microtus pennsylvanicus (except for occasional transgressors) than in the grassland (II) with Microtus pennsylvanicus. After a possible trapping bias is taken into account it is estimated that the difference is no less than 40% of the larger number. It is suggested that interactions with Microtus pennsylvanicus, possibly reinforced by the signs of that species, were responsible for the fewer movements of Clethrionomys gapperi into the grassland of enclosure II.