Competition and Niche Shifts of Mice Introduced onto Small Islands

Peromyscus maniculatus (Wagner) and Clethrionomys gapperi (Vigors) were introduced to a 0.8 ha island to which only Microrus pennsylvanicus (Ord) was native in order to study subsequent interactions. The populations were censused for the ensuing six summers. Although all three species occupied overlapping home ranges in some habitats, there were significant differences in their habitat preferences. Principal component analysis of number of captures of the respective species at 45 trap-sites showed that habitat utilization changed with time in response to changes in intraand interspecific densities. Multiple regression was used to estimate the magnitude of interspecific interference after removal of effects due to habitat selection. Clethrionomys displaced both Microrus and Peromyscus from wooded habitats, and Peromyscus in turn displaced Microtus from shrubby habitats. The magnitude of this interference increased with population density, and the inhibitory effects per individual at maximum observed densities were as follows: Clethrionomys on Microtus -0.649, and Clethrionomys on Peromyscus -0.443, and Peromyscus on Microtus -0.662. The reciprocal effects were close to zero. We suggest that the success of Microtus on islands accrues from its ability as an opportunist, while the failure of Clethrionomys on islands is attributable to its rigid and restricted habitat preferences.