Demographic Consequences of Movements in Subdivided Root Vole Populations

Abstract

We studied three types of movements: (1) movements leading to permanent transfer of individuals between habitat patches, (2) movements (excursions) into habitat corridors and (3) into a barren matrix area, and the demographic consequences in 12 enclosed populations of the root vole, Micrototus oeconomus. Each population was subdivided into two demes inhabiting one habitat patch each. The two patches were approximately two male home range diameters apart in a vole-hostile, devegetated matrix. While the patches were connected by a narrow (0.5 m) habitat corridor for six of the populations, the other six populations inhabited isolated patches. The experiment was initiated by introducing laboratory raised founder demes onto each patch in the beginning of July. The populations were thereafter monitored by live trapping for the next 4-5 months during the snow-free season. After the snow melted the following spring the experiment was terminated by removal trapping. The experiment was run over two years (1994 and 1995) with six population replicates each year. Movements into the corridors and permanent transfers of animals between patches were registered during ordinary live trapping at 15-d intervals. Movements into the barren matrix habitat were registered continuously during the snow-free season by activating edge traps along the fences of the enclosures every night. Except for males early in the summer, movements leading to transfer of individuals between demes were rare relative to mortality and recruitment, and transfer did not act to synchronize the dynamics between demes. Moreover, transferred animals possessed the same survival probability as those staying in their natal deme. Corridors slightly enhanced the rate of transfer in females, but not in males. Excursions into corridors and the matrix area took place much more frequently than transfer and most frequently in the first cohorts early in the season and more in males than in females. Movements in the hostile matrix had a considerable negative effect on survival and, thus on the demography of the populations. Predation by birds is the most likely cause of this movement related mortality which may play an important role in the dynamics of patchy vole populations during the snow-free season. Movements into the corridors did not have any independent effect on survival probability, and habitat corridors may thus act to transfer animals more safely from one patch to another.