SYNCHRONOUS DYNAMICS OF MICROTINE RODENT POPULATIONS ON ISLANDS IN LAKE INARI IN NORTHERN FENNOSCANDIA - EVIDENCE FOR REGULATION BY MUSTELID PREDATORS

We surveyed the densities of small mammals on 17 islands in Lake Inari in Finnish Lapland during 5 yr to estimate the degree of spatial synchrony in the dynamics of microtine rodents (Clethrionomys, Microtus) and shrews (Sorex). Microtine rodents have a 4-5 yr population cycle in Lapland, Most island populations of microtine rodents were in complete synchrony with the mainland populations and each other, and the same-island rodent and shrew populations showed largely synchronous population changes. The few exceptions included small and isolated populations of Microtus oeconomus fluctuating out of synchrony. The generally high level of spatial synchrony is unlikely to be caused by predation by nomadic avian predators, as has been suggested previously, because they are very scarce in the study area. We present and discuss results suggesting that the synchrony is due to predation by small mustelids, especially the stoat, which may also be the primary cause of the large-amplitude rodent oscillations.