Voles looking for an edge: habitat selection across forest ecotones

Abstract

We searched for the presence of "edge effects" in the occupation of adjacent boreal-forest habitats by red-backed voles (Clethrionomys gapperi). First, we reviewed four models that differ in their predictions of abundance at habitat boundaries. Three of the models predicted an edge effect, while the so-called matrix or habitat model served as a null expectation. We then developed a protocol to detect, objectively, the ecotone between two habitats that is necessary to differentiate among the four models. The protocol revealed both abrupt and gradual ecotones along transects crossing conifer to cutover and conifer to deciduous habitats. Though vole density was almost always higher on one side of the ecotone than on the other, we were unable to detect an edge effect of any kind. Vole density within ecotones was intermediate to that on each side (refuting the existence of an ecotone effect). There were also no differences in the pattern of density between abrupt and gradual ecotones (refuting the existence of a permeability effect), and no consistent pattern of vole density away from either type of ecotone (refuting the existence of a habitat-selection effect). Simulations that manipulated vole densities along the transects suggested, however, that the habitats may have been too similar to allow a habitat-selection effect to be detected. We suspect that our result will be common to moderately generalised species, and we recommend that controlled experiments be carried out to evaluate the conditions under which habitat-selecting species may exhibit edge effects.