Life on the edge : the role of habitat selection on vole density near forest boundaries

Abstract

Theories of density-dependent habitat selection predict pronounced gradients of population density near habitat edges. Population density in high-quality habitats should decline toward edges with lower-quality habitats, and population density in low-quality habitats should increase toward boundaries with higher-quality habitats. This pattern should be more obvious near abrupt boundaries than near ecotones where habitats gradually grade one into the other. 1 tested the predictions using the density of red-backed voles (Clethrionomys gapperi) along eight belt transects crossing edges between natural and anthropogenic boreal forest habitats in northwestern Ontario. Transects were classified as having either a gradual (70 m to 90 m ecotone) or abrupt edge (20m ecotone). Vole density varied consistently between pairs of habitats, but there was no detectable gradient in density at either abrupt or gradual edges. The absence of an edge effect may be related to errors in the assessment of habitat quality by individuals confronted with a matrix of patch types near edges separating similar habitats. Another possibility is that an, as yet, unidentified agent or process alters the quality of red-backed vole habitats near boreal-forest edges.