Corridors May Not Improve the Conservation Value of Small Reserves for Most Boreal Birds

Abstract

Building or maintaining corridors in fragmented landscapes may be an important method to conserve gap-sensitive species that avoid crossing gaps in forest cover. We tested the effectiveness of corridors by examining the changes in abundance of boreal birds pre- and post-logging in experimental 10-ha and 40-ha reserves that were isolated or connected by corridors, relative to their abundance responses in continuous forest (reference sites). Prior to the analysis, we categorized birds as to their predicted gap sensitivity based on two measures: their use of corridors and gap-crossing behavior in small-scale trials, and their habitat affinities (forest species vs. habitat generalists). The abundance of forest species as a group was consistently higher in reference reserves than in isolated or connected reserves after harvest, except for the first year after harvest, when crowding occurred in isolates. Habitat generalist species showed no differences in abundances across reserve types. As a group, resident species were more abundant in reference and connected reserves than in isolates in three of five years post-harvest, suggesting that corridors might benefit these species. None of the single species analyzed showed consistent evidence of benefiting from corridors. Although four species were most abundant in connected reserves after harvest, their abundances were not significantly lower in isolates than in reference sites. Behavioral classification (gap-crossing propensity) was not useful in classifying single species as to how gap sensitive they would be in response to our experiment: habitat affinity was a better predictor. We suggest that corridors may be useful to retain resident birds on harvested landscapes, but that corridors connecting small reserves of forest are unlikely to offset the impacts of fragmentation for most boreal birds. Assessments of the utility of corridors must, however, be done in the context of the full plant and animal communities that live in the boreal forest.