Corridors and Mammal Community Structure Across a Fragmented, Old-Growth Forest Landscape

We studied the influence of corridors on the community structure of old-growth forest mammals across a fragmented ecosystem, the Olympic National Forest, Washington, USA. This region of once contiguous forest has been transformed by logging into a mosaic of landscape features including clearcuts, second-growth forest, and old-growth forest patches and corridors. To assess corridor utility, we quantified among- and within-corridor variability in community structure, landscape indices, and habitat descriptors. Discriminant analyses showed that the four corridors studied differed significantly both in species assemblages (P < 0.05) and in habitat characteristics (P < 0.005). Changes along individual corridors were primarily associated with adjacent habitat. The proportion of adjacent old-growth forest significantly decreased along two of the four corridors, reflecting this system's fragmentation gradient of an increasingly disturbed landscape matrix. The number of forest species found in corridor sites was significantly correlated with this matrix: less old-growth forest surrounding a corridor site resulted in fewer forest-dependent species. Width of the corridor at a site and site isolation often acted singly or in combination with the fragmentation gradient as an influence on community structure. This was also the case for many individual species. Demographic measures suggested that, while reproduction is occurring along these corridors, levels are lower than in continuous forest. Finally, data showed that, while forest species richness and occurrence of specific forest species were consistently higher in corridors than in the surrounding matrix, these patterns were driven primarily by differences between corridors and clearcuts. These results suggest that, although these corridors appear to be effective and may possibly serve as demographic sources of individuals, they should not be considered equivalent to one another. Only by considering corridors individually can their respective value be determined. For the Olympic National Forest, this value is significant, with the use of a combination of different corridors comprising a viable supplement to maintaining continuous forest for long-term preservation of biodiversity.