Corridors and mammal community structure across a fragmented, old-growth forest landscape

We studied the influence of corridors on the community structure of old-gro wth forest mammals across a fragmented ecosystem, the Olympic National Fore st, Washington, USA. This region of once contiguous forest has been transfo rmed by logging into a mosaic of landscape features including clearcuts, se cond-growth forest, and old-growth forest patches and corridors. To assess corridor utility, we quantified among- and within-corridor variability in c ommunity structure, landscape indices, and habitat descriptors. Discriminant analyses showed that the four corridors studied differed signi ficantly both in species assemblages (P < 0.05) and in habitat characterist ics (P < 0.005). Changes along individual corridors were primarily associat ed with adjacent habitat. The proportion of adjacent old-growth forest sign ificantly decreased along two of the four corridors, reflecting this system 's fragmentation gradient of an increasingly disturbed landscape matrix. Th e number of forest species found in corridor sites was significantly correl ated with this matrix: less old-growth forest surrounding a corridor site r esulted in fewer forest-dependent species, Width of the corridor at a site and site isolation often acted singly or in combination with the fragmentat ion gradient as an influence on community structure. This was also the case for many individual species. Demographic measures suggested that, while re production is occurring along these corridors, levels are lower than in con tinuous forest. Finally, data showed that, while forest species richness an d occurrence of specific forest species were consistently higher in corrido rs than in the surrounding matrix, these patterns were driven primarily by differences between corridors and clearcuts. These results suggest that, al though these corridors appear to be effective and may possibly serve as dem ographic sources of individuals, they should not be considered equivalent t o one another. Only hv considering corridors individually can their respect ive value be determined. For the Olympic National Forest, this value is sig nificant, with the use of a combination of different corridors comprising a viable supplement to maintaining continuous forest for long-term preservat ion of biodiversity.