Forest Fragmentation in the Pacific Northwest and its Potential Effects on Wildlife

Fragmentation is the creation of a complex mosaic of spatial and successional habitats from formerly contiguous habitat. Loss of habitat and the less obvious phenomenon of habitatpatch isolation are aspects ‘of fragmentation that threaten the viability of wildlife populations, the components of biotic diversity. Fragmentation of forests on Federal lands in the Pacific Northwest is the product of staggered-set clearcutting of late-successional forest. Large-scale block clearcutting on private lands has minimized fragmentation simply by leaving few or no fragments. Less than 20 percent of the original old-growth forest remains, and the fragmentation of remaining old-growth stands may degrade the quality of these areas for plants and wildlife. Analysis of population vulnerability suggests that life-history characteristics, along with biotic and physical environmental factors, influence population structure and individual fitness. These factors determine the risk of local population extinction. We compiled a list of 93 species associated with late-successional Douglas-fir forests in the Northwest and rated the risk for each species of local extinction from fragmentation on a scale of 1 to 10 based on the frequency of occurrence, abundance, and variation in abundance data from the old-growth community studies (this volume), and on body size, vagility, and migratory status information from the literature. Over 80 percent of the listed species had high to moderately high risk scores (7 to 10); risk for these species was a function of the low frequency and abundance associated with three combinations of body size and vagility. Clearcutting in patterns that minimize fragmentation would reduce the impact of isolation and small patch size for 27 species, likely benefit another 20 species, but probably have little impact on 46 species.