Graduate Thesis Or Dissertation
 

Edge effects of clearcut harvesting on ground arthropod species composition and predator community structure in old-growth Douglas-fir forests

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  • Edge effects resulting from forest fragmentation are likely to alter the distributions and interactions of resident species. I evaluated changes in species composition, species turnover, and relative abundance of ground arthropods across replicated transects extending from regenerating clearcuts into old-growth Douglas-fir forests. Arthropods were collected from 3-August to 14-September in 1997 and 23-May 1998 to 31-July 1998 using pitfall traps. Pitfall traps were positioned at 75 m (in the clearcut), -25, 0 (forest edge), 50, 100 and 200 m into the forest in 1997. In 1998, traps were repositioned at 25, 0, 25, 50 ,lOO, and 200 m into the forest. Changes in species composition and relative abundance were compared using non-metric multidimensional scaling ordination. Two-hundred, five species representing 24,178 individuals were collected. Edge effects on species composition and relative abundance were apparent up to 100 m into old-growth forests. Species were characterized as edge-phobic (interior forest associates), edge-philic, edge-insensitive or as edge-input (clearcut associates). The majority of species were characterized as either edge-phobic or edge-input species. Seasonal patterns in activity are also reported. To further address the impacts of edges on community structure, changes in species abundance of predator taxa across the edge-forest gradient were compared to four models of resource partitioning. Observed patterns of species abundance did not differ across the edge-forest gradients and were consistent with a model of random assortment (or non-equilibrium) where an individual predator species utilizes resources independently of other predators. This pattern was consistent whether species abundance was expressed as numerical abundance or biomass. This study suggests that edge effects resulting from forest fragmentation alter species composition and may negatively affect interior forest species. Furthermore, although species composition changes across forest edges, this study suggests that resource partitioning by a trophic group such as predators remains unaffected by forest edges.
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