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UBC Theses and Dissertations

Stream invertebrates in northwest British Columbia : an assessment of the relative importance of forest harvesting and environment factors at local and landscape scales Bennett, Shauna Ann


Streams are strongly linked to their terrestrial environments through processes which influence habitat structure and food availability at several, nested scales. While forest practices can affect the hydrological, sedimentation and disturbance regimes, the relationship between community structure and forest practices across broad scales of natural environments is not well understood. In this study, the relative influence of environment and forest harvest attributes on stream community composition was examined in 143 unique 1st to 6th order streams, across a broad geographic scale in northwest B.C. Environment and forest harvest variables explained unique aspects of the community composition using canonical correspondence analysis (CCA), although landscape level environmental variables representing catchment topography and hydrology were selected first using a stepwise procedure. Using partial CCA, 20% of the variation in community structure between sites was explained by environment variables, 12% by forest harvest variables, and 4% was shared between the two sets of variables. The low proportion of shared variance suggested low redundancy between the two sets of variables. Variables describing recent forest harvest (15 years prior to sampling or less) explained unique aspects of the community composition compared to variables describing older forest harvest (more than 15 years prior to sampling), perhaps suggesting partial recovery of the stream community after 15 years. The 143 stream sites were sorted into three LAND USE groups based on the proportion of forest harvest within the catchment basin. Using catchment area as a covariate, relative abundance and rarified taxa richness increased by 30% and 20% respectively, while the proportion of EPT individuals decreased, in catchments with > 15% harvest compared to undisturbed catchments. Distance-based ordination scores (nonmetric multidimensional scaling) changed significantly between the three LAND USE classes of forest harvest on both axes. The effects of forest harvest on streams were confounded with natural environmental gradients. Generally, the catchment basins with >15% harvest were larger, with less topographic relief, more lakes and wetlands, and less rapid drainage characteristics when compared to undisturbed catchments.

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