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Population and habitat use characteristics of forest-dwelling small mammals in relation to downed wood Craig, Vanessa Joy

Abstract

Downed wood has long been considered an important habitat component for small mammals, although studies to date have generated equivocal results. To examine this relationship in an experimental manner, I removed and added downed wood on areas within two serai stages and two ecosystems, and monitored the population-level response of: a habitat generalist, the deer mouse (Peromyscus maniculatus), early serai specialists (meadow voles, Microtus pennsylvanicus and long-tailed voles, M. longicaudus) and an old serai specialist (southern red-backed vole, Clethrionomys gapperi). I also studied fine-scale habitat associations of red-backed voles on the treatment areas using radio-telemetry to determine how relationships with downed wood and other habitat components changed with the removal of downed wood from an area. This study was part of two multi-disciplinary silvicultural systems research projects in southern British Columbia. The first was the Opax Mountain Silvicultural Systems Project area, located in a Douglas-fir-lodgepole pine forest in a warm and dry ecosystem in the interior of British Columbia. The second was the Sicamous Creek Silvicultural Systems Project area located in a high-elevation cold, wet Engelmann spruce- subalpine fir forest. My study generated several unexpected results that have not been reported in the correlational studies published to date. The relationships of small mammals with downed wood and vegetation varied by species, as well as with ecosystem. At the Opax site, deer mouse populations responded positively to harvesting but not to downed wood manipulations. The highest densities were found on clear-cuts at the Opax site where downed wood had been removed. Within forested areas, higher densities were found in stands with lower canopy cover and higher shrub cover. At Sicamous, deer mice did not respond to harvest treatments nor downed wood manipulations on forested areas. Lower survival rates and higher rates of capture of mice on the edge of low treatment areas suggested that deer mice on clear-cuts might have been negatively affected by the removal of downed wood. Microclimate, and likely the amount of ground covered by vegetation, influenced patterns in abundance on both clear-cut and forested areas. Meadow voles were strongly associated with clear-cuts at Opax but did not respond to downed wood treatments. This was expected because meadow voles are typically found in grassland habitats, which normally do not have large quantities of downed wood. Unexpectedly, meadow voles were more abundant on sites with higher shrub cover. Long-tailed voles at Sicamous responded positively to the number of pieces of downed wood on clear-cuts. This study and other work suggested that downed wood is an important habitat component for longtailed voles, regardless of the amount of vegetation on the area. Downed wood was particularly important for red-backed voles at Opax, but less so at Sicamous, where abundant shrubs likely served a similar role as downed wood in providing cover. Retaining downed wood on Opax clear-cuts mitigated some of the initial effects of harvesting on red-backed voles. Downed wood at Sicamous did not serve the same function, probably because of the sparse vegetation on clear-cuts. Radio-telemetry of red-backed voles on forested areas at Opax indicated that voles were closely associated with downed wood at both the home range and the microhabitat scale. The abundance of vegetation was an important variable for many small mammal species studied, although there were instances where relationships with downed wood were strong and independent of vegetation. The response to habitat components by small mammals may vary with site conditions (climate, serai stage, disturbance), the spatial scale of investigation, and species.

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