Graduate Thesis Or Dissertation

Small mammal distribution, abundance and habitat selection in managed riparian habitats of Bear Valley, eastern Oregon

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  • Riparian zones are critical habitats for management because of their importance for both cattle production and wildlife, and a high potential for resource conflicts. Riparian management should address habitat and microhabitat features that sustain both livestock production and wildlife diversity. I conducted a study to determine how small mammal distributions and abundance differ among 3 structurally different riparian habitats in eastern Oregon. The 3 habitat types, herbaceous, discontinuous willow, and continuous willow, represent a range of habitats typical of riparian zones in central and eastern Oregon. I estimated small mammal population sizes in 9 trap grids placed in riparian zones using capture-recapture techniques. Four species of small mammals were captured during 7 trapping periods from August 1994 to September 1995. Small mammal distributions and population sizes of each species varied both within and among riparian pastures. Montane voles (Microtus montanus) were the most abundant species in all grids. Deer mice (Peromyscus maniculatus) had high population sizes in grids with low montane vole populations and moderate to high willow cover. Competition with voles appeared to occur in riparian habitats of Bear Valley, and influenced the distribution and numbers of deer mice. Western jumping mice (Zapus princeps) were captured at relatively low numbers and almost exclusively in continuous willow habitats. Vagrant shrews (Sorex vagrans) were captured on all grids, and population sizes were small. Haying negatively affected survival rates of voles. Biomass, vegetation height, and percent ground cover were the habitat variables most frequently associated with occurrence of montane voles in Bear Valley. The probability of occurrence of this species was positively correlated with these habitat variables. Deer mice probability of occurrence was positively correlated with percent ground cover, plant biomass, and soil moisture. Most deer mice captures occurred in continuous willow habitats, and the number of captures increased with willow basal cover (R²=0.7579, P<0.001). Western jumping mice occurrence was positively correlated with plant biomass and soil moisture, and negatively correlated with distance from willow and percent ground cover. Western jumping mice also were more frequently captured in willow than nonwillow vegetation associations (P=0.054) within continuous willow habitats. Vagrant shrews were positively associated with plant biomass, vegetation height, and soil moisture. Small mammal habitat associations from my study sites in Bear Valley are in agreement with the literature, suggesting relatively consistent patterns in species-habitat associations. Distribution of small mammal species differed among pastures probably as a result of historical management practices. Alterations of riparian zone structure from historical management practices were evident in the study area, including elimination or decreased willow density, channel entrenchment, and invasion of riparian areas by xeric vegetation. Western jumping mice are apparently the most sensitive species in this area, requiring dense willow stands, suggesting that jumping mice could serve as an indicator species for riparian habitat condition.
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