Small mammal micro- and macrohabitat selection in streamside ecosystems

During summer and fall of 1981 to 1983, mark-recapture wasconducted in riparian and upland habitats within old-growth andmature forests in the Cascade Range of Oregon. Number ofindividuals and species richness were greater in riparian thanupland habitat. For most species, the mean adult weights andthe percentages of reproductively active males and females weregreater in riparian habitat. Insectivora had higher numbers ofcaptures, densities, and biomass per ha in riparian habitat.Four rodents had higher numbers of captures, densities andbiomass per ha in upland habitat. An additional five rodentshad higher values for these parameters in riparian habitat.Both mustelids captured had higher numbers of captures,densities, and biomass in riparian habitat.
Three sympatric voles, Microtus oregoni, Microtus richardsoni, and Clethrionomys californicus, were studiedrelative to microhabitat separation because they are potentialcompetitors due to similarities in morphology and life history.Discriminant analysis identified significant separation basedon cover of lichen and deciduous trees and distance from thecreek. Specifically, C. californicus selected microhabitatswith a high percent cover of lichen and western hemlock and lowpercent cover of deciduous trees. M. oregoni selectedmicrohabitats high in deciduous and evergreen herbs anddeciduous shrubs. Microhabitats selected by M. richardsoni hada high soil exposure, greater length of recently fallen logs,and reduced cover of Douglas-fir.
To investigate patterns of competition, Peromyscus maniculatus, the most abundant small mammal, was removed fromfour grids in 1983. All species of the order Insectivoraincreased with the removal of P. maniculatus relative to mostparameters measured. Captures were significantly higher onremoval grids for Sorex monticolus, Sorex bendirii, Sorex trowbridgii, and Scapanus orarius. Two rodents, Zapus trinotatus and Glaucomys sabrinus, also increased with theremoval of P. maniculatus, suggesting responses to release fromexploitative competition. Tamias townsendii had significantlyfewer captures and individuals on experimental grids. Removalof P. maniculatus left T. townsendii the most abundant smallmammal on experimental grids. It is possible that predators "switched" to T. townsendii thereby effectively depressingdensities of T. townsendii on the removal grids.