Observations of Weasels in Second-Growth Douglas-Fir Forests in the Puget Trough, Washington

Abstract

Mustela frenata and M. erminea are common mammalian predators in Pacific Northwest forests, yet very little is known about their ecology in this region. Forty-five Mustela spp. were captured in >200,000 trap nights. Mustela erminea were most often captured in thinned stands with dense understory but little coarse woody debris; M. frenata were most often captured in unthinned stands with little understory development but high levels of coarse woody debris. Management history may influence Mustela spp. abundance and diversity. Eight M. frenata dens were located; 2 were in arboreal stick nests. Evidence from 4 yr of small mammal research in second-growth Pseudotsuga menzeisii stands, including recovery of 58 radio transmitters placed on Glaucomys sabrinus, suggests that Mustela spp. are important predators of G. sabrinus in the Puget Trough. Our knowledge of wildlife ecology in Pacific Northwest forests is rapidly growing. Substantial information has been accumulated on game animals, threatened species, management indicator species, and vertebrate communities (Szaro and others 1988, Ruggiero and others 1991). However, little information exists on species without special legal status or species that are not sampled effectively with techniques used for study of vertebrate communities, such as weasels (Mustela spp.). Two weasels, M. frenata (long-tailed weasel) and M. erminea (short-tailed weasel), are sympatric in the Pacific Northwest (Ingles 1965). Incidental records of road-killed M. frenata suggest they are relatively abundant in some areas of the Puget Trough (Buchanan 1987). Little is known, however, of the ecological roles played by M. frenata or M. erminea in this region. Weasels are predators and, sometimes, prey; they consume small to medium-sized mammals and birds and, in turn, can be preyed upon by larger predators (for example, owls, hawks, and canids; Fagerstone 1987). Mustelids prey on a variety of species (King 1983, 1989); the size of prey seems related to the size of the species of mustelid. Both M. frenata and M. erminea exhibit sexual dimorphism, with males much larger than females (King 1989). M. erminea are small (males are 70 to 206 g, females are 28 to 85 g; Fagerstone 1987), and can easily enter the burrows of small prey, thus M. erminea specialize in preying on small rodents, particularly microtines (Erlinge 1981, King 1989). The larger M. frenata (males are 160 to 450 g, females are 80 to 250 g; Fagerstone 1987) also prey on microtines but seem to be less specialized; they hunt above ground more often, and they prey on larger species (for example, squirrels and lagomorphs) than do M. erminea (Fagerstone 1987). Weasels have been shown to be arboreal, but most observations of arboreal activity describe escape behavior or pursuit of prey (Nams and Beare 1982). In areas of sympatry, M. erminea are reported to be more abundant than M. frenata (Fagerstone 1987). We have been live-trapping arboreal and forest-floor small mammals in the Puget Trough for 4 yr (Carey and others, in press b). We recorded incidental captures and occurrences of weasels during live-trapping, and during radio-telemetry studies of Glaucomys sabrinlls (northern flying squirrels). Our objectives here are to summarize our records of weasel captures and weasel predation in the Puget Trough, and speculate on the role M. frenata and M. erminea play in the region's forest ecosystems.