Survival and Predation of Weasels (Mustela erminea, Mustela frenata) in North America

Abstract

Predators may limit populations of small carnivores indirectly through exploitative competition for resources, and directly through interference competition, including predation. We evaluated direct effects of predation by large predators on small, specialist carnivores by estimating annual survival and cause-specific mortality. We used known fate models to estimate annual survival of 23 radio-collared short-tailed weasels (Mustela erminea) in western Oregon in 2012–2013. Further, we reviewed published and unpublished studies of radio-tracked weasels (Mustela spp.) in North America, to identify main predators of weasels and survival. If the study did not report survival but did report number of days tracked and causespecific mortalities, we extrapolated an annual survival index, based on daily survival rates (Mayfield index), to facilitate comparisons between studies. Estimated mean ± SD annual survival of short-tailed weasels in our study was 0.26 ± 0.08, and did not vary by season or sex of the weasel. Most (80%) predation mortalities, however, occurred in winter, and were attributed to predatory birds, suspected to be barred owls (Strix varia). For seven North American studies, including ours, we estimated the average annual survival index for M. erminea and Mustela frenata to be 0.37 ± 0.28. Across studies, most mortality was attributed to predation. Predators killed weasels > 2x more frequently in winter when most mortality (86%) was attributed to predatory birds. Mammalian predators, in contrast, killed weasels most frequently in summer (75%). Our results support the hypothesis that predation may play an important role limiting weasel populations in North America.