Predation on Microtus townsendii populations: impact and vulnerability.

The object of this study was to assess the impact of avian and mammalian predators on M. townsendii populations, especially during decline periods. In addition, Errington's hypothesis of differential susceptibility of certain classes of animals to predation was examined. The vole population went through a severe decline in the winter and spring of 1972-1973 and again in the winter and spring of 1973-1974. Intensive searches were made for all pellets and scats from February 1973 to May 1974. Of the tagged animals disappearing, less than 8% in the decline of 1972-1973 and less than 20% in the decline of 1973-1974 could be accounted for by predation. I conclude that predation is not necessary to initiate or maintain a decline. In the tagged animals, neither sex nor any weight class showed any consistent vulnerability to predation when compared with the rest of the tagged population. In the total sample of voles eaten by avian predators, the body weights were consistently lower than those in the trapped population. In the other life-history characteristics examined, voles were eaten at random. I conclude that no animal was more vulnerable to predation than another in the tagged population, but in the total population younger animals may have been.