Weasel Predation on a Cyclic Population of the Montane Vole (Microtus montanus) in California

(1) In the Sierra Nevada, California deep snow covers the ground from about late November to mid-April and voles (Microtus montanus) build subnivean, non-breeding nests. (2) Ermine (Mustela erminea) and long-tailed weasels (M. frenata) prey on voles under the snow and use vole nests as dens. Food habits of these carnivores and their predation on the voles were measured by examining vole nests in spring over four years. (3) Voles showed characteristics of a cyclic population; they declined in density between autumn 1965 and spring 1967, increased to peak density in mid-summer 1968, crashed in winter 1968-69 and continued to decline through summer 1969. (4) Ermine were common in summer 1966, decreased in winter 1966-67 and were scarce in summer 1967 and winter 1967-68. They increased in summer 1968 and were common in winter 1968-69. In winter 1967-68 only one ermine was present on 14 ha of meadows but the following winter four were present on the same area. Ermine territories were restricted largely to the meadows. Long-tailed weasels also varied in numbers but they could not be counted. Long-tailed weasels hunted over large areas of both meadow and forest. (5) Ermine and long-tailed weasels in winter preyed almost entirely on voles. Food habits of ermine did not vary with changes in density of voles, but evidence for long-tailed weasels was inconclusive. (6) The percentage of winter nests of voles occupied by ermine in different years varied from 4.9 to 27.7% and by long-tailed weasels from 1.5 to 3.5%. The number of voles eaten at each den (occupied nest) also varied, being highest when the percentage of nests occupied by weasels was highest. These two variables, combined as the number of voles eaten per 100 nests examined, provided an estimate of the minimum percentage of the vole population eaten by weasels. (7) Ermine ate 21.2% of the population in winter 1965-66, 54.3% in 1966-67, 5.9% in 1967-68 and 28.3% in 1968-69. Predation was heaviest when voles were at lowest density and very light as voles increased again to peak density. Predation by long-tailed weasels was always light reaching its highest level after the vole population peaked, when 11.4% of the voles were eaten. (8) In winter 1966-67 all losses of voles from the population between autumn and spring could be accounted for by predation, almost entirely by ermine, but in subsequent winters as voles increased and were at higher densities fewer of the losses were accounted for. (9) These results support Pearson's hypothesis that predation during and following the crash is responsible for the timing and amplitude of the microtine cycle including that of lemmings. Small weasels (M. nivalis or M. erminea) are present throughout almost the entire range of cyclic microtines and may be the main carnivores responsible for the cyclic changes in density of microtine populations.