Demography of the Spring Decline in Populations of the Vole, Microtus townsendii

Abstract

Population densities of small rodents typically decline in the spring at the start of the breeding season. Spring declines (18) of M. townsendii were monitored by mark-recapture methods on undisturbed grasslands near Vancouver, British Columbia [Canada]. The duration and size of spring declines was highly variable: sometimes there was virtually no decline, and at other times more than 90% of the population was lost in a spring decline that could last for 12 wk after breeding begins. Males usually disappeared more rapidly than females. The size of the spring decline was closely related to the rate of loss. In light-to-moderate spring declines, most of this loss was due to emigration; in severe declines most loss was due to death rather than movement. In males, the amount of rump wounding was highly correlated with the size of the spring decline. Spring declines in both sexes of M. townsendii are probably produced by spacing behavior. Whether differences in spacing behavior are sufficient to explain all variations in spring declines was discussed.