Does Predation Risk Constrain Maturation in Cyclic Vole Populations?

Abstract

Animals may delay reproduction under temporarily high predation risk. If temporarily high predation risk has a suppressive effect on prey reproduction on a large scale, there should be a negative association between the predation risk level and the proportion of breeding individuals in prey populations. We used long-term trapping data on two vole species and data on predator densities to find out whether there was a relationship between the predation risk level and the proportion of breeding individuals in a small mammal assemblage with cyclic fluctuations. In spring and, in the field vole (Microtus agrestis), in autumn, current predation risk from small mustelids could explain observed variation in the proportion of mature voles better than their own density, current or previous. The association between the proportion of breeding field voles and the current predation risk from small mustelids was negative in autumn, but not in summer when the current reproductive value is high relative to the residual value. For bank voles (Clethrionomys glareolus), which have a lower predation risk than field voles, there was a negative relationship between predation risk from mustelids and proportion of breeding individuals in early summer, but not in late summer or autumn. In late summer, the proportion of breeding year-born bank voles was negatively related to the vole density in spring. Our results indicate that the indirect effects of predators on vole populations may be of great importance on a large scale, and that these effects should be taken into consideration in studies on factors regulating vole populations