Factors affecting the population dynamics of the Skomer vole, Clethrionomys glareolus skomerensis

Abstract

This thesis describes a twelve year study of the Skomer vole (Clethrionomys glareolus skomerensis) which highlighted remarkable similarities between the voles' biology and that of other island rodents. Capture-recapture trapping provided demographic data. The population fluctuated annually with a trough at the beginning and a peak at the end of the breeding season. There was no regular multi-annual population cycle. Densities between 58/ha and 475/ha were recorded. Most young were produced by over-wintered animals; few animals bred in the year of birth. The numbers of adult males present during the breeding season varied little from year to year; the number of adult females was more variable. A removal experiment showed that interactions with adult females did not prevent the maturation of females in the year of birth. The dispersion of adult males and females was random. Intrinsic population regulation was by reduced litter size, a shortened breeding season and delayed maturation of both sexes. Extrinsic factors appeared to influence the timing of the breeding season and the maturation of animals in the year of birth. Factors affecting the over-winter survival of females and the survival of young of both sexes during the breeding season were thought to be important in regulating the population. The animals required dense cover and were closely associated with bracken (Pteridium aquilinum) and bluebells (Endymion non-scriptus). Dense grass under the bracken led to a reduced vole population. The voles' diet consisted mainly of bracken, bluebells and grass; they ate little animal material. They had an annual fat cycle with a winter peak. Predation may have exerted a local effect but probably did not regulate the population. There was no link between haemoparasitic infections and fluctuations of the vole population. Infections with pneumotropic viruses may have played a part in limiting the population of voles.