BODY-WEIGHT AND POPULATION-DYNAMICS - CYCLIC DEMOGRAPHY IN A NONCYCLIC POPULATION OF THE FIELD VOLE (MICROTUS-AGRESTIS)

Differences in body weight. individual growth rate, survival, and recruitment of young between years were analysed in a noncyclic population of field voles (Microtus agrestis) to determine the respects in which this population differed from cyclic ones. Large differences in body weight, survival of both sexes, and recruitment of young existed between years. Over 4 years, body weight was positively related to population growth (measured as the change in density over the reproductive season). Also, the recruitment of young was higher in years when female voles were heavier and survived longer. Thus, the occurrence of high body weights in years when density is increasing or high is not a unique feature of populations showing cyclic density changes. Our findings indicate that survival is the primary factor connecting body weight to population growth. Neither genotypic changes nor variable predation pressure were likely to account for the between-years differences in body weight, survival, and recruitment. Instead food quality and (or) quantity, possibly affected by density, seem to be the primary extrinsic factors influencing individual quality, thereby influencing population growth. We propose that this population of field voles does in fact carry the potential for cyclic density changes. although predation prevents the manifestation of such a pattern.