Population Regulation in the Wood Mouse, Apodemus sylvaticus. I. Density Dependence in the Annual Cycle of Abundance

Abstract

SUMMARY (1) Density dependence in the annual dynamics of Apodemus sylvaticus in woodland was investigated using live trap data for sixty-three complete or partially complete 14month periods from published sources. (2) Dynamics were remarkably consistent with numbers decreasing in spring, remaining relatively stable during early summer, and increasing in late summer and autumn, in 87%, 71% and 88% of the available data, respectively. Stability in population size in summer was produced by low densities rather than depressed population growth rates. (3) Instantaneous rate of population change per week was negatively correlated with initial population size during periods when numbers normally increased. Density dependence accounted for up to 61% of variation in instantaneous rate of population change. Intrinsic mechanisms contributing to this effect are likely to include variation in rates of reproduction and dispersal with density of overwintered males and females. (4) There was no evidence of density dependence in the instantaneous rate of population change during periods when numbers normally declined. Following greater peak abundance, however, the rate of decline over winter was less severe. This effect is attributable to the positive influence of increased food supply on abundance in autumn and early winter and the survival of mice overwinter. (5) There was no evidence of density dependence in the chronology of the termination of the spring decline or the onset of autumn increase. The end of the autumn increase, however, was negatively related to initial autumn density, i.e. peak abundance was later following lower densities in late summer. (6) Density-dependent effects during the increase phase of the annual cycle of A. sylvaticus, are sufficient to regulate population size by promoting population recovery from very low summer densities and limiting peak abundance in early winter. In years of