Demographic Mechanisms in the Fluctuating Populations of the Field Vole Microtus agrestis

Abstract

Demographic mechanisms in the fluctuating populations of the Central-Scandinavian field vole, Microtus agrestis (L.), are described predominantly on the basis of the author's own long-term studies in Southern Finland. The populations monitored and used as examples have shown abundance peaks at 3 to 4 year intervals. The survival schedules, growth patterns, reproduction rates, population structures and habitat dynamics are discussed in relation both to the season and the cyclic phases. The average life span of a field vole varies greatly with the time of birth, sex and cyclic phase, the overwintered peak-year cohort having the highest value, the early decrease-year cohorts the lowest. Nestling mortality is relatively high, ranging from about 20% in spring to more than 50% during the nutritional crisis. Individual growth pattern and attainment of sexual maturity are closely intercon- nected, especially in females. The fecundity rates of the females vary with the season and the cyclic phase. Post-partum oestrus and a pregnancy rate approaching 100% are the rule during the most intensive part of the breeding season in the early summer months. During the midsummer crisis both maturation and pregnancy rates decline. Towards the autumn they usually rise again, especially during the increase phase of the vole cycle. Litter size varies according to the season and the age (size) of the female, but is one of the least variable parameters in the course of the microtine cycle. Age-specific and cyclic differences in the reproductive performance are summarized in the form of mx and lxmx functions. The prevailing population structures reflect these fecundity schedules and the age and sex specific survival and dispersal schedules. The sex ratios at birth are close to the theoretical 1:1 ratio, but they become heavily weighted to the female side in the course of the breeding season, especially at peak abundance. This change in the functional sex ratio is predomi- nantly due to different dispersal of sexes . The chief production habitats of the field vole are permanent grassland patches with a high proportion of the preferred dicotyledon- ous food plants. The interactions between these and the other habitat types are discussed on the basis of two concrete examples, one representing a typical mixed agricultural habitat complex, the other a forestry-dominated taiga landscape.