Intraspecific Competition and Home Range Dynamics in the Field Vole Microtus agrestis

Abstract

General activity, spatial movement patterns, grouping behaviour and antagonistic relationships, i.e., contest competition, as well as food selection and food utilization, i.e., intraspecific scramble competition, in the field vole, Microtus agrestis (L.), are discussed mainly on the basis of the author's own studies on the experimental and field populations. In the treatment of home range characteristics, use is made of the concept of a fixed home range centre (HRC) and the density function of the observed points of capture, HR-sigma. Both breeding males and females seem to have relatively fixed home ranges, but only the male ranges seem to be defended territories. The movement patterns of the juvenile voles are easily described in terms of their mother's HR-sigma, while the subadults behave nomadically without any fixed home range pattern. General activity, HR size, and HR overlap (neighbour density) show seasonal changes, which (at least in the females) cannot be related to the overall population density, but seemingly run parallel to scramble competition for preferred food. These characteristics also show some differences between cycle phases. Maternal care of the young lasts a fortnight, after which the fate of the young is determined by their maturation patterns and the functional role they adopt in the population. The majority of early spring-born female young stay in or near the mother's home range, while the bulk of the males is forced to disperse by the aggressiveness of the dominant males. The high dispersal rate of the subadult cohorts at mid summer can also probably be ascribed to contest competition. A phenomenon typical of M. agrestis is the spontaneous dispersal of the reproductive females, frequently observed during the short spell between weaning and birth of the next litter. M. agrestis is highly selective in its food search. The preferred items, e.g., the leaves of clover and dandelion, also appear to be the most nutritive. High selection pressure on the favourite food items, and the simultaneous cessation of their growth, lead to hard scramble competition during the nutritional midsummer crisis, whose demographic effects are decreased litter size, abortions, increased nestling mortality, decreased growth and maturation rates, loss of weight in reproductive individuals, and even increased death rate of the reproductive adults. The adverse effects of scramble competition may be expected to continue during the winter following a peak year, when strong exploitation of winter-green food has been observed. It is concluded that scramble competition for food plays a more important role in the population dynamics of M. agrestis than is generally understood.