Interactions between the Field Vole Microtus agrestis and Its Microtine Competitors in Central-Scandinavian Populations

Abstract

The main competitors of Microtus agrestis (L.) in Central-Scandinavian populations are Microtus arvalis (Pall.), Clethrionomys glareolus (Schreb.) and Arvicola terrestris (L.); of these three species M. arvalis is restricted to Southern Finland. The habitat preferences of M. agrestis and its main competitors show some overlap. Interactions of M. agrestis on the one hand andM. arvalis orA. terrestris on the other are most likely to occur in permanent grassland habitats on mineral soil, i.e., favourite production habitats of M. agrestis. The ecological ranges of M. agrestis and C. glareolus are most likely to overlap outside the breeding season, e.g., in forest regeneration lots. The food niches of M. agrestis are rather similar to those of its sympatric competitors, M. arvalis and A. terrestris, during the reproductive season. Outside the breeding season its food niches overlap with those of M. arvalis and C. glareolus, while A. terrestris has clearly different food preferences. As to direct behavioural interactions, the competitors represent a clear gradient in regard to the severity of the encounters with M. agrestis. In the relationship between M. agrestis and C. glareolus, the former is considered physically dominant, but competition is mainly manifested in exploitation of common food resources. Between M. agrestis and M. arvalis strict territorial defence occurs during the breeding season but aggressiveness is much less pronounced in the non-breeding subadult-dominated populations. A. terrestris is physically dominant over M. agrestis, and sympatric coexistence generally leads to exclusion of M. agrestis from theArvicola-dominated area. Owing to exploitation of different strata, coexistence of these two species is possible during the winter months. The different types of species interactions are illustrated by a series of graphical models and the leaky vessel model by Grant is recommended as a starting-point for the quantification of the effects of interspecific competition on the population dynamics of M. agrestis.