WHO BEARS THE COSTS OF INTERSPECIFIC COMPETITION IN AN AGE-STRUCTURED POPULATION?

Abstract

Social and density-dependent life history processes may differ according to age and the reproductive history of individuals. Arvicoline rodents have a typical, season-dependent, bimodal, age distribution of breeding individuals within a population. This distribution may influence population fluctuations. In this study, we measured effects of interspecific competition from field voles (Microtus agrestis) on various fitness components of female bank voles (Clethrionomys glareolus) in an age-structured breeding population in large (0.25 ha) outdoor enclosures. We monitored survival, reproduction, and space use of experimental bank vole populations with females from two different age groups. Within the enclosures we manipulated the presence and absence of field voles. When coexisting with field voles, the year-born bank vole females had lower survival rates than the over-wintered ones did. Home ranges of bank voles were smaller in the presence of field voles than in their absence. Characteristics of litters of breeding females were not affected by the competition treatment. Reproduction of the youngest of the year-born females stopped earlier in the season than for over-wintered females although both age groups had reproduced prior to the trial. We did not observe indications of senescence, such as lower reproduction or survival of the over-wintered females in late summer. An earlier cessation of breeding in the younger females suggests an age-specific trade-off of summer reproduction and winter survival for parous females. Inter- and intraspecific competition seemed to work additively on the year-born females through direct interference. By selectively reducing survival of younger breeders, interspecific competition could influence age distribution and population growth of breeders toward the end of the breeding season. Because old, over-wintered females are not surviving to the next nonbreeding season, the lower survival of young breeders through interspecific competition might impact greatly the structure of the breeding population at the start of the next breeding season, thus altering population growth patterns.