Long-term Population Trends in White-Footed Mice and the Impact of Supplemental Food and Shelter

Abstract

Central to most models of population regulation is the idea that the degree of intraspecific competition is in some way proportional to the availability of limiting resources. Although laboratory research has demonstrated a number of proximate mechanisms by which behavior might affect population growth, little is known about the resources that are actually limiting population density in the field or about how animals might compete for them. Long-term field studies reveal that the white-footed mouse, Peromyscus leucopus , exhibits two types of pronounced changes in density: intra-annual (seasonal) and inter-annual. The former seem to be due ultimately to climate and the lack of winter breeding, but populations often decline sharply in late summer in the midst of plentiful food; fall recruitment of new animals is density-dependent and usually poor. Differences in peak densities from one year to the next as high as 13-fold have been recorded. Weather, shelter, and food are possible ultimate limiting resources. Food has received themost attention, but supplemental feeding experiments have yielded mixed results. A review of the social behavior of this polygnous species suggests that each sex is limited by different combinations of factors. Females may defend a food source and nest sites; males may search actively for females at low adult densities, covering large areas, and defend access to females at high adult densities. Additional long-term field studies are needed, both to provide direction to laboratory research on proximate mechanisms and to provide the data base for understanding the role of weather, food, and shelter as ultimate limiting factors. Enough short-term studies have been published to permit comparisons across habitats and through time which will give a better perspective on climate and habitat variables. Field experiments are necessary to demonstrate the operation of proximate behavioral, physiological, and genetic mechanisms of population regulation in natural populations.