Effects of Supplemental Food on Population Dynamics of Cotton Rats, Sigmodon Hispidus

Abstract

Variation in resource abundance affects population dynamics by altering demographic processes and interactions among individuals in the population. For small mammals, food is likely to be a critical resource. Population densities should vary directly with food abundance, but the underlying demographic changes are more difficult to predict. We experimentally increased food available to populations of hispid cotton rats, Sigmodon hispidus, to examine how rates of recruitment and disappearance varied with food abundance. We expected supplemental food to increase winter survival and to increase recruitment, perhaps advancing the date of first reproduction in the spring. We also thought larger, behaviorally dominant animals might dominate the point sources of supplemental food, altering age or size ratios and producing an excess of transient animals. Supplemental food increased population densities but did not dampen pronounced seasonal fluctuations. Supplemented populations contained proportionately more juveniles and small adults than did controls; social behavior may have limited increases in numbers of large adults. Survival rates did not change; density increases were due to increased reproduction and immigration. In supplemented populations, reproductive effort by females increased, but the proportion of reproductive females decreased. Reproductive females and nonreproductive animals of both sexes were less likely to be transients than were reproductive males. Transients constituted higher proportions of control populations because of successful settlement, primarily by juveniles and small adults, into supplemented areas. The strongly seasonal climatic conditions under which northern S. hispidus populations exist produce contrasting selective pressures which might favor season-specific foraging strategies. Our data support a scenario of territorial females and wandering males in the reproductive season, with females maximizing resource acquisition for production of offspring. During winter, both sexes may restrict foraging time (and exposure to predators and weather) to the minimum required for survival.