EFFECTS OF SUPPLEMENTAL FOOD ON POPULATION-DYNAMICS OF COTTON RATS, SIGMODON HISPIDUS

Variation in resource abundance affects population dynamics by alterin g demographic processes and interactions among individuals in the popu lation. For small mammals, food is likely to be a critical resource. P opulation densities should vary directly with food abundance, but the underlying demographic changes are more difficult to predict. We exper imentally increased food available to populations of hispid cotton rat s, Sigmodon hispidus, to examine how rates of recruitment and disappea rance varied with food abundance. We expected supplemental food to inc rease winter survival and to increase recruitment, perhaps advancing t he date of first reproduction in the spring. We also thought larger be haviorally dominant animals might dominate the point sources of supple mental food, altering age or size ratios and producing an excess of tr ansient animals. Supplemental food increased population densities but did not dampen pronounced seasonal fluctuations. Supplemented populati ons contained proportionately more juveniles and small adults than did controls; social behavior may have limited increases in numbers of la rge 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 repr oductive females decreased. Reproductive females and nonreproductive a nimals of both sexes were less likely to be transients than were repro ductive males. Transients constituted higher proportions of control po pulations 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 for aging strategies. Our data support a scenario of territorial females a nd 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 weath er) to the minimum required for survival.