The Limiting Effects of Natural Predation on Experimental Cotton Rat Populations

Abstract

Survival curves for 2to 6-month periods were plotted for 17 different populations of nonbreeding cotton rats (Sigmodon hispidus) introduced into isolated or enclosed areas of natural habitat. Eight populations were introduced onto predator-free islands or into a 1-acre, predator-proofed enclosure of old field habitat, and nine populations were released into 1-acre enclosures freely accessible to bird and mammal predators. Radioactive pins (cobalt-60 or zinc-65) inserted under the skin aided in determining the fate of cotton rats which failed to appear in live traps. Survival curves were linear or convex for introduced populations protected from predation and strongly concave for those open to natural predators. Abundant evidence for kills by hawks, foxes, and bobcats was found in the latter cases. A density of about 15/acre, the mean point of inflection in the survival curves, was considered to be a predator-limited carrying capacity since mortality was high above, and low below, this density when predators were active. The experimental study has special relevance to the fall and winter cotton rat populations in the southeastern United States. It is concluded that when diverse and highly mobile predator populations are present they are more important than food, social interaction, or weather in regulating cotton rat density. The role played by predation in the regulation of mammal populations is usually difficult to determine because survival of individuals and the exact causes of mortality are hard to ascertain in the field. Isolated or enclosed areas of natural habitat can simplify the evaluation of predation effects since emigration and immigration can be controlled and the survival of marked individuals can be followed with reasonable precision. The object of this investigation was to determine the effect of natural predation on the mortality of nonreproducing populations of the cotton rat confined to areas of natural habitat. Survival of groups of animals introduced to predatorfree islands and to a large predator-proof enclosure was compared with survival of similar groups occupying enclosures accessible to predators. In all, survival curves for 2to 6-month periods were plotted for 17 different populations. I am especially indebted to Drs. Eugene 1 Contribution from the Laboratory of Ecology, AEC Savannah River Plant supported by AEC Contract No. AT(38-1)-310. 2Present address: Tall Timbers Research Station, Tallahassee, Florida. 698 P. Odum and Frank B. Golley for help in organizing the manuscript; Drs. Richard G. Wiegert and Michael H. Smith also read the manuscript and gave helpful suggestions. John H. Harper and Dr. George F. O'Neill of AEC Savannah River Plant assisted with radionuclide labeling and John B. Gentry, Jim C. Mayenschein, Michael R. Pelton, and Dr. Howard Orr assisted in the field. Dr. Orr also permitted me to use his unpublished data to construct curve No. 4 (Fig. 8).