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The Limiting Effects of Natural Predation on Experimental Cotton Rat Populations

Jay H. Schnell
The Journal of Wildlife Management
Vol. 32, No. 4 (Oct., 1968), pp. 698-711
Published by: Wiley on behalf of the Wildlife Society
DOI: 10.2307/3799543
Stable URL: http://www.jstor.org/stable/3799543
Page Count: 14
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The Limiting Effects of Natural Predation on Experimental Cotton Rat Populations
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Abstract

Survival curves for 2- to 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.

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