|SREL Reprint #0170|
The Limiting Effects of Natural Predation on Experimental Cotton Rat Populations
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 concaved 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.
SREL Reprint #0170
Schnell, J.H. 1968. The limiting effects of natural
predation on experimental cotton rat populations. Journal of
Wildlife Management 32:698-711.