Effects of Intensive Gray Fox Control on Population Dynamics of Rodents and Sympatric Carnivores

The impact of continuously removing gray foxes (Urocyon cinereoargenteus) from upland Virginia poultry farms was studied over a 25-month period. Primary study areas were two sets offarm woodlots. Foxes were left undisturbed on one fann, and were intensively controlled for a 14-month period on the other. Demographic analysis ofrodent populations and enumeration of sympatric carnivores were penormed on both farms before, during, and after the period offox control. During fox control, weasels(Mustelafrenata) irrupted to significant (P <0.05) levels; numbers of skunks, opossums, and raccoons remained unchanged. Weasels disappeared upon the reestablishment of foxes during postcontrol. Rodent trapping yielded 631 small mammals, including 331 woodmice (Perornyscus leucopus), in 9,042 trapnights. Analysis of woodmouse population dynamics indicated that fox removal (and the resulting weasel irruption) did not affect overall density; however, all other parameters studied showed significant (P <0.05) alteration. On the Reduction Area turnover rate increased, mean longevity fell from 3.10 to 1.68 months, sex ratios shifted toward more females, age structure shifted toward more subadults, and fecundity increased through continuous rather than seasonal breeding. The enlarged weasel population apparently exerted more predatory stress upon woodmice than did the original fox population. This study shows that a sympatric predator can assume the predatory role ofa removed species. Implications are that predator removal studies may be invalid where sympatric predators are ignored, or where simple prey density is the only parameter used in assessing predator management.

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