A Population Study of the Gray Fox

Abstract

An attempt was made to analyze some of the component forces of some gray fox populations in northern Florida. The size of the populations studied was determined by the use of an age-ratio-reduction method. In this method the size of the population was indirectly derived by determining the magnitude of the effect on the age ratio of sterilizing a known number of females. A knowledge of the degree of reduction of the age ratio caused by sterilizing these females allowed the calculation of the entire female population. Application of the sex ratio permitted an estimation of the size of the entire fox population. It was possible to estimate the size of these same populations by several variations of the recapture method (Lincoln Index and Schnabel method) and the removal method. The recapture methods usually gave higher estimates than the age-ratio-reduction method. The estimates of the age-ratio-reduction method indicated a density of slightly less than 3 to 4 foxes per square mile. From recapture distances, a probable approximate diameter of two miles was estimated for the size of the home range of the gray fox in this region. Groups of foxes of family composition were captured in areas isolated from captures of other foxes and were termed family aggregations. Twenty-one family aggregations were found. The diameters of these family aggregations were in close agreement with the recapture data concerning the estimate of the size of the home range. This study explores the possibility of estimating the size of a gray fox (Urocyon cinereoargenteus floridanus) population by altering the breeding capacity of a known number of females and determining the effect on the age ratio of the population. The size of the population is deduced from the magnitude of this effect. The study also includes investigations of reproduction, mortality, and movement. The census method is similar to the methods used by Allen (1942), Kelker (1940;1945), and Riordan (1948) in estimating the sizes of populations of game animals that were selectively hunted according to sex. Their method was to compare preand postopen season sex ratios after which they computed the size of the populations by the use of knowledge of the number of male and/or female animals killed. Kelker's formulae estimated population sizes for each sex and fawns separately. The age ratio was used to determine the number of fawns, but the observed change takes place in the adults rather than in the young as in the method presented here. The recapture method (Jackson, 1939) derives an estimate from the proportion of marked individuals in the population. In contrast to the recapture method, the age-ratio-reduction method presented here does not require the recapture of a large number of the pre1 A portion of the author's doctoral dissertation presented to the Division of Vertebrate Ecology, Johns Hopkins University, School of Hygiene and Public Health.