Mark-Recapture Density Estimation for Animals with Large Home Ranges

Mark-recapture studies of animals with large home ranges typically involve many individuals that routinely traverse study area boundaries. Radio-collars can be used to determine the number of marked animals available for capture, but if the recapture period is long, some animals may be present only part-time; treating these as full-time occupants would overestimate density. A density estimator proposed in this paper weights each animal by the proportion of time it spends on the study area during the recapture period. Summing these animal-equivalents yields an estimate of the average number of marked animals simultaneously residing in the designated area. Likewise, animal- equivalents can be calculated for unmarked animals that are captured and radio-collared. These individuals may spend less time in the trapping area than previously marked animals, either because they live near the edge or are merely passing through. Probabilities of capture, thus, may differ between marked and unmarked animals, as well as among marked animals, depending on the size and location of their home range, and consequent differing lengths of exposure to traps. Defining each parameter in the Petersen equation in terms of summed animal-equivalents compensates for these differences. The need for this modification increases with increasing home range size relative to the size of the recapture area. An application of this technique with black bears (Ursus americanus) is presented.