The Relationship Between Burrow Abundance and Area as a Predictor of Gopher Tortoise Population Size

Abstract

Because gopher tortoises (Gopherus polyphemus) occupy high-value lands in the southeastern United States, are declining throughout their range, and stir public passion when they are entombed through currently legal take, refined conservation measures for this species are needed. Of particular interest is a determination of how many individuals constitute a population and what sized areas are required by such populations so that animals currently in harm's way might be moved to designated reserve areas. We use surveys of burrows of gopher tortoises from a large sample of areas of differing sizes to test for a relaxation of the linear relationship between area and number of burrows expected at the edge of a population of gopher tortoises. For measures of burrow abundance, some of our data are from sites on which estimates were based on transect samples; on other sites, complete counts were made. The relationship between area and abundance was elevated for transect samples compared to similar data from complete counts, indicating that transect data yield inflated counts when extrapolated over large areas. For complete counts, our data indicate a threshold point at which a linear relationship between area and abundance ends and a loss of this relationship for larger areas begins. This threshold point suggests that, on average, populations of gopher tortoises consist of 444 burrows, cover 755 ha, and contain 240 tortoises. These figures describe features of tortoises inhabiting a variety of lands that differ in current management goals and past land-use history. Our results suggest that many current reserve areas for gopher tortoises are likely to be too small. Our results also provide an example of how key conservation variables can be generated for long-lived species.