Data from: Techniques for estimating the size of low density gopher tortoise populations

The gopher tortoise (Gopherus polyphemus) is a candidate species for range-wide listing under the U.S. Endangered Species Act; hence reliable population estimates are required to aid management and guide policy needed to recover the species. Line transect distance sampling has been adopted as the preferred technique to estimate population size. However, when tortoise density is low, it can be challenging to obtain enough tortoise observations to reliably estimate the probability of detection, a vital component of the method. We suggest a modification to the method, based on counting usable tortoise burrows (which are more abundant than tortoises) and separately accounting for the proportion of burrows occupied by tortoises. The increased sample size generated by using burrows, rather than tortoises, can outweigh the additional uncertainty generated by needing to account for the proportion occupied. We demonstrate the method using surveys conducted within a 13,118 ha portion of the Gopher Tortoise Habitat Management Unit at Fort Gordon Army Installation, Georgia. An additional feature of these surveys was that we employed a systematic random design to obtain more precise estimates using a newly-developed systematic variance estimator. Individual transects consisted of pseudo-circuits, a design which greatly improved sampling efficiency on this large site. The burrow density was 0.091 ± 0.011 burrows/ha (CV = 12.6%, 95% CI = 0.071-0.116); 25% of burrows were occupied by a tortoise (CV = 14.4%), yielding a tortoise density of 0.023 ± 0.004 tortoise/ha (CV = 19.0%, 95% CI = 0.016-0.033) and corresponding population size of 297 (95% CI = 210-433). These techniques are applicable to other studies and species: surveying burrows or nests, rather than animals, can produce more reliable estimates when it leads to a significantly larger sample of detections and when the occupancy status can reliably be ascertained. Systematic line transect survey designs are practical to implement and, now, to analyze, and yield better precision.FGtort_NE and NW datasetsTwo distance sample projects - Download Program distance at http://distancesampling.org/. Tutorial in the readme for loading data and running analysis.FGtort_data.zi