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.

哥法地鼠龟（Gopherus polyphemus）是美国《濒危物种法案》（U.S. Endangered Species Act）下拟开展全域保护名录列入程序的候选物种，因此为辅助该物种的恢复管理与政策制定，亟需获得可靠的种群数量估算结果。线样带距离抽样法（Line transect distance sampling）已被选为估算种群规模的首选技术。然而，当龟类密度较低时，难以获取足够的龟类观测样本以可靠估算检测概率——而检测概率正是该方法的核心参数之一。本研究提出对该方法的改进方案：以可观测到的龟类洞穴（数量远多于活龟）作为计数对象，并单独估算洞穴被龟类占用的比例。相较于直接计数活龟，通过计数洞穴可大幅提升样本量，其样本增益足以覆盖因需估算占用比例而引入的额外不确定性。本研究以佐治亚州戈登堡陆军基地哥法地鼠龟栖息地管理单元内13118公顷的调查区域为例，验证了该改进方法的有效性。本次调查的另一特色在于，采用系统随机抽样设计，并结合新开发的系统方差估计量，以获得更为精准的估算结果。单条样带采用伪回路（pseudo-circuits）设计，该方案大幅提升了大型调查区域内的抽样效率。调查结果显示，洞穴密度为0.091±0.011 个/公顷（变异系数（coefficient of variation, CV）=12.6%，95%置信区间（confidence interval, CI）=0.071~0.116）；其中25%的洞穴被龟类占用（CV=14.4%），由此推算出龟类密度为0.023±0.004 只/公顷（CV=19.0%，95%CI=0.016~0.033），对应种群总规模为297只（95%CI=210~433）。该改进方法可推广至其他研究与物种：当通过计数洞穴或巢穴而非活体动物能显著提升检测样本量，且可可靠判定其占用/筑巢状态时，该方案能够获得更为可靠的估算结果。系统线样带调查设计不仅易于实施，且如今可依托完善的分析方法获得更高的估算精度。