Can Distance Sampling be Used to Estimate the Abundance of Mountain Ungulates? A Comparison of Distance Sampling and Double-Observer Survey Methods

Robust population surveys and monitoring of ungulates in mountainous areas are hindered by complex terrain and limited access to remote regions. Distance sampling, a widely used method for surveying ungulates in non-mountainous habitats, faces challenges in such environments due to violations of key assumptions. In this study, we examine the assumptions underlying distance sampling and explore how they can be addressed in mountainous terrain. We then applied distance sampling to estimate the abundance of bharal (Pseudois nayaur) on the Tibetan Plateau and compared the results with estimates from the double-observer survey method, which is commonly used for mountain ungulates. Both methods produced highly comparable estimates—2189 from distance sampling and 2038 from the double-observer survey—approximately twice as high as the total count (963). Distance sampling yielded a wider confidence interval (95% CI: 1207–3972) than the double-observer survey (95% CI: 1405–2670). We recommend using distance sampling when human resources are limited or when the closed population assumption is difficult to meet. Otherwise, the double-observer method is preferable due to its narrower confidence intervals and greater ability to detect population changes.