A comment on Horcajada-Sanchez and Barja (2015): a cautionary tale about left truncation and density gradients in distance sampling

摘要

Distance sampling is one of the most widely used methods to estimate density and abundance of wild populations. Horcajada-Sanchez and Barja (2015) presented a critical assessment of two field methods to implement distance sampling surveys of roe dear, "surveys on foot in the evening and nocturnal surveys by car". In particular, they concluded that line transect distance sampling surveys conducted from a car at night would outperform distance sampling on foot by day. However, they noted that for nocturnal driven surveys, left truncation was required due to "an imbalance in the detection function at zero distance". The recommended method produced density estimates more than double those of foot surveys. While I agree that such evaluations of different techniques are desirable and lead to valuable insights, I believe that the justification for left truncation of the nocturnal driven transect data is invalid and its use produces a potentially biased population estimate.