Direct counts could underestimate the size of mountain ungulate populations

Mountain environments pose challenges for accurately assessing the population size of ungulates, which may hamper confidence in using abundance estimates for decision-making. Thus, evaluations of the accuracy of methods using known-size populations are valuable. Here, we tested the accuracy of direct counts and distance sampling to assess the size of a fenced population of Iberian ibex (Capra pyrenaica) of known size. We evaluated the influence of observer expertise (beginners and experts) on the detection error of female and male ibexes and whether training improved accuracy. The ibex population comprised 18 individuals (ten males and eight females) living in a 17-ha enclosure in northeastern Spain. After 27 surveys, experts detected 16% more ibexes than beginners. Male ibexes were ~ 13% easier to detect than females, and experts were more accurate than beginners in sexing individuals. Additionally, the detection error in absolute counts was similar among beginners but different among experts (> 10%). Despite reducing detection error over subsequent surveys, under-detection was > 50% in all events (> 85% and > 67% for beginners and experts, respectively). Our data indicate a systematic underestimation of direct counts of our mountain ungulate populations and the contribution of expertise to improving accuracy. Our results show that managers assessing mountain ungulate populations for management purposes should consider alternative methods.