Random encounter model to estimate density of mountain-dwelling ungulate

Abstract

Methods for estimating population densities of unmarked species using camera traps are still under development. One such method is called ‘random encounter model (REM)’ and, to our knowledge, has never been used to estimate densities of mountain-dwelling ungulates. In this study, we tested the REM method to estimate the density of Balkan chamois (Rupicapra r. balcanica) in a Mediterranean habitat, Mt. Biokovo. To meet the assumptions of REM, we systematically placed 25 camera traps throughout the known range of the population (approximately 65 km2) at the intersections of 2-km grid cells. Prior to data collection, population density was estimated by visual counts on sample plots in August 2020. Cameras were operational between July 2020 and September 2020 and active throughout the 24-h period. All parameters for REM (i.e. average movement speed, angle and radius) were estimated using exclusively camera trap data. We obtained 279 independent events of chamois from 2503 camera trap days. The density estimate obtained by REM resulted to be 20.65 ± 5.27 ind. km−2, slightly higher than the reference value obtained by visual counts: 17.33 ± 0.98 ind. km−2. Other parameters required to calculate density were speed (1.62 km·day−1 ± 0.21), detection radius (5.56 m ± 0.21) and detection angle (1.16 + 0.05 radians). Therefore, REM has shown comparable results to visual counts and may have potential for estimating density of mountain ungulates, especially in rugged and inaccessible mountainous areas with low detectability where other approaches are inadequate or impossible.