Double-observer approach with camera traps: Towards an unbiased density estimation of unmarked animal populations

Abstract

Camera traps are a powerful tool for wildlife surveys. However, camera traps may not always detect animals passing in front. This constraint may create a substantial bias in estimating critical parameters such as the density of unmarked populations. We proposed the double-observer approach with camera traps to counter the constraint, which involves setting up a paired camera trap at a station and correcting imperfect detection with a hierarchal capture-recapture model for stratified populations. We performed simulations to evaluate this approachs reliability and determine how to obtain desirable data for this approach. We then applied it to 12 mammals in Japan and Cameroon. The results showed that the approach could correct imperfect detection as long as paired camera traps detect animals nearly independently (Correlation coefficient < 0.2). Camera traps should be installed to monitor a predefined small focal area from different directions to satisfy this requirement. The field surveys showed that camera trap might miss animals by 3 %-40%, suggesting that current density estimation models relying on perfect detection may underestimate animal density by the same order of magnitude. We hope that our approach will be incorporated into existing density estimation models to improve their accuracy.