Do occupancy or detection rates from camera traps reflect deer density?

Camera trapping is a powerful tool for studying mammal populations over large spatial scales. Density estimation using camera-trap data is a commonly desired outcome, but most approaches only work for species that can be individually recognized, and researchers studying most mammals are typically constrained to measures of site occupancy or detection rate. These 2 metrics are often used as measures of relative abundance and presumed to be related directly to animal density. To test this relationship, we estimated density, occupancy, and detection rate of male white-tailed deer (Odocoileus virginianus) using camera-trap data collected from 1,199 cameras across 20 study sites. Detection rate and density exhibited stronger positive linear correlation (r² = 0.80) than occupancy and density (r² = 0.27). When hunted and unhunted paired areas were compared, detection rate and density showed the same trend between paired sites 62.5% of the time compared to 87.5% for occupancy and density. In particular, agreement between estimates was lowest for pairs of sites that had the largest differences in surrounding housing density. Although it is clear occupancy and detection rate contain some information about density, models suggested different ecological relationships associated with the metrics. Using occupancy or detection rate as proxies for density may be particularly problematic when comparing between areas where animals might to move or behave differently, such as urban-wild interfaces. In such cases, alternate methods of density approximation are recommended.