Evaluating relative abundance indices for terrestrial herbivores from large-scale camera trap surveys

The ability to directly monitor animal populations across time and space is a key element of wildlife conservation and management, but logistically difficult to achieve. Photographic capture rates from camera trap surveys can provide relative abundance indices (RAIs) for a wide variety of medium- to large-bodied wildlife species. RAIs are less complex than other estimation methods and are commonly used when true abundance is difficult or costly to measure. However, this method is controversial as it does not account for potential bias arising from imperfect detection. Here, we evaluate the reliability and precision of RAI estimates drawn from a large-scale camera trap survey for ten African herbivore species by comparing them against preexisting aerial survey data. RAIs correlated strongly with independent estimates, particularly when indices were derived from counts of all photographed animals. RAIs were most reliable for species that were nonmigratory, occurred in open habitats and had high rates of daily movement. Increasing survey coverage and duration both had strong but comparable effects on improving RAI precision. Our results suggest that RAIs from camera traps hold substantial promise as a tool for monitoring herbivore relative abundances, and we provide guidelines on the utility of this approach for ecological inference.