Towards good practice guidance in using camera-traps in ecology: influence of sampling design on validity of ecological inferences

The development of camera-traps has provided an opportunity to study ecological relationships and population dynamics of species that are rare, difficult to observe or capture. Their use has seen a major increase recently, particularly with the recent progress in methods adapted to species for which individuals cannot be identified. We took advantage of extensive camera-trap data sets from large spatiotemporal-scale studies of a diverse assemblage of avian and mammalian scavengers in subarctic/arctic tundra to determine sampling designs that minimize detection errors (false-negative) and to evaluate the influence of sampling design on estimation of site occupancy. Results showed that raw error rates in daily presence varied between 5 and 30% among species when using time-triggered cameras with a 5-min interval. Using movement-triggered cameras resulted in larger raw error rates, between 30 and 70%, as well as a lower number of daily presences detected. Increasing the time interval from 5 to 20min greatly increased the raw error rate in daily presence, but it had negligible impacts on estimates and precision of occupancy and detection probability. Occupancy estimates were mostly influenced by variation in the number of days included during the sampling period. For most species, a threshold of between 20 and 30 problem-free days (i.e. without camera-related technical problems) was required to stabilize occupancy and detection probability, as well as to maximize their precision. Based on the results, we discuss guidelines for establishing sampling designs according to the different ecological questions researchers might want to answer. To our knowledge, our study is the first to directly test the influence of sampling design in camera-trap studies, providing guidelines that are likely to be directly applicable to a large range of species and ecosystems.