Placement matters: implications of trail- versus random-based camera-trap deployment for monitoring mammal communities

Optimizing protocols to assess and monitor mammal communities is essential to meet the current biodiversity targets of halting species loss. Camera-traps are the most effective tool for multispecies monitoring, yet their deployment strategy is debated, with two main strategies adopted: trail- and random-based camera deployment. To date, few studies have compared these two strategies and reached contrasting recommendations. Here, by simultaneously deploying 60 camera-traps for each placement strategy in a National Park in central Italy, we aimed to assess differences in species richness and composition, photographic rate, detection/occupancy probabilities, also in responses to environmental and anthropogenic variables, and temporal activity. Site species richness was greater on than off-trails, with elusive carnivores mainly detected on trails. Community composition was different, with a smaller proportion of ungulates on trails, and lower detections of carnivores off-trails. Photographic rate, detection, and occupancy probabilities were higher on trails for almost all mammals. Occupancy responses to environmental variables did not match, possibly due to the different behavioral strategy adopted by mammals (trails for movement, off-trails for resting and foraging). Thus, a mixed approach with cameras located both on- and off-trails is recommended when studying habitat use. We also found a consistent negative response of occupancy and site-use intensity to human frequentation, with mammals avoiding both highly frequented trails and adjacent random sites. Temporal activity curves were similar between designs, suggesting that the choice of the sampling strategy would not bias the inference. However, nocturnal behavior was higher on trails for some species, indicating varying degrees of temporal avoidance of humans. With faster data accumulation, easier accessibility of sampling sites, and the ability to record human activity, on-trail cameras are more efficient than off-trail cameras for monitoring mammal communities.