The road less travelled: assessing variation in mammal detection probabilities with camera traps in a semi-arid biodiversity hotspot

Abstract

Camera traps are an increasingly popular tool for monitoring medium to large mammals, but the influence of camera trap placement on the detection probabilities of different species has seldom been investigated. In this study we explore the influence of roads on the detection probability of medium to large mammals in three vegetation types in the Little Karoo, an arid biodiversity hotspot. We placed cameras in nine 100 m-long transects, running perpendicular from roads within a conservation area. The camera traps were spaced at ~25 m intervals, and were active for an average of 88 days each. Detection probabilities relative to distance from roads showed extensive variation between species and habitat types. There was no clear relationship between distance from the road and the detection probability of most species and guilds, although carnivore detection probability declined significantly as distance from roads increased in all vegetation types. Our results suggest that there is considerable inter-specific variation in detection probability that is significantly influenced by camera trap location relative to roads. Therefore studies that seek to maximise the detection rates of particular species or guilds (e.g. carnivores) by placing cameras on prominent roads and trails are unlikely to provide reliable estimates of the relative abundance of the broader range of sympatric species; a trend observed elsewhere but hitherto untested in arid environments. We recommend that future studies employ a mixed design of cameras located on- and off-roads to provide better estimates of biodiversity in general and predators specifically.