Evaluating predator control using two non-invasive population metrics: A camera trap activity index and density estimation from scat genotyping

Context: Invasive mammalian predators are a threat to biodiversity and agriculture globally, yet management outcomes for lethal predator control remain difficult to monitor and evaluate. Understanding whether changes in activity indices correspond to true changes in population density will help inform effective monitoring and management programs. Aims: The aim of this study was to evaluate the effect of poison baiting on invasive red fox (Vulpes vulpes) populations using two alternative population metrics: fox activity from camera trap surveys and density estimation from scat genetic analysis. Methods: We conducted before-after control-impact studies in two regions of semi-arid Australia (Wimmera and Mallee) by monitoring paired non-treatment and treatment sites during unbaited and baited periods. We estimated the effects of poison baiting on: (1) a monthly fox activity index, derived from an array of 10 off-road camera traps per site; and (2) fox density. To estimate density, we collected fox scats along 14-km transects, identified individuals using polymorphic microsatellite DNA markers and fitted spatially explicit capture-recapture models. Key results: Fox activity remained consistently low at all sites except the Mallee non-treatment. The top-ranked models of fox activity and density contained an interaction between treatment and period, with an interactive and additive effect of region, respectively. However, there was little evidence that baiting reduced fox activity or density. In the unbaited period, fox densities ranged from 0.69 (95% CI: 0.47-1.0) to 1.06 (95% CI: 0.74-1.51) foxes km-2 and were similar across regions. Conclusions: Camera traps have the potential to provide continuous index-based measures of fox populations but may not record sufficient observations to detect change. Indices can also be confounded by variations in animal behaviour. Scat genetic analysis is a viable option for providing direct estimates of population change at specific snapshots in time; however, this approach is considerably more expensive, and large sample sizes may be required if genotyping success is low. Implications: Our study presents a rare example of multiple concurrent - and non-invasive - monitoring techniques to evaluate the effectiveness of predator control. We highlight the value of rigorous study designs and high-quality density information for designing predator management and monitoring programs.