Monitoring methods influence native predator detectability and inferred occupancy responses to introduced carnivore management

Abstract

Context Management actions that suppress introduced predator densities can benefit the population recovery of native species. Nevertheless, ensuring that predator management produces measurable population-level benefits can be influenced by multiple factors affecting species detection. Monitoring designs using multiple survey methods may perform better than increasing sampling effort with single-method protocols. Aims This study aimed to estimate individual and cumulative detection probabilities and site occupancy estimates from the use of five different monitoring methods to survey a native mesopredator, the lace monitor (Varanus varius). Second, we assessed the effect of lethal red fox (Vulpes vulpes) baiting on lace monitor detection probabilities and site occupancy estimates collected from each monitoring method. Methods Multi-method sampling for Varanus varius occurred at 76 sites across lethal fox baited and non-baited habitats in East Gippsland, Victoria. Bayesian site occupancy models were used to estimate the effects of detection method and fox-baiting treatments on Varanus varius detection probability and site occupancy. Key results Method-specific detection probabilities (P = 0.00–0.12) and site occupancy estimates (Ψ = 0–0.53) varied considerably among methods, but combinations of multi-method monitoring improved lace monitor detection probability (P = 0.11–0.18) and site occupancy (Ψ = 0.87 ± [0.66–0.93]−0.91 ± [0.76–0.97] mean ± [95% credible intervals]) above any single method. However, there was extreme heterogeneity in the size and direction of the introduced predator baiting effect on method-specific lace monitor detection. Three methods (box traps and two different visual search surveys) all indicated lace monitor detection probabilities increased in fox-baited sites. However, sand pads reported a decrease in lace monitor detection at fox-baited sites, whereas pipe traps obtained no detections. Conclusions Combining detection data from all methods led to the inference of a positive fox-baiting effect, albeit with a smaller magnitude and better certainty than that estimated using a reduced method monitoring design, which had fewer detection data after excluding biased detection from sand pads. Implications Using a multi-method monitoring approach improved lace monitor detection and reduced sampling effort. However, depending on sampling methodology, the management effects on lace monitors can change.