The dispersion and detection patterns of mtDNA-assigned red fox Vulpes vulpes scats in Tasmania are anomalous

Inferring the distribution and demography of an invasive species from sighting data: the red fox incursion into Tasmania

A recent study has inferred that the red fox (Vulpes vulpes) is now widespread in Tasmania as of 2010, based on the extraction of fox DNA from predator scats. Heuristically, this inference appears at first glance to be at odds with the lack of recent confirmed discoveries of either road-killed foxes--the last of which occurred in 2006, or hunter killed foxes--the most recent in 2001. This paper demonstrates a method to codify this heuristic analysis and produce inferences consistent with assumptions and data. It does this by formalising the analysis in a transparent and repeatable manner to make inference on the past, present and future distribution of an invasive species. It utilizes Approximate Bayesian Computation to make inferences. Importantly, the method is able to inform management of invasive species within realistic time frames, and can be applied widely. We illustrate the technique using the Tasmanian fox data. Based on the pattern of carcass discoveries of foxes in Tasmania, we infer that the population of foxes in Tasmania is most likely extinct, or restricted in distribution and demographically weak as of 2013. It is possible, though unlikely, that that population is widespread and/or demographically robust. This inference is largely at odds with the inference from the predator scat survey data. Our results suggest the chances of successfully eradicating the introduced red fox population in Tasmania may be significantly higher than previously thought.

Degradation and detection of fox (Vulpes vulpes) scats in Tasmania: evidence from field trials

Context The Tasmanian Government is attempting to eradicate foxes from Tasmania and carnivore-scat surveys using humans and dogs combined with DNA testing are the main methods of detection. Understanding the rate that scats degrade is a key component for estimating the power of monitoring for detecting cryptic predators and will contribute to a broader understanding of the use of scat monitoring for informing eradication programs. Aims To estimate the degradation rate of fox scats and derive an estimate of the abundance of scats available to observers monitoring for fox presence. Methods In total, 486 fresh fox scats were placed at nine sites within three bioregions in Tasmania and left to degrade for up to 126 days. Scats were observed periodically by both humans and dogs to determine when they became unrecognisable and/or undetectable. Key results Recognition of scats by humans declined faster in summer than in winter and did not vary systematically among bioregions. Median survival times of scats were 19 days in summer and 26 days in winter. Recognition of scats by dogs was higher in summer than in winter, with dogs recognising scats past the time they became unrecognisable to humans. Using estimates of scat degradation derived from human observers, the equilibrium abundance of detectable scats within a fox home range was estimated to be 179–243 scats. However, the abundance of detectable scats on linear features subject to monitoring was estimated to be 10–15 scats. Conclusions Using our estimate of the abundance of scats on linear features, the current distribution of fox scats detected in Tasmania may not be as anomalous as has been suggested by others. However, fox detection from scats will be highly dependent on deposition patterns and distribution of scats on linear features and this should be critically reassessed in Tasmania. Implications Fox scats are not expected to exhibit systematic regional differences in degradation rates that might have an impact on monitoring strategies. Estimates of the abundance of scats detectable by observers are critical for assessing the effectiveness of scat-monitoring programs. We advocate that a rigorous assessment of future scat-monitoring programs in Tasmania be undertaken to determine their power to detect foxes.