Improving detectability of an invasive rat: Pacific rat (Rattus exulans)

Abstract

The ability to detect and identify invasive species is essential for detecting invasion and reinvasion, the planning of species-specific management strategies, and the monitoring of eradication and control progress. Detectability is the probability of a monitoring device detecting an animal given that the animal is present in the survey area. It is influenced by the probability of an animal encountering and interacting with the device. In New Zealand, Pacific rats (Rattus exulans) are particularly difficult to detect when they coexist with other invasive predators and persist in low densities. I examined the detectability of Pacific rats and identified factors influencing the detectability on D’Urville Island, where Pacific rats co-exist with multiple invasive predators. Detectability of the Pacific rats were low and influenced by a range of factors, including the types of monitoring devices, vegetation types, seasons, inter-specific interferences, and population densities. Camera trapping was the most reliable methods for detecting Pacific rats, and indicate low rat population densities and high inter-specific interferences from stoats and weka (Gallirallus australis) that might have reduced rat detectability. I then conducted a field-based experiment on the lure preferences of Pacific rats on Slipper Island, where Pacific rats are the only existing invasive predators. Detectability by social lures appeared to be higher than food-based lures, with mouse scent having the highest detection rate. Lastly, I investigated the possibility of distinguishing tracking tunnel footprints between Pacific and ship rats using statistical modelling techniques. Prediction accuracies of the linear discriminant models were high (>90%), suggesting reliability of the methods for species discrimination. Majority card classification could further improve model applicability, i.e. assigning species classification of a tracking card by applying a threshold value on the percentage of footprints predicted as Pacific or ship rats. Overall, this study reveals factors that might influence the detectability of Pacific rats, but further research is required to validate the potential ways for improving detection and monitoring efficacy of Pacific rats that persist in low population densities, and for the effectiveness of social lures on populations that have been exposed to other invasive predator species.