Detection probabilities and surveillance sensitivities for managing an invasive mammalian herbivore

Abstract

Invasive mammalian herbivores can have significant unwanted impacts on indigenous ecosystems and production landscapes. Their impacts are often mitigated using sustained lethal control or eradication. If eradication is the chosen option, it is critical to confirm where the pest population is distributed and prove that eradication has been achieved following the removal intervention. This form of pest management requires robust and quantitative estimates of surveillance. We compared the detection probabilities, derived surveillance system sensitivities (under standardized conditions), and cost of surveillance for four survey methods for invasive Bennett’s wallabies (Notamacropus rufogriseus) in South Island, New Zealand: helicopter surveys with observers, helicopter surveys using a thermal imaging camera, a ground hunter with detector dogs, and camera traps. We found camera traps and ground hunters with dogs were the most sensitive surveillance methods (82% and 45% chance of detecting a wallaby assuming full search coverage of a 100-ha area, respectively) and were most cost-effective (62% and 85% cheaper than helicopter surveys with observers, respectively). However, although these methods are useful for surveying small areas (~1000 ha or less), capacity and time issues make them impractical for surveying large areas. The helicopter-based methods will likely be more suitable for monitoring large areas (e.g., 10,000–50,000 ha) that need to be completed more rapidly than could be achieved using ground-based methods. The model that we used for evaluating detection probability and surveillance sensitivity for different survey methods is highly applicable for managing a wide range of mammalian pests and confidently declaring their absence from an area.