Abstract
The New Zealand native fauna has developed life history traits because of evolving in the absence of predators that make them vulnerable to invasive predatory animals, such as gigantism, reduced predator aversion, and low reproductive rates. Norway rats (Rattus norvegicus) are efficient predators of native invertebrates, frogs, and avifauna. To date, no studies exist that assess Norway rat resource selection and detection in the New Zealand high country, namely the Mackenzie Basin and the Te Manahuna Aoraki project area (TMA). Research has primarily focused on offshore islands; however, we cannot assume that Norway rat ecology will be the same in the different habitats. The aim of this study was to better understand Norway rat distribution in relation to habitat types present in the Mackenzie Basin, and to develop appropriate methods for detecting this species in this landscape.
Pest detection devices are a critical tool for monitoring Norway rats. To date, studies show that irrespective of what device is used, Norway rat detection is generally low. I used the Kaplan Meier estimator to assess differences between devices in time to detection. I compared cumulative incidence of rat detections per device, and extrapolated detection rates to assess any differences in device-specific probabilities of Norway rat detection for tracking tunnels, trail cameras, and chew-cards. I used generalized linear models to determine if device placement relative to distance-to-water, and vegetation density affected Norway rat detection. I found no significant differences between device time to detection and found no effects on detection in relation to device distance-to-water and vegetation density. There were small positive effects of chew-cards and distance-to-water (p=0.06) and vegetation density (p=0.08), suggesting a possible disparity in chew-card Norway rat detection and these variables. All three devices detected Norway rats after one to three monitoring nights, which challenges the assumption that all wild Norway rats will be neophobic and reiterates that we cannot assume Norway rat ecology will be the same between different habitats.
Data collected from Department of Conservation trapping operations in the TMA area over 14 years were used to assess resource selection by Norway rats. Five ecologically significant habitat variables for Norway rat presence were measured at traps that had ever caught a Norway rat. I used a generalized linear models to assess the degree of association rat presence had with each habitat variable, which was then projected onto a habitat map to identify sites with a high probability of Norway rat presence. All predictor variables were significant (P<0.05); however, land type had the strongest influence. Hardwood forests, landslides, and tussock grassland and shrubland were most influential, and water bodies, mines and dumps, and aquatic vegetation were least influential on the probability of Norway rat presence. It was surprising that I did not find an association of Norway rat presence and water, however this could be due to the absence of ship rats and lack of subsequent competition in the TMA area permitting rats to utilise a wider range of habitats. It was expected that Norway rats would be associated with hardwood forests, and tussock grassland and shrubland because of the likelihood that this habitat will provide food sources.
These results will better inform pest controllers of Norway rats prior to a control operation. Pest controllers can use this information about detection devices both pre, and post control to reliably confirm Norway rat presence and absence. Future research is required to further assess uncertainties in Norway rat ecology in the New Zealand high country, including diet, dispersal, and the most efficient eradication method.