Urban Rats in Wellington: Estimating Home Ranges, Population Densities and Detection Probabilities

The ship rat (Rattus rattus) and Norway rat (Rattus norvegicus) are prolific pest species with a near- global distribution. Their spread has had serious public health repercussions as carriers of disease and by causing considerable agricultural losses. They are also invasive to many native ecosystems, degrading ecosystem processes, and preying upon native species, resulting in significant losses to biodiversity.

This study aims to guide more effective rat management strategies through an increased understanding of the spatial ecology of rats in an urban environment. Three separate studies were conducted, all located in Wellington, New Zealand:

1) A radio-telemetry study looked at the home range and spatial behavior of 10 urban ship rats. Results showed comparatively small home ranges (0.01 - 0.45 ha at 100% minimum convex polygons) with maximum linear distances within a home range of 19-74m. There was significant spatial overlap between home ranges– up to 90% (between two adjacent home ranges); co-nesting behavior between both sexes; frequent diurnal activity amongst ship rats (9 of 10 rats); and two longer distance dispersal events (~120m) by ship rats. Implications for rat management include: a need for tighter spacing of devices in urban habitats for control and detection of survivors, potentially every 20-25m if eradication is the goal.

2) A capture mark re-sight study to estimate the minimum density of ship rats in an 0.63 ha urban bush fragment. A total of five rats were live caught in cage traps and uniquely marked before release. An additional eight wild rats were uniquely identified on cameras based on distinctive features of their appearance. A conservative Lincoln-Petersen estimate was used to estimate the number of rats within the bush fragment: this produced an estimate of 14.6 rats with 95% confidence intervals [7.69-55.6], which translates to a density of 23.2 rats/ha [12.2-88.25]. These densities are significantly higher than those found in most mainland studies and more comparable to those in island habitats. This could be because ship rats are subsidizing their diet with human-derived foods, although this was not confirmed here.

3) A detection probability study investigated the sensitivity of three devices (wax tag, chew card and bait station) to ship rat presence and examining age-related differences in detection. The bait station was found to have the highest detection probability (0.5 detections/sighting) followed by the wax tag (0.44 detections/sighting) and chew card (0.37 detections/sighting) although results were based on data retrieved from a low sample size of devices (n=2 of each type). The bait station showed a sharp difference between the adult (0.1 detections/sighting) and adolescent populations (0.89 detections/sighting) detection probability. Furthermore, this difference in detection probability was found, although less pronounced, in both the wax tag and chew card. Implications for rat management include: a recommendation that wax tags be used as the primary means of ship rat monitoring; a need for further behavioral studies looking at detection probabilities across a range of kill and monitoring devices so that the most effective ones can be identified; and the development and testing of devices that are attractive to adult rats that may have become “trap shy”.

These three studies together provide useful insights into urban rat ecology with implications for pest management. However, a more comprehensive study with larger sample sizes is recommended to fully substantiate this work.