Ecology of brushtail possums in a New Zealand dryland ecosystem

The introduced brushtail possum (Trichosurus vulpecula) is a major environmental and agricultural pest in New Zealand but little information is available on the ecology of possums in drylands, which cover c. 19% of the country. Here, we describe a temporal snapshot of the diet and feeding preferences of possums in a dryland habitat in New Zealand's South Island, as well as movement patterns and survival rates. We also briefly explore spatial patterns in capture rates. We trapped 279 possums at an average capture rate of 9 possums per 100 trap nights. Capture rates on individual trap lines varied from 0 to 38%, decreased with altitude, and were highest in the eastern (drier) parts of the study area. Stomach contents were dominated by forbs and sweet briar (Rosa rubiginosa); both items were consumed preferentially relative to availability. Possums also strongly preferred crack willow (Salix fragilis), which was uncommon in the study area and consumed only occasionally, but in large amounts. Estimated activity areas of 29 possums radio-tracked for up to 12 months varied from 0.2 to 19.5 ha (mean 5.1 ha). Nine possums (4 male, 5 female) undertook dispersal movements (≥ 1000 m), the longest of which was 4940 m. The most common dens of radio-collared possums were sweet briar shrubs, followed by rock outcrops. Estimated annual survival was 85% for adults and 54% for subadults. Differences between the diets, activity areas and den use of possums in this study and those in forest or farmland most likely reflect differences in availability and distribution of resources. Our results suggest that invasive willow and sweet briar may facilitate the existence of possums by providing abundant food and shelter. In turn, possums may facilitate the spread of weeds by acting as a seed vector. This basic ecological information will be useful in modelling and managing the impacts of possum populations in drylands.