Habitat-specific densities of urban brushtail possums

Invasive mammalian pests threaten biodiversity globally across a diverse range of habitats. The unique combination of resource subsidies and disturbance in cities can provide favourable conditions for invasion. Recent interest in urban biodiversity enhancement has increased the demand for effective urban pest control, but efforts are often hampered by a lack of understanding of the ecology of urban invasive mammals. The Australian common brushtail possum (Trichosurus vulpecula) has invaded most New Zealand landscapes, including urban areas, and is a nationally significant pest species. Recent shifts in national pest control and conservation priorities demand an assessment of the capacity for urban areas to harbour possum populations. We estimated the density of possums across three representative habitat types within the city of Dunedin, New Zealand: an urban forest fragment and two residential areas of varying vegetation quality. Possums were live-trapped and camera-trapped over eight days at each site in late summer to early autumn. Spatially explicit capture-recapture methods were used to estimate density at each site, and “minimum number alive” estimates were also calculated. Our estimate suggests that the forest fragment supported possums at a density (3.1 ha⁻¹) capable of inflicting harm on resident native wildlife, but this density was low compared with non-urban estimates in the same forest type, suggesting a possible influence of disturbance from human activity in and around the fragment. Few possums were caught at the two residential sites (0.1 ha⁻¹ at each), and behavioural avoidance may have reduced capture success there. Our estimates confirm that urban areas are an important habitat for possums, and our study provides the first rigorous estimates of urban possum density, which can be incorporated into predictive modelling and other methods of control planning.