Population density estimates of brushtail possums (Trichosurus vulpecula) in dry grassland in New Zealand

The introduced brushtail possum (Trichosurus vulpecula) is the most important wildlife host of bovine tuberculosis in New Zealand and is considered to be a major environmental and agricultural pest. Dry grassland ecosystems in New Zealand include some of the least protected and most threatened native biota. Drylands cover 19% of the country, but there is little published information on the population density of invasive brushtail possums in these environments, and previous estimates are not based on quantitative methods. We surveyed possum density in two open dry grassland habitats on hilly slopes in the southern South Island. One site had higher shrub and rock cover than the other. We used a spatially explicit capture–mark–recapture method to estimate possum density. Possum densities were slightly higher at the more shrubby and rocky site; estimates (±SE) from capture–mark–recapture from maximum likelihood were 0.69 (±0.05) per hectare, compared with 0.44 (±0.03) per hectare in the less shrubby and rocky site. Density estimates were similar to those recorded in radiata pine (Pinus radiata) and beech (Nothofagus spp.) forests in New Zealand, but were 5- to 29-fold lower than those in podocarp–broadleaved forest. Maximum likelihood home range estimates derived from spatial trap data were larger in the less shrubby and rocky site (54.07 ha cf. 36.19 ha). These home ranges are much larger than those recorded in native forest, and probably reflect the lower abundance and more patchy distribution of food resources and shelter in dryland environments. The ecological information provided here is useful for wildlife managers planning to target possum control in dryland environments.