Home-range changes by brushtail possums in response to control

Common brushtail possums (Trichosurus vulpecula) are intractable pests in N ew Zealand. The effectiveness of local control can be limited by immigratio n, some of which has been attributed to a 'vacuum effect' directed movement s induced by the control itself. To characterise the vacuum effect we exami ned changes in the home ranges of trapped possums following control in a 6- ha block at one end of a 13-ha forest patch on farmland near Dunedin, New Z ealand. We also monitored a sample of possums by radio-telemetry. After con trol, the density was 3 ha(-1) inside the removal area and 16 ha(-1) outsid e. During the year after the removal, 29% of possums within 100 m of the bo undary of the removal area (n = 38) shifted their range centre at least 50 m towards it. The effect diminished rapidly with distance: only 1 of 28 ani mals moved more than 200 m from the boundary. The size of the previous rang e was a significant predictor of movement among males, but this may be part ly a sampling artifact. We measured a net flux of 69 possums km(-1) across the boundary in the 12 months after control, and possums settled on average 44 +/- 6.9 m inside the boundary. The vacuum effect in brushtail possums a ppears largely confined to home-range adjustments by individuals with range s overlapping the area of reduced density. This limits its potential role i n population recovery.