Common brushtail possums
(Trichosurus vulpecula) are intractable pests in New
Zealand. The effectiveness of local control can be limited by immigration,
some of which has been attributed to a ‘vacuum effect’ –
directed movements induced by the control itself. To characterise the vacuum
effect we examined 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 Zealand. We also monitored a sample of possums by
radio-telemetry. After control, the density was 3
ha–1 inside the removal area and 16
ha–1 outside. During the year after the removal,
29% of possums within 100 m of the boundary 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 animals
moved more than 200 m from the boundary. The size of the previous range was a
significant predictor of movement among males, but this may be partly 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 appears largely confined to home-range
adjustments by individuals with ranges overlapping the area of reduced
density. This limits its potential role in population recovery.
Mini-PCNL with vacuum effect: A case series
