Spatiotemporal changes in relative rat (Rattus rattus) abundance following large-scale pest control

We examined spatiotemporal changes in rat tracking indices following large-scale (>10 000 ha) pest control using aerial applications of sodium monofluoroacetate (1080) baits in Tararua Forest Park, North Island, New Zealand. Population control of rats appeared effective, with few to no rat tracks recorded in treatment areas during the 6 months after control. However, the rat tracking index increased rapidly after that, and 24–30 months after control, rat tracking indices in treated areas exceeded those in the non-treated areas. Rat tracking indices first increased at the treatment margins (6–12 months post-control), with rat recovery in the centre of controlled areas delayed by 24–30 months. The best supported statistical model of rat tracking indices included an interaction term between time since treatment*distance to non-treatment area, which indicated that overall increases in rat tracking after control were highest at monitoring lines located in the interior of the control zone, with a negative growth rate estimated for lines located outside of the control area. This suggests a competitive release for rat populations in the interior of the control zone. The observed delay in rat recovery on the interior lines compared with lines located at the control margin implies that rat population increase following control was initiated by rats migrating into the treated area from adjacent untreated forest areas. Treatment persistence, therefore, might be increased by increasing the size of pest control areas; aligning pest control boundaries with immigration barriers, such as large water bodies and/or alpine zones; or implementing intensive pest control around treatment boundaries to intercept immigrating rats