Rodent eradication scaled up: clearing rats and mice from South Georgia

The Subantarctic island of South Georgia lost most of its birds to predation by rodents introduced by people over 2 centuries. In 2011 a UK charity began to clear brown rats Rattus norvegicus and house mice Mus musculus from the 170 km long, 3,500 km² island using helicopters to spread bait containing Brodifacoum as the active ingredient. South Georgia's larger glaciers were barriers to rodent movement, resulting in numerous independent sub-island populations. The eradication could therefore be spread over multiple seasons, giving time to evaluate results before recommencing, and also reducing the impact of non-target mortality across the island as a whole. Eradication success was achieved in the 128 km² Phase 1 trial operation. Work in 2013 (Phase 2) and early 2015 (Phase 3) covered the remaining 940 km² occupied by rodents. By July 2017, 28 months after baiting was concluded, there was no sign of surviving rodents, other than one apparently newly introduced by ship in October 2014. A survey using detection dogs and passive devices will search the Phase 2 and Phase 3 land for rodents in early 2018. Seven (of 30) species of breeding birds suffered losses from poisoning, but all populations appear to have recovered within 5 years. The endemic South Georgia pipit Anthus antarcticus was the first bird to breed in newly rat-free areas, but there were also signs that cavity-nesting seabirds were exploring scree habitat denied them for generations. Enhanced biosecurity measures on South Georgia are needed urgently to prevent rodents being reintroduced.