Expansion of invasive carabids across elevation and habitats on sub-Antarctic South Georgia

Up-to-date information about the extent and underlying drivers of biological invasions is necessary to improve conservation strategies. The flightless, predatory carabid beetles, Merizodus soledadinus and Trechisibus antarcticus, were introduced to the sub-Antarctic island of South Georgia after 1960. The most recent but opportunistic monitoring of the species' distributions in the late 2000s indicated significant range expansions. We systematically assessed the presence and numbers of carabids and the associated communities at >200 locations on the northern coast of the island in 2023. The ranges of both species have continued to increase over the past 10–15 years, and they are now present from lowland coastal areas to the tops of valleys (>300 m above sea level). We modelled the detectability and abundance of carabids, finding that they occurred across all habitats examined but with greater abundance in dense vegetation. Our survey suggests that T. antarcticus is less abundant when M. soledadinus co-occurs. While a complete assessment of carabid impacts on native communities is not feasible with our data, we found that some native insects were considerably rarer within the carabids' range than beyond it. Ongoing climate change will likely provide opportunities for carabids to expand their range and become more abundant, highlighting the need for continued monitoring and biosecurity. We recommend regular application of systematic hand searches in favourable habitats, underlain by quantified estimates of search effort. Our transferable research methodology can optimise monitoring of introduced species in regions where field surveys are challenging.