Invasive species and thermal squeeze: distribution of two invasive predators and drivers of ship rat (Rattus rattus) invasion in mid-elevation Fuscospora forest

Abstract

Global climate change could alter the range, abundance, and interactions of species, potentially favouring invasive species and harming endemics. Ship rats (Rattus rattus) are one of the world's worst invasive predators but are typically absent from Aotearoa New Zealand's native Fuscospora cliffortioides (mountain beech) forest above 1000 m. Stoats (Mustela erminea) are another damaging invasive predator in Aotearoa New Zealand and prey on ship rats. We analyse community trapping records 2007–2020 to investigate the spatial and temporal distribution of ship rats and their key predator stoats at Craigieburn Forest Park. We document an invasion of ship rats after 2010 at Craigieburn and hypothesised two drivers of the increase in rat abundance: (1) more frequent mountain beech high-seed years providing more food for rats; and (2) warming winter temperatures allowing rats to invade areas that were previously too cold. We were unable to test a third possible driver (stoat trapping resulting in top-down meso-predator release) due to the nature of the data available. Rats were more common at low altitudes near streams, and stoats were more common at higher-altitudes on forest edges. Average winter temperature, but not seedfall, increased significantly at Craigieburn mid-elevations since 1972. The best predictor of annual rat catch was higher average winter temperatures interacting with high seedfall. This shows a key interaction between two global change drivers: warming temperatures have allowed exotic ship rats to expand into areas where they were previously absent, increasing the resultant "thermal squeeze" of predation on sensitive endemic birds at higher-altitude sites.