Eradication of an invasive species from an island must be justified by strong evidence of their negative impacts on the ecosystem, and confidence that those impacts outweigh any unexpected surprise effects which might occur. For invasive rats and mice these impacts have been already well documented globally. In some cases impacts of mice may be as strong as those of rats. Certainly mice can have profound impacts on birds on some islands (e.g. Gough Island mice parasitizing baby albatross), but generally the impacts of mice are more subtle, such as on plants and invertebrates. However, these impacts are no less devastating, particularly if you are a botanist or entomologist. On Antipodes Island, the evidence of supressing effects by mice on plants and invertebrates are clear.Mouse on Antipodes Island (Photo: James Russell)
The distribution and abundance of invertebrate Orders on Antipodes Island have been studied by pitfall trapping; a small cup placed in the ground with liquid to capture ground-dwelling invertebrates. Although the entire main Antipodes island is invaded by mice, structuring in invertebrate diversity is clear by habitat, even in winter (see graph). On offshore Bollons and Archway Islands, where mice have never been recorded, the diversity of invertebrates is huge, as demonstrated by comparisons with the main Antipodes Island published in Polar Biology. Furthermore, a species of weta unknown to science has previously been recorded, but not yet collected, on Bollons Island, protected by its westerly exposure making landing difficult except in the finest conditions. Mice have elsewhere even recently been linked to geomorphic events, coincidentally recently observed on Antipodes Island.
The entire Antipodes Island terrestrial food-web is a complex network of interacting species. Where mice are absent on offshore islands, seabirds drive invertebrate diversity. After mouse invasion, although birds may not be strongly affected, plants and invertebrates are, which is bad for the species in and of themselves, but also has flow-on effects to birds which rely on plants and invertebrates for habitat and food, such as the parakeets, pipits and snipe found on Antipodes Island. To more precisely understand these relationships, and the magnitude of response we might expect to see following mouse eradication, we are continuing pitfall trapping studies as well as undertaking a stable isotopes assessment of the terrestrial community to identify where species are located in the food-chain and how they respond to one another.