Do mice matter? Impacts of house mice alone on invertebrates, seedlings and fungi at Sanctuary Mountain Maungatautari

The advent of mammal-resistant fences has allowed multi-species eradications of mammals from ecosanctuaries on the New Zealand mainland. However, maintaining eradication of house mice (Mus musculus) has proven difficult, and at some fenced reserves they are the only exotic mammal present and reach a high population density. Over 5 years we examined the impacts of mice alone on biodiversity at Sanctuary Mountain Maungatautari by comparing forest blocks with relatively high and low numbers of mice. We managed two independently fenced sites within the sanctuary to achieve high mouse numbers (up to 46 per hectare) at one site and undetectable mouse numbers at the second site. We then reversed these treatments by eradicating mice from the first site and allowing their numbers to increase at the second. We found strong evidence that mice reduced the abundance of ground-dwelling invertebrates, in particular caterpillars, spiders, wētā, and beetles, and reduced the mean body size of some taxa. In addition, earthworm abundance, biomass and species richness increased with a decreasing mouse population in one study block. Overall, there is substantial biodiversity gain from eradicating the full suite of pest mammals other than mice. However, Mice may be catastrophic in ecosanctuaries that focus on the recovery of invertebrates or lizards. We expect that mouse control tools will steadily improve so that in the future mice can be eradicated and excluded from forest reserves such as Maungatautari.