The effects of mice on stoats in southern beech forests

Introduced stoats ( M ustela erminea ) are important invasive predators in southern beech ( N othofagus sp.) forests in N ew Z ealand. In these forests, one of their primary prey species – introduced house mice ( M us musculus ), fluctuate dramatically between years, driven by the irregular heavy seed‐fall (masting) of the beech trees. We examined the effects of mice on stoats in this system by comparing the weights, age structure and population densities of stoats caught on two large islands in F iordland, N ew Z ealand – one that has mice ( R esolution I sland) and one that does not ( S ecretary I sland). On R esolution I sland, the stoat population showed a history of recruitment spikes and troughs linked to beech masting, whereas the S ecretary I sland population had more constant recruitment, indicating that rodents are probably the primary cause for the ‘boom and bust’ population cycle of stoats in beech forests. R esolutions I sland stoats were 10% heavier on average than S ecretary I sland stoats, supporting the hypothesis that the availability of larger prey (mice verses wētā) leads to larger stoats. Beech masting years on this island were also correlated with a higher weight for stoats born in the year of the masting event. The detailed demographic information on the stoat populations of these two islands supports previously suggested interactions among mice, stoats and beech masting. These interactions may have important consequences for the endemic species that interact with fluctuating populations of mice and stoats.