Stable isotope analysis reveals variable diets of stoats (Mustela erminea) in the alpine zone of New Zealand

The alpine zone of New Zealand covers c. 30% of public conservation land and is home to a high diversity of endemic species. Predation by introduced stoats (Mustela erminea) is identified as a major threat to alpine fauna. However, a lack of biological information, such as what stoats eat in different settings, hinders efforts to focus control measures in time and space in order to achieve the greatest conservation gains. We used a biochemical tool, stable isotope analysis, to estimate stoat diet across three time-periods in the alpine zone of three national parks. We then assessed possible drivers of dietary variation that could lead to greater per capita consumption of native species by stoats. Our models indicate that mammal prey items formed the largest contribution to the metabolic requirements of stoats in long-term estimates (47–90%), but the mid-term (spring and summer) estimates show a greater reliance on insects. The estimated proportions of prey consumed did not differ with elevation, sex, or age, but were significantly different between sites and seasons. Both stoat and ship rat (Rattus rattus) abundance were significant in explaining the proportion of mammals consumed. Higher stoat trap-catch and an absence of ship rats at one site (Nelson Lakes National Park) coincided with a greater range of prey being regularly consumed by stoats; this was the only site to record substantial proportions of birds (26%) and skinks (33%) in stoat diet. Conservation managers should be aware of the potential for marked differences in per capita rates of consumption of threatened alpine species by stoats, possibly linked to differences in abundance of mammalian prey. While this study confirms that stable isotope analysis can be useful to assess the diet of stoats, further research is needed to determine specific isotope enrichment values and to confirm the accuracy of these results.