Individual specialists within a generalist niche: variable diet of stoats and implications for conservation

Conservation programmes aiming to suppress or remove invasive small mammal populations that threaten endemic fauna assume that eliminating an individual predator has the same effect as eliminating a conspecific in terms of decreasing risk to the prey species. However, marked between-individual variation in prey take could, at times, lead to uneven predation pressure. Such variation in the diets of introduced predators has long been hypothesised in New Zealand, suggesting that some observed rates of predation are not typical of the prey population as a whole. We used stable isotope analysis to estimate the isotopic dietary niches of stoats Mustela erminea (n = 51) caught in three New Zealand National Parks. We modelled δ13C and δ15N ratios from five tissue types to estimate the isotopic niche for each individual stoat, and for each population. The isotopic niche used by the population of stoats in Nelson Lakes National Park was substantially larger (5.3 ‰2) than for those at either Mt Aspiring National Park (3.1 ‰2) or Fiordland National Park (2.8 ‰2). Despite this, a measure of individual specialisation (proportional niche use) in stoats at Nelson Lakes National Park (0.4 ± 0.3) was similar to the other two sites. Collectively these data suggest a comparable degree of individual specialisation even within a more generalist population niche. This variability in the range of prey items taken by small carnivores, between populations and individuals of the same species, has consequences for how we design and monitor small mammal control programmes; for example, the threatened species outcomes observed in one area or time period may not be applicable to another.