Modelling the distribution of stoats on New Zealand offshore islands

Establishing the factors that correlate with the distribution of invasive species on islands provides a means of evaluating invasion potential and pathways, and enables the invasion risk for specific areas to be predicted. These island risk profiles then provide a basis for conservation managers to prioritise conservation areas, and to create appropriate monitoring regimes. In New Zealand, stoats (Mustela erminea) are invasive predators responsible for numerous local extinctions of birds, and they present a significant invasion risk for offshore islands. Here we examine the distribution of stoats on New Zealand's offshore islands in order to establish what factors are correlated with their distribution, and predict which islands display a high invasion risk. Data on the distribution of stoats were compiled, along with the characteristics of New Zealand's islands and their settlement history. The distribution of stoats on these islands was regressed on island characteristics using a logistic generalised linear model. Species interactions were examined by including the distribution of a range of other introduced mammalian species. The distribution of stoats was found to be strongly affected by the distance offshore and by the area of the island. No significant correlations with the extent of human settlement or the presence of other animals were detected. The model indicates that the invasibility of islands by stoats has been underestimated. Islands within 1 km of the mainland have a very high stoat invasion risk, which is consistent with known invasions, and for large islands (≥ 500 ha) there remains a significant risk to at least 3 km offshore. This model provides a framework for assessing risks of stoat incursion and makes specific predictions of islands with a significant invasion risk that have not had stoats detected before.