The invasion ecology and molecular ecology of stoats (Mustela erminea) on New Zealand's islands: Detecting and assessing migration

Abstract

Stoats (Mustela erminea) are small carnivores that were introduced to New Zealand in the 1880s in an unsuccessful attempt to control introduced European rabbits (Oryctolagus cuniculus). Since then they have spread throughout New Zealand, contributing to the local extirpation of many indigenous bird species. The stoats remain one of the greatest threats to the current terrestrial fauna of New Zealand. I undertook a range of different studies of the invasion ecology of stoats, which I present in this thesis – with the primary aim being to better understand stoat population dynamics, in order to improve management programmes. First, in a study of the genetic diversity of stoats in New Zealand I found that stoats in New Zealand are more genetically diverse than the present day stoat population in Britain – from which it was founded. I infer from this observation that British stoats probably lost genetic diversity due to myxomatosis decimating their primary prey in the 1950s. The second study I present involved creating a predictive model of stoat population presence on islands, based on the physical, biological and human settlement characteristics of the islands. I found that the distance offshore and island size were the only predictive factors for stoat presence on islands, and from this result I infer that swimming is the primary invasion pathway. I also deduced that previous estimates of the invasion potential of stoats significantly underestimated their swimming ability. In two further studies, I evaluated the origin of stoats caught on two different islands after control programmes on each island had commenced using genetic assignment and clustering techniques. I discovered that a stoat caught on Rangitoto Island a year after the eradication programme originated from the opposing mainland, which required a 3 km swim or human assisted transport. On Secretary Island, I found that both in-­‐situ breeding by animals missed by trapping, along with immigration from the mainland was occurring. Furthermore, I investigated stoat population connectivity among mainland sites in the Auckland Region, and to nearby islands (also estimated using genetic methods) finding low recent migration between distinct forest-­‐patches, and extremely low migration rates to two large islands several kilometres offshore. The primary conclusion of my research as a whole is that stoats can swim further, and do so more regularly than previously thought, and that they need to be managed in a metapopulation context, with an understanding of inter-­‐population connectivity.