Control Of Stoats In New Zealand

The recent documentation of the damage to native fauna due to stoats (and, to a much lesser extent, common weasels) fully justifies the conclusion that the introduction of stoats was one of the worst mistakes ever made by European colonists in New Zealand. However greatly one might admire stoats as individual hunters making a living against all the odds, or as examples of evolutionary adaptation at its most impressive (and we both do admire them, as is clear from previous chapters of this book), they do not belong in New Zealand, and conservation authorities are under great pressure to find better ways to control them.

The danger from stoats to important threatened native fauna in New Zealand is taken very seriously. Throughout the 1990s, the Department of Conservation (DoC), the government agency responsible for protecting native fauna and landscapes, increased its investment into research on stoat biology. The groundwork had been laid in the 1970s but, at that time, the information on stoats could not be coordinated with the necessary observations on the birds. Only in the mid-to late 1990s were several key papers published on the impact of stoat predation on threatened species (Elliott 1996b; McLennan et al. 1996; Wilson et al. 1998; Basse et al. 1999).

As a direct result, in 1999 the New Zealand government accepted the argument that only an extensive, nationally funded campaign could protect endangered ground and hole-nesting birds from stoats (Hackwell & Bertram 1999). It therefore granted NZ $6.6 million to instigate a 5-year program to find more cost-effective and sustainable approaches to stoat control. The new funding stimulated an explosion of new research, as summarized in the annual reports of the Stoat Technical Advisory Group and published by the Department of Conservation (Murphy & Fechney 2003). For a short period, far more money was being spent on stoat research there than in any other country in the world, for example, NZ $1,352,000 in 2000-2001 (Parkes & Murphy 2003).

Obviously, effective control of any animal population is not merely efficient killing, although it may involve that. It is the translation of ecology into management policy. A rational stoat control program must (1) ensure that stoats are at least one of the main reasons for the observed damage and remove all cause for suspicion that any improvement in the birds could be due to something else; (2) understand the normal population ecology of stoats and how to disrupt it by the most humane and efficient method possible; and (3) monitor the effectiveness of a program and abandon it if it has no impact on the target population. These requirements are easy to understand but hard to meet in practice, especially as they require different strategies in different habitats.

For example, in North Island mixed forests Brown (1997) filmed 65 nests of New Zealand robins and tomtits in the 1993-1994 breeding season, of which 72% were lost to ship rats. No stoats were seen at the nests during this study, so Brown concluded that stoats were probably not important predators of forest passerines in nonbeech forests when rats were present. Likewise, intensive monitoring of nests of another species living in nonbeech forest, the North Island kokako, recorded many rats and possums, but only one stoat (Innes et al. 1999). Stoat control, therefore, needs to be focused on protecting species where stoats are usually the important predators of eggs or young (Table 13.1), such as yellowheads (Dilks 1999), kaka (Moorhouse et al. 2003), and robins (Etheridge & Powlesland 2001) in beech forests, and kiwi (Gillies et al. 2003) anywhere.

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