The trend toward implementing more humane treatment of all animals, including pests (Chapter 12), has stimulated intensive research not only on more humane traps but also on cost-effective, nonlethal methodology. Interest in this subject is worldwide, but there is probably more concern about it in New Zealand than anywhere else in the world. What happens on those remote islands may well affect how pest mammals are treated in many other countries in the future.
There are good reasons to be cautious about lethal control policies. Invasive pests cannot be treated in isolation from the rest of the ecosystem with which they interact. When pests are removed without reference to the other species in the same community, unexpected consequences tend to follow (Zavaleta et al. 2001).
For example, unless the two most serious pests of nesting birds (rats and stoats) are removed together, there is a risk that removal of stoats alone might permit rats to increase in numbers (King 1984b; Barlow & Choquenot 2002). Rats are also serious predators of birds, and live at much higher densities than do stoats. In the Eglinton Valley after two heavy seedfalls in consecutive years (1999, 2000), stoats and rats increased rapidly in numbers together (Dilks et al.
2003). Even though modeling suggests that stoats cannot prevent rodent irruptions (Blackwell et al. 2003; Ruscoe et al. 2003), the possibility that the unprecedented response of rats in the Eglinton could have been helped by a decade of stoat trapping has not yet been eliminated. In the same years, the Eglinton population of native long-tailed bats suffered greater predation and lower survival than in nonseedfall years, correlated with variations in the density index for rats (Pryde et al. 2005).
Conversely, after successful control operations in North Island forests that affected rats more than stoats, stoats switched to eating more birds (Murphy et al. 1998). Another interaction that complicates the picture is that mice increase in numbers whenever rats are removed (Innes et al. 1995), which is a concern since mice are important food for stoats and, therefore, affect their numbers (Chapter 10).
As confidence and technology improve, management policies in New Zealand are looking toward more ambitious goals—beyond mere eradication of pests, including stoats, to broader ecosystem restoration, at least in patches (Towns et al. 1997; Atkinson 2001). Stoats, common weasels, and ferrets might not have been among the aliens that must now be tackled if the upper chamber of the New Zealand Parliament had not allowed to lapse in 1876 the bill (already passed by the lower chamber) that would have prohibited the introduction of mustelids (Galbreath 1989:126). But, so long as the modern political will supports the public yearning to put right that historic mistake, some hope remains for the threatened native fauna.
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