across north america and Eurasia, both stoats and least/common weasels are native predators that may attack introduced game birds; in New Zealand, they are introduced predators that often attack native birds, many of them endemic. This simple fact makes a world of difference between the attitudes to small mustelids in the northern and southern hemispheres (McDonald & Murphy 2000). Regardless of their sins in chicken coops and on game estates, stoats and common weasels in their native countries have an intrinsic value that they do not have in New Zealand.
Biogeography also makes a difference. The inshore islands of the northern hemisphere are not very different from the mainlands, and those separated by only shallow (<100 m) channels have been connected to the mainland during periods of low sea level in the past. The larger ones are inhabited by weasels, and many of the smaller, closer ones are visited from time to time, since all weasels, especially the large ones, are good and confident swimmers (Chapter 6). The native faunas of those islands have evolved alongside weasels for millennia, so include no species that cannot coexist with them over the long term.
Matters are very different, however, when weasels reach an island further offshore or in the open ocean that has never been connected to a continental mainland. History does not prepare the inhabitants of such a place to deal with predators, and the consequences are likely to be catastrophic. New Zealand is by far the best-documented example of this tragic process.
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