Rabbits were also the key players in the second example cited by King and Moors (1979a), but which had the opposite result. The common weasels and stoats introduced into New Zealand both survived and spread, and now, over a hundred years later, stoats are common in virtually all forested areas. Common weasels, originally imported in much greater numbers than stoats, are now among the rarest of all New Zealand's mammals.
Why did the pioneering stocks of the two small mustelids respond so differently to their new environment? The hypothesis suggests that it was because the radically unfamiliar conditions in New Zealand changed the balance of their relative advantages. Although rabbits and rats are abundant in New Zealand, voles are absent and other smaller prey are scarce; feral house mice are the only rodents under 50 g in body weight (Chapter 5).
Because common weasels are so strongly specialized to prey on small rodents, especially voles, they are handicapped in New Zealand. Their main advantage over stoats, their greater efficiency as specialist hunters of small rodents in tight spaces, is of little use to them where small rodents are always hard to find. The only alternative small prey, mostly large native insects and lizards, were insufficient substitutes for voles. Stoats, on the other hand, have the range of large and small prey that they prefer (see Figure 5.4), and they have hardly any larger predators to avoid. The consequence was that the balance of advantages was permanently tipped in favor of stoats.
Common weasels are never likely to be abundant in New Zealand but, even after decades of disadvantage, they are not yet extinct either. The reason they have survived despite the lack of voles is probably that the two main islands of New Zealand are very large and diverse, giving them plenty of room to disperse to find habitat patches with sufficient mice, and to avoid stoats. Where stoats are being effectively removed, weasels seem to be becoming more common.
Matters were different, however, on Terschelling, a much smaller island (680 km2) off the Dutch coast. As in New Zealand, many more common weasels were released than stoats, and like New Zealand, Terschelling has no voles. By 1934 the common weasels had disappeared, while the stoats were well established. The size distribution of prey available on Terschelling was much better for common weasels than was that in New Zealand (King & Moors 1979a), which suggests that common weasels might have survived on Terschelling if there had been no interference from stoats, but on such a small island, a weasel could not always avoid meeting a stoat in a tight corner.
The same hypothesis also offers an explanation for the puzzling distribution of stoats and common weasels on the offshore islands of Britain. Eleven of these islands, including relatively small ones (<60 km2) are, or have been, occupied by stoats, but only four by common weasels—all large (>380 km2) or connected to the mainland by bridges.
This is odd, because one might expected the small common weasels to survive better than stoats in a restricted area. But in fact, for common weasels, the size of an area is less important than its accessibility. Almost all populations of common (or least) weasels and stoats become locally extinct periodically, and depopulated habitats have to be recolonized from further afield. On small islands with voles but without stoats, common weasels are too vulnerable to local extinction to last long. On larger islands they might be better off, except that they are vulnerable to interference from stoats.
Only if the island is large enough to support diverse habitats, or is easily recolonized, can populations of common weasels survive. Terschelling, one of the few islands on which we know that both arrived in numbers sufficient to found a population, is twice the size of the Isle of Wight (380 km2), the smallest British island on which both may be found together, but like New Zealand, Terschelling has no voles.
Ireland is much larger than Wight, and the absence of common weasels there is a mystery. One suggestion (King & Moors 1979a) is that common weasels were there in earliest postglacial times, along with a whole fauna of hardy northern species including stoats and lemmings (Yalden 1999), but that they disappeared later when the lemmings became extinct and were not replaced by voles. Yalden rightly points out that there is no evidence for this idea, which is true, but in the fossil record, more than in most places, absence of evidence is not evidence of absence.
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