Foods Of Stoats And Common Weasels Introduced Into New Zealand

One of the constant themes of this book is the close relationship between weasels generally and the various species of northern hemisphere voles with which they evolved. So close is this relationship that one might predict that no species of weasel could survive where there are no voles. In New Zealand, history has provided a test of this idea.

The stoats and common weasels transported from Britain to New Zealand in the nineteenth century joined a simple community of introduced mammals. The range of body sizes of prey available was different from anywhere in the northern hemisphere: fewer small mammals of mouse size (0 to 50 g), and more of rat (80 to 200 g) and rabbit size (500 to 2,000 g). The only competing predators were feral cats (then still scarce), and the native morepork owls and bush falcons. So, the forests of late nineteenth-century New Zealand were not only well stocked with food for stoats, but also offered them shelter from the harrier hawks and ferrets that preferred more open country.

Not surprisingly, stoats moved into the New Zealand forests at once. Even though their new home contained a radically different array of potential meals, compared with their native land, stoats adjusted quickly and thrived. Common weasels, however, were (and still are) less adept at hunting grown rabbits than stoats, and they found the few available prey of small size (feral house mice, native lizards, wetas) an insufficient substitute for voles. New Zealand has proved inhospitable for common weasels: They are not extinct, but they are scarce and patchy in their distribution compared with stoats.

It would be interesting to know what the first colonizing stoats ate, but that, of course, is now not possible to discover. Almost the only thing we can be sure of is that the Australian brushtail possums, introduced for their fur and now a widespread and important source of carrion, were not on the menu, because they were still only just getting established when the mustelids first arrived. Fortunately, it is just as interesting to learn what contemporary stoats eat, because this gives us unusual insight into how a predator adjusts over time to totally unfamiliar prey.

A survey of the biology of stoats during the 1970s, in which King and Moody (1982) collected 1,599 stoats from all ten national parks then gazetted in New Zealand, showed that the most important single class of food for stoats was birds (all species pooled, found in 43% of the 1,513 guts examined). The remaining categories of food were feral house mice, rabbits (and, occasionally, hares), possums and other carrion, rats (mainly ship rats but possibly some Norway rats or, in one area, a very occasional Polynesian rat), geckos and skinks (small lizards), freshwater crayfish, and insects. In forests, birds and insects between them constituted more than half the items eaten, and rats, mice, and possums were also very important (Figure 5.4). In open tussock grasslands, lizards and lagomorphs substituted for possums and rats. Hedgehogs are very often found dead on roads in New Zealand (King 2005a), but in this survey they were the only animals within the expected range of prey sizes that stoats avoided (found only twice).

Insects, mostly weta (native Orthoptera), were found in an astonishing 41% of the total guts collected in the 1970s. Stoats in New Zealand eat insects all year

Figure 5.4 Some representative examples of the food habits of local populations of stoats in New Zealand. (a) Podocarp/mixed forest (44 guts, Rickard 1996); (b) podocarp/mixed forest (52 guts, King et al. 1996); (c) podocarp forest (129 guts, King & Moody 1982); (d) coastal habitat (75 guts, Alterio & Moller 1997b); (e) high-elevation shearwater colony (788 scats, Cuthbert et al. 2000); (f) alpine/southern beech forest habitat (93 guts, Smith 2001); (g) southern beech forest, average (75 guts, King & Moody 1982); (h) southern beech forest, mice few (62 guts, Purdey et al. 2004); (i) braided riverbed habitat (196 scats, Dowding & Elliot 2003); (j) southern beech forest during mouse population high (54 guts, Murphy & Dowding 1995); (k) southern beech forest during mouse population low (54 guts, Murphy & Dowding 1995); (l) "bush" habitat (124 guts, Fitzgerald 1964). Prey designated as in Figure 5.1A.

Figure 5.4 Some representative examples of the food habits of local populations of stoats in New Zealand. (a) Podocarp/mixed forest (44 guts, Rickard 1996); (b) podocarp/mixed forest (52 guts, King et al. 1996); (c) podocarp forest (129 guts, King & Moody 1982); (d) coastal habitat (75 guts, Alterio & Moller 1997b); (e) high-elevation shearwater colony (788 scats, Cuthbert et al. 2000); (f) alpine/southern beech forest habitat (93 guts, Smith 2001); (g) southern beech forest, average (75 guts, King & Moody 1982); (h) southern beech forest, mice few (62 guts, Purdey et al. 2004); (i) braided riverbed habitat (196 scats, Dowding & Elliot 2003); (j) southern beech forest during mouse population high (54 guts, Murphy & Dowding 1995); (k) southern beech forest during mouse population low (54 guts, Murphy & Dowding 1995); (l) "bush" habitat (124 guts, Fitzgerald 1964). Prey designated as in Figure 5.1A.

round, much more often than their relatives do in the northern hemisphere (McDonald et al. 2000). This is understandable; even though a weta supplies only a small parcel of food for a stoat, it is still large enough (2 to 4 g) to be worth snapping up in passing, and available all year round. In the cold parts of their enormous ranges, in North America and Russia, stoats and longtails do eat insects in summer, but only occasionally.

Large prey (possums, lagomorphs, and rats, all introduced) clearly supplied half or more of the total food value for stoats. Birds and mice together supplied only a third, while insects supplied only about a tenth, despite their frequent consumption. House mice, the only small mammal prey available to stoats in New Zealand, are obviously no substitute for the voles that are their mainstay in the northern hemisphere.

Since that first national survey, numerous studies have confirmed that birds generally comprise over half the prey captured regularly by stoats in New Zealand (King et al. 1996; Alterio & Moller 1997b; Murphy et al. 1998). A second large national-scale survey of the foods eaten by stoats collected in 1999-2000 is now awaiting analysis (R. A. McDonald and D. Smith, in prep.), and should provide an interesting comparison with the older data.

An intriguing comment on the use by stoats of road-killed carrion was supplied by Murphy and Dowding (1994) from radio tracking work in the Eglinton Valley, one of the study areas that had been surveyed in the 1970s. They tracked several male stoats that actively preferred to hunt along a road, and were often found denning near road-killed possums. One managed to drag a possum (a deadweight of five to ten times its own body mass) to a nearby den.

Stoats are versatile predators, able to switch their attention to whatever prey are most abundant. Prey switching can bring relief from stoat predation for their staple prey (see Figure 13.3), but only if the alternative is nutritious enough. For example, throughout the massive swings in abundance of mice following a heavy seedfall of southern beech, New Zealand stoats still eat as many birds as usual (King 1983b; Murphy & Dowding 1995)—perhaps because, as experiments in enclosures have shown, stoats cannot gain much net energy by hunting house mice (Raymond et al. 1990). Only when house mice reach plague numbers, and then only for a short period and in only one area identified so far, can stoats afford to concentrate totally on mice and give birds a break (Purdey et al. 2004; White & King in press).

Ship rats are also abundant in some forest types, and they also take many birds and their eggs, so to protect birds, ship rats are regularly removed from some areas (Innes et al. 1995). After one successful program to remove ship rats, stoats replaced the rats lost to their diet with birds (Murphy et al. 1998). Stoats have affected populations of many bird species in New Zealand (McDonald & Murphy 2000), and we will return to the problem of how to meet the challenge stoats present to conservation of the vulnerable endemic fauna of the New Zealand archipelago in Chapter 13.

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