New Zealand

For weasels, home range area clearly does not depend only on body size of the hunter, but also on the type and abundance of the prey. Such a generalization can often be tested by watching how animals react to new situations. The introduction of stoats to New Zealand provides just such an opportunity. The home ranges of stoats have been studied in several different ecological communities there, supporting prey resources completely unlike anything in the northern hemisphere. The story that emerges is, in fact, mostly nothing new, and is important because of that.

Murphy and Dowding (1994, 1995) radio tracked stoats in a southern beech forest during two summers when feral house mouse populations boomed and then busted. When mice were abundant in 1990-1991, three males had mean ranges of 93 ha, and four females, 69 ha, and the stoats were often active during the day. In the same area in the next summer, when mice were scarce, the numbers were 204 ha for four males and 124 ha for five females. In another southern beech forest when mice were scarce, the home ranges of four males averaged 223 ha, and of seven females, 94 ha (Alterio 1998). The home ranges of females were evenly spaced and overlapped slightly at a few extreme data points. The same was largely true of males, except that the home range of one male did overlap the home ranges of the others. In yet another habitat, a coastal grassland with grazed and ungrazed portions interspersed with dense scrub, the home ranges of six male stoats averaged 133 ha and of two females 83 ha. In this area, stoats preferred the ungrazed patches intermixed with coarse, woody vegetation (Alterio et al. 1998; Moller & Alterio 1999), where they hunted both by day and by night (Alterio & Moller 1997a).

In two podocarp forests in south Westland, New Zealand, a habitat completely different from either beech forest or coastal scrub, Miller et al. (2001) fitted 27 stoats with radio transmitters and tracked them from July 1997 to May 1998. They collected extensive data on 19 animals, and analyzed them by an unusually sophisticated series of methods—two for home ranges (minimum convex polygons and restricted edge polygons) and a different one for core areas (hierarchical cluster analysis). The mean size of the male home ranges across all seasons calculated by the minimum convex polygon method, most useful for comparing with other studies, was 210 ha, and for females, 89 ha. Males extended their ranges during the breeding season (256 ha) compared with the nonbreeding seasons (149 ha), but females did not. Home ranges overlapped within and between sexes in all seasons, most extensively between male and female ranges.

These comparisons suggest that the large stoats in New Zealand generally do have larger home ranges, closer to those recorded in Scotland by Pounds (1981) (where a single male used 254 ha in 10 days, and three females averaged 114 ha) than those of the smaller stoats of Quebec farmland (20 to 40 ha in males). However, these comparisons must be made with caution, because differences in prey density are critical. New Zealand forests of both types have no voles, and when feral house mice are scarce, that is, in most years, these forests may offer only marginal habitat for stoats (Murphy & Dowding 1994). In years of heavy seedfall in New Zealand beech forests, high densities of mice are quickly followed by high densities of stoats, which live on smaller home ranges (Murphy & Dowding 1995) and are nonterritorial (Alterio 1998). Even during a postseed-fall mouse plague, the large male stoats of New Zealand could not survive for long on a Quebec-sized home range of only 20 ha.

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