Control of a population can be indirect, by manipulating its habitat and food supplies, or direct, by removing more animals each season than can be replaced. Indirect control is usually a side effect of some other process. For example, myxomatosis removed the main prey of British stoats, and thereby achieved a spectacular reduction in their numbers (see Figure 10.2). A similar though more moderate effect was noticed in New Zealand when real control over rabbits was first achieved (by aerial distribution of poison baits) from the 1950s onward (Marshall 1963).
Unfortunately, deliberate habitat manipulation usually does not work for predators that, like stoats, normally live almost anywhere they can find food and cover. For example, at one stage it was suggested that a "grass wall" surrounding the nests of yellow-eyed penguins in southern New Zealand might deter predators from reaching them. In fact, not only did the tall grass fail to keep the stoats out, but it also supported so many mice that it actually attracted stoats to hunt there (Alterio et al. 1998; Ratz 2000).
Actively removing the local rats or rabbits might work better, but may have dangerous side effects if it is done so quickly that hungry predators then switch their attention to birds (Alterio & Moller 1997b; Murphy et al. 1998). Where indirect control is inadvisable or not possible, direct control (what Graham Caughley  called "frontal assault") is the only option.
The great difficulty of achieving effective, long-term control of small, shortlived mammals with high annual productivity like stoats ("r" strategists, see
Chapter 14), is that the control measure(s) must add to, not replace, the natural mortality rate. Weasels of all species are naturally resistant to control, because of their normally high mortality rates—around 70% in stoats (Chapter 11)—and their skill at recolonization. Productivity of stoats is variable but can also be very high, depending on food supplies during the previous breeding season (Chapter 9).
Ideally, the females should be brought to very low numbers before they have given birth, but that is at present very difficult or, in many places, unattainable. One promising alternative is to use trained "stoat dogs" to find breeding dens, and then, depending on the location and construction of the den and the number of exit holes, the whole litter can be humanely dispatched (Theobald & Coad 2002). This is much more efficient than waiting until they are independent and having to catch them all individually, but such a skilled resource is not available everywhere.
These aspects of their population dynamics make the large-scale management of stoats a severe challenge, but a lot of progress has been made in the last decade. Biologists in New Zealand are finding practical ways to reduce stoat numbers, to monitor the results with statistical rigor (Brown & Miller 1998), and to model both current and future control options (Barlow & Choquenot 2002; Barlow & Barron 2005).
New Zealand may be the only country in the world where stoats have become, by a historic human misjudgment, such a widespread and serious invasive pest, but the rapid advances in understanding of stoat biology there are of interest to students of small mustelids everywhere. The earliest days of stoat control to protect wildlife in New Zealand benefited from the ecological knowledge and practical skills developed over centuries both by North American fur trappers (Seton 1926) and by British gamekeepers (Anon. 1981). Now, New Zealand is the world center for research on the ecology and management of small mustelids, and New Zealanders are beginning to reverse the previous flow of information.
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