Why is eradication of invasive mustelids so difficult?

We conducted two field experiments to explore the reactions of feral ferrets (Mustela furo) to traps and bait dispensers set on pastoral farmland in central North Island, New Zealand. First, in 2004 we showed that only six of 13 radio-collared ferrets resident near four observation stations approached to within 8 m of two stations, and only three of the six entered over 8 days of observation. Five of the 15 ferrets available on the 6000 ha study area eluded recapture, although all remained present. Second, in 2006 we monitored the survival of 23 radio-collared ferrets before, throughout and after a 5-week field experiment, using toxic bait deployed in 20 automated bait dispensers distributed over 2554 ha. Eight ferrets entered a bait dispenser: four entered but did not take the bait; two did not visit but were killed by secondary poisoning; and nine never entered a bait dispenser. After the experiment, intensive live trapping guided by repeated radio-location surveys retrieved only two of 13 collared ferrets that were definitely still alive on the study area. Inefficiency of trapping wide-ranging mustelids such as ferrets, stoats (Mustela erminea) and mink (Neovison vison) is probably commonplace, due to lack of opportunity (if animals take longer to find or enter a trap than it remains available) and/or to active avoidance (refusal to enter traps or to take bait). Our results provide confirmed examples of both, and help explain why short-term or seasonal control of invasive mustelids is often very inefficient, and eradication unlikely.