Effects of rat-poisoning operations on abundance and diet of mustelids in New Zealand podocarp forests

This study aimed to quantify the changes in numbers and diet of stoats, weasels and ferrets following rat and possum poison operations in two podocarp-hardwood forests between 1989 and 1995. Poison operations were classified according to their success in reducing rat numbers, and if they used an acute toxin (1080) or an anticoagulant (brodifacoum or pindone). Stoat catch rates followed the same seasonal patterns as rat footprint tracking rates, and stoat catch rates were positively correlated with rat catch rates. Rat numbers in spring had no significant relationship with the number of juvenile stoats caught in summer. Stoat catch rates did not vary significantly with poison-operation type over a six month period, but all three successful anticoagulant operations resulted in lower stoat catch rates than did unsuccessful operations. Brodifacoum in bait stations may have lowered stoat numbers by secondary poisoning for the first 2-3 months, but thereafter there was no apparent effect. The sex ratio of stoats caught varied significantly amongst the poison operations. The fewest females were caught following anticoagulant operations. Stoat stomachs and intestines contained mostly rats, and some birds and mice. Weasels ate mostly mice, while ferrets predominantly ate lagomorphs and invertebrates. Male and female stoats ate similar proportions of rats, but females ate more mice. Both sexes, but particularly females, ate fewer birds in autumn and winter than in spring and summer. Stoats shifted between eating rats and birds, depending upon the abundance of rats. Thus successful rat-poisoning operations resulted in higher bird consumption than unsuccessful ones. Combining the numerical and functional responses of stoats into a 'bird predation index' showed that stoats are likely to have the greatest effect on birds after successful 1080 poison operations. Diet shifts could not be demonstrated in weasels or ferrets because sample sizes were too small for quantitative assessments. The risk of increased predation pressure on birds from diet-shifting by stoats must be balanced against the predation pressure on birds and other ecological impacts of rats and possums from different poison operations.