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

Abstract

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 Z98003 Received 9 January 1998; accepted 22 June 1998 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.

Advances in New Zealand mammalogy 1990–2000: European rats

“…Ship rats are a staple diet item of stoats in New Zealand podocarp-hardwood forests, found in 22-71% of stomachs (Murphy & Bradfield 1992;King et al 1996c;Rickard 1996;Murphy et al 1998;Murphy et al 1999). They are also eaten by feral cats, weasels and ferrets (King et al 1996c;Murphy et al 1998).…”

“…Net outcomes (the balance between so-called 'costs' and 'benefits') of rodent population control must be measured at the community level, because non-target deaths, secondary poisoning, diet switching, and other unexpected responses may counter-intuitively negate the benefits of reducing ship rat numbers (Murphy & Bradfield 1992;Spurr & Powlesland 1997;Murphy et al 1998;Powlesland et al 1998;Innes & Barker 1999). In contrast, scientists looking at invertebrate responses to pest control often study communities rather than single species (Hunt et al 1998;Hutcheson 1999).…”

Brodifacoum residues in target and non‐target animals following large‐scale poison operations in New Zealand podocarp‐hardwood forests

“…Mustelid trapping had been on-going at Mapara since 1990, using 142 Mark 4 Fenn traps set 150-300 m apart along a 24 km trapline (Murphy & Bradfield 1992). Trapping continued until May 1995.…”

“…These predators in forests include stoats (Mustela erminea), weasels {Mustela nivalis), ferrets (Mustela furo), cats (Felis catus), and moreporks (Ninox novaeseelandiae). The effects of poisoning programmes on predators can include changes both in diet and in numbers (Murphy & Bradfield 1992;Murphy, Clapperton, Bradfield & Speed in press). The latter can be caused indirectly by the effects of reduced numbers of prey, or directly by secondary poisoning.…”

The potential for biological control of stoats (Mustela erminea)

“…Effective stoat control may lead to unwelcome responses in rodent populations, which are themselves also destructive predators in New Zealand. Conversely, developing better control methods for rodents (including biological control) may have benefits in the long term for suppressing stoat abundance, provided the ecological ramifications of doing so are acceptable (see Murphy & Bradfield 1992;Murphy et al 1998). Australian research on biological control of mice is well advanced (Robinson et al 1999) …”