Timing of Predator-Control Drops Key to Stopping Rat Rebounds, Study Finds

Aerial drops of the poison 1080 sharply reduce populations of rats and stoats across New Zealand's conservation land, but the predators — especially ship rats — can rebound to higher numbers within a few years, according to two studies published in the New Zealand Journal of Ecology. The researchers, led by Susan Walker, say repeat 1080 operations need to be timed to local conditions and the species being protected, because the speed of rebound varies sharply between regions.

The first study analyzed more than 591,000 tracking-tunnel records collected from 1998 to 2022 across 13 conservation regions. It found that 1080 is highly effective at reducing ship rat and stoat numbers in the one to two years after use, consistent with improved survival among vulnerable species such as whio (blue duck), pīwakawaka (fantail), kiwi and kākā. Mouse tracking rates recovered within six to 12 months, then declined again as ship rat populations rebounded to higher levels than in unmanaged areas over the following two to three years.

The size of these rat rebounds varied greatly by region, with much larger rebounds in warmer, low-elevation northern forests than in cooler southern and eastern South Island regions — a pattern the researchers say likely reflects the food surplus available to rats that survive control operations. Stoat activity recovered more slowly than rodent numbers, requiring the longest periods of control in cooler, drier South Island regions. The researchers found that indigenous species are likely to be protected by 1080 only if operations are repeated at frequencies suited to the local predator, giving the example that kākā populations in South Westland would grow if 1080 operations were carried out every two or three years to prevent stoats from recovering.

A second study, using the same two decades of tracking data, examined whether rat populations are expanding into cooler, higher-elevation forests where low temperatures previously limited their presence — a shift that would threaten refuges for native birds and bats. The researchers found no overall trend from 2006 to 2022, despite high-elevation rat outbreaks following large mast seedfall events such as the 2019 "mega-mast," and said longer time series are needed to detect any emerging pattern given the large year-to-year fluctuations in rat numbers.

The second study also found that rodent dynamics differ between eastern and western forests: mice dominated population surges after mast seedfalls in eastern beech forests, while rats and mice irrupted together in western forests and stayed more active between mast events. Unexpectedly, the highest stoat tracking rates were found in wet, mid-elevation western forests where ship rats and mice are usually scarce, a pattern the researchers say is not yet well understood and warrants further study.