News release

From: New Zealand Institute for Bioeconomy Science

Study 1: Pest management and population recovery across New Zealand conservation land

This study describes the effects of aerial 1080 predator-control operations on abundance of ship rats, mice, and mustelids across New Zealand forests, using more than 591,000 tracking tunnel records collected between 1998 and 2022 across 13 different conservation regions.

The data show that 1080 is highly effective at reducing numbers of ship rats and stoats in the 1-2 years following its use, consistent with studies showing improvements in nest, chick and overall survival of vulnerable species such as whio/blue duck, pīwakawaka/fantail, kiwi and kākā.

Mouse tracking rates recovered early (within 6 to 12 months) but then decreased again as ship rat populations rebounded to higher levels than in unmanaged areas over the subsequent 2-3 years. The size of rat rebounds varied greatly by region, being much larger in warmer zones and in northern regions with more diverse low-elevation forests than in cooler southern and eastern South Island regions. The size of rat rebounds probably reflects the size of the food surplus available to individuals that survive control operations. Stoat activity recovered more slowly than rodent tracking rates, with the longest durations of control in cooler and drier South Island regions.

Unless predators can be locally eliminated, the work shows that indigenous species populations are likely to be protected by 1080 only if operations are repeated regularly at frequencies that are appropriate to protect the target species from its key predators in a particular geographic region. As an example, the findings of this study concur with other research that kākā populations in South Westland would grow if 1080 operations were undertaken every second or third year to prevent stoat recoveries.

Study 2: Long-term trends in rodent and mustelid activity across New Zealand conservation land

This study models long-term patterns in ship rat, mouse, and mustelid activity using more than two decades of tracking tunnel data from conservation land across New Zealand. The authors investigated how rodent populations have increased or declined over time, how tracking rates vary between different places including between warmer and cooler environments, and whether there is evidence yet that climate change is influencing long term trends. The study also explores differences between rat and mouse dominated pest outbreaks and how mustelid populations respond to changes in rodent abundance.

There has been concern that ship rats are expanding into cooler, higher-elevation forests where low temperatures previously limited their presence, posing risk to important refuges for threatened native bird and bat species. However, this study found no overall trend between 2006 and 2022, despite high elevation outbreaks in rat populations in response to large mast seedfall events such as the 2019 ‘mega-mast’. Because of the large interannual fluctuations, longer time series will be needed to detect any emerging trends.

The study showed that rodent dynamics differed between eastern and western forests. Mice have dominated irruptions in response to mast seedfalls in eastern beech forests, whereas in western forests rats and mice irrupted together and maintained higher activity between masts. Fewer rodents in eastern forests in the aftermath of mast seedfalls may explain greater predation by stoats and cats on indigenous species such as kea.

Another new finding is that geographic patterns in mustelid activity differ from those of rodents. Unexpectedly, the highest stoat tracking rates were in wet western mid-elevation forests where ship rats and mice are usually scarce. The mechanisms controlling the distribution of stoats remain incompletely understood and warrant further study.