Science1 publisher2 min readPublished
Ship rats rebound on regional clocks after aerial 1080 drops
Twenty-four years of tracking-tunnel data from New Zealand conservation land show 1080 suppressing rats and stoats for a year or two, then rebounds arriving faster and higher in warm northern forest than in the cool south.
The Scientist · Science desk
What happened
- Two papers in the New Zealand Journal of Ecology analysed more than 591,000 tracking-tunnel records gathered from 1998 to 2022 across 13 conservation regions of New Zealand public conservation land.
- Aerial 1080 cut ship rat and stoat numbers sharply in the one to two years after each operation, the clearest and most consistent result in the dataset.
- Mouse tracking rates came back within six to twelve months, then fell again as ship rats rebounded over the following two to three years to levels above those in unmanaged forest.
- The second paper found no overall trend in rat activity between 2006 and 2022, even with high-elevation outbreaks after the large 2019 mega-mast seedfall.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- decision A single national reapplication frequency stops being the planning unit: each region paired with each key predator implies its own interval, so an averaged schedule over-protects some sites and under-protects others.
- cost Because 1080 suppresses rather than locally eliminates, protection is a standing recurring line item for as long as the species needs it, not a one-off operation with a durable result.
- exposure Kea and other eastern-forest species can become more exposed exactly when the rodent index looks reassuring, because post-mast rodent scarcity may push stoats and cats onto native prey.
- constraint Managers cannot yet cite climate-driven upslope rat expansion to justify spending on high-elevation refuges, because interannual swings are large enough to hide any such trend in the current record.
The papers attribute rebound size to the food surplus available to the animals that survive an operation [7]. The geography fits that reading: the largest rat rebounds came from warmer zones and northern regions with more diverse low-elevation forest, the smallest from cooler southern and eastern South Island regions [6].
Stoats run on a slower clock. Their activity recovered more slowly than rodent tracking rates, and suppression lasted longest in the cooler, drier parts of the South Island [8]. That asymmetry is what sets an interval. The authors' recommendation is that repeat drops be timed to local context and to the animals being protected [18], and their worked example is South Westland kaka, where they say their findings agree with earlier research that operations every two or three years would let the population grow by keeping stoats from recovering [10].
Put the dataset in per-unit terms: 591,000 records across 24 years and 13 regions averages roughly 1,900 records per region-year [17]. That supports a regional rebound curve over two decades, but it does not support conclusions about any single valley in any single year. It is also about where the recommendation sits, at the region rather than the individual operation.
The interval math says nothing about how many birds an interval buys. Tracking tunnels count footprints, not fledglings, and the survival gains reported for whio, fantail, kiwi and kaka come from other work that the 1080 result is consistent with [3]. A rodent index is also a poor stand-in for a mustelid one: the second paper found geographic patterns in mustelid activity differ from those of rodents, with the highest stoat tracking rates in wet western mid-elevation forests [16][15].
A "rodent outbreak" is not one event either. After mast seedfalls, mice dominated irruptions in eastern beech forest, while in the west rats and mice irrupted together and held higher activity between masts [13]. The predator you are timing against changes with the forest, which is the sense in which a national calendar is the wrong instrument.
What to watch
- Whether operational plans for public conservation land start setting different reapplication intervals by region and target predator rather than one national frequency.
- Whether the next big mast year, added to the series, turns the 2006-2022 null on high-elevation rat activity into a detectable trend.
- Whether monitoring adds stoat-specific measurement in places where rodent tracking rates track mustelid activity poorly.