Goodbye possums
Using novel techniques to drop 1080 poison, a research group has shown it’s possible to kill every possum and rat in an area. Will Harvie reports.
There are many problems with current standard 1080 drops to control rats and possums. One is that they aim only to suppress pests, not eliminate them. As a result, the animals eventually re-populate treated areas and repeat applications of 1080 poison are needed every three to five years.
A group of researchers wondered what it would it take to eliminate possums and rats completely from an area using 1080 poison.
To find out, they went to a 2300 hectare wedge of land at the confluence of the Jackson and Arawhata rivers about 50 kilometres south of Haast in Westland.
It’s a classic forest of podocarp and beech trees. Close to where the rivers meet, the wedge is flat. Towards the back, the Thomson Range rises steeply to almost 1200 metres.
There were an estimated 18,400 to 34,500 possums living on that wedge, according to a survey done in June. That’s eight to 15 possums per hectare, or with a little maths, about 11.5 possums per international rugby pitch (a rugby pitch is roughly 1ha).
That’s considered ‘‘high’’ by Zero Invasive Predators (Zip) – a partnership between the Department of Conservation and the philanthropic Next Foundation that’s pioneering techniques to rid New Zealand of rats, possums and stoats and which conducted this trial.
Surprisingly, there were almost no rats in the wedge, fewer than two per rugby field.
In July, Zip spread 1080 from the air, using a number of novel techniques.
At the end of August, more than two months after the survey, not a single possum or rat was detected alive in a representative sample plot within those 2300ha. Not one.
‘‘The complete removal of possums and rats appears to have been successfully achieved within the treatment area,’’ concluded a Zip technical report on the trial.
‘‘This is possibly the first time that aerial 1080 has been shown to successfully remove all rats and possums from a site.’’
These novel 1080 techniques might be used one day to help rid all of New Zealand of possums and rats. It could be one tool in the toolbox that gets the country to being predator free by perhaps
2050.
‘‘If we are successful, and we also successfully develop techniques to prevent possum and rat invaders from re-establishing, then the large-scale repeated application of aerial 1080 may no longer be necessary to protect New Zealand’s biodiversity,’’ another Zip report on the trial concluded. Zip called the trial 1080 to Zero. As 1080 was used, other animals were also found dead in the sample bloc – three deer and seven birds, two of them native tomtits.
‘‘We’re not pretending those numbers are the only things we killed,’’ said Zip chief executive Al Bramley in an interview. Rather, those are the numbers that were picked up in the sample bloc.
The reports also note that no kea were found dead after the 1080 application, although their numbers within the wedge were low.
Zip started the trial with two ‘‘pre-feeds’’ of non-toxic 1080 lookalike bait. It ‘‘teaches’’ possums and rats that the food is safe so that they’ll more readily eat the toxic version of the bait when it is applied. In standard current
1080 drops, one pre-feed is applied. This is one of the novel techniques.
Another technique was to use a 50 per cent overlap of the bait swathe . ‘‘Effectively, bait was applied everywhere twice,’’ the technical reports states. Again, this isn’t generally done in standard drops.
A third novelty was to sow toxic bait to the edge of the rivers, which isn’t normally done to prevent toxin from entering the water. The point was to remove any ‘‘exclusion’’ zones within the wedge so that every animal present was exposed to the bait.
The fourth novelty will likely alarm some: Zip used ‘‘twice the amount of toxin compared to a standard 1080 operation’’.
People including those with
anti-1080 sentiment sometimes focus on the methods and not on the goal, says Bramley. The goal is to clear New Zealand of these predators to preserve and then grow biodiversity, he says.
For 55 days after the last 1080 drop, rangers monitored a ‘‘core detection area’’ of 394ha with cameras, chew cards and tracking tunnels that reliably turn up evidence of live possums and rats.
This was the representative sample plot and worked something like political polls that sample small populations to extrapolate what the entire voting public thinks about politicians.
And like polls, there’s room for error. Finding no live possums or rats in the sample plot doesn’t mean they weren’t there, just that they weren’t detected. It’s also the case that possums and rats may have been alive outside the sample plot.
But representative sampling is statistically valid and Zip thinks it got to fewer than one possum per
400ha, the point where the population is ‘‘no longer functionally viable’’.
‘‘For possums it looks like it’s doable [complete elimination],’’ says Bramley. ‘‘For rats we’re not so sure and that’s largely because in Haast our starting population was very low.’’
With the experiment over, it is presumed possums and rats have started re-invading the wedge.
But Zip has plans for another trial that will attempt to permanently defend this area from re-invasion. The location is still to be confirmed. Bramley expects rivers will do most of the defending, especially for possums.
But Zip will also deploy trapping knowledge and techniques it has been refining at a research facility in Marlborough for several years. That research shows possums moving into a new area will be caught by traps placed about every 50ha.
Zip will also expand the drop zone to about 10,000ha, an area about four times larger than was targeted this year.
Zip tends not to publish its research in scientific journals but posts reports and results to its website, zip.org.nz.