28 Aug 2018

Eliminating mainland predators

From Afternoons, 1:15 pm on 28 August 2018

The ZIP project is on its way to eradicating predators from 12,000 hectares of Westland, using a combination of rivers, AI cameras, animal psychology and egg mayonnaise.

A possum caught on a DoC motion sensor camera.

A possum caught on a DoC motion sensor camera. Photo: DoC

The Zero Invasive Predators, (ZIP) project was set up to develop innovative technologies to completely remove rats, possums and stoats from large mainland areas, and then defend those areas from reinvasion.

ZIP chief executive Al Bramley says they've selected the Perth Valley in the southern West Coast, a few minutes drive north of Franz Josef, and they're hopeful of eradicating possums shortly - with rats and stoats to follow.

He says the area was chosen because of its rivers.

"We tried to see how many possums are actually happy to cross rivers and to our surprise very few are prepared to take a swim to get to the other side.

"Down there [there's] lots of rain and lots of naturally big rivers so in some ways it lends itself to being a naturally defensible place to work."

The Perth Valley area being used as a testing site.

The Perth Valley area being used as a testing site. Photo: DOC / ZIP

- See the map up close

It's also because of the specific flora and fauna in the area.

"Possums love rata, so left unchecked they basically kill the rata trees and we lose our big canopy tree out of that forest.

"What we're trying to do is come up with a way of completely removing the possums so that not only the trees take off but all those other things that possums eat - whether it be weta or birds' eggs - also get protected as part of the parcel."

Remove and protect

He says their plan is to remove all the possums from the area first, then protect the area to prevent more coming in.

"What we think we can do there is use the mountains as a kind of indefensible back door, and take a big branch in two of the river systems and use that as the sort of wedge that we clear possums from, and keep it clean."

"To be honest one of our main tools is aerial 1080 - we like to use it to clear - but once we've used it once we're hoping to get to the point where we don't need to use it again unless it's for spot treatment or something that has an incursion going on."

The remove and protect approach, if successful, will make it possible to:

  • Carry out predator control in terrain where it is neither desirable nor possible to construct predator fences
  • Reduce our dependence on the repeated wide scale application of toxins at chosen sites
  • Enable progressive expansion of a protected area as funds and confidence allow
  • Create an environment on the mainland where, in time, ecological integrity could rival that of predator-free offshore islands

The first six months of the project so far has been largely about setting up the infrastructure, Bramley says.

"Because it's a research and development research site we need to have people in there 24/7, measuring and monitoring and trying to ensure that we know what we're doing and whether we need to tweak things.

"We've put about 60km of route in there, three temporary bivvies and I suppose we've been getting an understanding of what's the starting conditions."

ZIP field team leader Duncan Kay with a new trap that notifies trappers when it's deployed.

ZIP field team leader Duncan Kay with a new trap that notifies trappers when it's deployed. Photo: RNZ / Conan Young

He says they had hoped to clear the valley of possums this winter, but learned something new which meant they had to push that deadline back.

"Possums are up above the snowline. I suppose maybe we anthropomorphised a bit too much, and we thought 'it'll get cold, they'll come down into the bush,' well, they don't.

"They stay under the snow, so when we had more than a metre of snow on the ground, the possums were still living in the tussocks.

"It makes it really hard for us to expose it to an aerial toxin, so we decided 'nah, reset, let's go again next March when we're just going into winter."

AI cameras and egg mayonnaise

Bramley says they're setting things up to scope out how many rats and possums there are, how strong the rivers are, and what strength of river is needed to act as a barrier.

"This is where the electronics comes in … in the not too distant future there will be cameras that have AI, which just sit in the forest at really light density waiting for our key predators to incur, and then we know they're there.

From left to right, a stoat, rat, and possum.

From left to right, a stoat, rat, and possum. Photo: 123RF

"So the ranger can wake up in the morning, have a look at their cell phone and see a text message or jump on a web server and see that last night, 2am, a possum or a rat has incurred.

A tree-mounted automated lure dispenser in the Perth Valley.

A tree-mounted automated lure dispenser in the Perth Valley. Photo: Supplied / ZIP

"What we're trying to get ot is the position where we use low-powered radio so we can talk to our devices anywhere in the most rugged of landscapes.

"Once you know it's there we can act pretty quick."

The cameras are also being used to track rats and stoats, with food being used to lure in the animals - particularly stoats, which are too wily for other methods.

"Stoats are the one that we're having to scratch our heads a lot about. In some ways they're easy because they only breed once a year, but in other ways they're hard because they're the smartest.

"We spent quite a bit of time trying to work out if we can use the bedding of female stoats in season as a lure, and what we found is it's a brilliant lure but what it appears unfortunately is that the stoats suss out that it's a fake and once they come to it once we couldn't get them to come back.

"Right now we're using egg mayonnaise, so across the Perth Valley, seven and a half thousand hectares, we have 82 lure dispensers putting out a little dose of egg mayonnaise every night and a camera trained on that dispenser and we've been able to detect the stoats that are really low numbers on that landscape."

Using pests' behaviour against them

Bramley says some of their tests on possums and rats have yielded some useful information about their habits.

"A few things actually help us, the first thing is when possums are lonely, they don't just live in their normal home ranges of about one to two hectares, they push out to about 100 hectares."


Removing possums from the Perth Valley is a first step for the ZIP project.   Photo: 123RF

The animals' wider range means they are then more susceptible to bait and poison over wide areas. The rats are harder to gauge, but there's some lessons there too.

"Rats are really hard to detect as individuals, but when we whacked some GPS transmitters on rats and looked at what they did, we discovered that the most sensible thing to do is to let the mum breed and then catch the first family, because that has a much bigger footprint that's still containable.

"You catch one rat, you look at it, and you go 'oh, 55 gram juvenile, what are we going to do', and you know that if you treat 100 hectares or so then you can remove it.

Rolling out and scaling up

Bramley says the project is now in a position where they have funding and willpower.

"We are fortunate to have some very good backers for what we're doing and I suppose we run it a bit like a business: if we don't make good progress then we won't exist.

"Unlike, maybe, traditional science funding we're now in the mode where we're about deliverables - meeting this milestone, and then charging on and getting to the next scale."

He says they can figure out how to carve up the country into sections once they know how effective a river is as a pest barrier.

"Our initial trials we started with just small rivers - about three to five cubic metres a second - and they were still significant barriers for possums.

"I think there's real hope, because we might even find that the drain at the back of the farm is actually a reasonably effective barrier for possums provided it's not bridged."

Read more about ZIP:

They are working towards the predator-free by 2050 goal.

"We were fortunate to start before those goals came along. Our team feels like we're in a position to remove possums quite soon and now the Perth Valley is kind of our first scale test.

"Can we do it at about the 10,000ha scale? If we can do it at that scale then the year after or the year after that we'll just take the next chunk of country and clear that as well."

He says they've grown from being 7 staff to 23, which he says is about right.

"What's a little bit unusual with us is about half of our effort goes into engineering, so when we learn and we think we know what we're doing we make stuff that lasts and works in the back country.