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September 2018 Issue
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NZ could be possum-free by 2030

Possums can be eradicated, but maybe just not yet, says James Russell

Recent developments in pest control mean possums could be eradicated from New Zealand by 2030, but ecologist Dr James Russell says it might not be a good idea just yet. 

Russell, who is associate professor of biological sciences at the University of Auckland, said developments in the use of 1080 means possums can effectively be eradicated from large areas of forest, while remote monitoring of traditional traps could eradicate possums from other areas.

The number of possums had already halved since the 1980s, from about 70 million to 30 million, due to concerted pest control programmes. But Russell said new 1080 techniques are now making eradication possible. 

“Previously, the mentality was that we can’t eradicate possums from an area because they would just come back,” Russell said. “So we were trying to kill about 95 per cent of possums using the lowest amount of poison possible, and doing that more regularly. But we’ve been learning from island eradications and have been investigating ways to wipe them out in mainland areas.”

Dropping a non-toxic bait prior to a 1080 drop, called a pre-feed, has seen a significant increase in kill-rates, as possums become less suspicious of the poisoned pellets.

The other development involves doing a double drop of 1080, two weeks apart. When these techniques are combined, 99.9 per cent of possums have been killed in trials. The few that survive are so dispersed that they are unable to find a mate to breed, and the population can be eradicated. 

In areas where 1080 can’t be used, like urban and rural land, Russell said traditional leghold traps, also known as gin traps, have proven to be effective. Previously large-scale use of the traps has been limited as animal welfare laws required the traps to be checked every 24 hours. But with recently developed remote monitoring, traps only have to be checked once they’ve been activated.

The other factor is possum biology. Compared to rats and stoats, possums are slow breeding and are therefore easier to wipe out. It took possums about 150 years to spread throughout NZ – in 1950, 54 per cent of the country was possum-free, but now they are found throughout the mainland.

But Russell said just because we can eradicate possums now, doesn’t mean we should.

“It’s still quite expensive using these tools. What if we wait another 10 years, there may be another breakthrough that reduces the cost 10 times?”

Eradicating possums could also cause a boom in rat numbers, which would have an even greater impact on birdlife.

“Possums and rats are in a competitive relationship. If we take out possums we will see rat numbers increase because there will be more food for rats. I see rats as the biggest threat to birds, so it might be better to wait.

“There are no possums on Great Barrier Island, but birds there are still devastated by rats.”

Dealing with rats will likely require another scientific breakthrough. Russell is working on a project to develop species-specific poisons, which would only be toxic to the target species and could be safely used at a large scale.