the Predator-free 2050 policy is explicit that the eradication of stoats, rats and possums cannot be achieved without advanced, yet-to-be-developed, technologies like gene editing.

OPINION: Gene editing technologies have been getting a huge press recently for an enormous range of applications but especially in human health and food production.

It has become a hot topic in New Zealand too, though not for medicine or farming, but for conservation. Some are proposing that we release animals, like rats, into the wild that are genetically engineered to produce infertile offspring.

Indeed, the Predator-free 2050 policy is explicit that the eradication from NZ of stoats, rats and possums cannot be achieved without advanced, yet-to-be-developed, technologies like gene editing. Some in government and science are advocating for it. Money is being invested to explore the possibility.

But you can't pick a fight with evolution and win.

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* The technology behind plans for a predator-free New Zealand

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The offspring of parents carrying a synthetic infertility gene can develop genetic immunity to it and breed anyway. Not only are they fertile but their offspring are more likely to produce fertile grandchildren even when their future mate also has genes for infertility. This was shown recently in laboratory experiments with mosquitoes.

Any gene, therefore, that reduces fertility, even a synthetic one that is "driven", will eventually be made impotent by adaptation and evolution.

Immunity is not just genetic, however. Immunity can be ecological too. Behaviour, particularly choice of a mate, complicates sex for mammals much more than it does among insects like mosquitoes (any sexually active, or hopefully active, person can tell you how significant behaviour is to sex and getting more of it!).

Gene-edited mammals may be detectably "different". These small differences might not be apparent to people but they can be to other rats. This can be because gene editing introduces other, unintended errors into the genome that are also expressed and inherited.

Potential mates in the wild that think animals with synthetic genes aren't sexy will prefer to mate with fertile peers. And because they will have more offspring, their genetic or learned ability to detect a genetically synthetic "fraud" is spread through the population – thus rendering gene-edited animals evolutionary "losers".

The chances of immunity developing are also greater if gene-edited animals spread through a wild population slowly. It will take many generations after their release for animals with synthetic genes to become widespread and common enough to work their infertility 'magic'. If immunity can develop among a few hundred mosquitos in a one litre laboratory flask after five generations, as recently shown, then immunity is highly likely among the millions upon millions of rats spread across New Zealand.

Two ideas to prevent immunity developing have been suggested. One is to edit many genes so that if immunity develops in one the others still cause infertility. The second is to release so many gene-edited animals that the population is overwhelmed and declines rapidly before immunity develops.

The problem with the first idea is that gene editing is not perfect. It generates other changes and errors in the genome. There is debate among scientists about how many unintended errors occur but it is certain that they do occur.

The more gene editing we do the more errors will occur, thus increasing the chances that the organism is "different" and unattractive to wild mates or that we engineer a genetic variant that has unintended consequences.

The problem with the second idea is that quickly overwhelming wild rats on a national scale will require the release of very large numbers of genetically engineered animals. Is it environmentally sensible to substantially increase rat numbers in our cities and towns, and on farmed and conservation landscapes, for the several generations necessary to eventually suppress their numbers?

The raised rat numbers would increase rat predation of native biodiversity and elevate their existing risk to human and livestock health. Larger, exotic predators that eat rats, like cats, stoats, ferrets and weasels, would also benefit and increase in number, only to then switch to also killing native biodiversity when the rats decline.

The massive release of genetically engineered predatory pest species at a national scale is just an obviously silly idea and I am aghast that we are considering it.

Gene editing also poses societal and political risks that I haven't discussed. And they make the technology even less likely to be applied.

Putting our efforts into 'silver bullet' technologies for small mammal pest control, like gene-drive, that cannot deliver what advocates of it are promising, is the wrong approach to biodiversity conservation. There are better biodiversity goals than eradication of select pests. And there are more effective technologies and strategies for biodiversity recovery already available or nearly developed.

Gene-drive cannot solve the single most significant impediment to nationwide predator eradication, which is killing the last few, elusive animals.

Gene editing will have many uses in the future, but it is not a panacea for mammal pests.

Dr Wayne Linklater is Associate Professor of Conservation Science at Victoria University and director of its Centre for Biodiversity and Restoration Ecology.