With teeth, claws and a strong sense of smell, introduced mammals cause huge problems in New Zealand. To eliminate them once and for all, should we start tweaking their genes?
Illustration by Joanna Beckett
I was about 10 years old when I was first exposed to predator control in New Zealand. I was in Queen Charlotte Sound with my family, at the top of the South Island, when my siblings and I came across a tiny brushtail possum in our room.
We didn’t know it had been deemed an ‘invader’ or a ‘villain’. Actually, we were delighted to see this little creature. It was adorable, terrified and alone; I was completely under its spell and feeling rather protective of it. Unfortunately, that spell was broken when a local man killed the possum with a baseball bat.
“It’s a pest,” he said, with an air of superiority. “We have to kill them.”
From that day, I've had an interest in predator control in New Zealand — not just the technical side to the issue, but also the ethics and controversies that come with it. We are a country known for swerving our cars at possums and holding bunny hunting contests. Last year, a school in the North Island made international news for supposedly drowning joeys after an annual possum hunt.
At first glance, it may seem that New Zealanders are conditioned to kill these mammals. But I have just completed a master's thesis looking at what young adults think of predator control, and one thing that stood out above almost everything else was that people are very concerned about the welfare of these targeted animals.
A common brushtail possum meets its end on a New Zealand road. Jon Sullivan/Flickr (CC BY-NC 2.0)
The issue at hand is one that has plagued New Zealand for nearly 800 years, ever since humans stepped off their canoes onto this new land, bringing with them the islands’ first four-legged, furry creatures. Five hundred years later, another wave of mammals arrived: feral cats, stoats, rats and mice were soon roaming across New Zealand.
The problem is that New Zealand’s native flora and fauna evolved largely in the absence of mammals. Only two species of native, terrestrial mammals exist here, and they’re both bats. Our native animals aren’t well equipped for defending themselves against these new mammals, with their teeth, claws and incredible sense of smell. Many of New Zealand’s native birds are nocturnal, slow breeders and have lost the ability to fly. As such, many spiralled towards extinction.
Over the last 1000 years, New Zealand has lost 40% of its native bird species, at least half of its frog species, and an unknown number of lizards and invertebrates. Today, New Zealand has one of the highest proportions of threatened or endangered species in the world.
The solution to this problem is far from a simple one. It starts with controlling the numbers of introduced mammals and eradicating them in certain areas. The first island mammal eradication took place in 1912 when rabbits were eradicated from Ngawhiti Island. Then, in the early 1960s, rats were eradicated from Maria Island through the use of rodenticide. As eradications became more frequent, they became progressively cheaper and refined. Rats have now been cleared from 117 of our islands. New Zealand is considered a world leader at eradicating mammals from offshore islands, and, with all this success, attention is turning to the mainland.
Large, flightless and slow breeding, the takahē is a classic New Zealand bird. Until its rediscovery in 1948, it was thought to be extinct.Today, the species is classed as endangered, with only 347 known individuals. Bernard Spragg/Flickr (public domain)
“Basically, we’ve got three options,” says Dan Tompkins, a wildlife ecologist for Predator Free 2050 Ltd. “We can keep using 1080, we can let our native species go extinct, or we can try something new.”
Tompkins is leading a strategy team that is researching predator control in New Zealand. Their aim is to find methods that are both effective and socially acceptable.
Sodium fluoroacetate, or 1080, has been used to control introduced mammals in New Zealand since the 1950s. It is a relatively cheap and effective pesticide, but also highly controversial. Application of 1080 requires poison to be literally rained from the sky in bulk aerial drops. Although mammals are much more susceptible to this poison than birds, the method is seen by many as inhumane. In 1995, a hunter from Taupō hijacked a helicopter and held the crew hostage on Mt Tongariro at gunpoint. They remained there for five days until police caught up with them. His reason? He was protesting the use of aerial drops of poison and suspected the men on board were dropping pellets. Over the following two decades, further protests against the poison have been commonplace.
My research showed that young adults were unsupportive of aerial poison drops as a form of predator control, possibly due to the animal welfare concerns or because of other alleged issues. Other well-known methods, such as trapping, bait stations and shooting, were met with wider support. However, these methods are expensive, require considerable manpower and are impossible to use in areas inaccessible to people.
Tompkins and his strategy team are now deciding which predator control methods they will pursue over the next seven years. Their goal is to eradicate some mammal species (possums, rats and stoats) from New Zealand by 2050. This is an ambitious goal, described as New Zealand’s very own ‘Apollo programme’, and will require a new method of mammal control, one that is both effective and socially acceptable. Alongside technologies such as host-specific toxins, better pest lures, and wireless surveillance networks, one of the multiple options being investigated for the Predator Free 2050 goal is ‘gene drive’.
“If it could be developed, pest control based on gene drives could tick a lot of boxes,” says Tompkins. “It has the potential to be humane, requires minimal manpower and could drive a pest population down to zero.”
Gene drive technology is being explored as a control measure for New Zealand's introduced mammals, including stoats, rats and possums. Kevin Law/Wikimedia Commons (CC BY-SA 2.0)
There are two parts to gene drive technology: the ‘drive’ and the ‘cargo.’ Because chromosomes come in pairs, and offspring inherit one chromosome from each pair, an ordinary chromosome has a 50% chance of being inherited. The ‘drive’, in this case, would come from what we call selfish genes. A selfish gene purposefully damages its equivalent on the other chromosome, so that the non-damaged gene (i.e. the selfish gene) is copied into its place. This means that selfish genes are inherited by more than half of the offspring, and therefore can spread through a population more quickly.
“An example of this occurred in the common fruit fly in the 1950s,” says Tompkins. “A random mutation in a population, somewhere in the world, developed what we called the selfish p element, and, within 40 years, every population worldwide had that mutation. Many overseas are currently researching an artificial drive mechanism, known as CRISPR/cas9.”
Then there’s the cargo part. “These selfish genes could also carry a trait that would have a negative effect on their carriers," says Tompkins. "Researchers are looking at the ‘cargo’ being a sex ratio distortion, like, say, 80% of offspring being male and only 20% female. This would spread through the population with the selfish gene. Theoretically, we could release half a dozen individuals carrying these genes, and, over several generations, the population would decrease and disappear.”
The thought that we could eliminate populations of pests without shedding a drop of blood is appealing. But with all the research that has taken place over the last few decades, the question no longer begins with ‘can we?’ but ‘should we?’ Should we be editing genes so that a species will drive itself towards extinction? Even if we think we understand this technology, history is full of cases of good intentions gone wrong. A New Zealand example would be the release of stoats onto our shores in the 1880s. They were originally brought over to control feral rabbit populations but, unfortunately, they enjoyed the taste of our native birds much more.
The idea of editing genes so that a species would drive itself to extinction was first proposed back in 1968, and, since then, most of the focus has been on malaria-carrying mosquitoes. In fact, if this currently theoretical approach comes to fruition, the first gene drive will most likely be used on mosquitoes, by a non-for-profit research consortium called Target Malaria. This group could use gene drive in one of two ways — either to make mosquitoes immune to malaria, or to drive mosquito populations to extinction.
Gene drive technology will most likely be used first to control mosquito populations in regions prone to malaria and other mosquito-borne diseases. Simon Kutcher, AFAP/Flickr (CC BY 2.0)
Another use for gene drive would be to target crop pests that have developed resistance to herbicides — it could be used to reverse such resistance. Research is at the point where gene drive works on lab fruit flies and mosquitoes, so invertebrate pests might not be too far away. And groups throughout the world are looking at ways of using CRISPR/ cas9 to engineer plants to build up resistance against pests. So why couldn’t it be used on introduced mammals in New Zealand and other islands around the world?
While we expect eradication will take a combination of different approaches, having this new option to reduce pest numbers at a large scale would be invaluable. Given how far genetic engineering has come in the last five years, this option may no longer be just theoretical.
One issue, however, is the potential for one of these genetically modified individuals to get overseas and cause worldwide extinction. If a genetically modified possum got back to Australia, it could be disastrous for a species that is fully protected just across the ditch. And, every year, there are hundreds of rats that come and go from New Zealand as stowaways on ships.
“If a modified individual were to get overseas, we’d have violated the Cartagena Protocol on Biosafety, which is an international agreement that GMOs are not to cross national borders," says Tompkins. "But, say that a modified individual had an impact on a native population, we may have violated the UN convention on biological weapons. We can’t use it until we are confident that we understand it.”
A major concern with gene drive technology is that affected individuals may stowaway to neighbouring populations. Jan van der Crabben/Wikimedia Commons (CC BY-SA 2.0)
There are a few different ideas that are being explored to control gene drive. One option, called Daisy Chain gene drive, would mean that the gene drive would be designed to go to a peak level and then crash and disappear. Another option is looking at the specific genetic components of island populations and seeing if those unique elements can be targeted. This has potential for islands with strict biosecurity rules. Clearly, there’s a lot of work to be done, but Tompkins believes researchers might have something working for mice in captivity by 2020.
“It won’t be here,” he explains. “It’ll be somewhere overseas, but mice are the obvious mammal to start with and quite a tricky pest in their own right. It’s very hard to eliminate mice from eco-sanctuaries. Getting gene drive to work on rats would be harder, since no-one yet has the tools to genetically manipulate them like they can mice.
But there’s a general consensus that research should continue. In fact, there’s a global partnership called Genetic Biocontrol of Invasive Rodents (GBIRd), which is researching the potential use of gene drive, not just the technological side but also the social and political parts to it. Invasive rodents are a problem for two main reasons. New Zealanders are interested in getting rid of them for conservation purposes, but, globally, the biggest problem with rodents is food security. They cause crop loss on a massive scale.
Of course, we need to consider the social acceptance of gene drive in New Zealand. The majority of New Zealanders oppose the use of GM applications, so would releasing genetically modified mammals into the environment be accepted by the public? In December last year, it was widely reported that gene drive research was being partially funded by the US military, and that New Zealand islands were being sized up for trialling this technology. However, my research showed that young adults may not be against the use of gene drive as a predator control method, or at least they acknowledge that it has potential. In total, 49% of respondents were supportive of gene editing as a possible control method, whereas 22% opposed it. This was very different to the view of aerial poison drops, which 57% opposed.
Gene editing is just one option in the toolbox of predator control methods. There is no silver bullet that will save New Zealand’s birds. Our species are disappearing at rapid rates and it’s highly unlikely that we’ll have them all at the end of the century. But to save as many as possible, we need to explore every avenue we have.
Edited by Andrew Katsis
