Shooting Times & Country Magazine
Birth control and bullets
Matt Cross looks into the latest technological advances that are promising to help curb the impact of problem non-native species
Afriend of mine had a mink in his farmyard last week. It was a bold little critter — after casually eating the eggs out of a wild pheasant nest, it posed for photos then slipped off into the nettles. Happily, his neighbour is a gamekeeper and the mink will not be around for much longer. In a strange way, the mink was a trip down memory lane for him. In the 1980s, when his grandparents had the farm, mink were abundant.
They rolled about in the river pool below the house and ate their way through the duck, oystercatcher and plover eggs. Huge efforts to control them, combined with the return of the more formidable otter, saw numbers tumble but, obviously, they were never eradicated.
Mink are far from the only unwanted addition to our fauna. The classic non-native problem is the brown rat, which first reached the UK in the 1730s; the grey squirrel invasion began in the late 1870s; muntjac first bred in the wild in 1901; and, more recently, we have added parakeets and, hidden from view, New Zealand flatworms and signal crayfish.
Each of these species has had a real impact: the grey squirrel has pushed the red to the brink; muntjac have been instrumental in the decline of the nightingale; parakeets compete for nest spaces with native species; flatworms eat earthworms; and the signal crayfish are wreaking havoc along our riverbanks.
By international standards, we have got off relatively lightly. On Gough Island in the South Atlantic, a plague of invasive giant mice ate seabird chicks alive; in Hawaii,
mongoose released to kill invasive rats did for the rats, but then set about the local ground-nesting birds and small mammals. The world, the UK and my friend’s farm have a problem.
If you had mink in your yard you would probably reach for a shotgun or a DOC trap, but many people are not so closely acquainted with the reality of managing wildlife, often demanding a step away from lethal methods of population control and towards something kinder. There certainly seems to be one alternative — contraception. Could birth control take over from bullets as the main tool of invasive species management? The biochemistry of this is quite well worked out, and it has a real history of managing problem non-native species.
Wild West
Like longhorn cattle, cowboy hats and battered pickup trucks, wild horses — mustangs — are an iconic part of the American West. But they only reached the Americas with Spanish settlers in the 16th century and growing herds have had an enormous effect on biodiversity. When managing their numbers by shooting them or rounding them up and sending them to slaughterhouses became politically unpalatable, a set of federal agencies started looking for an alternative.
The first approach was to try to use chemical sterilants to make the stallions infertile. When that didn’t work well enough, they turned to silicone implants for wild mares.
Not entirely unexpectedly, an approach that required catching and restraining large numbers of wild horses was not completely without its problems. So they tried an approach called Porcine Zona Pellucida vaccination or PZP. It uses an extract from a pig ovary to trigger an immune response in the horse. The antibodies
“Mongoose released to kill invasive rats set about the local ground-nesting birds”
produced then block the sites where a sperm could fuse with an egg, rendering the horse infertile. Foaling rates in one herd dropped from 60% to just under 7%; in another herd, the change was from 63% to 18%; and even a less effective form of the vaccine halfed the horses’ fertility rate. It has been met with delight from wild horse campaigners.
The Wild Horses of America Foundation says: “We believe Porcine Zona Pellucida (PZP) is the best tool available to control the growth rate of wild horse and burro herds. As is the case with most drugs, PZP is not perfect. However, we feel that any side effects (real, perceived or hyped) pale in comparison to the negative impacts imposed by round-up and removal strategies.” PZP has been tested on a range of species, from horses to sea lions, and is effective for a lot of different mammals.
There is also another, more recent, innovation in injectable contraceptive vaccines. This uses a copy of a hormone called gonadotrophin-releasing hormone or GNRH and teaches the immune system to produce antibodies to the real version. Doing this disrupts the ‘hypothalamus-pituitarygonad linkage’. In other words, it pulls out one link in the complex chain of hormones that leads to the production of eggs. Like PZP, GNRH works, and studies have found that it significantly reduces fertility in a range of wild mammals, including horses and deer.
On the surface, it looks like we have a solution. We can give injectable contraceptive vaccines to problem species and if it’s kept going for enough generations, populations will finally become unviable. But it’s far from a perfect solution. Wild
horses are not typical of invasive non-natives. They are large animals that gather together in herds and many are relatively easy to approach so getting within the 50m range of a dart gun is feasible. Individuals are also reasonably easy to identify, so it is easy to record how many doses each horse has had — they need two every two years. Imagine trying that with muntjac, never mind mink.
When these approaches have worked with deer it has been with
“An oral product is available for mice and experiments with squirrels are under way”
deer that are not ‘wild’ in the way we would think of it, but with small island populations or, in one case, “on a fully fenced corporate-office campus in suburban New Jersey”. There is also price to consider. Once all the costs are added together, each PZP vaccination costs between $300 and $2,600 (£250 to £2,100) depending on how you do it. It is simply not a scalable solution for controlling large numbers of small animals.
Right dose
Injected contraceptives are not the only method though. There is also a range of oral contraceptive products. Indeed, an effective oral contraceptive is already available for mice and experiments with squirrels are under way. But these only solve the problem of how you get the contraceptive agent into the body, not how you ensure that the right individual gets the right dose or that the wrong animal doesn’t.
Contraceptives also have a curious and somewhat counter-intuitive effect. They don’t flatten populations nearly as well as you might expect. Giving birth to and nurturing young is very physically demanding, so once females are freed from this biological burden they live much longer, meaning fewer young are needed to sustain a population.
As things stand, contraceptives do not offer anything like a replacement for lethal control. But is what we are doing now working? In the mid2000s, I joined a ‘red squirrel ranger’ for his rounds in northern England. He was shooting and trapping greys to protect the reds. “Do you think this will work?” I asked him. “Can you actually stop the greys pushing out the reds?” “Oh definitely,” he replied. “Just as long as someone does this every single day, for the rest of time.”
Therein lies the rub. It is easy for us to dismiss the fantasies of those with little practical experience, but we are at best holding the line. When — and I suspect it is when, not if — the technology evolves to the extent that contraceptives become a useful part of how we manage invasives, it would be foolish for us not to add them to our arsenal.