In a mossy woodland on the northwest coast of Wales, Craig Shuttleworth pulls off a dirt road and parks his battered Land Rover. Leaping over a stone wall, the tall, wiry biologist checks a trap, where a gray squirrel paces anxiously. Shuttleworth kneels, calmly slides a sturdy plastic sack around the trap’s door, and blows into the cage. The squirrel, fearing the human scent, darts into the bag. The biologist quickly rolls up the sack to immobilize the animal. “I don’t like doing this,” he says, picking up a heavy stick worn smooth with use. “But they don’t belong here.”

THWACK! THWACK! The bludgeoning fractures the squirrel’s head. It is another casualty in a long war against one of the world’s most invasive animals, the Eastern gray squirrel. In the 140 years since the species was introduced from North America, the gray squirrel has spread across most of the United Kingdom. Along the way, it has muscled out the native red squirrel, which is considered endangered in the country.

Shuttleworth, a conservation biologist with the Red Squirrels Trust Wales, and other scientists appear to be finally turning the tide. In 2015, the trust declared the Isle of Anglesey—separated from mainland Wales by a narrow strait—free of grays, thanks to an eradication project that the 45-year-old Shuttleworth led there for 18 grueling years. This summer, culling will begin in earnest here on the mainland. “Red squirrel conservation is blossoming, because we’ve got proof that we can eradicate gray squirrels from the landscape,” he says.

The red squirrel’s range spans northern Europe to Asia, but it is especially beloved in the United Kingdom. Prince Charles, for one, thinks it should be a national mascot. Perhaps its popularity is due to Beatrix Potter, who wrote a children’s book in 1903 called The Tale of Squirrel Nutkin, set in the Lake District. Or maybe it is memories of Tufty Fluffytail, a cartoon squirrel that for decades taught road safety to children. Whatever the reason, Brits are enamored with the creature. “People have a real pride and passion for them,” says Zoe Davies, an ecologist at the University of Kent. “There’s a huge amount of excitement and determination to protect the red squirrel.”

In the United Kingdom, the species needs all the help it can get. Not only do gray squirrels normally outcompete the reds for food and habitat, they also carry a deadly virus called squirrelpox. Gray squirrels are immune, but when the reds catch it, they quickly succumb to the gruesome disease. There are no reliable estimates of overall populations, but grays likely outnumber reds 200 to one. Perhaps 135,000 reds live in Scotland and northern England, a fraction of earlier numbers. Farther south, a few thousand persist mainly on islands free of gray squirrels, such as Anglesey and the Isle of Wight. Conservationists have defended the northern refuges with large culls, despite adamant opposition from animal rights groups.

Yet even the most ardent advocates admit that victories are fleeting; without constant counterattacks, gray squirrels advance inexorably. Some advocates hope that the recovery of the pine marten, a relative of weasels and badgers that preys on gray squirrels, might provide long-term relief for the reds. Scientists caution, however, that much about the pine marten’s resurgence and ecological impact remains unknown.

The plight of the United Kingdom’s red squirrels is a cautionary tale for the rest of Europe. The gray squirrel has colonized nearly 2000 square kilometers of northwest Italy. Delayed by lawsuits from animal rights groups, biologists there missed the chance to eradicate it, giving grays an opening to spread into France and Switzerland, and ultimately to devastate red squirrels across much of their range. “The real lesson is that it’s very hard to stop this invasive species,” says Colin Lawton, a mammal ecologist at the National University of Ireland, Galway. “The opportunity is to catch them early before they become established.”

Gray squirrels first gained a foothold in the United Kingdom in 1876, when a wealthy silk manufacturer released a pair on his estate in Cheshire. Bigger, bolder, and easier to spot than the secretive red squirrels, the grays charmed aristocratic collectors. The most ardent enthusiast by far was the 11th Duke of Bedford, Herbrand Russell. In 1890, he set 10 free on his estate about 65 kilometers northeast of London. He also dispersed the species by giving away offspring, including six pairs as a wedding present to a friend who released them from his castle in Ireland. (All Ireland’s grays are descended from those squirrels, genetic studies have shown.)

By the early 20th century, biologists knew that populations of gray squirrels were booming. And they soon noted problems: The grays were damaging young trees by stripping bark with their claws, digging up flower gardens, and raiding bird nests. “I know of more than one patriotic Englishman who has been embittered against the whole American nation on account of the presence of their squirrels in his garden,” an ecologist wrote in 1931. In 1937, the U.K. Parliament banned the introduction and possession of gray squirrels.

Even earlier, scientists sounded the alarm over a troubling phenomenon: Where gray squirrels established colonies, red squirrels sooner or later vanished. Although rarely aggressive toward red squirrels and no more prolific as breeders, grays appear better adapted to broadleaf woodlands. That’s primarily because grays can digest acorns, an ability they evolved in the oak-hickory forests of eastern North America. But in 1930, a University of Oxford ecologist proposed another reason for the reds’ decline: The grays might be transmitting a disease.

That hunch was right. In 1981, researchers identified the culprit as a Parapoxvirus (the taxonomy is not settled), and experiments 20 years later confirmed that the virus kills red squirrels while sparing grays. Grays can shed the virus in scat and from scent glands, and reds somehow pick it up, perhaps through their own scent glands as they mark territory. Fleas can also spread the virus, which may happen when grays investigate red squirrel nests. Once the virus slips into a population of reds, it spreads quickly.

Gray squirrels presumably evolved immunity in North America. But red squirrels are defenseless. The virus causes weeping sores, particularly around the digits and face. The eyelids can crust over completely with scabs. Most squirrels die within a few weeks, baffling researchers. “No one really understands why it’s causing mortality,” says Colin McInnes, a virologist at the Moredun Research Institute in Penicuik. One idea is that sick squirrels can’t eat or drink, but some dead animals have been found hydrated and nourished. Another theory behind the population collapse is that lethargic, sensory-deprived animals may be an easy target for foxes, raptors, and

other predators.

Whatever the reason, the virus was decimating the red squirrel, says Peter Lurz, an independent biologist based in Randersacker, Germany, who has studied red and gray squirrels in the United Kingdom for more than 25 years. As reds succumb, gray squirrels quickly take over the habitat. When the disease is present, their range can expand by as much as 34 square kilometers per year—25 times faster than when the red squirrels are healthy, Lurz and colleagues have found.

Squirrelpox’s grisly symptoms boosted public sympathy for the red. “You see the animal you cherish die a horrible death,” Lurz says. But the sole practical remedy—killing gray squirrels en masse—disturbs animal rights advocates. Some challenge the premise that reds, as a native species, deserve more protection than grays. Animal Aid, a U.K. animal rights organization, adds that humans have themselves to blame for worsening the red squirrels’ plight; they were once considered a pest, and foresters killed untold numbers in the 1900s.

But the grays are now the real enemies. In the 1950s, a government bounty hardly dented the population. More recent eradication attempts, such as a 3-year experiment in Thetford Forest in Suffolk, also failed to push back the grays. It’s not for want of trying. In Northumberland, Rupert Mitford, the 6th Baron Redesdale, has claimed to have had more than 23,000 gray squirrels killed on his estate and beyond. Prince Charles has ongoing culls on his properties in Scotland and in Cornwall, where he hopes to reintroduce red squirrels. To have a chance at success requires more than persistence. “You need a situation that is defensible,” says Chris Thomas, an ecologist at the University of York. “If you can’t do control to the point of exclusion, you might be throwing good money after bad.”

The only unalloyed victory against the grays has been on Anglesey. The 714-square-kilometer island is fairly secure, because squirrels can reach it only by scampering across bridges. Grays first invaded it in the late 1960s. By 1998, just 40 or so red squirrels remained. Then, an avid 87-year-old conservationist named Esmé Kirby began a campaign to remove the grays and hired Shuttleworth, not long out of graduate school. By 2010, Shuttleworth’s team had trapped and killed more than 6400 grays. As the population thinned, virus prevalence dropped, Shuttleworth and colleagues reported in 2014 in Biological Invasions . The team caught about a dozen gray squirrels in 2012 and just one the next summer. “It’s amazing what they’ve done,” Lawton says. Reds have bounced back, aided by translocation from zoos, and now number at least 700.

The next step is to defend Anglesey with a 165-square-kilometer gray-free zone on the mainland. Funding will come from Red Squirrels United, an umbrella group of 32 organizations that has several million pounds in grants from the European Union and the U.K. Heritage Lottery Fund. Anglesey is not their only point of attack. The group will train 1250 volunteers to trap and kill grays, including in northern England’s Kielder Forest, which has many reds. In addition, the group aims to secure 128 square kilometers of red squirrel habitat in Northern Ireland.

By far the largest redoubts are in Scotland and northern England, which together hold the vast majority of the population. These regions are dominated by lodgepole pine, Sitka spruce, and Scots pine—trees more to the liking of red squirrels than gray. The red squirrels may also benefit from new plantations that connect once-isolated forest patches, boosting the squirrels’ genetic diversity. Another major advantage for Scotland is that the squirrelpox virus didn’t arrive there until 2005, so reds have been largely spared the devastating crashes seen to the south.

A collaboration called Saving Scotland’s Red Squirrels (SSRS) in Edinburgh has a three-pronged defense. The Scottish Wildlife Trust and environment agencies are killing infected gray squirrels in southern Scotland to curtail viral outbreaks. Second, they are working with landowners to wipe out grays in a highland line in order to defend red-only habitat to the north. And staff and volunteer trappers are blitzing the region around Aberdeen, which has the only gray squirrel population north of the highland line; grays were released there in the 1970s and have not yet spread widely, which means that eradication is feasible. “Things are going well, but I don’t underestimate the challenges,” says Mel Tonkin, project manager of SSRS.

An unexpected piece of good news has come with the return of the pine marten. The enemy of farmers and gamekeepers because of its taste for chickens and pheasants, this cat-sized predator was nearly exterminated in the 20th century. After receiving full legal protection in 1988, the species began to rebound, and several thousand pine martens now roam the highlands. In 2007, Scottish foresters near Perth noticed that gray squirrels were less common in places where pine martens turned up. Farmers in the Irish midlands noticed a similar trend. Following up this clue, Lawton and his former Ph.D. student Emma Sheehy detailed the first known population crash of invasive gray squirrels in Biodiversity and Conservation in March 2014.

Red squirrels appear to be quickly recovering in the places where grays are gone, Lawton says. “It gives me faith that the red squirrel has a future.” One explanation for the pattern: Gray squirrels might be easier for martens to catch. They tend to hunt on the ground, where grays search for beech nuts and acorns. Reds tend to stay in trees and nibble cones.

To boost the number of predators, the Vincent Wildlife Trust has set 20 martens free in Wales. The animals appear to be thriving; last month, researchers spotted five kits. The trust plans to release another 20 adults this fall. It is unclear how many pine martens would be needed to permanently keep gray squirrels in check, or whether they might end up ravaging reds as gray squirrels dwindle. And red squirrel advocates worry that the pine marten could be a false hope, promising a free and uncontroversial solution that could threaten funds for culling. Lawton agrees: “The real concern is that everyone stands back and assumes everything is fine.”

For the time being, gray squirrel control remains in human hands. For Shuttleworth, that means more long hours prowling the woodlands in search of invaders. Walking along a dirt lane with a bloody sack slung over his shoulder, he reaches his Land Rover and tosses several furry carcasses onto a heap of traps in the back. He’s had a productive day, but knows that the dozens of other traps he has set will soon claim more victims. “It’s like fighting the undead,” he says. “They just keep coming.”

From this vantage on the mainland, Anglesey’s medieval castle, built by Edward I to conquer the Welsh, can be seen across the Menai Strait. Red squirrels now have the run of the woods near the ruins. “I like the idea that my children will have a chance to see these creatures,”

Shuttleworth says. “We won’t give up on them.” The odds are daunting, but he is committed to slaying the gray invaders and safeguarding the island sanctuary.