How Long Should a Species Stay on Life Support?

At about 3:30 a.m., four hours into our drive, Travis Livieri’s phone began to thrum. “I’ve got a ferret for you,” a voice crackled through the static. The animal in question was one of North America’s most endangered mammals, for which the next hour might be the strangest of her life; for Livieri, the wildlife biologist tasked with saving her, it would be one of thousands of interventions he’s made to prevent her kind from permanently vanishing. Over the past 28 years—through two graduate degrees, a marriage, the founding and running of a nonprofit, and multiple cross-state moves—he has thought of little else.

Livieri coaxed his Chevy Silverado off the bumpy stretch of South Dakota grassland that he and I had been circling and headed toward a designated meeting point, where Maddie Hartlaub, a biologist at Livieri’s conservation organization, Prairie Wildlife Research, handed him a crimped black tube. Inside was a black-footed ferret that needed a vaccine.

With the young ferret secured in the back seat, Livieri steered us toward his vaccination headquarters: a white trailer, its packed interior jury-rigged into a laboratory workbench. Inside, Livieri strapped on an N95 mask (a precaution to protect the ferrets) and—with a paper-towel roll duct-taped to the handle of a broomstick—nudged the two-pound kit, who was snarling, hissing, and chattering, out of her tube and into a makeshift anesthesia chamber. Four minutes later, she was asleep. With her black-tipped paws and tail outstretched, canines peeking from beneath her upper lip, she suddenly resembled a plushie more than a wild predator.

Livieri combed her back for fleas and inspected her ears for ticks; he sampled her blood, her fur, the cells that lined her cheeks. He poked a microchip between her shoulder blades. The main event was the vaccines, one into each hip, each a Crystal Light pink. The shot on the left guarded against canine distemper, the shot on the right against plague—a flea-borne bacterial disease that kills virtually every unvaccinated black-footed ferret it infects.

Roughly a century ago, scientists estimate, up to a million black-footed ferrets scampered the plains of North America; nowadays, just 340 or so of the weasels are left in the wild, fragmented across 18 reintroduction sites. And plague “is their No. 1 nemesis,” Dean Biggins, a grassland ecologist with the U.S. Geological Survey, told me. If ferrets were facing only habitat destruction or food insecurity, multiplying them in captivity might be enough to replace what nature has lost. But each time conservationists have added ferrets to the landscape, plague has cut down their numbers.

To keep the species from dying out, researchers have deployed just about every tool they have: vaccines and captive breeding, but also insecticides, artificial insemination, and a medley of safeguards for prairie dogs, the weasels’ primary prey. In 2020, black-footed ferrets even became the first endangered animal in North America to be successfully cloned for conservation purposes. Still, those efforts are not enough. Mike Lockhart, the U.S. Fish and Wildlife Service’s former black-footed-ferret recovery coordinator, once thought that, this far into the 21st century, ferrets “would be downlisted at least, maybe even recovered,” he told me. But their numbers have been stagnant in the wild for about a decade. Without new funds, technology, or habitat, the population looks doomed to only decline.

Ferrets’ woes are “absolutely our fault,” Biggins told me. Humans imported plague to North America more than a century ago, unleashing it on creatures whose defenses never had the chance to evolve. That single ecological error has proved essentially impossible to undo. Today, black-footed ferrets exist in the wild only because a select few people, including Livieri, have dedicated their lives to them.

Within an hour, the freshly vaccinated ferret was on her way back to her burrow in her species’ last remaining paradise. Livieri and his colleagues have counted roughly 150 ferrets—almost half of the individuals estimated to remain in the wild—currently living in South Dakota’s Conata Basin and the nearby Badlands; this stretch of cactus-studded grassland is the only place where researchers are certain that ferrets have been sustaining themselves for decades without regular infusions of captive-bred kits. “If something happened to Conata Basin, we would be done, literally done, with the recovery program for the species,” Steve Forrest, a biologist who has been working with black-footed ferrets since the 1980s, told me.

Conata Basin/Badlands is also the ferret habitat where humans have intervened the most. Over the past two decades, Livieri, who is 52, estimates that he has vaccinated 1,500 black-footed ferrets in the region—many of them twice, the gold standard for plague shots. “He’s caught more ferrets than anyone,” Kristy Bly, the manager of black-footed-ferret restoration for the World Wildlife Fund’s Northern Great Plains Program, told me.

Mainly because of Livieri, who has been working with ferrets full-time since the ’90s, about 95 percent of the modern Conata Basin/Badlands ferret population has received at least one plague-vaccine dose. Every August since the late aughts, whether it meant traveling from his home in Colorado or from his next residence in Wisconsin, Livieri has trekked down to South Dakota to spend two to three months “on ferret time.” By day, he grinds away at data sets and gets what sleep he can in a camper on a friend’s bison ranch. By night, he dons his weasel-catching uniform—a periwinkle work shirt and a gray cap, stamped with Prairie Wildlife Research’s ferret-centric logo—and blasts Van Halen, scouring the landscape with a spotlight fastened to the top of his truck, and planting a cage over every burrow where he’s seen the green glint of a ferret’s eye.

The weasels were once easier to find. In 2007, more than 350 ferrets roamed the region, enough that researchers were scooping up kits to augment populations elsewhere. Then, in May 2008, the prairie-dog corpses began to pile up—and researchers realized that plague had arrived. By 2013, the local ferret population had plummeted to 49; at the nearby Lower Brule Indian Reservation, Shaun Grassel, the Lower Brule Sioux Tribe’s former wildlife biologist, watched a community of about 60 dwindle to a single breeding pair.

Scientists had initially assumed that plague wouldn’t trouble black-footed ferrets, because early studies had suggested that their cousins, domestic ferrets and Siberian polecats, were largely immune. By the end of the ’90s, researchers saw how wrong they’d been. Plague killed ferrets without fail, filling their lungs with fluid and their abdomen with blood; prairie dogs—which make up as much as 90 percent of the weasels’ diet—were extremely vulnerable too. When epidemics broke out, whatever ferrets didn’t succumb to the disease generally starved instead. During the first big outbreaks in ferret territory, researchers sometimes didn’t realize that the scourge had begun to spread until thousands of acres of prairie-dog town had gone quiet, the burrows caved in, their entrances lidded with cobwebs.

As plague outbreaks intensified across the Mountain West and Midwest, Livieri told me, some researchers seemed resigned to letting the disease run its course. But he had already dedicated the past decade of his life to black-footed ferrets. “It wasn’t within me to just walk away,” he said. He got his hands on a plague vaccine, still being tested by the Geological Survey and the Fish and Wildlife Service, and recruited a vet friend in Texas to teach him how to take tricky blood samples in the field. “Everyone said, ‘There’s no way you can vaccinate all the ferrets,’” Livieri told me. “I said, ‘Try me.’”

To immunize wild ferrets, Livieri and other weasel-chasers (as some call themselves) must drive through the night, often for weeks, keeping themselves alert by chain-snarfing candy and caffeinated drinks. The ferrets are not keen on being caught: Even though a team might place a trap on the same burrow night after night, Livieri estimates that at most half of the cages end up filled. Many other sites lack the resources and reinforcements to immunize half their ferrets in a given year with even a single plague-vaccine dose.

Livieri stumbled into ferret conservation fresh out of college, eager to work with wildlife. In 1995, he helped discover the first litter of wild-born kits to be seen in South Dakota in at least two decades. A couple of years later, he and a colleague released a cohort of captive ferrets and watched 80 percent of them survive. He started to see how he could contribute, year by year, to the ferrets’ future. The success drew him in, but his growing attachment to the slinky, mischievous mammals kept him coming back, even as plague culled the populations that he and his colleagues had built. He can still remember the four-digit numerical codes of animals he snared in the ’90s; he married a fellow ferret lover and has six domestic ones at home.

Through all of this, he has been hoping for some breakthrough that might render his work obsolete: He knows that he alone cannot determine the species’ fate. But “I don’t know,” he told me, “who’s going to be the next me.”

In the 1980s, conservationists gathered near the small town of Meeteetse, Wyoming, to scoop up the last 18 black-footed ferrets remaining in the wild, all presumably from the same extended family. Only seven bred. Every modern member of the species is their descendant, each the genetic equivalent of a half-sibling to the rest, Bridget Baumgartner, the director of research and development at the conservation nonprofit Revive & Restore, told me—with three exceptions.

To preserve what genetic diversity is left, a conservation geneticist at the Smithsonian logs every captive ferret’s relatedness in a stud book to determine each spring’s optimal mating pairs; a reproductive biologist at Georgia State University has banked cryogenically frozen ferret semen since the ’90s, as an option to supplement natural matings. Still, captive ferrets bear inbreeding’s toll: lower birthing success, poorer sperm quality, the occasional crooked tail.

Two female ferrets born last spring, named Noreen and Antonia, may be key to slowing their species’ decline. Both are clones of Willa, a Meeteetse ferret that died in the 1980s and left no living descendants. They are the result of years of effort by Revive & Restore, partnering with Fish and Wildlife. The plan is for at least one of these clones to breed this spring and add a much-needed eighth genetic founder to the species. But ferret health can be finicky, and cloning from old tissue samples is still a fairly new scientific pursuit. Another Willa clone, Elizabeth Ann, born in 2020, had just one kidney and a malformed uterus that had to be removed before she had the chance to breed.

Captivity is, in one sense, good for ferrets: Plague does not reach them there. But that safety has trade-offs. Ferrets are normally solitary hunters, but in captivity, the animals spend their days in metal cages, often alongside dozens of their own kind; the staple of their diet is a factory-made horse-meat blend, fed to them by humans, whom the animals learn to rely on. Biggins, the USGS ecologist, told me that in the 1990s some of the first ferrets to be reintroduced into the wild were so flummoxed by their surroundings that they were picked off by coyotes, owls, and other predators within days. Alarmed, scientists began to train captive-born kits in outdoor pens, where they could learn to treat burrows as havens and hunt live prairie dogs before their release. Those boot camps increased survival about tenfold. But today’s captive-borns still lack some basic skills, Grassel told me. They never fare as well as ferrets born in the wild.

Initially, the intent of the captive-breeding program was “to do ourselves out of business,” Lockhart, the former U.S. Fish and Wildlife recovery coordinator, told me. But as plague’s threat has grown, so has conservationists’ dependence on breeding ferrets—including in ways that could permanently alter them. In collaboration with scientists at MIT, Revive & Restore is working on a heritable plague vaccine that, once written into black-footed-ferret DNA, could enable the animals to sire a line of ferrets that can generate their own immunity. Meanwhile, scientists at the Smithsonian-Mason School of Conservation and Penn State have been hunting for genes in other, more plague-resistant weasels that could theoretically be stitched into the black-footed-ferret genome.

Even if they bred a legion of plague-proof ferrets, though, conservationists would still need to find more places for them to live. Black-footed ferrets currently occupy 300,000 acres of North America; to leave the endangered-species list, they will likely need three times that amount, the WWF’s Bly told me. Last year, researchers had to scramble to find homes in the wild for all 231 captive-born kits—close to a record—that Fish and Wildlife had dispatched to them for release. (Fish and Wildlife did not respond to requests for comment.) Wild habitats are simply too scarce, Bly said: The captive-breeding program has effectively outstripped what’s left of nature’s capacity to benefit from it.

To save a species, scientists must save its way of life—which, for black-footed ferrets, means preserving prairie dogs. At night, the weasels sneak into the rodents’ burrows to prey on them, suffocating them while they sleep with a lethal bite to the neck; their vacant chambers also double as dens in which ferrets eat, rest, breed, and raise their kits. Lose prairie dogs, and ferrets will die too.

One morning, Livieri, fresh off a night of spotlighting, offered to show me what the ferrets’ world looks like by day. We drove through a series of grazing pastures, carefully opening and closing gates, until we reached a stretch of prairie pockmarked with burrows, each wide enough to stick an arm down. Several burrows had bright-red grain piled at their entrance. This bait, laced with a drug that turns prairie dogs’ blood into a flea-killing insecticide, could be a slightly less labor-intensive alternative to painstakingly spraying pesticide on individual burrows. But both strategies have their drawbacks—and no single intervention has yet been enough to protect prairie dogs. In recent decades, the prairie-dog species that black-footed ferrets most depend on have declined so precipitously that conservationists and researchers have petitioned to get them federally listed as threatened.

Fish and Wildlife so far hasn’t budged. And prairie dogs do remain abundant enough to rankle many of the ranchers and farmers whose grasslands and crop fields they wreak havoc on. “They’re like moles on steroids,” Gene Williams, whose family has ranched in South Dakota for decades, told me. Across the Mountain West and Midwest, poisoning of prairie dogs remains common, and in several places, it is carried out by the U.S. Forest Service. Where they’re allowed to, ranchers shoot the rodents.

Williams is among the ranchers who appreciate that without prairie dogs, black-footed ferrets—a species he’s admired since childhood—have no hope. It helps that the rodents make him money: He runs a prairie-dog-centric tourist attraction—complete with a giant prairie-dog statue—where visitors can buy prairie-dog-themed shirts and mugs, and toss unsalted peanuts to a small colony adjacent to his parking lot. Other ranchers have accepted financial incentives from government to host the animals on their property. Some, though, oppose just about any prairie-dog presence in their pastures.

In 2009, the Forest Service designated at least 18,000 acres of Thunder Basin National Grassland—a 550,000-acre expanse of federal, state, and private lands in northeastern Wyoming—as protected prairie-dog habitat, laying the groundwork for ferret reintroductions. That area is widely considered one of the “most promising” uninhabited spots for ferrets left in the U.S., Bly told me. But the proposal sparked backlash from local ranchers such as Ty Checketts. In 2016, a population boom blanketed about a third of his property with colonies of prairie dogs, which so thoroughly stripped the vegetation that his cattle went hungry, wild deer and elk migrated away, and swaths of his land succumbed to weed overgrowth and erosion. When plague wiped out most of the prairie-dog population, “it was a blessing from God,” he told me. In 2020, the Forest Service amended its plans for Thunder Basin, paring back prairie-dog protections.

Finding suitable sites for ferret reintroductions is only getting harder as droughts brought on by climate change exacerbate conflicts between ranchers and prairie dogs, and as demand for wind, solar, and other renewable energy sources pushes development deeper into the Mountain West. Maintaining new sites would require still more investment: Fish and Wildlife supplies sites with captive-born ferret kits for reintroduction but does not routinely follow those infusions with funds for plague management; sites must find money for those measures themselves. Of the 34 North American habitats where ferrets have been released, about half no longer have any of the animals, primarily because of plague. The two dozen ferrets I saw over my three nights in Conata Basin may be more than some sites will ever host.

On the second night of my visit, Livieri and I prepped one of those ferrets for release beneath a nearly full moon—a time, weasel-catchers believe, when the animals are especially active. From inside his pet carrier, the young kit scuffled impatiently. But when Livieri popped open the cage’s front, the animal paused and locked eyes with us. On his chest was a freshly drawn blue stripe, running from the base of his left ear to the top of his right shoulder, denoting his first plague shot. His chances of survival were far higher now; with the shot’s defensive molecules teeming within him, his body was also no longer entirely wild.

Livieri often compares himself to an alien abductor, swooping in with his giant spotlight, poking and prodding and microchipping the animals, then dropping them back at their burrows to wake, groggy and unsure. If all goes well, each benefits and then forgets anyone was there. But this particular ferret existed because humans intervened in his species’ past—capturing those last 18 ferrets from the wild, planting some of their descendants here. He may even have existed because this specific person looming above him had vaccinated one of his direct ancestors.

Conservation dogma generally holds that the best version of a species to preserve is the most untouched one. But meddling with ferrets’ lives may be necessary for as long as plague is around and humans want black-footed ferrets to be, too. Our species has so thoroughly changed the world that we now have little choice but to retrofit ferrets to a version of nature that is no longer hospitable to them.

At his burrow’s entrance, the ferret hesitated again, before vanishing in a cream-colored flash. He had no way of knowing that ideally, he would be caught again. For his best chance at survival, he still had to earn another blue stripe, right ear to left shoulder—the second half of Livieri’s hand-drawn X.