Original publication by Janet Marinelli for e360.yale.edu 5 July 2022
With a growing number of studies demonstrating the importance of large mammals to healthy ecosystems, scientists are proposing concrete plans to reintroduce these animals to the wild. The return of just 20 species to native habitats, they say, could be a boon to biodiversity.
For thousands of years, bison herds thundered freely throughout the Chihuahuan Desert on both sides of what is now the U.S.-Mexico border. In November 2009, after three frantic months of chasing down the required permits, Rurik List and Nélida Barajas watched as 23 bison from Wind Cave National Park in South Dakota arrived by tractor-trailer at the Santa Teresa international cattle crossing in southeastern New Mexico.
The animals, 20 females and three males, galloped through the dusty stockyards, across the border, and into the state of Chihuahua. A permanent herd of wild bison had been missing from Mexico for more than 150 years. “It’s hard to describe the feeling,” says Barajas. “We were bringing the bison back home.”
Two weeks later, 140 miles southwest of the border crossing, the bison were released from a quarantine corral at El Uno ranch, a 46,000-acre oasis of recovering grasslands in a Chihuahuan Desert landscape severely degraded by the overgrazing of domestic livestock. List, a conservation biologist at Mexico’s National University who had drafted the bison restoration plan for northern Mexico, and Barajas, a Nature Conservancy scientist and the ranch manager at the time, were joined by 700 government officials and local ranchers and farmers and their families to witness the event. When the gates opened, a bull led the herd into an iconic Western tableau of big sky and luminous sweeps of golden desert grasses backed by the rugged peaks of the Sierra Madre Occidental.
Research underscores the importance of large mammals as ecosystem engineers, shaping natural processes and sequestering carbon.
Bison, which can reach six and a half feet at the shoulder and weigh as much as 2,000 pounds, are critical to the continued recovery of the desert grasslands. Unlike cattle, which graze grasses to the root, bison roam while they graze, leaving enough of each plant to enable it to continue to grow. They also wallow, sculpting depressions in the ground where water can accumulate and sustain healthy stands of grass.
In the past two or three decades, research has underscored the importance of large mammals like bison as ecosystem engineers, shaping and maintaining natural processes and sequestering large amounts of carbon. But the world’s large herbivores and predators continue to suffer alarming losses. Researchers estimate that almost two-thirds of the world’s large carnivores are threatened with extinction. Fewer than 6 percent of 730 ecoregions worldwide studied by scientists still have the extensive, intact large-mammal communities that were dominant 500 years ago.
After several decades of research refining the understanding of the importance of large mammals to healthy ecosystems, scientists are now proposing a concrete plan about which herbivores and predators to reintroduce and where, and how this might best be done, given the challenges.
In a paper published earlier this year, a global team of researchers led by the U.N. Environment Programme World Conservation Monitoring Centre and the U.S. nonprofit organization RESOLVE proposed a detailed strategy to reverse the global decline of large mammals and the ecoregions they once inhabited. The rewilding of large mammals is an essential but too often omitted component of current restoration efforts, they point out, and “should become a global imperative in the decade ahead.”
According to the study, published in the journal Ecography, reintroducing just 20 large mammals — 13 herbivore and seven predator species — can help biodiversity bounce back around the world and tackle climate change in the process. Among these candidates for rewilding are brown bears, bison, wild horses, jaguars, reindeer, Eurasian beavers, elk, moose, wolverines, tigers, and hippopotamuses.
The researchers also identify 30 priority ecoregions on five continents that meet key criteria: They lack no more than one to three of the large herbivores and predators historically present, provide extensive habitat, and can feasibly be restored in the coming decade. These areas range from the flooded grasslands of South Sudan and the dry puna of the Central Andes to the xeric grasslands and shrublands of the Chihuahuan Desert, where intact communities of large mammals could be restored in the next five to 10 years, the scientists say.
At roughly 200,000 square miles, the Chihuahuan Desert is the largest desert in the Western Hemisphere, sprawling across six Mexican states, the southeastern corner of Arizona, southern New Mexico, and much of western Texas. It is also the most biologically diverse. Historically, the Chihuahuan Desert was one of the few places where grizzly bears, wolves, and jaguars could be found in the same locality.
The past two centuries, however, have not been kind to many of the desert’s 130 wide-ranging mammals. Wild bison were wiped out in Mexico by the second half of the 19th century, and other than some private herds, bison no longer roam widely on the U.S. side of the Chihuahan Desert. The Mexican gray wolf once ranged far and wide across parts of Mexico, Texas, Arizona, and New Mexico. But it was extinct in the wild by the late 1970s, slaughtered by an aggressive campaign of hunting, trapping, poisoning, and removing pups from their dens. According to List, the cattle carcasses laced with poison that were used to exterminate wolves also led to the extirpation of the grizzly bear, an opportunistic scavenger. Overhunting and habitat loss brought other large animals such as pronghorn and bighorn sheep to the brink of extinction.
Restoring intact communities of large mammals such as these won’t be easy. Throughout history people have feared large animals, particularly predators, justifying politically expedient measures to minimize their numbers — or even eliminate them altogether. Oregon State University researchers Christopher Wolf and William J. Ripple calculate that 64 percent of the world’s remaining large carnivores are at risk of extinction and 80 percent are declining. According to Jens-Christian Svenning, a professor of ecology at Denmark’s Aarhus University and co-author of the Ecography paper, the state of the world’s large herbivore species is almost as dire, with 59 percent of the 74 species of large herbivore species weighing 220 pounds or more threatened with extinction.
The body of scientific literature documenting the importance of top predators and herbivores has revealed how their loss destabilizes and even unravels ecosystems. In the absence of predators, for example, populations of herbivores often explode. In the eastern U.S., deer were once kept in check by wolves and mountain lions. Today, booming deer populations are preventing keystone species such as oaks from reproducing and have literally devoured the understory habitat of hooded warblers and other birds.
The intact communities of herbivores and predators that existed centuries ago are now largely gone.
Research has also demonstrated that healthy animal populations play an important role in sequestering carbon. Yale School of the Environment ecologist Oswald J. Schmitz notes that even if we could completely stop all our emissions, switch to renewables, and stop deforestation, it wouldn’t keep global temperature rise under the tipping point of 1.5 degrees Celsius. “We have to draw out a significant amount of CO2 and store it on the planet to stabilize the temperature,” he says. “Animals can help us get to this goal a lot faster.” Schmitz and colleagues in the Global Rewilding Alliance calculate that rewilding, restoring, and conserving endangered and threatened animals could increase carbon uptake by 1.5 to 3 times or more around the world.
From 2003 to 2010, Carly Vynne, director of the biodiversity and climate team at RESOLVE and lead author of the Ecography paper, studied maned wolves, pumas, jaguars, tapirs, giant anteaters, and giant armadillos in what she calls “one of those special places in the world,” the Brazilian Cerrado. “This was a place that really got me thinking about what it takes to keep the full assemblages of species in place,” she says. She and her colleagues decided to update a previous paper on the world’s remaining communities of large mammals to add a critical missing dimension to current efforts to strengthen global biodiversity targets. As they got into it, she says, they decided to not only update the paper but make it more forward-looking by focusing on “where we might be able to feasibly restore large mammal assemblages.”
Vynne and her coauthors point out that most of the earth’s land surface still has some large mammals, but the intact communities of herbivores and predators that existed centuries ago are now gone. Areas with more than three missing species, they conclude, are likely to be degraded, or the threat of hunting may be severe. To accelerate reintroduction planning, they identify those 20 key species with the greatest potential to quickly increase the amount of land globally with intact large mammal communities. Nine of the priority herbivores and predators are globally threatened, and nearly all are species of conservation concern at national or regional levels, so reintroducing them would not only stabilize and restore the integrity of ecosystems but help save them from extinction.
Rewilding actions with the greatest potential impact, according to the scientists, include reintroducing the European bison, Eurasian beaver, reindeer, wolf, and lynx in Europe. Returning wild horses and wolves to the Himalayas, they calculate, could increase the intact large mammal coverage in that region by 89 percent. In Africa, rewilding the hippopotamus, cheetah, common tsessebe antelope, African wild dog, and lion could expand coverage by 108 percent. Reintroduction or other measures to improve conservation of brown and American black bears, American bison and wolverine could more than double the area in North America with intact large mammal communities. And in South America, reintroduction of jaguar, pacarana, pampas deer, marsh deer, and white-lipped peccary would expand the presence of healthy large mammal assemblages over hundreds of thousands of square kilometers.
Their proposed strategy, they believe, can boost the percentage of land globally with intact communities of large mammals from 15 to 23 percent. How high should this percentage be to stabilize ecosystems and wildlife populations around the world? “Personally” says conservation biologist Reed Noss, an early advocate for rewilding and a coauthor of the paper, “I think that the 50 percent of the Earth that many people now agree should be protected should also be rewilded with large animals.”
Successful rewilding efforts typically have involved a concerted effort to promote coexistence between people and wildlife.
The story of a wolf called Mr. Goodbar is emblematic of the potential stumbling blocks that lie ahead. The young Mexican gray wolf — a critically imperiled, relatively small, reddish-brown subspecies of the more familiar gray wolf — was born in a Kansas zoo. In 2020 he was released into the wild in Arizona as part of a captive breeding and reintroduction program for the species. Last year he left his pack in eastern Arizona, presumably in search of his own territory and a mate. He was observed pacing back and forth along a stretch of the 30-foot border wall in New Mexico for nearly five days before giving up and returning north.
In January, the lanky two-year-old was spotted again, this time dragging a rear leg badly fractured by a gunshot wound. The veterinarian at the Albuquerque BioPark who amputated Mr. Goodbar’s leg said he had probably been struggling with the injury for a few weeks before being rescued by biologists. When the wound healed, he was returned to the wild, where scientists say his odds of surviving are good.
Mr. Goodbar has been more fortunate than many of his kind. U.S. Fish & Wildlife Service data show that 105 of the 185 Mexican gray wolves that died between 1998 (when the reintroduction program began) and 2019 were killed illegally.
Hunting pressure is just one of the impediments that have prevented rewilding from being incorporated into conservation planning. In the words of Noss, a retired professor at the University of Central Florida who is now president of the Florida Institute for Conservation Science, “Mainstream conservationists thought rewilding of large carnivores and herbivores was impractical and politically suicidal, since most people fear these animals and ranchers and other politically powerful large landowners particularly hate them (with some important exceptions).” He and the other researchers stress that such challenges need to be addressed before reintroduction programs commence.
Successful rewilding efforts typically have involved a concerted effort to promote coexistence between people and wildlife. From the beginning of her time at El Uno, for example, Nélida Barajas envisioned the ranch as not just a living laboratory for researchers, but also an educational center where neighboring ranchers could learn about new, sustainable grazing practices. She set out to show that “we were not crazy biologists from cities against livestock” but scientists and land managers “who seek solutions for all.” Today, the El Uno bison are a seed herd providing animals for other areas of the desert.
Meanwhile, more than two decades after they were first returned to Arizona and New Mexico, the prospects for the Mexican gray wolf have finally begun to brighten. At the end of last year, about 196 Mexican gray wolves ranged across the Blue Range Wolf Recovery Area straddling the Arizona-New Mexico border. Mexico’s National Commission of Natural Protected Areas (CONANP) recently released two wolf pairs in the state of Chihuahua, bringing the country’s wild wolf population to 45. This is thanks in part to measures to ease, if not eliminate, the acrimony over the predator’s comeback. For example, the U.S. Fish & Wildlife Service has been installing electric fences, and range riders monitor wolves with radio collars to keep wolf packs and cattle apart. Just as important, ranchers are compensated for the loss of their livestock.
Five years ago, Carrie Trudeau of the El Paso Zoo, which has contributed wolf pups to the reintroduction effort, was high up in Arizona’s Gila Mountains repairing fences designed to reduce wolf-cattle encounters. After an exhausting day of pounding in posts and stringing barbed wire, she and her coworkers were gathered around a campfire, listening to one of their guides tell a story. Suddenly, he stopped. “And then we could hear the wolves howling,” Trudeau recalls. “It was haunting, kind of made you shudder all the way through.” This, she says, was their reward for a long day of backbreaking work. “I know it is anthropomorphizing, but it was like the wolves were saying thank you.”