What Can One Disease Do to a Landscape?

After the vicuñas in Argentina’s San Guillermo National Park caught mange from domesticated llamas, the world around them changed.  

A puma, vicuñas, and a condor, with blue squiggly lines linking them
Joe Riis / The Atlantic

The past two years have shown just how badly a disease that originated in wildlife can upend the human world. But epidemics can also move in the opposite direction, with equally dramatic results. Argentina’s San Guillermo National Park, for example, was once dominated by the puma—a top predator that, by controlling grazing animals, determined the patterns of plants across the landscape. But the puma was ousted from its role as the park’s chief terra-former by a disease, which radically reshaped the entire ecosystem in a few short years.

Julia Monk of Yale and Justine Smith of UC Davis were well placed to watch the outbreak unfold. For years, they had been studying the park’s wildlife, including its vicuñas—smaller relatives of llamas. San Guillermo was home to 5,000 to 10,000 vicuñas, before their numbers began to crash in 2017. In 2019, Monk saw just 80 of them. “We went from driving around and seeing them everywhere to going for days without seeing any,” she told me.

The vicuñas had been decimated by a disease called sarcoptic mange, which is caused by a microscopic mite that burrows into an animal’s skin. In domestic dogs, the mites cause itching and hair loss; in vicuñas, the consequences are more severe. These animals have some of the finest wool on the planet, which allows them to survive on cold and windy mountains. By robbing them of their hair, mange deprives them of warmth. In advanced cases, the disease also locks their joints, paralyzing them. Smith remembers seeing mange-ridden vicuñas in a snowstorm, exposed to the elements and unable to walk to shelter.

Local vets and farmers had never seen mange afflicting San Guillermo’s vicuñas in any of the past five decades. So where did the disease come from? A team led by Marcela Uhart of UC Davis found an important clue: The mites that had infected the dead vicuñas were all almost genetically identical, which suggested that they had all come from a single source. Around the world, livestock and pets are known to pass mange mites to wild species; in San Guillermo, the most likely culprits were domestic llamas, which had been introduced into the surrounding area in 2009 and would wander in and out of the national park. Some had been diagnosed with mange.

The vicuñas’ absence rippled through the rest of the park’s wildlife. They used to be the major prey of San Guillermo’s pumas, which then left behind carcasses that fed Andean condors—huge vultures that, with 11-foot-long wingspans, are the world’s largest flying birds. Vicuña carcasses made up almost 90 percent of the condors’ diet, and as the mange outbreak spread, the condors flew elsewhere in search of food. “In earlier years, you’d see condors flying overhead every day, no question,” Monk said. “In 2019, we saw condors once or twice in three and a half months.” If the exiled condors try to scavenge livestock carcasses from the closest farmlands, they risk being poisoned, either by pesticides in the meat or by poisons that farmers deliberately add to keep the birds away.

The park’s vegetation has also changed. In the past, the vicuñas used to avoid the park’s canyons in favor of its open plains, where they could more easily spot approaching pumas. As a result, the canyons were grass-covered but the plains were barren, as “the plants had been eaten down to almost nothing by these vicuñas,” Smith told me. But mange erased the difference between the canyons and the plains, increasing grass cover in the latter by up to nine times. The plains went from “completely barren ground, nothing but gravel, to this highly productive grassland,” Smith said.

The forces that shape the land are now different. Pumas used to dominate, patterning the park’s plants by concentrating the vicuñas in the plains. By killing the vicuñas, mange nullified the pumas’ influence. (Monk said she isn’t sure whether the number of pumas has declined; their scats suggest that they’re weathering the loss of their major prey by shifting to smaller targets.)

Having brought mange to San Guillermo, humans could conceivably do something about it. The disease can be treated with—ahem—ivermectin, and “especially when the population is close to nothing, it wouldn’t be too hard,” Smith said. But it’s unclear whether vicuñas would then rebound. The grasses that have sprouted in the plains are feeding invasive European hares, which seem to have at least doubled in numbers since the mange outbreak began. If the pumas start eating hares, they might sustain themselves at a level that prevents the decimated vicuñas from eventually bouncing back.

For decades, scientists have shown that the presence or absence of predators can trigger dramatic food-chain chain reactions—or “trophic cascades,” in the lingo of ecologists. Parasites and diseases should have similar effects, but to prove that they do, scientists need to collect data about an ecosystem before and after an outbreak occurs. And since “we rarely know when and where that will be, we tend to lack ‘before’ data,” Julia Buck, an ecologist at the University of North Carolina at Wilmington, told me. By good fortune, Monk and Smith had arrived in San Guillermo before mange did, and they knew what the park’s baseline was in the pre-epidemic years.

Wildlife epidemics are becoming more common. In 2015, a bacterial infection wiped out two-thirds of the world’s saiga, a big-nosed Asian antelope. An unknown epidemic killed songbirds across the eastern and midwestern United States last year. The fungus that causes white-nose syndrome is hammering North American bats. Contagious cancers are killing Tasmanian devils. In a few cases, the ripple effects of these diseases are clear. In 2013, a mystery illness disintegrated the starfish of America’s Western Seaboard: These unlikely predators are the coastal equivalent of pumas, and in their absence, their sea-urchin prey were free to devour offshore kelp forests. A deadly fungus that humans inadvertently carried around the world has ravaged the planet’s amphibians, wiping out 90 species and leaving more than 100 others close to extinction; the snakes that eat those amphibians have also dwindled.

“My guess is that this happens more often than we have evidence for,” Smith said. As humans and our livestock expand into the ranges of wild species, we create more opportunities for diseases to spill over in both directions. And as San Guillermo’s mange outbreak shows, the growing threat of wildlife epidemics can’t be managed by simply segregating animals in protected spaces. The park is six hours away from the nearest town. It sees almost no tourists. “It’s really remote and it was still exposed to this disease that was probably anthropogenic,” Smith said. If mange can reshape San Guillermo, “what places are really safe?”