In early 2014, wildlife ecologist Arthur Middleton and photographer Joe Riis boarded a helicopter in the foothills of the Absaroka Mountains, in northwest Wyoming. Middleton had hired the pilot for collaring elk—an intense and somewhat absurd process that requires flying above a herd so “muggers” can shoot some of the animals with nets, enabling a ground team to tackle and fit them with GPS collars then release them.

Middleton and Riis planned to use the GPS data to follow and document the herd’s migration to its summer range, 50 miles away in the remote southeastern corner of Yellowstone National Park. But first, they wanted to see how tough traveling along the migration route might be. The first leg goes up and over Needle Mountain, a massive crag of rock that geologist Arthur Hague described a century ago as “the most imposing single occurrence of any intrusive mass in the Absaroka Range.” Neither Middleton nor Riis had seen the mountain in person.

“The pilot didn’t even want to go close to the summit,” Riis said. “We were, like, halfway up—I couldn’t even see the top of it, from a helicopter.”

“We basically saw it and said, ‘Holy … Jesus,’” Middleton said.

Figuring out how to traverse Needle Mountain proved only one of many obstacles Middleton and Riis would face in an ambitious quest to tell the story of elk migrations in the Greater Yellowstone Ecosystem, the roughly 20 million acres of mostly undeveloped land with Yellowstone National Park at its heart. Over the past two summers, the duo rode 1,500 miles on horseback through the Absarokas, following the elk’s path to experience their travel firsthand.

Middleton and Riis already had been part of a burgeoning movement in Wyoming to protect some of the longest land migrations on the continent, but their GYE elk project amounted to a wholesale scaling up of migration studies—not just in terms of learning animal behavior from a scientific perspective, but in revealing an ancient and largely unseen phenomenon that sheds light on how ecology functions in and around America’s first national park.

“With elk in the GYE,” Middleton said, “migration is the engine of the whole goddamned system.”

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Much of the current research on migration taking place in Wyoming stems, at least in part, from the work of Hall Sawyer, who grew up hunting and fishing in the state and has studied its wildlife for more than 15 years. In the late 1990s, Sawyer tracked the migration of a herd of pronghorn from Grand Teton National Park more than 100 miles south into the Upper Green River Valley. Then, in the mid-2000s, he stumbled upon a revelation when he was tracking mule deer herds in a southwestern part of the state called the Red Desert.

“We thought the mule deer just resided year-round in the desert or migrated short distances out there,” he said. “That’s when we discovered that half of those deer were migrating some 150 miles.”

The migration Sawyer revealed is the second longest recorded land migration in North America—only Arctic caribou go further. But these mule deer do not travel across barren tundra. Between the herd’s winter range in the windblown Red Desert and summer habitat among the granite and cool timber of the Hoback Canyon, they travel through oilfields and skirt residential developments, swim reservoirs and finger lakes, and cross three highways and more than 100 fences. The migration traverses a complex patchwork of public and private land controlled by people and agencies with diverse, sometimes conflicting interests.

That a migration of such epic length existed in the first place—unknown, under science’s nose, in the 21st century—struck Sawyer as amazing. But as the excitement subsided, he started to think about how difficult it would be to ensure that such a migration could persist. He knew, however, doing so would be a worthy task. Wildlife biologists have long understood that migration is crucial to the health of ungulate herds: Animals grow fatter and produce more calves when they can follow the green growth of spring into higher country as the weather warms—what scientists call “surfing the green wave” of vegetation—and when they can retreat to the lowlands as winter snows cover forage in the mountains. When their herds are robust, they, in turn, support healthy populations of carnivores and scavengers—essentially, entire ecosystems.

Because individual herds pass down knowledge of migration routes from one generation to the next, migration’s success depends on the land remaining mostly free of obstructions—once a migration route is blocked, animals do not simply learn another way, but tend to cease migrating altogether. In the case of the Red Desert mule deer, Sawyer wondered, how would it be possible to get all the necessary parties on board to help keep their long routes intact?

“We knew sustaining migration was important, but in the past, we followed the typical path that scientists have: You do research and you publish papers in scientific journals, and you just assume that the science reaches the managers and the policymakers and the public,” Sawyer said. “After a decade of that, it became clear to me that that standard model really wasn’t working. … I decided to take a little different approach.”

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Sawyer wanted to tell the story of the Red Desert mule deer’s journey in a way that might better get people’s attention. By the mid 2000s, conservationists had begun to use migration data to try to protect corridors animals travel, but no one had tried in earnest to capture images of what these migrations looked like.

So Sawyer called up Riis, the photographer. Back in June 2009, Riis had worked with Emilene Ostlind, a writer, to document the pronghorn migration Sawyer had tracked in the late ’90s and illustrate the obstacles the animals face. Riis and Ostlind, both of whom were in their mid-20s at the time, had followed Sawyer’s maps and the pronghorns’ hoof prints in the mud south out of Grand Teton National Park, backtracking the 100-mile spring migration on foot. Ostlind eventually walked the route five times and wrote a cover story about it for High Country News. Riis lived mostly in his truck for two years, supported by a National Geographic Young Explorers grant, spending months at a time at locations where he thought the landscape might funnel the pronghorn together for a good shot.

Riis’s photos were the first and best images to depict a large ungulate migration; they showed the phenomenon intimately, as no one had ever before seen it. The photos, which illustrated Ostlind’s magazine story and helped Riis get a job with National Geographic magazine, were displayed in museums and shared widely online. And conservation groups used them to raise awareness of threats to the pronghorn migration. Thanks to subsequent efforts of various nonprofits and government agencies, the animals now clamor above highways atop pronghorn overpasses and slip under specially altered fences throughout their route.

Building off this storytelling model, Sawyer and Riis developed a three-prong approach to raise awareness and support for protecting the Red Desert mule deer’s migration. For government agents, private landowners, and NGOs, they produced a glossy magazine-like publication that guides readers section-by-section through the entire 150-mile migration and the obstacles the mule deer face. For Wyoming residents, they put together an exhibition of Riis’ photographs that toured museums and libraries around the state. And for the public at large, they produced a four-minute online video. At first, they hoped a few hundred people in Wyoming would watch; now, more than 4 million people have viewed it.

Sawyer and Riis soon realized that what they thought was a local conservation issue could, in fact, capture the attention of people around the world.

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Arthur Middleton was among the people the mule deer project inspired. The Yale-educated ecologist had come to Wyoming in 2007 to study interactions between wolves and elk, but shifted his focus to elk migrations. With a population of 30,000 to 40,000, elk are by far the most common ungulate in the Greater Yellowstone Ecosystem, which is comprised of the national park, several national forests, and other public and private lands. The elk maintain large populations in part by migrating into the highlands of the national park during the summer, and to lower elevations—often on massive private ranches—for the winter. But while the GYE seems an unpeopled wilderness compared to the rest of the United States, counties around Yellowstone experienced some of the fastest rates of residential development in the nation over the past 40 years—an incremental yet insidious threat to the elks’ annual movements.

Middleton conceived a project modeled after Sawyer and Riis’ work. What he envisioned was a story that challenged perceptions that a national park is an isolated wilderness in a box. “The migrations are transiting across this landscape, crossing all these lines and divides between people, literally connecting ranchlands to park lands routinely as a matter of fundamental biology,” he said.

Middleton approached Riis about the undertaking, and together they won the inaugural Camp Monaco Prize, which supports biodiversity research and education related to the Greater Yellowstone Ecosystem. In June 2014, they and a team of outfitters rode from the foothills of the Absaroka Mountains to follow the elk migration route into the Thoroughfare, a grizzly-infested swath of forest and grasslands surrounded by sheer and complex topography—winding drainages as deep as the Grand Canyon cut through volcanic rock, crumbly peaks capped year-round by snow fields, colorful banded cliffs, high-altitude plateaus that become lush in the summertime and host abundant wildlife. Geographically, there’s a point in the Thoroughfare that’s further from any road, paved or unpaved, than any other place in the lower 48 states.

The caravan had the benefit of their GPS-collar data, which pinpointed the elks’ movements down to a sixteenth of a mile, and the generations-deep knowledge of accomplished local hunting guides. Still, they immediately encountered difficulties: river crossings too swollen with meltwater runoff to navigate, summer snowdrifts still too deep to push horses through, electrical storms that chased them off summits, and trails too steep and muddy to climb. Where the elk travelled, the humans often could not pass. The migration begins with a pair of rollercoaster ascents and descents—6,000 feet in elevation each—across two river drainages. The team failed to follow the route over Needle Mountain, forcing them to go around. And both Middleton and Riis were novice horsemen. Riis remembers chasing down a mule that ran off into the forest while he was packing it, scattering his camera gear as it trotted away.

After three weeks, the team emerged from the wilderness, having traversed the migration route from beginning to end and back. But this was just the start. Over the remainder of that summer and the next, Middleton, Riis, and the outfitters—along with a rotating cast of artists, scientists, journalists, and other collaborators—continued to ride in and out of the Thoroughfare. After the initial journey, the team’s main purpose for trips was to set and maintain Riis’ camera traps. Using slightly more direct paths than the actual migration route, the most remote of these locations was 15 miles from a road. They visited it 25 times.

With each trip, their understanding of the migrations grew. They came to understand the central role of these movements in the ecosystem. “I recall being on Thoroughfare Plateau in summer 2014, it’s where I realized what the migrations mean,” Middleton said. “You’re up there at 11,000 feet, it’s raining, the sun’s shining, you’re looking at expansive miles of grass, patches of melting snow. That’s where the thought struck me: This is where this ecosystem starts. Here’s the sun, the water, it’s growing this grass, the elk are standing here grazing it, they’re making fat, they’re nursing calves. And when I think about the 30,000 elk that are doing that, that’s where it gels for me, that’s where it clicks. They’re taking all this fat and muscle that’s in this system back out into the rest of this landscape.”

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Today, visitors to the National Geographic Society in Washington, D.C., and the Buffalo Bill Center for the West in Cody, Wyoming, can view fruits of these efforts. Invisible Boundaries: Exploring Yellowstone’s Great Animal Migrations is on display concurrently at both museums. Alongside texts by Middleton and photos by Riis, the exhibition includes a 3D animated map of every elk migration in the Greater Yellowstone Ecosystem and paintings by illustrator and naturalist James Prosek. Its opening coincided with the release of a documentary film by Jenny Nichols, Elk River.

“Joe’s photos almost reintroduce to the world animals that we’ve taken for granted,” Middleton said. “An elk doesn’t just have to be a bugling six-by-six bull standing next to the willows and breathing foggy breath into the autumn air. They’re on these incredible journeys and they have this hidden life.”

Riis said most people think they know what deer, elk, and pronghorn look like, and he takes pleasure in changing their perspective. “They’re common animals, like a mallard,” he said. “People see them on the side of the road. But when people think they understand something, and then you show them a photograph of something different—like pronghorn swimming a river, or an elk deep in a migration in the most remote corner of the United States huffing and puffing up a mountainside—it makes them look twice.”

Riis likes to say that migration is what makes the animals he photographs wild. The name of the exhibition, Invisible Boundaries, is a nod to the fact that migrating animals do not consider the jurisdictions humans have imposed upon the land. They do not respect quarrels between federal agents and local landowners, or hunters and environmentalists, though the future of their migrations depend on the buy-in of all parties.

“[Scientists and conservationists] need to stop only talking to each other and working with people who are comfortable and easy to work with,” Middleton said. “Mapping out an area that wildlife need to use needs to go from bad news for the people who live in that area to being good news, or at least neutral news. … You cannot conserve these migrations over the long term if you can’t get people from many walks of life interested, excited, and willing to help out in some way. You need everyone.”