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?