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Chris Geremia was surprised. After considerable effort, and substantial risk to life and limb, he and his colleagues finally had the results from their decade-long experiment, and those results were both clear and unexpected: Bison do not surf.

Specifically, bison (or buffalo) don’t follow the waves of new shoots that burst from the ground every spring. This phenomenon, known as surfing the green wave, allows animals to eat plants at their most nutritious, when they’re full of nitrogen and proteins and low in indigestible matter. Such freshness is fleeting, and so grazers undertake large migrations to track the new greenery as it crests across the landscape. Over the past decade, scientists have shown that mule deer, barnacle geese, elk, elephants, Mongolian gazelles, and a dozen other species all do this. Geremia wanted to see whether bison, which once formed the largest grazing herds in North America, follow the same pattern.

Beginning in 2005, he and his colleagues started putting GPS collars on bison in Yellowstone National Park, home to the largest remaining herd in North America, and the only one that truly migrates. Their sociable nature makes for an impressive spectacle, but also creates a problem: When you tranquilize one of them, the others tend to surround their fallen herd-mate. “It took a few years to learn the confidence to walk into this group of a hundred animals, each weighing between 1,000 and 2,000 pounds, and put a collar on the one that’s sleeping,” says Geremia, who works at the National Park Service. “Most of the time, the others just move away.”

Once it had collared enough bison, the team used satellite images to see whether the animals’ movements matched the appearance of new greenery. “They really didn’t,” Geremia says. “They start to surf, but then they stop,” allowing several weeks’ worth of fresh vegetation to pass them by.

Confused, the team followed the bison in person, and collected dung samples to see whether the animals were suffering from a nutritional deficit because of their lax migrations. The poop, however, revealed that the bison were still consuming as much protein as if they had continued to surf the wave. “It threw us for a complete loop,” Geremia says. “How can they fall behind but still have an incredibly high-quality diet?”

He found out by fencing off small patches of land along the bison migration route. By comparing the plants within and beyond the fences, the team learned that bison graze so intensely that they freeze plants in early spring for weeks at a time, preventing them from maturing and forcing them to continuously produce young shoots. Other North American mammals like mule deer can’t do this, because they travel in small-enough groups that plants can still outgrow the effects of their grazing. Bison, however, gather in the thousands. By moving in synchrony, they don’t have to surf the green wave. Uniquely, they can also create it.

Their actions change the landscape. In areas where bison graze, plants contain 50 to 90 percent more nutrients by the end of the summer. This not only provides extra nourishment for other grazers, but prolongs the growing season of the plants themselves. And by trimming back the plant cover in one year, bison allow more sunlight to fall on the next year’s greenery, accelerating its growth. When Geremia’s team looked at parts of Yellowstone where bison numbers have fluctuated, it found that the green wave grew in intensity and crested over a longer period as the herds grew larger. The bison engineer and intensify the spring. And astonishingly, they had a stronger influence on the timing of plant growth than weather and other environmental variables. They’re equivalent to a force of nature.

That force would have been even more powerful in centuries past, when 30 to 60 million bison roamed North America. “They would have been everywhere,” says Matthew Kauffman of the University of Wyoming, who led the new study. “The productivity of those grasslands would have been radically different because there are that many bison, trampling, eating, defecating, and urinating.” These herds must have changed the path of the green wave, and inadvertently governed the fates of other animals that surf it, from deer to elk to bighorn sheep. What happened, then, when European colonizers virtually eliminated the bison? By 1900, fewer than 600 remained.

When we lose animals, we also lose everything those animals do. When insects decline, plants go unpollinated and predators go unfed. When birds disappear, pests go uncontrolled and seeds stay put. When herds of bighorn sheep and moose are shot, their generational knowledge disappears and migration routes go extinct, as Kauffman showed last year. And when bison are exterminated, springtime changes in ways that we still don’t fully understand.

They’ve rebounded somewhat, but still occupy less than 1 percent of their former range. There are probably about 500,000 bison around today, but the majority are part of privately owned herds. Only 20,000 or so live on public lands, and only 8,000 of those can move freely. And of those unfenced bison, about 5,500 live in Yellowstone. “This large population can change how spring happens,” Geremia says, “but there aren’t a lot of other places today where bison have the landscape that they do here.” It’s not enough to preserve bison numbers without also conserving bison behavior. If the animals exist, but aren’t allowed to migrate, there will still be a bison-shaped hole in the world.

That hole was felt most keenly by Native Americans, many of whom see the buffalo as a sacred symbol and a crucial part of their culture. Now indigenous groups are leading efforts to reintroduce free-roaming buffalo throughout their lands. At least 30 tribes across the U.S. and Canada have signed a treaty that commits to restoring the animals across 6.3 million acres, and some hope to have several thousand-strong herds within 50 years.

“The near extinction of the buffalo left a major gap,” said Leroy Little Bear, a Blood Tribe member and a retired professor at the University of Lethbridge, in a statement. “The treaty on buffalo restoration aims to begin to fill that gap and once again partner with the buffalo to bring about cultural and ecological balance.”

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