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.