Herds of bison milling through Yellowstone National Park may seem aimless to the average visitor, but a new study reveals the animals are hard at work engineering their ecosystem. By rigorously mowing and fertilizing their own patches of grassland, the big herbivores essentially delay spring until late summer.

Researchers suggest that most ungulates in the western United States, including elk, mule deer and pronghorn, migrate during the spring following the “green wave,” when plants and grasses awake from their winter sleep and sprout up. This allows the animals to have a constant supply of newly emerged vegetation, which is often the most nutritious. Mule deer can migrate up to 200 miles in the spring as they “surf” the wave, following the greener grass into higher elevations and higher latitudes.

For the new study on bison in the journal PNAS, researchers wanted to follow bison as they “surfed” the green wave in Yellowstone National Park. Scientists began tracking the herd’s movements using GPS collars in the mid-2000s. But researchers noticed something odd about the 4,500 bison in Yellowstone: tracking data showed that while other ungulates moved with the wave to higher elevations, the bison lingered behind, reports Tom Bauer at the Missoulian.

“They surf the green wave early in the spring,” co-author Jerod Merkle, a migration ecologist at the University of Wyoming says, “but at some point, they stopped. [M]any bison did not reach their highest summer ranges until well after the green wave had passed.”

Even more surprising, when researchers analyzed the bison dung, they found that the animals experienced no nutritional deficits when letting the green wave wash by them. “It threw us for a complete loop,” study co-author Chris Geremia, a National Park Service scientist, tells Ed Yong at The Atlantic. “How can they fall behind but still have an incredibly high-quality diet?”

For the new study, researchers analyzed 13 years of GPS data, measured the plants the bison ate and inspected the bison’s dung. They found that when bison stop chasing spring, they actually create their own "green wave." By constantly mowing down the emerging vegetation and fertilizing it with their dung and urine, they ensure a steady stream of young, nutritious shoots growing through May, June and into July, reports The Associated Press.

“We knew that bison migrated, we figured they followed the green wave, but we didn't know that their influence on the landscape could affect the entire way that spring moves through the mountains and valleys of Yellowstone,” Merkle says in a press release. “They are not just moving to find the best food; they are creating the best food. This happens because bison are aggregate grazers that graze in groups of hundreds, or more than a thousand animals.”

Yong reports that bison herds’ massive size that can number in the thousands are what make the endless spring possible. Other grazers, like mule deer, simply don’t gather in large enough numbers to have the nibbling power to prolong spring time growth.

While it might seem like getting constantly nibbled would weaken the grasses the bison graze over time, that’s not the case. The team set up exclosures, or areas where the bison were not allowed to graze, and compared the vegetation to the area that was heavily impacted by the bison. “The mowed-down forage had higher ratios of nitrogen to carbon, a standard measure of nutritional quality,” says co-author Matthew Kauffman of the U.S. Geological Survey based at the University of Wyoming.

When the bison do finally move into the higher elevations in August, their “grazing lawn” is allowed to enter a belated “spring.” The study shows that by the end of the summer those heavily grazed plants contain 50 to 90 percent more nutrients than the grasses allowed to grow untouched. They also survive longer into the fall.

Yong reports that this has long-term impacts; when the green wave returns the following year, the heavily grazed areas have a more intense spring that lasts longer than in other areas. According to the press release, the effect is so pronounced that researchers can see the difference between heavily grazed and lightly grazed grasslands on satellite maps.

The impact of the bison herd is so profound, it raises questions about how grassland ecosystems worked in pre-settlement times. At one point, it’s estimated 30 to 60 million bison roamed between the Appalachian and Rocky Mountains. By 1884, unrestricted hunting reduced that number to just 325 wild animals, including 25 in the Yellowstone region.

“It makes us think a lot about how this grassland system worked at continental scales, when we had tens of millions of bison roaming around,” co-author Mark Hebblewhite, an ecologist at the University of Montana, tells Bauer.

Hopefully, people will be able to see some of the landscape-wide effects bison can have over the next few decades. Hannah Osborne at Newsweek reports that there are currently half a million bison in the United States, though less than 15,000 are free-ranging.

That’s starting to change, with bison reintroduction projects happening across North America. Just last month, Badlands National Park opened 22,000 new acres to bison. In 2017, Banff National Park returned bison to the area after a 130-year absence. In 2016, the Blackfeet Nation re-introduced the descendents of the last wild bison in Montana, which were sold to the Canadian government in the 1870s.

“Today there is growing effort to restore bison to habitats they once roamed,” Geremia tells Osborne. “As we seek to reestablish bison, this study shows us what large bison herds are capable of when they are allowed to seek out the best forage and move freely across large landscapes.”