Scientists Tracked Migrating Bisons in Yellowstone for 6 Years — and Found Something Incredible

It is not unusual for the visitors of Yellowstone National Park to spot a herd of bison milling around in a meadow, chewing grass. However, each time of the year shows them indulging in a different activity. Bison here spend their winters hoarding up on calories by eating all day, swinging their necks to rip out little plants shooting from the snow melts. Autumn is spent in rutting, a word animal scientists use to refer to their official mating season. In late summer and early fall, while the males move into the “bachelor herds,” the females graze around the meadows, feeding their calves with one older female guarding the herd as its leader.

Bisons, the beasts 

Every year, millions of visitors arrive to capture the herds of bison grazing and migrating. All of this might sound like a marvelous postcard-worthy spectacle, but scientists believe it is much more than that. In a new study published in Science, a team of researchers reveals that these bison contribute a bounty of lushness and fertility to the park’s landscape while they migrate each season. When NASA satellites hovering in space investigated the park’s wildlife, they recorded a stunning shift in the grassland dynamics each summer. Initially, it seemed to be a fluke in the local weather, but when researchers dug deeper, they were amazed.

Positively manipulating the landscape

When the rutting season ends and it’s time for them to migrate westward to escape the piling snow, bison gravitate towards hotspots like Lamar Valley and Roosevelt Arch to hibernate and feed. They stop by rivers and roam around hot spring basins, bellowing in tall grasses, concentrating in the river valleys, and snacking on wildflowers. And while they feed their bellies, they make an unintentional contribution to the entire planet. Every pinny grass they eat unknowingly manipulates the landscape by sprinkling lushness and abundance.

According to NPS, Yellowstone houses the “nation’s largest bison population on public land.” Whether it is congregating during the breeding season or fighting to compete for mates, the wild behaviors they’ve inherited from their ancestors have enabled the successful restoration of a population that was on the brink of extinction just over a century ago. In Yellowstone alone, thousands of bison travel about 1,000 miles each year, moving back and forth along a 50-mile migration route running through the park.

Carving patchworks of habitats

In this research, researchers from Washington and Lee University partnered with the National Park Service and the University of Wyoming to find out how and whether these behaviors enhanced the health of the ecosystem or degraded it. The team deployed an array of field experiments, satellite images, and GPS collar data to compare the grazed and ungrazed plots. To their utter astonishment, they found that these animals are positively re-shaping the ecosystem, carving patchworks of habitats that enhance biodiversity while keeping the soils healthy and fertile.

Speeding up the 'nitrogen cycle'

While they pull out plants and grasses from the park’s grounds, bisons speed up the “nitrogen cycle” of the plants. Each blade of grass they eat increases the volume of microbes in the soil, which not only enhances the nitrogen content of the soil but also fertilizes it, enabling a rapid propagation of plants. “What we’re witnessing is that, as bison move across the landscape, they amplify the nutritional quality and capacity of Yellowstone,” Bill Hamilton, a leading researcher from the team, explained in a press release.

He added, “Their grazing likely has important consequences for other herbivores and for the food web as a whole, similar to the changes that occurred in the Serengeti when the wildebeest population recovered.” He remarked that humans are lucky to have bison as Yellowstone couldn’t have been better than it is in their feral presence.

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