A bison group on the move at sunrise in Lamar Valley.

The migrating herds of bison enrich – not impoverish – the northern grasslands of Yellowstone National Park, according to a recent study.

That conclusion was examined by the Park’s lead bison biologist, Chris Geremia, Ph.D., during an April presentation at the Buffalo Bill Center of the West. Along with studying grassland ecosystems, he heads the effort for transferring Park bison to Native American tribes.

Another conclusion from the study showed bison follow the so-called green wave, the emergence of plants in spring, from lower to higher elevations, then pause mid-summer. At that point they gather in groups and re-graze certain areas, Geremia said.

Although such intense grazing resulted in dramatic differences between the plants’ heights in open versus fenced areas, both the grazed and ungrazed plants contained the same high value.

“Grazing is enhancing diet quality,” Geremia said. Further, “the more intense grazing with a larger bison population caused the same areas to green up earlier, more intensely and for longer each year.

“We know now that large herds of bison migrating together shape the entire way that spring comes to the Park.”

That broad view emerged from observations of the same sites over 20 years, during which the central herd grew from 300 to 1,000 head and the northern herd from 600-800 to 4,000-plus. Despite those increases in bison counts, the numbers comprise a fraction of the 30-60 million that once roamed the West, Geremia noted.

By the late 1800s, he said, only 200-400 bison remained, with some in Yellowstone. Today there are about 400,000 in the U.S. and Canada, kept mostly for meat production. The 20,000 maintained for conservation in 60 different herds are constrained and no longer migrate. 

The mass migrations and wildfires of precolonial days shaped the West’s grassland ecosystem, when the bison “were truly a force of nature,” Geremia said.

The current northern herd grew from a captive breeding program begun in 1902 when the Park brought in 21 bison to the Buffalo Ranch in the Lamar Valley. There, they were fed in the winter and hazed into the high country in the summer “to try to teach them what to do,” Geremia said.

Simultaneously, the native, central herd of 20 in Pelican Valley, north of Yellowstone Lake, migrated toward the captive herd in the summer. Gradually, over the decades, the two herds gathered together in the summer and then separated in the winter.

“There’s nowhere in the world where we actually have learned the story of an animal relearning its migration,” Geremia said. “This is an unprecedented story.”

In the 1980s and ’90s, as their numbers rose, the northern herd began migrating toward the Park’s north boundary. When bison began leaving Yellowstone, “that created a ton of controversy,” Geremia said. “People thought the Park was overgrazed.”

Bison should be thought of “as a migratory animal,” he said. “As this animal relearns the landscape, they move in tempo with resource availability.” 

In the winter, they learned to head north to avoid the deep snow. In the summer, they sought out the highest quality and quantity of forage, which translates to plants that green until mid-summer. As plants age, they become more indigestible.

According to the tracks of bison fit with GPS collars, they moved with the green wave in early spring, fell behind it in mid- to late-spring and then months behind it at high elevations. After examining their feces, researchers found “the bison are surfing the green wave even after they let it pass them by,” he said.   

To explain that phenomenon, Geremia said that although plants lose water as they grow, grazers can make plants’ water use more efficient by cropping them closer to the ground. 

“Bison also return an immense amount of nutrients to the soil through their urine and dung,” he added. “The decomposition rate of animal waste is much more rapid than plant decomposition.”

Further, he said, cropped grass produces new shoots that are high in nitrogen, while the nitrogen-rich dung produces nitrogen-rich soil. 

“Through these direct linkages, plants may be able to regrow after being grazed,” Geremia said.

The researchers also examined sites that offered rich and poor nutrition and how various grazing intensities affects them.

“Plant communities were able to compensate for grazing, regardless of the timing and amount of leaf tissue removed,” Geremia said. “As a result, bison grazing sustained the energy flow through the ecosystem.

“We found no support that grazing lowered productivity.”

In fact, in the rich sites, plants not only were able to compete but also did better, he said, calling that “a really important concept. That means that although things look very different, the energy flow is at least sustained in this ecosystem.

“I feel very confident that bison don’t lower the productivity of this landscape.”

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