Yellowstone bison double as arborists

The picturesque Lamar Valley in Yellowstone National Park is a stronghold for wild bison.

Spread out across the verdant grasslands, the animals allow visitors from around the world to imagine what it must have been like 200 years ago when bison thrived across the Great Plains.

“When visitors are asked: ‘What is the single most important thing for you to see when they come to Yellowstone,’ the number one is bison, followed by Old Faithful (geyser),” said Cam Sholly, Yellowstone’s superintendent.

OSU research

A recently published study by Luke Painter of Oregon State University said Yellowstone’s bison in places like the Lamar Valley are affecting the regeneration of aspen. Bison bulls will trample small aspen or rake their horns on the trees, stunting their growth or killing them.

“I’ve studied the response of aspen in northern Yellowstone to the reduction in elk after the wolves were brought back and found that during this time, bison increased and have begun to affect aspen,” Painter said in a university news release. “Now we’re showing strong evidence of a previously unreported behavior of bison bulls breaking aspen saplings,” possibly related to “displays of aggression” during the mating season.

Aspen revival

The revival of aspen stands in portions of Yellowstone National Park following the reintroduction of wolves in 1995-97 was touted by some scientists as evidence of trophic cascade.

The term was used to explain how, when wolves killed elk and reduced their population, there were fewer elk eating aspen trees. As a result, more aspen trees grew, providing a home for songbirds. The trophic cascade was caused by the return of a key elk predator, wolves, and the resulting cascade of ecosystem effects that followed.

The topic of bison effects on aspen came up at a recent webinar during which park staff answered questions regarding the recently released bison draft management plan. When asked about the large animal’s effects on riparian areas, the park’s scientists said areas like the Lamar Valley had changed, and undoubtedly will continue to change.

“When we look across the entirety of Yellowstone, these impacts are limited to pretty small areas relative to the entire park,” said Chris Geremia, the park’s senior bison biologist.

Sholly defended the Park Service scientists working in Yellowstone, saying it’s not unusual for there to be disagreements regarding science.

“I get it, not everyone agrees with — name the science — but I think that the team is doing a really good job in making sure we have the best scientific material to inform decisions,” he added.

Ecosystem engineers

A bull bison can weigh up to 2,000 pounds. Females weigh about half as much. Because they have few predators, the population can grow by about 15% every year. This past year, the population of bison utilizing the northern portion of Yellowstone grew to more than 4,400 animals. With that many large ungulates roaming around, eating and defecating, there’s bound to be some effects.

“Bison are ecosystem engineers, which means they will modify the environment for their own benefit,” Geremia said. “They influence the grassland by changing how nutrients cycle through the system by altering the plant communities that are there.”

Painter said the large population of bison in the Lamar Valley are hindering some aspen stands from replacing dying trees. Yet Geremia noted, “We’re not trying to bring it back to where it was 150 years ago.”

Elk effect

P.J. White, Yellowstone’s chief wildlife biologist, said it’s “widely accepted” that elk, which grew to a population of around 19,000 animals before wolf reintroduction, had an effect on the aspen, willows and cottonwood trees they fed on.

“Those effects essentially changed it from a community along the rivers where you had beavers and those riparian vegetations to more of a grassland community without beavers,” he said. “But that was done before bison were restored to more abundant numbers on the northern range.”

Painter, the Oregon State scientist, praised the recovery of bison as a “big success story,” but said the recovery of aspen and other woody plants was also worth celebrating.

“Thus, one important conservation goal is affecting another important conservation goal,” he said. “Researchers are only beginning to understand how these conservation goals have overlapped and affected each other. We reported a piece of this complex puzzle, describing and quantifying one way that bison shape their habitat by suppressing trees.”

Always changing

White pointed to last summer’s historic flood and how that had changed the landscape in places like the Lamar Valley — carving new channels, eroding streambanks and depositing gravel and silt — as evidence of continued natural effects on the Yellowstone ecosystem.

“It is our park policy to manage for ecosystem function, not necessarily historic integrity,” Geremia said.

He noted invasive plants took root in the park following the reintroduction of bison in the early 1900s. They continue to spread, yet the ecosystem is still functioning.

“We’ve got something very complicated in the Lamar going on, and we want that complication to continue into the future,” he said.