Bull stampede: Bison behavior hinders aspen recovery in Yellowstone

In the northern region of Yellowstone National Park, a historically large bison herd is causing an unexpected hindrance to the recovery of quaking aspen saplings. This crucial species in the American West ecosystem is trying to bounce back from decades of over-browsing by elk. 

The discovery was made by a research team led by Luke Painter of Oregon State University. The study follows up on a paper Painter published five years ago in the journal Ecosphere, which indicated that the reintroduction of wolves in Yellowstone had sparked a recovery of aspen both within and outside the park. 

Focus of the study

“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,” said Painter. “Now we’re showing strong evidence of a previously unreported behavior of bison bulls breaking aspen saplings.”

Painter noted that while some saplings were tall enough to avoid being browsed by elk and therefore had a higher chance of growing into trees, bison were breaking them off at lower heights. Additionally, other saplings were killed when bison scraped off the bark with their horns.

Aspen recovery 

The quaking aspen predominantly reproduces through root sprouts, a process known as suckering, and aspen stands often consist of a single organism connected by a common root system. Fire stimulates aspen reproduction from both roots and seeds. 

Throughout much of the 20th century, aspen sprouts were unable to grow into trees due to consumption by elk during winter. 

However, with the reintroduction of wolves and an increase in the population of other large predators like grizzly bears and cougars, elk numbers in northern Yellowstone decreased, providing relief to the aspen. 

New twist in the story

“Some young aspen began growing into saplings – young trees taller than 2 meters – which was an indication they were no longer being consumed by elk and were likely to grow into mature trees,” said Painter. 

“It was a trophic cascade that changed the Yellowstone ecosystem, creating conditions that could bring it closer to what it was historically, with more aspen, willow, and beaver, which depend on these plants. But the tremendous increase in bison over the last two decades has added a new turn to the story.”

Bison behavior 

Painter added that bison have long been known to exert significant effects on their environment, including removing and suppressing shrubs and trees by eating, trampling, and breaking them. 

With the considerable increase in bison numbers in northern Yellowstone over the past 20 years, their impact on plants has also escalated. In areas with large bison populations, such as the iconic Lamar Valley, the bison are preventing some aspen stands from replacing their dying trees.

What the researchers discovered 

The researchers examined a random sampling of plots in 87 randomly selected aspen stands, revealing that 18 percent of saplings had been broken. 

Although they may resprout from their base, the lost height of the saplings and the vulnerability of new sprouts to being consumed by bison or other herbivores are concerning, note the researchers.

Painter and his colleagues, Robert Beschta and William Ripple from the OSU College of Forestry, found multiple lines of evidence to attribute the breakage to bison. 

“Most broken saplings were in areas of high bison density and low elk density, and they were broken in summer when elk wouldn’t have been foraging on them,” Painter said. 

“Plus we directly observed bison breaking aspen saplings. The purpose of the behavior doesn’t seem to be about accessing food, and we observed only bulls engaging in this behavior, so it may be related to displays of aggression.”

Rising bison numbers

The management of Yellowstone bison involves an agreement with the state of Montana that requires bison to remain in or very near the park. Bison that stray are either killed, captured, or hazed back into the park, primarily due to the risk of brucellosis – a bacterial infection that poses a threat to Montana’s cattle industry. 

Although elk also carry brucellosis and have transmitted it to cattle, the same restrictions are not applied to them. Consequently, unlike other wildlife, bison are not permitted to disperse to other areas as their numbers and density increase.

Overlapping conservation goals

Painter noted that the conservation of bison in Yellowstone, whose numbers drastically declined nationally due to over-hunting in the 1800s, is a significant success story. Similarly, the recovery of aspen and other deciduous woody plants that began with the comeback of the park’s large predators is also a success. 

“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.”

This study highlights the complex and interconnected nature of ecosystems and the challenges that arise in managing and conserving them. 

The findings emphasize the need for a better understanding of the interactions between different species and their environment to develop effective conservation strategies.

The research is published in the journal Ecology and Evolution.

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