Study: Yellowstone bison mow, fertilize their own grass

A study of bison grazing behavior in Yellowstone National Park found that bison can continue grazing in one area, rather than chase

Hundreds of bison grazing in an area stimulates the growth of nutritious grasses, in part because their waste acts as a fertilizer, according to research published this week in the Proceedings of the National Academy of Sciences.

“They add fertilizer through urinating and defecating, they drop nutrients back on the landscape, which are then available to plants,” Yellowstone scientist Chris Geremia said Wednesday.

“It’s almost like the bison become this giant fleet of lawnmowers moving back and forth across the landscape,” he said.

When more bison grazed an area more intensely, the area greened up earlier and faster and the grass stayed greener and had a higher nutritional quality for a much longer time, Geremia said.

From 2012 to 2017, researchers fenced off plots of grass along bison migration corridors and compared them to the grazed areas.

“The data showed that grasses heavily grazed by bison were more productive compared to exclosures where bison were not allowed to graze,” said Matthew Kauffman, unit leader of the U.S. Geological Survey’s Wyoming Cooperative Fish and Wildlife Research Unit at the University of Wyoming. “The mowed-down forage had higher ratios of nitrogen to carbon, a standard measure of nutritional quality.”

Trampling and nibbling by the bison kept the plants shorter and denser and forced the plants to keep growing, giving the bison a steady supply of fresh, nutritious grass.

“During most of May and June and part of July ... they are grouped together repeatedly grazing the same area,” Geremia said.

While the effect of aggregate grazers was known on a small scale, sensors on NASA satellites can detect how grassland dynamics differ between areas that are lightly or heavily grazed by bison, the research showed.

“Our work shows that bison are capable ecosystem engineers, able to modify grasslands in a way that enhances their own grazing,” said Mark Hebblewhite, professor of ungulate habitat ecology at the University of Montana.

The research could bolster efforts to rebuild herds of bison around the West, including in Banff National Park in Alberta, Canada, and on the Blackfeet Indian Reservation in northern Montana, Hebblewhite said.