American bison were once so numerous that in 1889 the Superintendent of the National Zoo wrote that trying to count them would be like counting “the number of leaves in a forest.” It’s in large part why the exact ecological impact of North America’s largest land mammals was never measured, before settlers hunted them to near extinction in the 1800s. But current efforts to reduce them to their historic range have confirmed what conservationists and Native Americans have been saying for decades: bison are critical to prairie health.
New research on the long-term benefits of reintroducing bison shows that their presence makes the country more biodiverse and resilient to drought. An article published in the magazine this week PNAS measures the ripple effects of the giant grazers on the tallgrass prairie ecosystem that used to stretch from modern Texas to Minnesota and cover 170 million acres of North America. Today, only about 4 percent of the old prairie remains, mostly in the Flint Hill region of Kansas where the study took place. The data, spanning several decades after the bison’s return, are unequivocal: The herbivores more than doubled the number of native species in tall-grass habitats.
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“Bison is the kind of organism you would expect to have a big impact,” said Zak Ratajczak, a biologist at Kansas State University and lead author of the study. “They are very large, travel long distances and can consume plant species at a scale that changes the competition.”
They also specialize in eating large blue stems and other tough grasses that are more likely to be passed over by other herbivores, including non-native livestock. Growing fast and tall, these grasses obscure other plants that perform a wide variety of functions, such as wildflowers that support pollinators and legumes that fix nitrates in the soil. Given enough time, Ratajczak says, “the cumulative, cascading effects [of the bison] are big.”
Since the 1980s, scientists at the 8,616-acre Konza Prairie Biological Station in Kansas have documented changes in plant biodiversity with the reintroduction of the bison herd, whose numbers have stabilized between 275 and 300 in recent years. By comparison, they tracked. also the health of areas of tallgrass prairie that were eaten by livestock, as well as parts that were left completely untouched.
In addition to the clear positive impact of the bison, they found a few other important differences. First, while cattle grazing was not even half as effective as bison grazing, it was better for biodiversity than not grazing at all. And second, the bison-occupied prairie was better able to withstand periods of drought, thanks to a greater variety of plant species and newly stimulated growth through grazing.
“It’s encouraging to see that it can withstand some degree of warming,” Ratajczak said, noting that this will be especially important with the predicted increase in intensity and frequency of extreme heat in the near future due to climate change.
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Eric Patterson, the chief keeper of the Tallgrass Prairie National Preserve, sees that the park’s herd of about 100 reintroduced bison has a similar impact on the variety and abundance of local plant species. He tells visitors to the nearly 11,000-acre site, which is in the same Flint Hills region as the research station, that “grazing is a fundamental part of maintaining balance in the prairie ecosystem,” along with moisture, fire and human activity. usage.
Still, both Patterson and Ratajczak emphasize that while bringing bison back to the prairie is fantastic in terms of biodiversity, it’s not a miracle cure for conservation. Historically, the megafauna likely played a central role in balancing life on the tallgrass prairie, but only about 4 percent of that ecosystem remains intact. Today, ranching, agriculture, and urban development dominate the Great Plains.
“I could see how people could see this as a cattle versus bison story,” Ratajczak says. “But one important thing that I hope doesn’t get lost is that livestock can also positively impact native species.”
To that end, Patterson says some biologists are using the knowledge they gain from studying reintroduced bison to develop livestock grazing practices that mimic the effects of the wild herbivore. He and Ratajczak also point out that in recent decades ranchers have helped maintain the critical burn regime previously caused by lightning and Native Americans.
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“Bison deserves every credit they get, but there aren’t many more,” says Patterson. Findings from stretches of intact prairie like the Kansas Flat Hills need to be adapted to the grazers — and landscapes — we still have.
“Places, like [the Tallgrass Prairie National Preserve] are great,” adds Patterson. “But 11,000 acres is just a museum artifact if we fail in the larger mission of encouraging better management of everything else.”