Plant Diversity Booms When Bison Return to the Prairie

The mammals increase species richness by 86% compared to areas where they don't graze.

Great Plains bison in field of yellow flowers

Jill Haukos (Kansas State University, Manhattan, Kansas

Reintroducing bison to prairies doubles plant diversity and boosts resilience to extreme weather, a new study finds.

Somewhere between 30 to 60 million bison likely roamed North America before the mid-1800s, mostly on the Great Plains. But the massive mammals were nearly hunted to extinction.

Today, there are only about 30,000 bison in public and private herds in North America. Approximately another 400,000 bison are raised as livestock.

“Plains bison, previously one of the most dominant and widespread species of megafauna in North America, were nearly driven to extinction in the late 19th century and currently occupy less than 1% of their pre-European range,” study author Zak Ratajczak, an assistant professor of grassland biology at Kansas State University, tells Treehugger.

“Like other megafauna, bison are thought to have an outsized impact on ecological dynamics because they can exert strong control on vegetation and tend to form a large web of interactions with other species.”

For their study, Ratajczak and his colleagues explored if bison can still have a large impact on tallgrass prairie and, if so, what does that say about what effect they likely had in the past?

World Wildlife Fund

Prior to European colonization plains bison are estimated to have numbered between 30 million-60 million animals and were the widest-ranging large mammal in North America. Bison were (and remain) central in the lives and traditions of many Native nations and an umbrella species for many plants and animals sharing its habitat. By 1889, only 512 plains bison remained after the ravages of westward expansion, market demand, and a deliberate effort by the U.S. Government to eliminate the bison to subdue the Native people that relied so heavily upon them.

Three Decades of Data

Researchers considered about three decades of data from the Flint Hills region of Kansas. They analyzed information that started in the late 1980s and early 1990s, but took a second look after noticing droughts that happened in 2011 and 2012.

“These droughts resemble the climate extremes that will probably become more common in the near future, due to climate change,” says Ratajczak. “Therefore, the ecosystem response to these droughts might give us a measurement of whether we can expect the ecosystem to be resilient in the future.”

The region they studied was divided into three sections. In one, there were no megagrazers, one had reintroduced bison grazing year-round, and the other had cattle grazing from April to November, which is the growing season.

They found that the region with bison had an 86% increase in the richness of native plant species compared to areas that had no bison grazing. Cattle grazing also increased plant species but by less than half of the amount linked to bison.

The results were published in the journal Proceedings of the National Academy of Sciences (PNAS).

Researchers say it is one of the largest increases in diversity ever caused by a grazing animal, and the prairie growth gains were resilient to extreme drought, which is likely to become more prevalent.

“Tallgrass prairie (where the study took place) is among the most productive grasslands in the Great Plains. There are a handful of very dominant grasses that can take advantage of this, becoming incredibly abundant, which leaves very little room or resources for other species,” Ratajczak explains.

Bison prefer to eat those dominant grasses and can consume large quantities. That leaves more space for other plant species, which leads to an increase in diversity.

Bison also help the ecosystem by wallowing. That’s when they repeatedly roll on the dry ground to shed winter coats or fend off insects. The behavior creates a depression in the earth–called wallows–that can assist plant diversity.

Resilience Is Key

It was critical to do a long-lasting study because some environmental responses to a change in land use can take years or even decades to appear, researchers say. In this study, they saw increases in plant diversity over three decades. Had they ended the research earlier, they would have thought that bison had a much smaller impact than they do long term.

“Like many grasslands, tallgrass prairie exists in an area with a variable climate. This is ‘natural’ for the ecosystem and many of the organisms in this ecosystem are adapted to a variable climate,” Ratajczak says. “However, we expect that the ecosystem will have to cope with an even more variable climate, therefore, it’s important to have a long record of data to understand how these ecosystems function over ecological and evolutionary time scales.”

Megafauna, like bison, are missing from many, or even most, ecosystems.

“I kind of thought that perhaps the species that evolved to do well alongside bison might have gone locally extinct by now. Some probably have, but clearly many have not. This means that reintroducing bison to some tallgrass prairies might really increase their plant biodiversity,” Ratajczak says.

“I think the resilience we saw in this study is especially important. These drought events can be really disconcerting, and while we can’t take this resilience for granted, I was heartened to see that the plant community was able to bounce back fairly quickly after a very extreme drought.”

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View Article Sources
  1. "Frequently Asked Questions: Bison." National Park Service.

  2. study author Zak Ratajczak, an assistant professor of grassland biology at Kansas State University

  3. Ratajczak, Zak, et al. "Reintroducing Bison Results in Long-Running and Resilient Increases in Grassland Diversity." Proceedings of the National Academy of Sciences, vol. 119, no. 36, 2022, doi:10.1073/pnas.2210433119

  4. "Bison Bellows: What's Wallowing All About?" National Park Service.