- In a recent study, a team of biologists found that the release of American bison (Bison bison) on a small section of North American grassland led to declines in a species of bird called the bobolink (Dolichonyx oryzivorus), likely because the density of the bison in the fenced-in study area was too high.
- Bison once numbered in the tens of millions on the Great Plains, but hunting drove the population down to around a thousand by the end of the 1800s.
- Concerted efforts to protect remaining wild populations and reintroduce the animal to parts of the Plains has resulted in a resurgence, and the species is no longer in danger of imminent extinction.
- However, the results of the bobolink study reveals that bison releases and reintroductions must be done carefully to avoid negative impacts on the broader ecosystem.
The story of the American bison is both tragic and uplifting. Once dappling the prairies of North America in the tens of millions, hunting winnowed the number to perhaps a thousand survivors by the end of the 19th century. Nearly wiping out the continent’s largest surviving land animal was a bloody stain on the century-old country that had been driven in part by a campaign by the U.S. government to bring the bison-dependent First Nations of the Great Plains to heel.
Today, bison dot the vast grasslands once again, though their numbers are nowhere near what they once were. Federally protected herds roam wild in places like Yellowstone National Park, and the practice of releasing small groups, often on privately held lands, has grown in popularity. Through the will of individuals and organizations determined to hang on to this iconic animal — it’s now the United States’ national mammal — the survival of Bison bison is no longer in doubt.
“It’s very exciting because they almost went extinct,” avian ecologist and conservation biologist Nico Arcilla told Mongabay. “It was purely because of human effort. People said, ‘We’re going to protect these last animals.’”
Given the bison’s recovery, it might seem that returning the continent’s largest land animal to the landscape at every opportunity would be a logical step toward restoring the ecosystem of the Great Plains. But research by Arcilla and her colleagues suggests that reintroducing bison can have unintended consequences.
In a study looking at 18 years of data published Sept. 10 in the journal Animals, they report that the population of a small songbird call the bobolink (Dolichonyx oryzivorus) went down significantly in the years after bison were released on a small section of grassland in the Platte River Valley of the U.S. state of Nebraska. The bobolink is considered a bird of conservation concern by the U.S. Fish and Wildlife Service, because its abundance has been declining as more of its grassland habitat has disappeared.
The team was surprised that the return of bison would precipitate a dip in the local bobolink population, said Arcilla, who is also the president and research director of the International Bird Conservation Partnership in Monterey, California.
“It’s a native animal you’re bringing it back,” she said. “It just makes sense it would make things better.”
Bring back a keystone species — especially an ecosystem engineer like the bison — and nature will begin to heal itself, so the thinking goes. Conventional wisdom held that returning bison to the plains would benefit birds. Restless bison weighing in at more than a ton (900 kilograms) churn up the soil. They also chomp on grasses heavily in a specific area for a time and then move on. This grazing “on the hoof,” as biologists call it, creates a patchwork of different habitats on the deceptively uniform plains, providing shelter and potential nesting sites for a variety of bird species.
The trouble is that there’s little research to back up the dictum that bison benefit bird populations. A few studies have found that certain species do better, such as the grasshopper sparrow (Ammodramus savannarum), which adapt better to bare ground, Arcilla said. But the available data don’t demonstrate sweeping positive effects on bird diversity and abundance in general. Indeed, the dearth of such evidence impelled Arcilla and her colleagues to investigate whether bison improve the lot of grassland birds.
In this case, the number of bison in the relatively small, 2,400-hectare (5,930-acre) plot of private conservation land appears to be at the root of the problem for the bobolinks. A herd of 53 animals had been released in 2015 onto a 140-hectare (346-acre) section of that grassland. Four years later, there were more than twice as many living in a larger, but still small, 336-hectare (830-acre) pasture. Between 2015 and 2019, the number of adult bobolinks fell by nearly a third. The team found that, in the same four-year span, bobolink numbers remained stable in other grasslands in the area.
Bobolinks prefer tall, thick grass to shield them from keen-eyed predators and the blazing sun during Nebraska’s stifling summers, Arcilla said. They also nest in these clumps.
The team found an even steeper, 84% decline in young bobolinks, suggesting that overgrazing and trampling by the bison may affect juvenile survival, though Arcilla said more research was needed to understand these “mechanisms” driving the decline.
Arcilla said bison probably weren’t as disruptive to bobolinks when they plied the fenceless prairies that existed through much of the 1800s.
“They were able to roam over this enormous area as they pleased,” she said. Now, “They can’t do that … There are just too many bison on this little piece of land for certain birds to make it.”
If too many bison are fenced into the study area, then why not move some of them? That’s easier said than done, Arcilla told Mongabay. Herding them to new areas or loading them onto trucks like their more docile cattle cousins stresses them out, creating a potentially dangerous situation for people and animals.
Another option is what biologists call “lethal removal”: culling the herd to maintain a sustainable number of animals on the landscape. Grand Canyon National Park held its first bison hunt in September and October. But safely carrying out a cull in a way that is also socially acceptable isn’t easy and is often beyond the capacity of smaller landowners.
Several Native American tribes on the Plains have returned bison to their lands with the intention of allowing limited hunting and harvest of the animals. In October 2020, the Sicangu Lakota Oyate nation worked with WWF and the U.S. Department of the Interior to release 100 bison on tribal land in South Dakota. The Sicangu Lakota plan to expand the herd to 1,500 animals, but the size of the pasture available to them will be five times the size of the study area in the Platte River Valley.
These initiatives bring back a culturally significant animal that Native Americans consider to be part of their family, proponents say. The return of bison also restores a source of food that was stolen when hunters pushed bison to the edge of extinction in the 19th century.
A 2019 study led by ecologist Kate Wilkins found that reintroducing bison to the prairies of northern Colorado boosted the connection that visitors had with the site. The research did not turn up a noticeable impact on birds two years after the bison returned. That said, Wilkins told Mongabay, reintroductions have to be done with careful planning.
“Long-term ecological monitoring at bison reintroduction sites is extremely important, especially if land managers want to understand how to reach goals focused on grassland conservation,” she said in an email.
Wilkins, a postdoctoral fellow at Colorado State University who was not involved in the research in Nebraska, said the bobolink study demonstrated that the effects of reintroductions on “sensitive” bird species must be taken into account. More broadly, she added, bringing bison back cannot be viewed as the final piece of the restoration puzzle, as vital as that piece might be.
“Bison remain an integral part to grassland conservation because they have the potential to help restore lost ecological function and repair severed cultural connections for Native American tribes,” Wilkins said. But, she added, “Successfully reintroducing bison to meet ecological goals requires managing herds to ensure they create heterogenous landscapes that support different species of grassland birds and other animals.”
In a 2003 paper, policy analyst and historian Michael Black likened the restoration of natural ecosystems to the story of Humpty Dumpty. In the well-known children’s nursery rhyme, Humpty Dumpty — often in the form of an egg, thanks to an illustration in Lewis Carroll’s Through the Looking-Glass — topples from his perch on a wall. Try as they might, the hapless king’s guard have no luck mending Humpty.
“Ecosystems, like Humpty Dumpty, are vastly easier to preserve than they are to reassemble,” Black writes, which the bobolink study demonstrates.
Still, Arcilla said the resurgence of the bison is evidence that, with effort, humans can affect positive environmental change.
“We can almost destroy things totally, or we can save things and make room for things,” she said. The question now is whether that capability will be limited to a single species or be brought to bear on the restoration of a landscape that would include other members such as the bobolink.
“Do we want a healthy ecosystem?” Arcilla asked. “Or do we just want to see bison again?”
Banner image of a bison herd grazing at sunset by Nico Arcilla.
John Cannon is a staff features writer with Mongabay. Find him on Twitter: @johnccannon
Correction: A previous version of this article misstated the number of years of data that Kate Wilkins and her colleagues had on birds after the return of bison. It was two years, not one.
Black, M. (2003). Can we design ecosystems? Lessons from the California rivers. International Journal of Engineering Education, 19(1), 213-226.
Kaplan, R. H., Rosamond, K. M., Goded, S., Soultan, A., Glass, A., Kim, D. H., & Arcilla, N. (2021). Bobolink (Dolichonyx oryzivorus) declines follow bison (Bison bison) reintroduction on private conservation grasslands. Animals, 11(9), 2661. doi:10.3390/ani11092661
Wilkins, K., Pejchar, L., & Garvoille, R. (2019). Ecological and social consequences of bison reintroduction in Colorado. Conservation Science and Practice, 1(2). doi:10.1111/csp2.9
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