Influence of Temperature and Drought on the Migration of Plains Bison

It epitomizes the Great Plains in both spirit and form: a 2,000-pound tank on hooves, covered in a winter-tested shaggy coat, and draped with horns that serve as both a warning and a weapon.

Even its scientific name, Bison Bison Bison, seems to evoke an echo worthy of its majesty. However, the robust appearance of the plains bison – the national mammal of the United States and the largest on the continent – belies the vulnerability of its history, which has seen its hordes decimated from tens of millions to just a few hundred in just a few spans. colonial centuries.

Conservation efforts have pushed its number closer to 20,000, and its status from threatened to near threatened. But a recent study led by Nick McMillan of the University of Nebraska-Lincoln hints that continued conservation of plains bison will require a reckoning with the climate — especially the number of scorching days and powdery landscapes — that it encounters as it moves forward.

In their study published in Ecology and evolutionMcMillan and his colleagues at Oklahoma State University found GPS-supported evidence that extreme temperature and drought can drive movement among herds of plains bison. Continued increases in both, combined with the fact that most bison herds are now confined to parts of the land they once roamed, could present challenges to managing the popular species, Macmillan said.

“When we think about reintroducing a bison or any large animal to a landscape, the landscape that the animal inhabits is likely to be a lot smaller than it was in the past,” said McMillan, assistant professor of agricultural engineering and horticulture at Nebraska. “In 1491, if there was a drought in northeastern Montana, the bison had the entire Great Plains to escape from that drought. They could move as far as they needed to.

“In this time when we’re seeing more extremes – more extreme air temperatures, more extreme droughts – maybe we need to rethink how we organize these landscapes, and whether or not they’re really meeting the basic physiological needs of these animals.” .

The team reached its conclusions after analyzing movement data for 33 plains bison from two different locations in Oklahoma: the Wichita Mountains Wildlife Refuge in the southwestern part of the state, and the Talgrass Prairie Preserve near its northern border. Both bison wore GPS-equipped collars that tracked their location every 12 minutes over several years, resulting in a total of 715,344 measurements. Correlating this data with temperature, precipitation, wind, and other variables recorded by nearby weather stations, as well as drought readings that reveal desiccation in the soil at both sites, allowed the team to look for links between bison movement and weather.

Air temperature explained differences in bison movement better than any other factor the researchers analyzed. When the temperature ranged from a few degrees below zero to 83 degrees Fahrenheit, bison movement increased by 92.5% for every 18-degree rise, so that movement nearly doubled when the temperature rose from, say, 65 degrees to 83 degrees. Above that limit — from 83 degrees to at least 112 degrees, the highest temperature on record — an increase of 18 degrees would instead correspond to a decrease in motion of 48.5%.

The team said the temperature-related increase in movement indicated that plains bison were foraging for grasses that grew better amid the extreme heat, especially since bison get the majority of their water from foraging. Meanwhile, low movement in the face of excessive heat indicates that they rested and cooled themselves in places where standing water and the shade of nearby trees prevented the equivalent of heatstroke.

“When you think about (that) this is the first study of plains bison across multiple herds — and then we find the same relationship between herds, in two completely different regions — that’s a big problem,” said McMillan, who noted that this trend also parallels a study On wood bison in Canada.

In looking for possible effects of drought, the team turned to sensors that measure soil moisture at two depths: 5 centimeters, where drought indicates moderate drought, and 25 centimeters, where it indicates severe drought. Although bison appeared mostly unaffected in the first case, enhancing their ability to tolerate moderate drought by taking up water through vegetation, they moved more dramatically when faced with the severe drought that hit Oklahoma in early 2010.

“There’s a lot of (previous) research that suggests bison are basically drought-tolerant,” McMillan said. “They’re like these tanks on the prairie that don’t need anything. They can just take whatever comes along, no big deal. At least, that’s the dogma in the world of reintroduction.

“So I think that’s really interesting, because we’re showing that, hey, they’re still not immune to dehydration. They have this potential threshold where they can’t handle it.”

“I was just hooked.”

When bison covered the Great Plains, Macmillan said, they were not only a vital food source and cultural touchstone for indigenous peoples, but also a species on which many animals and plants depended for survival. After their population dwindles and eventually moves to protected areas, plains bison may or may not serve the same ecological purposes they once did. Regardless, Macmillan said their place in the history of the continent and the country must be preserved in the animals themselves.

“They’re incredibly culturally important and represent the cultural identity of all plains tribes, at least,” Macmillan said. “But it also represents one of the most attractive animals we have in North America. So it’s really important to our identity as a country and all the people who live here.”

It resonates with McMillan on another frequency, too. As the son of a plant biologist and ecologist who hosted a national television show that aired on PBS, the South Carolina native tagged his dad on a visit to Nebraska when he was 15. Badlands of western Nebraska, would ignite his “deep attachment” to the Bisons and help define the course of his career.

“We came to the Great Plains,” he said, “and I was hooked.” “I never really thought about anything else after that. I didn’t really originally envision myself necessarily being a scientist or studying bison as a scientist, but I was always fascinated by how they were in the landscape, and giving them back — that really emotional story. So for me, it’s very personal. “.

McMillan said he hopes the team’s findings will help manage Plains bison in places like Yellowstone National Park, which has the largest wild herd of this species. He said these findings underscore the need to pay attention to what animal behavior tells ecologists and conservationists. Although the size and natural beauty of national parks and other protected areas may seem to offer everything a bison could want on the plains, the fact that the animals are still trying to leave suggests otherwise.

And if the extreme heat and drought that seem to encourage plains bison to move becomes more common, McMillan said, the area once considered sufficient may not be. This problem can be exacerbated by the fact that ensuring the diversity of their habitat — that they have access to grasslands, as well as trees, as well as standing water — seems to be becoming more important as that habitat shrinks.

“So it’s an ethical question for us,” Macmillan said. “Are we really ethical if we force these animals into a landscape that might not be suitable for them in the future? Whether bison were in Yellowstone historically has nothing to do with their likelihood of continuing there in the future. Because today is completely and fundamentally different from yesterday.”