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ÈÕ±¾avÊÓƵ researchers calibrated thousands of years of bison existence records with future projected weather patterns to forecast a significant northwest shift in suitable living conditions for the North American bison. 

The Great Plains has functioned as an ideal habitat for the North American bison for thousands of years. But according to new research from ÈÕ±¾avÊÓƵ, the grasslands of South Dakota and North Dakota may no longer be the national mammal's model habitat by the end of the century.

Earth's climate has changed throughout deep history, with periods of both warming and cooling. Currently, the North American climate is seeing an increase in temperatures and variability in precipitation. This change is causing some species to shift their range as living conditions become unsuitable.

The research team's findings, , suggest that the center of suitable climate conditions for the North American bison will shift from the Saskatchewan-Montana/North Dakota border significantly to the northwest, near the Alaska/Canada border, by the year 2100. While Canada and Alaska will become more suitable for bison, much of the contiguous United States and all of Mexico — once home to vast herds in the northern part of the country — will fall underneath the suitability threshold.

"Our findings demonstrated that much of the North American continent, where bison were largely distributed over the last 160,000 years, will have a much smaller suitable living condition range by the end of the century," said Alex Shupinski, postdoctoral researcher at ÈÕ±¾avÊÓƵ and the lead author on the study.

Recovery from near extinction

At one time, bison could be found in the rich grasslands everywhere between Alaska and the Gulf of Mexico with a population estimated between 30 and 60 million in the early 18th century. But overhunting, mass slaughter, novel diseases from livestock and human encroachment brought the population down to just 750 animals by 1889.

Since then, coordinated recovery efforts have brought the species back from near extinction, and today, there are roughly 420,000 bison found in tribal, private, public and nonprofit conservation herds. The four sectors make up what Jeff Martin, one of the country's leading bison experts and an assistant professor of bison biology and management in ÈÕ±¾avÊÓƵ's College of Agriculture, Food and Environmental Sciences, describes as the Bison Management System. Notably, nearly all bison are no longer free roaming, and some are considered as privately-owned captive livestock, but all bison remain "undomesticated," unchanged from their ancestral wild form.

"We aim to support ongoing restoration and management of bison across the entire Bison Management System," Martin said. "Our study identifies regions of North America where bison suitability will be lost and gained."

The vast historical range of bison allowed the research team to gather a wealth of data surrounding what types of weather patterns and living conditions are most ideally suited for bison. Martin and Rachel Short, assistant professor in ÈÕ±¾avÊÓƵ's Department of Natural Resource Management, have spent over 15 years collecting evidence of bison existence records throughout North America. They now have over 160,000 years of evidence, which includes fossils, archaeological records and historical sightings.

"Collecting this mountain of evidence took a lot of work from a lot of different people," Short said. "We visited museums around the country to find fossil and archeological records of bison and searched through digitized journals to find evidence of historical sightings."

This comprehensive database, totaling more than 6,000 specific bison occurrences over the 160,000,00-year-period, contributed 1,775 specific bison occurrences over the last 21,000 years that were calibrated with historical climate data, allowing them to gain a clear understanding of what constitutes suitable living conditions for bison throughout recorded history.

"Because bison are captive and fenced-in, where they are located today is not reflective of their preferred climate," Shupinski said. "We need to look to the past to get a true perspective of what their preferred climate is and the range of temperatures and precipitation that creates suitable living conditions."

The team identified three variables (from 19) that were the stronger predictors of past bison distribution: the warmest temperature of the warmest month, temperature seasonality and precipitation of the coldest quarter.

The range of data helped the researchers create an evidence-based living condition suitability threshold. If all three variables veered out of certain thresholds, the region would be considered "unsuitable" for bison to live in, based on the team's database and weather patterns. The team then leveraged this threshold alongside a future global weather pattern model — The Hadley Centre Climate Model — to determine what regions will become more suitable, or unsuitable, for bison in the future.

A northwest shift

The results paint a grim picture for bison suitability in the Great Plains. Different scenarios were modeled, but each show the once ideal conditions of South Dakota, North Dakota and other states in the Great Plains region becoming increasingly unsuitable by 2060. Favorable conditions will shift to the north and west throughout the second half of the century, with Yukon Territory, Alberta, parts of British Columbia, Northwest Territories and Alaska becoming the ideal habitat for bison by 2100.

"Alaska and northern Canada are predicted to have significant increases in winter temperatures as well as summer and winter precipitation levels over the next 40 years," Martin said. "Increases in temperature and precipitation have the potential to increase net primary productivity by extending the growing season for vegetation, thereby providing greater food resources for bison."

On the contrary, much of the bison's historical range in the U.S. and Mexico is expected to become increasingly hotter, particularly in the summer and winter months, while also becoming drier and more arid with less precipitation expected. Less precipitation may create water availability challenges and will reduce the grasses and forbs available for grazing.

Because bison are no longer free-roaming, anticipating these changes will be key in ensuring the future long-term viability of the species, particularly in the Great Plains. Temperature changes will be of particular importance for the Bison Management System. Bison respond directly to temperature increases. As it becomes seasonally warmer, on average, the body size of a bison decreases. This phenomenon, which demonstrates the bison's adaptability, has already been documented in other related studies by Martin in several herds across its historical range. South Dakota and North Dakota are expected to see average temperature increases between 7 and 10 degrees Fahrenheit in the next 60 years, concentrated mainly in the winter months, which may reduce the average body size of herds being maintained in these areas.

"By the end of this century, bison body size is predicted to decline by nearly 50% if climate trajectories follow the ‘business-as-usual’ worst-case scenario," Martin noted. "The bison we have in the U.S. in the future are likely going to be drastically smaller than they are now."

What effect will body size have on the bison's overall population? This is the great unknown, Martin said, but there are two leading theories on what may happen, both related to calving. Beginning at the age of 3, bison will have a calf each year. With decreased mature body size, one possibility is that bison could see an increase in twinning, with two calves born each year — as long as there is adequate nutrition. This phenomenon is seen in white-tailed deer, an animal who shares similar preferred living conditions with the bison but are much smaller in terms of body size.

"The issue is here that we have documented evidence that if and when bison have twins, they often abandon the second one," Martin said. "They don't have the learned behavior in how to care for two calves."

The second leading theory is that bison will begin to have calves every other year.

"This potential theory is based on the nutritional component," Martin said. "If there is sufficient energy, protein, water — these macronutrients — bison could continue to have calves annually. But if there's not, we may see a change in that pattern."

What can producers do to prepare?

For producers, conservationists and other stakeholders, these research findings are key to beginning to prepare and develop management strategies that can be used across the Bison Management System. Because bison will not be able to naturally disperse to their preferred living conditions, management strategy modifications will be required for the long-term viability of the species. For producers, these strategies may include hay bale grazing, mineral placements on the landscape and water infrastructure investments, Short said.

For the research team, they will be looking at herds currently living in areas underneath the suitability threshold to see how bison have already begun adapting. As Martin theorizes, there may be other genetic adaptions — besides reductions in body size — from bison in hotter and drier locations that could help herds in more northern areas more quickly adapt to the changing living conditions, suggesting that targeted translocations from more southern herds to more northern herds as a sensible option to consider.

But how can the average person help ensure the future viability of the largest land mammal in North America and an iconic species of the American West?

"Go to your national and state parks with bison and other wildlife and pay those gate fees. Enjoy the wildlife. Those gate fees help pay for the stewardship of these animals," Martin said. "And better yet — at the local restaurants — eat a bison burger, which is direct investment in the bison industry and supports the conservation of the animal. By eating one bison, you are contributing to the conservation efforts to save six bison."

Contributing authors on this study include Joshua Rudnik, ÈÕ±¾avÊÓƵ assistant professor; the late Brent Brock, formerly of Indigenous Led; Erik Otárola-Castillo, faculty member at Purdue University; Matthew Hill Jr., faculty member at the University of Iowa, and Chris Widga, faculty member at Penn State University.

Funding for this research was provided by Indigenous Led, the South Dakota Agricultural Experiment Station, the ÈÕ±¾avÊÓƵ Center of Excellence for Bison Studies, the National Science Foundation and the U.S. Department of Agriculture's National Institute of Food and Agriculture.