Climate Change

The Incredible Shrinking Bison, an Unexpected Impact of Climate Change

Bison graze on Ordway Prairie, owned and managed by the Nature Conservancy. The site has a USFWS grassland easement protecting it in perpetuity. Photo © USFWS Mountain-Prairie/Flickr

Here’s a climate change impact you probably never considered: bison diets.

As the climate warms, bison in North America are likely to shrink, as documented in research published by Joseph Craine and colleagues.

The reason they shrink is because as grasslands warm, grasses and other plants accumulate less protein. Bison are then forced to eat plants that are less nutritious.

This raises a related question: what plants to bison actually eat?

The answer to this question could help conservationists manage for plant species that are higher in protein and preferred by bison – ensuring healthy herds on warming grasslands.

The Bison Story in Context

The near-loss of our bison herds is one of our most-told conservation stories. The great herds were reduced to near extinction, the survivors rescued with no time to spare. Bison have recovered – a remarkable success – but the animals are now scattered in much smaller herds on national parks, nature preserves and ranches.

Not only are there fewer bison roaming the Great Plains, they no longer migrate. Two or three major bison migrations once occurred on North America’s grasslands, on a scale that is difficult to comprehend today.

“Everyone thinks of the Serengeti as the home to a major migration, but it would have been dwarfed by the migration of bison on the Great Plains,” says Craine. “The scale of the plains is huge. Millions of bison would have traveled 1000 miles every year. They moved based on the nutritional quality of plants.”

Today, with most bison in fenced reserves, such movements are not possible. Bison need to meet their nutritional needs within the bounds of that reserve. If grasses with lower protein dominate, the bison have nowhere to move.

But how do bison diets shift in a warming climate? To answer this question, Craine researched actual bison diets in two herds on grasslands that differ in mean annual temperature by 6 degrees Celsius: the Samuel H. Ordway Jr. Memorial Preserve in South Dakota and the warmer Konza Prairie Biological Station in Kansas.

Bison. Photo © Matt Miller/TNC
Bison. Photo © Matt Miller/TNC

A mature bison in South Dakota averaged 500 pounds larger than one in Kansas. But researchers had a poor understanding of exactly what bison were eating in each place.

What he found, using sophisticated DNA analysis of fecal samples, contained some major surprises about what bison ate – including disproving the long-held belief that bison are strictly grazers.

Follow that Pooping Bison

To analyze bison diets, researchers first needed fecal samples. Mary Miller, the Conservancy’s Northcentral South Dakota conservation manager, led collection on the Ordway Prairie. Finding a specific bison “patty” on the prairie seemed like a straightforward task but at times could feel like a wild goose chase.

Miller and her team were tasked with collecting fecal samples from mature cows and mature bulls. “Finding bison can be a lot harder than you think it should be,” she says. “This was complicated by the fact that you actually had to see a specific animal defecate. You had to make sure you had an animal in the right age range and then observe it for signs that it was getting ready to go.”

Bison. Photo © Matt Miller/TNC
Photo © Matt Miller/TNC

Miller has a lot of experience observing bison, so she could usually pinpoint an animal. But the fact that they are usually roaming in herds could complicate the search.

“Sometimes you’re approaching a herd, and all of a sudden they all stand up and poop right before you get there,” she says. “You know it’s going to be a while before they defecate again, so you’re going to be watching one bison for a long time.”

The fecal samples are then analyzed for the DNA from bits of undigested plant material, allowing for a rapid assessment of all the plant species consumed by the animal. “It’s using CSI techniques for conservation,” says Craine. “It’s a powerful way to determine what bison actually eat, which it turns out is quite different from what we thought they were eating.”

Grazers and Browsers

Bison are supposed to be strictly grazers: herbivores that focus exclusively on grasses. At least that is what biologists have long thought, and verified by extensive field observations.

The DNA evidence, though, casts serious doubt on this idea. In fact, the bison in Craine’s study ate a far greater variety of plants than anyone had realized, including shrubs – what ecologists know as browse.

“There are 400 to 500 plant species in a typical healthy grassland,” says Craine. “And bison rely on a lot of them. They have a very diverse diet, relying on a lot of plants other than grasses.”

Photo © Matt Miller/TNC

This was especially true in the warmer grasslands of Kansas, where grasses were lower in protein. Here bison relied a lot more on shrubs and forbs for protein. The paper by lead author Craine suggested that as the climate warms, bison may shift more to browsing than grazing.

What does this mean for managers?

Traditionally, bison management has been all about the grass. Bison management has essentially been grass management.

“When thinking about herd health, managers have to promote plant diversity, not just grass,” says Craine. “If managers worked to promote grasses at the expense of nutritious forbs and shrubs, bison would suffer. Knowing what species bison rely on gives us the opportunity to promote and manage for those species that would help bison the most.”

For instance, Ceanothus – a nitrogen fixing shrub commonly known as Jersey tea – has never been considered important in bison management. Craine’s research, though, found this shrub to be an important source of protein for bison.

Of course, small preserves can complicate the picture. “We don’t know if bison eat certain plants because they’re nutritious or if they eat them because they’re there,” says the Conservancy’s Mary Miller.

Photo © Matt Miller/TNC

The DNA analysis will allow for a more accurate picture, and help bison conservationists adapt management strategies to a warming climate.

“We can’t make up for the fact that they can’t migrate to areas with more nutritious plants,” says Craine. “We know that as grasslands warm, the grasses become lower in protein. With these two facts, we could be looking at bison with lower growth rates and lower birth rates. But we could also compensate in our management, focusing on plant species that are higher in protein that bison will eat.

“Unfortunately, we are running out of time,” he continues. “we need to collect a lot more poop from a lot more bison if we are going to have a broad enough understanding of bison diet to stave off some of the effects of warming. We not only want to keep bison on the prairie, but also massive bison – the kind that make the earth shake when they run by.”

Join the Discussion

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  1. This is consistent with Whitetail deer and brood cows being larger than those in the south.
    1 Soils are more eroded and leached by rainfall; hence less fertility
    2 Plants in the South have higher moisture content and lower nutrition on both an as fed and dry matter basis thus requiring higher proportion of rumen/body weight which would tend limit mature size.

    This is why I’m convinced climate change is a big deal for grazing animals and the plants in their diets. I expect the type of brood cow and its forage will have to change as warming proceeds, especially if more fertile lands are moved from grazing.

  2. Oops! If I’d read the journal first before posting…………………………………..

  3. Would burning the prairie thatch be of help in enriching the soil or is there no thatch left now that bison are restricted to one area.

    1. That’s a great question. Throughout the Great Plains, periodic burning often promotes higher nutrient concentrations in grasses, which promotes the weight gain in grazers. Bison are often attracted to recently burned areas to feed on the nutritious new growth. Many Nature Conservancy properties have been instrumental in demonstrating this. Prescribed fire is something that can be applied more broadly to help bison.

  4. Whatever you can do to restore these majestic animals is most welcome. They have been decimated to near extinction and need to be restored. They are part of our American history and deserve a place for peace and quiet in our world.

  5. No surprise things are far more complicated than believed, and therefore more difficult to try to correct or at least improve. The good news is that the bison are still with us by way of massive efforts to save them. They are as beautiful as ever, a significant thread connecting us to a past that is all too often ignored, misunderstood, or forgotten.

  6. Wouldnt putting Bison on nature conserancy properties help out the Bison? Say like the flat ranch in island park idaho that the nature conservancy has cows on? Yellowstone Park is killing Bison left and right! Shouldnt we be finding more places for them to roam? The Bufalo feild campain is doing this shouldnt the nature consevancy do the same? Just a thought.

    1. Yellowstone Park is not killing the Buffalo as Ms. Shults erroneously implies. The Buffalo are
      prevented from mixing with domestic cattle because they sometimes carry brucellosis which
      causes cows to abort instead of carrying to term. The Buffalo field campaign is a bunch of
      hippies interfering with government efforts to get rid of excess Buffalo by shipping them to
      native American peoples. Yellowstone Park is checking on brucellosis among Buffalo, but
      it also carried by elk who don’t recognize Park boundaries.

  7. Buffalo should be monitored in different areas. For instance, what do they eat on Catalina island? What do they ingest in Yellowstone or the Kiamichi area of Kansas, or is it Oklahoma?
    It would seem that different forbs would be available on the Kaibab plateau or in the Palo Duro
    Canyon of Texas. What about the Woods Buffalo of Manitoba? Men who are credited with
    saving the Buffalo are Charles J. Jones of Garden City Kansas, Charles Goodnight of the JA
    ranch and Walking Coyote. Who is responsible for the Buffalo on Catalina Island? Is it Zane Grey
    who had a home there and was a friend of Charles Jones?

  8. Any news or comment on Ted Turner’s project to establish a contiguous corridor from Canada to Texas (I believe it was) to allow for bison migration?

  9. Warming climate is affecting all livings things. Many do not believe that this fact can be proved. Keep up the good work to save and improve the many ecological systems for all. Most animals are limited in what they can do to protect themselves. It is our responsibility to be change makers and educators.

  10. The Earth is changing wether we want it to or not, should we let the Bison adapt naturally or force them into a box, that should our efforts fail, they could never get out of?

    1. This also has me wondering about the emerging awareness of climate change adaptation and wondering whether a desire to keep bison big is a wise use of scarce resources for a species that’s quite-possibly adapting to climate change even faster than we are. After all, if relict Pleistocene-era mega-fauna aren’t sustainable on a warming planet, is it good science or nostalgia that we try keeping them as big and brawny as in a romanticized western landscape painting hanging in the Smithsonian?

      It’s a sad but true fact: more often than not, what’s desirable isn’t what’s sustainable.

    2. This is a good question. I cannot speak for TNC, but often our climates are changing too fast for organisms to adapt. In the extreme case, we are often faced with the choice of extinction or intervention. It seems like we should choose intervention.
      For bison, the types of changes we would have to make in managing them are relatively minor. But, first we need the knowledge, which is the slow part.

  11. This just makes me question more as to why we are messing around in the heavens when we do not know athird of what is going on right here; and were a cause of most of the problems.

  12. Tha Beautiful Bison are Necessary and they deserve to be here and run free… They have been around for hundreds of years… They are loving parents they have feelings and a heart and they are a soul just like we humans… Pls leave them alone to live in Peace n raise their families… Thank you

  13. 500 pounds! That is a massive difference and making the SD bison less massive. Do they eat the thistle in the photo? I assume is it one of our many native thistles and not a weed. Good hunting to the pooper scoopers!

  14. To simply say I care is not the same as actually getting my hands dirty working out solutions.

  15. Love the article and love the Bison. Do you know if the there were brush and the like for Bison to browse back when the herds migrated or did they just migrate to follow the grasses?

    1. Almost all of the species we see now were on the Great Plains when the bison migrated. We do not know for sure, but most were there for the bison to eat before. From other work, we know that grass was more nutritious in the past. Rising atmospheric CO2 has pushed down protein concentrations in grasses over the past 100 years. They have also promoted woody species more than grasses. More than likely, bison in the past relied a lot more on grass than they do now.

  16. I have read that the larger a mammal is, the more able it is to tolerate very cold weather. If our climate is warming (of course it is), perhaps the bison do not need to be as large. Could that also affect their current size???

    1. Great question. An old observation is that animals in the north are larger than those in the south in the Northern Hemisphere. Larger body size does confer benefits in a colder climate, but that is unlikely to be causing the N-S variation in body size that we see. We suspect this is true because when bison get transplanted from southern to northern grasslands in the spring they accumulate mass faster in the north than south. This happens before it turns cold, so winter severity is unlikely to be driving this. There is also little benefit to animals in the south not to put on weight and grow as large as they can (especially when males have to compete for females).
      The most parsimonious explanation is that the higher protein concentrations of plants in the north promotes animals to grow larger than in the south.

  17. A warming climate is certainly a factor in the bison’s environment and required nutrients. However, it should also be taken into consideration that most bison today are not genetically pure as were the bison who roamed the plains in the 1800’s. It is fact that today’s bison have DNA that is mixed with cattle DNA, thus having had an effect on both their size and robustness.

    1. We certainly do not know much about what effect the cattle DNA in bison is having on the animals, but only a very small proportion of the DNA in modern bison come from cattle. Most bison do not have cattle mitochondrial DNA as they have been selective culled out of herds. The amount of nuclear DNA has to be very low–likely just a few percent. This is because the first male hybrids between bison and cattle were sterile. In order to get a viable male hybrid, required a lot of backcrossing with bison. As such, many modern bison likely have some cattle nuclear DNA, but it’s not that much.

      Still, a good point for future research. It would be fascinating to know what cattle genes might be in some animals and if it affects their ecology at all.

      1. The study by Derr at Texas A&M did show that bison with cattle genes tended to be smaller. How does the weight of the bison in this study compare with the Blue Mounds herd in Minnesota?

        1. Thanks for the question, Deanne.

          We do not have any weight data from Blue Mounds, so I just don’t know. When I was doing my synthesis awhile back, I seem to remember, they didn’t have a scale.

          The question of whether cattle mitochondria affect weight gain in bison is still open for debate.

          The one study that examined the effects of cattle mitochondrial DNA on bison performance shows little impact. Derr et al. (2012) examined weight gain and heights of bison from Catalina Island for animals with cattle mitochondria and those without. For animals on Catalina Island, weights were 16 lbs (2%) less for animals with cattle mtDNA. This 16 lbs difference in weight was generally present in calves and then maintained with age, meaning that weight gain was not different among animals with and without cattle mtDNA. This is a bit strange because you’d expect the differences to become greater over time.

          In that study, for animals in a feedlot setting, weights of calves with cattle mtDNA were less than those bison mtDNA, but there were no differences in weight gain for the two categories. Heights averaged just 2 cm shorter for cattle mtDNA bison. That’s really not much.

          In all, I’d say that there is insufficient research to know of the presence of cattle mitochondria would affect weight gain.

          Derr, J. N., P. W. Hedrick, N. D. Halbert, L. Plough, L. K. Dobson, J. King, C. Duncan, D. L. Hunter, N. D. Cohen, and D. Hedgecock. 2012. Phenotypic Effects of Cattle Mitochondrial DNA in American Bison. Conservation Biology 26:1130-1136.

  18. Very good research and a worthy project. Do you have volunteers willing to assist in the collection and monitoring (watching) the animals as they eat and poop? If so, could I volunteer to assist here in California? I am retired and have the time available to assist in this worthy project.

  19. Livestock ranchers want all the buffalo gone. They want the land for them self. They will start a government sanctioned slaughter of these beautiful animals. After the Bald Eagle I feel the Buffalo is our 2nd US emblem. This should not be allowed to go forward.

  20. Thanks Matt for this article. I am a rancher on Central Montana running our livestock primarily on native cover. We have been working with professional botanists to develop a taxonomy for plants on our 14000 acres forage base. We felt pretty proud-maybe less so now-that we had identified up to 300 species, including the expected invasive plants that have naturalized in the area. Dr. Craine’s reference to 400 to 500 plant species being readily available for bison grazers, seemed a bit of a stretch, at least for our part of the world. We also have noticed that cattle that move frequently, are also very fond of a variety of forbs and small shrubs. Also, I think it has been documented with other foraging wildlife, like deer and elk, tend to be larger the farther north they occur. I would appreciate your thoughts on the observations, I have shared.

    Thanks, Bill Milton

    1. Thanks for the work you are doing, Bill. People getting to know their own backyard is the most important step.

      400-500 species is the number we use for southern prairies. it seems perfectly reasonable that central MT would only have 300 or so. That’s still a lot!

      We’ve started working with Texas A&M to look at cattle diet across the Great Plains. First paper on that should be coming out soon.

  21. Giantism is a little understood phenomenon in any species. The link between high protein intake and gross body size is certainly demonstrated in humans, and possibly in dogs. Since cats have an almost all protein diet, there may not be as much correlation.

    Question: Do we want massive bison, or massive numbers of bison? Your research may have to develop an optimum number of bison per acre (or per concentration of specific crops) for any particular body mass index.

    Is the aesthetic preference of your researchers for massive bison shared by the Native American tribes who are custodians of bison on their reservations? Is it shared by the general public who will fund the non-reservation bison preserves? Is there a market for locavore cultivation of a few bison for specialty restaurants? What size animal would they prefer to raise and take to the local slaughterhouse? And I am sure that a few minutes of free-range thinking would raise a lot more questions.

    This is great research and I look forward to seeing more of your results over the next 10 years. As I am 80, I probably will not see much more than that.

    1. All great questions, Teddy.

      Agronomists have spent a lot of time working on understanding nutritional constraints on growth. Bison and cattle are closely enough related that the basics of nutrition can be used to understand bison growth. There, we see that (up to a point), having a higher protein diet leads to greater weight gain.

      Bison are still different than cattle, though. For example, their metabolism drops in the winter. You can feed them all you want, they just aren’t going to be putting on weight like cattle do.

      Big animals make for tough meat, regardless. But that’s not a good reason for not helping the bison.

      We’ll try to get more research out on the animals. We might be slow to write papers, but that shouldn’t stop anyone from going out and appreciating the animals. They are amazing regardless.

  22. then there is the possibility that larger animals are more efficient conserving heat. No need to be bigger?

  23. Thank goodness someone is concerned about the Buffalo!! They are so magestic & you can not help but want to just hug one. They so resemble cattle. Trying to keep them in grasses & shrubs that will be healthy for them is awesome. I am happy there are folks who really care about these beautiful creatures.

  24. Birds spread their food preferences with seeds in ready-made fertilizer. For instance, berries are popular among birds, and plants with berries are found growing where birds poop. I wonder, do bison also spread their food preferences?

    1. Great question.

      If you take a close look at bison poop, it’s full of seeds. And many of these do germinate later.

      Bison hair is also a tangle of seeds.

  25. The government needs to stop harassing, capturing, & slaughtering the buffalo. Let them roam to find better grass.

  26. Thanks so much to Matt Miller, Joseph Craine, Mary Miller, and all the researchers who are bringing this data to light. I’m a former staff member, donor, Legacy Club member, and long-time fan of TNC’s work, and believe in this science-driven approach to conservation. I’m also an artist who loves bison! Here’s a link to one of my paintings that brings attention to the importance of grasslands. If you click on the image and look closely in the lower left corner, you can see the image of a bison within the grasses.

    Thanks for all you do, TNC!

  27. Very interesting article, and further proof of how climate change is affecting the world around us in so many ways. While Republicans seem to be the only ones in the world who don’t believe in what is happening, the rest of the world is beginning to get the message, and will hopefully put in place the necessary steps to protect our planet and future generations of humans, animals and plants alike.

  28. Dear Matt, Just wanted to say Many Blessings & Thanks!!! For All of the ‘poopy’ Important Research that You are doing!! Buffalo’s are/in my opinion /one of God’s Most Beautiful animals He created!!! Stay safe & warm. Again, Blessings from north New Mexico mtns.!! Sincerely, Jeanie Phillips

  29. I for one think the animal knows what to eat. Why must people inter fear with this wonderful animal. They are not pets you feed as you like. The world and all of us creatures are changing to fit our climate and resorses. If you want to spend your money on something, help the homeless.
    Get clean water to people who have lead in the water, or no water at all. I think you get the jest of what I’m saying. Thank you for wonderful photos.

  30. This article was very interesting as I’ve become more interested in the Bison of Yellowstone & the Midwest. I currently live in Nebraska so the prairie is important to our current & past history.
    I think research on birds could be quite helpful in this matter. Birds are opportunistic, however, if given even a small variety of different seeds they will eat the seeds with the most nutrition leaving the rest behind. After exhausting natural & provided bird seed, the lower nutritional value seeds will only be eaten if needed. Birds eat different seeds depending on the season. For example more fat in the winter. I would image it’s the same for Bison & of course they would eat a variety of foods as they migrated.

  31. Great article, Matt! Looking forward to getting my results back from Dr. Craine on bison feces analysis in Manitoba. It will be interesting to compare our results with the southernly studies!


  32. Very interesting article and I’ve enjoyed following the discussion. For over 20 years, I have raised bison in the Ozarks. My favorite place is in the middle of the herd on a 4wheeler, watching them eat, play and interact. I can share with you their favorite browse, as well as plants that they NEVER touch.

    I question, however, the thought that protein is the limiting factor. Doc Ken told us that bison have a unique ability to recycle protein rather than eliminate it through urine. Excess protein in bison can interfere with minerals and other nutrients. Also, excess energy through free choice grain damages the liver similarly to alcohol in humans, limiting time they can be kept on feed.

    I can’t argue on the subject of protein differences in various grasses, but my field is full of white clover, as well as red clover and the smaller yellow clovers. In addition, many of the weeds have as much as 20% protein (along with possible noxious substances that make them less palatable). I do not believe that protein is the limiting factor.

    On the subject of size, I learned in grazing school to “don’t put Cadillac cows on a Ford pasture.” Larger animals need higher maintenance and can eat as much as 30% more food, limiting the number of animals that can be sustained on a given pasture. When higher needs are not met, they will be less likely to breed back. The larger cow still only gives one calf per year, the same as the smaller ones.

    Because bison grow slower than cattle, and meat can get tough with increased age, they will not reach a truly big size before slaughter age without supplementing with grain. For those like myself who prefer a grass fed operation, that means the steaks and other cuts may be smaller than food service chefs prefer. (supersizing is the American way these days) However, my meat customers who come here to the farm have no problem with the smaller servings, especially with the higher cost of bison meats. You are welcome to follow the farm at

    1. Thanks for taking part in the discussion. 20 years of raising bison is an amazing accomplishment. I’m sure you have gotten to see a lot of interesting things. My family and I just spent spring break in the Ozarks. It would be a great place to watch bison. That was the eastern part of their great migrations. I tried to imagine a million bison working their way through the hills.

      On the topic of nutrition, I remember 15 years ago talking to the late Dr. Jerry Stuth about bison nutrition. He had founded the Grazer Animal Nutrition Lab and knew more about cattle and bison nutrition than anyone I’ve ever talked to. He said, “Bison are amazing creatures, but they cannot violate the laws of nutrition”. I didn’t know much about bison nutrition then, no less what the “laws of nutrition” were, but I remember reading up on all the research that was out at the time to understand what these laws were.

      Comparing bison and cattle nutrition is longer than can be done in a post, but the statement that bison can tolerate lower quality grasses than cattle has always intrigued me. We know that bison are unique from cattle. But how? I am not aware of any studies that show that bison can grow better on low-protein forage than cattle. There is no evidence that bison recycle nitrogen better than other grazers, either.

      This doesn’t mean it’s not true–there just isn’t any evidence to say that it is true.

      The lack of understanding of the unique adaptations of bison to life on the prairies has always been a hindrance to their conservation and commercial use. I think with the work that has been done with TNC bison, we’re getting closer to understanding the ecology of bison better.

      As they say, the best time to plant a tree is 20 years ago. Second best time is now.

      Same goes for bison research, I suppose.

  33. Excellent article as well as the fine and serious work being done on this little-known topic. I would like to receive future info and articles on this. Thank you.

  34. I got native americans in my family why don’t you ask the real true americans that were born here and ask how they can save thier true buffalos from dying no one knows them better but a true native american cause it’s part of thier culture

  35. Very interesting article on the science of bison management and the effects of climate change on the long term health of bison heards.

  36. The bison now are feeding on worn out cattle grazed pasture. The vegetation has changed and the soil is deficient in nutrients from past abuse.