Figuring out what’s happening with wildlife based on annual counts is not easy. So many factors affect where and when they travel, from predators to weather, habits to habitat.
So when Yellowstone National Park did its summer count of bison this year, it was somewhat surprising to see that although the animals suffered their second highest culling and killing since 2000 — 1,274 animals — it was the untargeted central herd that shrank substantially and not the northern herd, which is the main target of the bison removals.
It’s unknown what happened, but something changed in Yellowstone this spring that made a whole bunch of bison from the park’s central herd decide not to migrate back to their traditional Hayden Valley home. That was revealed in the park’s summer bison counts, which showed the central herd declining by 42 percent.
“So something really big happened this year to change the distribution dynamic on the landscape,” said Rick Wallen, the park’s lead bison biologist.
The first thought was that maybe those central herd animals were disproportionately killed during the winter by hunters outside the park’s boundaries and through the Interagency Bison Management Plan agreement that ships a portion of the herd to slaughter. The winter slaughter program, which donates the animals to tribes for meat, is an attempt to reduce the bison population and slow outmigration from the park into Montana.
Nearly 7,500 Yellowstone bison have been killed since 2000. Yet the park’s bison have, once again, proven to be incredible survivors. Between 2000 and 2016 the population more than doubled, rising from 2,600 to 5,400 animals. Between 2008 and 2017, the northern herd has increased by 275 percent.
“They just know how to survive on our landscape,” Wallen said.
That doesn’t seem to be the case with the central herd, though. Since 2008 that group of bison has exhibited “a lower potential for population growth,” according to Yellowstone’s survey. Maybe they are migrating to the Gardiner area in winter and being slaughtered, killed by hunters, or simply not returning to the central herd’s usual haunts after winter.
Strangely, out of 12 collared central herd bison, 11 survived the winter and six wintered on the western side of the park, not to the north where the slaughter program takes place.
“So it appeared they had high survival,” Wallen said. “It makes you wonder how many migrated to the Northern Range.”
More than 100 years ago, only 23 free-ranging wild bison remained alive in Yellowstone’s remote Pelican Valley. Giving that remnant herd U.S. Army protection was one leg of the park’s historic bison restoration, but it wasn’t believed to be enough. So the Army imported 21 more bison that were essentially treated like cattle.
That herd started out in the Lamar Valley, where buildings from the Buffalo Ranch still stand to mark that early era in bison management. By the 1930s, with the Lamar herd having grown to more than 1,000, the National Park Service decided to set the bison free and let them roam the park as wild animals. Wallen said some of those bison were moved to create the central herd, which now occupies the Madison, Gibbon, Firehole, Hayden and Pelican valleys.
According to research by Texas A&M University, for about 40 years the Northern Range animals, which inhabit the Lamar Valley, Little America and higher elevations in the region, were largely separated from their central herd cousins.
Now that seems to be changing for most, but not all, of the bison.
“There was some stirring in the pot of genetics,” Wallen said.
Bison No. 3225 is a central herd stalwart. She was collared 12 years ago and has remained free of the disease brucellosis, which can cause cattle to abort. Brucellosis is the main reason cited for sending bison to slaughter. The slaughter is a cooperative attempt by the many federal and state agencies to try and keep Montana’s cattle herds from being quarantined should a cow test positive for exposure to brucellosis.
Not everyone agrees with the tactics of slaughter, noting that elk roam free between Yellowstone and Montana and also test positive for exposure to brucellosis. But for now, this is the compromise the many agencies have worked out.
No. 3225 has also remained faithful to the old migration route out of Hayden Valley toward West Yellowstone.
“I don’t remember her ever going on a different movement pattern than Hayden to West,” Wallen said. “She has stuck to that longstanding pattern of the central herd.”
Wallen guessed 3225 could be 15 years or older, and so may be one of the oldest bison in that herd.
“There are not a lot that live beyond 12,” he said.
One of the reasons bison populations can grow steadily is they have few predators. Wolves and grizzly bears will kill a few, mostly calves but also a few injured or aged adults. Most die after getting injured — either from falling or being hit by vehicles — or contract parasites and can’t convert their food into enough energy to survive. One dead bison, which may weigh 1,000 pounds or more, can feed a lot of predators.
Bison 3225’s ancestors tried to pioneer outside the park into the West Yellowstone area in the 1980s but weren’t welcomed by the state of Montana until 2015 when Gov. Steve Bullock agreed to let the animals stay year round on about 400 square miles of mostly Custer Gallatin National Forest. Prior to that agreement, bison were annually hazed back into the park by helicopters and horsemen every spring.
Would the central herd have expanded into that forest area and grown in population if it hadn’t been repeatedly hazed? Wallen thinks so, and speculated that maybe after being turned back so many times more bison began migrating to join the northern herd.
“There’s never been a year we didn’t get migrants,” Wallen said. “The question is how long was it happening?”
A removal of about 750 bison in 1997 seems to indicate central herd animals either moved in to fill the missing Northern Range bison’s vacancies, or had migrated over in winter and were slaughtered, since the roughly 750 bison in that northern population, in theory, should have been wiped out by such a removal, Wallen said.
“They are such social animals, it’s easy to see how some would stay with the group” after hanging out together during the winter, he said.
Yet park biologists rarely see a northern herd animal migrate to the central herd’s summer pastures, so the flow seems to be largely one-way traffic.
In hopes of providing more information to researchers, Yellowstone will collar an additional 15 or so bison before this winter. By being able to track their migration in real time with GPS, the Park Service could reduce its capture of bison for slaughter when those depleted members of the central herd arrive in the Gardiner Basin.
“The question is: How many bison does that one collar represent?” Wallen said.
The agency has also asked, as it has in the past, that tribal and sport hunters limit their harvest outside Yellowstone’s western boundary, which is all central herd bison. Park officials are also recommending that no more than 1,250 bison be killed by hunters or shipped to slaughter this winter.
In the future, Wallen said he’d like to see the bison use the landscape how they’d like.
But for now, the interagency plan is guiding management as the annual debate over bison slaughter and the carrying capacity of Yellowstone’s grazing lands continues. A 2000 court-mediated agreement between the secretaries of agriculture and interior and then-Gov. Brian Schweitzer requires Yellowstone to reduce the bison population in Yellowstone to around 3,000 animals. The park has a way to go to reach that figure.
The aerial count of bison in August was 4,816. Of that total, 3,969 were counted in the northern herd and 847 in central Yellowstone. Overall, the population decline from last year was about 12 percent, with 389 bison killed in the Gardiner area and 97 on the western side of the park.
So what’s going on? Why is the central herd declining even though fewer are harvested? Why are more central animals migrating north instead of west like they used to do? In the future, will the central herd disappear after animals like 3225 die and take with them the habit of moving west? Will bison ever occupy the land west of Yellowstone that has been opened to them?
“It’s unlikely to see new exploratory patterns to the west if that subgroup is declining in abundance,” Wallen said. “You need more animals to develop a migratory pattern out to the west to explore some of those new areas.”
Bison populations and their migratory habits are always going to change, he added.
“That’s part of preserving the ecological processes, letting them figure out how to live in here.”