A number of environmental organizations, Western Watersheds Project, The Buffalo Field Campaign, and Friends of Animals, have petitioned the U.S. Fish and Wildlife Service to list the Yellowstone bison under the Endangered Species Act (ESA). Some may be baffled why any bison deserve listing under the ESA when there are at least 500,000 bison found in North America.

However, the Yellowstone Park sub-population of bison has been subjected to severe slaughter and culling as part of an effort to prevent transmission of brucellosis, a disease that can cause abortion in cattle. There is a lead wall (meaning bullets) that attempts to bottle bison up in Yellowstone Park when they migrate to lower elevations outside of the park in search of food during harsh winters.

In addition to the direct killing of animals by Montana Department of Livestock, park rangers, and hunters, bison have also been captured and slaughtered.

In recent years, more than a thousand bison have been killed near the Park borders as the bison attempted to migrate out of the park.  In 1996-1997, culling of Yellowstone bison removed 57% of the entire Northern subpopulation and 20% of the Central subpopulation.


The proposed listing would use a feature of the ESA called a Distinct Population Segment (DPS) designation. DPS permits the FWS to protect unique populations of a species that may have special conservation significant.

In the case of the Yellowstone park bison, its conservation values are both significant and unique. The bison in Yellowstone Park are free of cattle genes. Most other bison herds have some degree of hybridization with domestic livestock.

Furthermore, Yellowstone’s bison are the only known bison population that has been continuously wild (though for a few years they were fed hay in the Lamar Valley to build up their numbers). This is particularly important as Bozeman wildlife biologist Dr. Jim Bailey argues in his book American Plains Bison—Rewilding an Icon, Yellowstone’s bison are genetically unique.

There are only four sources of bison without cattle genes, the Henry Mountains of Utah (originally bison transplanted from Yellowstone), a private herd in New Mexico and in Canada. However, the largest cattle-gene-free bison herd is in Yellowstone.

Despite this obvious rarity, the U.S. Fish and Wildlife Service has denied two previous petitions to list the Yellowstone bison under the ESA. The new law suit asks the US FWS to do a full review of the bison, especially in light of new genetic and other scientific evidence suggesting the Yellowstone bison population is both genetically unique and threatened by on-going culling and slaughter.

Ironically this slaughter continues despite the fact that the bison was recently designated our “national animal.”


There are hundreds of thousands of bison being raised commercially in the United States. However, these herds are treated like livestock. That is, they are bred, artificially fed, inoculated, culled, and otherwise to one degree or another raised like domestic livestock.

There are 44 conservation bison herds in the US, numbering about 17,000 animals. These conservation herds, though more wild than commercial herds, still have a significant degree of animal husbandry. Most reside behind fences, and typically experience some artificial population control through culling or slaughter. And many herds are small and likely suffer from genetic inbreeding and genetic drift.

Even herds with as many as 2000-3000 animals will lose about 5% of their genetic diversity over a hundred-year time period according to Dr. Bailey.  Only two herds exceed 2000 animals, with Yellowstone possessing the largest number.  And we must remember that even the herds in Yellowstone have gone through a genetic bottleneck when the park’s bison numbered as few as 25 animals.


Yellowstone’s bison lives were, until recently, largely affected by natural selection. In other words, bison died from disease, predators or starvation without interference from humans. These natural evolutionary processes have produce the wild bison we have today in Yellowstone.


As most people know, for more than two decades now, bison migrating from Yellowstone National Park have been killed to prevent the spread of brucellosis.

Brucellosis is a disease that can cause abortion in the first born calf of domestic cattle. The disease was originally introduced into North America via imported domestic livestock.

Cattle primarily get brucellosis by licking an aborted fetus with active brucellosis bacteria.

The Brucella bacteria can cause what is known as undulant fever in humans. Efforts by the federal government to control brucellosis in cattle was originally justified and funded as a public health issue. However, since most humans got undulant fever by consuming unpasteurized milk, with the advent of wide-spread pasteurization, the human health threat has largely been dissipated.

Nevertheless, due to lobbying by the livestock industry, taxpayers still foot the bill for brucellosis control, even though the primary beneficiaries are those in the livestock industry.

As part of this control effort, all sources of brucellosis are considered a threat to the industry. A percentage of the wild bison in Yellowstone carry the disease, as do elk and other wildlife. The livestock industry and the Montana Department of Livestock (DOL) has effectively lobbied Congress to force the control wild bison either by hunting and/or capture and slaughter.

The likelihood of transmission is exceedingly small. So small that there is not one example of bison transmission of brucellosis to livestock under natural field conditions has been documented. However, a number of examples of elk transmission have occurred.

As will be explained below, neither bison bulls, yearlings, or calves can transmit the disease, though that hasn’t stopped the Montana Department of Livestock from killing thousands of such animals. Only bison cows are likely to transmit the disease and so far, there is no documented transmission of brucellosis from wild bison to domestic livestock under field conditions.

Despite the lack of evidence that bison pose any risk to livestock, more than 6000 of Yellowstone’s bison have been slaughtered in one fashion or another since 2000.


For cattle to become infected with brucellosis a host of unusual circumstances need to exist.

Cattle have to come in contact with live brucellosis bacteria.  The proposed means of transmission typically would involve the abortion of an infected fetus by a wild bison. However, this is rare in the wild because an infected cow bison usually reabsorbs an infected developing fetus before it can be aborted.

Most abortions of fetuses in wild bison occurs between February and May—at time when domestic livestock are not found on the public lands adjacent to Yellowstone Park.

Even if aborted, the bacteria must remain active. Since the brucellosis bacteria is sensitive to heat and dehydration, and/or the fetus is consumed by coyotes, ravens and other scavengers. One way or another live bacterium don’t usually last long in the environment.

Next assuming there is an aborted fetus and the bacteria is still alive, a domestic animal must come along and lick the dead fetus or birthing fluids.

For this to happen, there has to be physical geographical overlap between bison and cattle. Since bison typically migrate out to lower elevations in the winter, and move back into the higher elevations of the park for the summer, there are few cases of actual physical overlap.


The transmission of brucellosis from bison to cattle has never been documented, and as explained above, the chances for transmission, while not zero are effectively insignificant. Furthermore, there are livestock management options that further reduce the threat. For one, most of the public lands grazing allotments outside of the park are no longer used by domestic livestock as a result of permit retirement and other means. So the opportunity for geographical overlap is small.

In addition, as previously noted, neither bison bull, yearlings, or calves can transmit the disease, yet these individuals are regularly killed.

And despite the fact that all known cases of brucellosis transmission from wildlife to livestock has been attributed to elk, there is no similar slaughter or curtailment of migratory elk.

The real reason the livestock industry fears wild bison is that bison and cattle have similar diets. The industry fears that the public might demand that wild bison should have priority when it comes to grazing public lands.


The last remaining wild bison herd large enough to be evolutionarily significant resides in Yellowstone Park, yet the DOL, and state of Montana has repeatedly shown an unwillingness to accept wild bison outside of Yellowstone Park. Instead they are using artificial means of control that includes capture and slaughter, and hunting just outside of the park. Both means of population control are artificially selecting against the wild genes of the animals.

As with wolves, grizzly bears, spotted owls, sage grouse, and other species that impinge or even appear to impinge upon traditional economic interests, the state governments in the West are incapable of managing such controversial species due to the political pressure from local industry and politicians. This is why the Endangered Species Act was created, and why we need it now to save Yellowstone’s wild bison. The on-going tragedy is too reminiscent of the earlier attempt to wipe out wild bison on the plains as a means of controlling Plains Indians.

Only the courts can force the federal government to do its legal obligation to protect Yellowstone’s wild bison,and stop the shameful slaughter of an American icon.

About The Author

George Wuerthner

George Wuerthner is an ecologist and former hunting guide with a degree in wildlife biology

3 Responses to Yellowstone Bison Petitioned Under ESA

  1. avatar Ida Lupine says:

    Not allowing migratory animals to do what they do naturally, migrate, has got to be one of the worst crimes against nature. And hiding behind a nonexistent risk for brucellosis also.

    If we must take habitat from other of the land’s inhabitants for our ever-increasing ‘needs’ and numbers, can’t we assist them in some way – such as highway overpasses, etc. as is done in some places? Why is killing them the only answer and how can we rationalize killing in any form? Don’t human beings pride ourselves on having the finer emotions such as empathy and understanding? It seems to only apply to ourselves.

    I’m glad that the man who had the encounter with a grizzly is ok, but what do we suggest, killing off all wildlife so that humans can never be at risk? Or keeping them at such low numbers that their genetics are at risk?

  2. avatar rork says:

    “As will be explained below, neither bison bulls, yearlings, or calves can transmit the disease, though that hasn’t stopped the Montana Department of Livestock from killing thousands of such animals.” Irrelevant. It’s population control.
    “The last remaining wild bison herd large enough to be evolutionarily significant resides in Yellowstone Park” Debatable. Those other fairly pure populations you mentioned hold genetic diversity too. If we could safely move a few animals, it might help.
    “Both means of population control are artificially selecting against the wild genes of the animals.” Disingenuous, if I even understand what is being said. I think our author means that ANY killing of bison is “selection”.

    I’m all for more bison there. Need to find ways to make that work well for landowners in the area, and/or buy them out. Reduced grazing on public lands. Something prescriptive, not just courts.

  3. avatar Craig says:

    Yellowstone bison are not threatened with extinction. The Park contains a viable population. There are now at least 5 herds outside of the park started with Yellowstone bison. Yellowstone bison are not genetcally unique. The herd has input from animals taken from northern Montana and Texas. These bison were ranch-raised for over a half century in the Lamar Valley and have interbred with the remnant Yellowstone bison. While the current management of bison is deplorable and better strategies are available, the reality is bison numbers need to be managed.

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October 2016
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‎"At some point we must draw a line across the ground of our home and our being, drive a spear into the land and say to the bulldozers, earthmovers, government and corporations, “thus far and no further.” If we do not, we shall later feel, instead of pride, the regret of Thoreau, that good but overly-bookish man, who wrote, near the end of his life, “If I repent of anything it is likely to be my good behaviour."

~ Edward Abbey

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