Not Out of the Woods, Yet—Genetic Extinction (Part 3)

We all have issues.  The strategies and means employed to preserve the wild bison genome and promote genetic diversity is no exception.  As discussed in the previous blog—Not Out of the Woods, Yet—Genetic Extinction (Part 2)—several issues are involved in working toward these objectives.

Inbreeding and Genetic Drift:

The common strategy to avoid inbreeding depression and genetic drift is to create large herds.  It is estimated that herd sizes of 2000 to 3000 minimum are required [1].  Wild, free-ranging bison need to forage over large swaths of land.  For a herd size of 1000 animals it is estimated a land parcel of 100,000 acres or approximately 156 square miles would be needed [2].  Achieving the minimum herd sizes, then, would require land areas from 300 to 500 square miles.   For 500 square miles, this would be a square with each side having a length of 22.4 miles.  The only large conservation herd that meets both requirements for minimum herd size and land is the Yellowstone herd.  The herd of approximately 3500 roams over 3500 square miles [3].  However, much of that is mountainous and so does not represent the actual land available for exploitation by the bison, raising another issue—habitat requirements.  It is not enough that sufficient amount of land is acquired.  It must be terrain that can be exploited by the bison. 

Relatedly, especially where private lands are acquired, restoration of the terrain may be necessary.  Typically, private lands have been plowed-over and fenced-in for farming and ranching practices.  Any fencing has to be removed to allow for movement of the bison.  Other fencing, suitable for bison, has to be established along the perimeter of the reserve or refuge.  Any dams built to retain water for livestock would also have to be removed [4]. 

Then there is the issue of money [2].  The cost to acquire the necessary land and place a herd of 1000 onto that land may run well over $ 1 million. Reaching the minimum requirements to preserve and promote the bison genome could then run $2 to $3 million per herd. Significant funding raising efforts will be needed.

So what land is possibly available?  Bailey concludes that land east of 98 degrees longitude—Minnesota, Iowa, Missouri, eastern Oklahoma, eastern Texas and all points east—is  too fully developed to allow for the necessary land areas.  Between farmland and cities is there is no land parcel large enough to support the minimum herd size.  This leaves the plains—lands west of 98 degrees west longitude to the Rocky Mountains—available.  Perhaps some parts of Nevada and Oregon could be utilized [5].  There are still large tracts available in this region to promote such herds.

Cattle-Gene Introgression:

A potential problem has been identified in regard to purifying bison herds of cattle-gene introgression.  Removing bison with cattle genes may inadvertently remove genes of common ancestry.  Authors Kathleen O’Neal Gear and Michael Gear [6] raise the question: Did bison interbred with any prehistoric species of the Bos side of the Bison-Bos family, and if so, is this the source of the cattle genes?  No one really knows. Removing bison having only genes from domesticated cattle requires the DNA testing to differentiate between those genes belonging to both cattle and bison ancestors from genes belonging only to domesticated cattle.  This would require a complete mapping, or sequencing, of the bison genome [Gears], which to date has not been performed.  Except for the Yellowstone and the Henry Mountains herds, and more recently the American Prairie Reserve herd, all bison most likely have at least some cattle genes.  Derr has found cattle genes in approximately 64% of US federally managed herds [7].

The Gear position, though, does not address the cross-breeding that did take place on private ranches in the US and performed by the Canadian government into the 1960s. There is no doubt the cross-breeding occurred and a few studies have suggested that introgression has been detrimental to bison [8].

Purifying the herds of cattle-gene introgression along with the movement to list wild bison under the Endangered Species Act presents another potential issue if such listing would succeed, according to the Gears.  Some are arguing that the scarcity bison without cattle ancestry qualifies wild bison as an endangered species.  Under the ESA the sale or transporting of bison free of cattle genes could be punishable by a $50,000 fine and one year in prison per charge.  Ranchers or farmers owning bison without cattle ancestry could find themselves being charged under the ESA if they would try to sell or move their bison.  The argument to list wild, pure, bison as endangered, then, could lead to conflicts with current legal definitions governing the status of bison. 

Yet, legal recognition of plains bison as wildlife is required if the wild genome is to be restored on federal lands.  But this seems unlikely at this time.  Most states do not recognize wild bison (see March 2019 post, Legal Status of the American Bison), and the federal government will not restore wild plains bison without support from the affected states.  This could change if the Fish and Wildlife Service would recognize the threat domestication represents to the wild genome, and lists the plains bison as a threatened or endangered species [9].

In any event, the greater goal is to restore wildness to the bison genome.  Reducing cattle-gene introgression to low levels and letting nature takes it course, may over time swamp the cattle genes.  Achieving absolute purity may not be needed if the other actions to promote the wild genome are taken [10].

Artificial Selection:

Purifying the conservation herds of cattle-introgression, though, is not enough to preserve the wild genome and promote genetic diversity.  Artificial selection, caused by human intervention, must be minimized as much as possible.  The complete elimination of human intervention may not be feasible.   No matter how large the land parcel may be, fencing will still be required to keep bison out of private lands.  Handling, needed for testing, culling and transporting of animals, will also be involved in implementing the other objectives. 


Various mechanisms threaten the existence of the wild bison genome, requiring various strategies to thwart the threat.  These strategies and their implementation, however, present conflicting objectives, which may require trade-offs, and issues, which demand solutions.  But the restoration of the wild genome and the promotion of genetic diversity cannot wait until all issues have been fully resolved to all interested parties’ satisfaction.  Fortunately, efforts are proceeding to realize the necessary objectives (e.g., The American Prairie Reserve, the Buffalo Field Campaign, etc.) while work continues to resolve the obstacles still in the way.

End Notes:

[1] Hedrick, Paul W. “Conservation of Genetics and North American Bison (Bison bison).” Journal of Heredity 2009: 100(4): 411-420.

[2] Heidebrink, Scott, Bison Restoration Manager, American Prairie Reserve.  Email to author 03-Oct-2019.

[3] Bailey, 180. Baily, James A. 2013. American Plains Bison: Rewilding an Icon. Sweetgrass Books. Helena, MT.

[4] American Prairie Reserve Bison Report 2016-2017.  Retrieved 10-Oct-2019 from  Also, Bailey, 207.

[5] Bailey, 207.

[6] The Gears are well-known authors of over 50 novels.  They may be best known for their People of the Earth series.  In addition to writing novels, they raise bison.

[7] O’Neal Gear, Kathleen and Gear, Michael W. August 2010.“Bison Genetics—The New War Against Bison.”

[8] Geist, Darrell, Habitat Coordinator.  Buffalo Field Campaign.  Email to author 19-Sep-2019.

[9] Bailey, 220.

[10] Bailey, 214.

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