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The Future of American Plains Bison: Domesticated or Wild?
By James Bailey

“But doesn’t Ted Turner have a lot of them?” a salesperson once asked me.  She’d noticed an embroidered bison on my hat. Her comment typified the very limited knowledge that most Americans have about the past and present status of these iconic animals that once roamed the country by the tens of millions.

In early historic times, bison were common in much of what is now the eastern United States. Spanish records noted bison in Florida. Daniel Boone found several thousand of them grazing the bluegrass of Kentucky. Before the revolution, George Washington killed “the king’s” bison in West Virginia. The past abundance of bison on the Great Plains is well documented, but bison were also common throughout the Rocky Mountains. They seem to have been extirpated by Native Americans from the northern Great Basin, but an isolated population persisted in eastern Oregon and northeast California. 

Today’s popular media claim that plains bison are “secure” because there are populations in all 50 states. But the vast majority of these — more than 220,000 bison — are private, commercial herds that are managed as domestic livestock. By contrast, on native range south of Canada there are about 17,000 bison in 45 “conservation herds” managed by government agencies or belonging to two private conservation organizations. 

Wild bison are products of millennia of natural selection in wild environments.  Consequently, they are strong, agile, mobile, competitive, energy-efficient, disease-resistant animals. (Bison may endure to minus 40 degrees without increasing metabolism to maintain body temperature!)  On their own, bison are far superior to domestic livestock. 

We don’t pass bison on to future generations — individual bison die. We pass the bison genome to our descendants. Our obligations require us to consider the past, present and future of bison genes — and even a casual review suggests the future of wild bison is in jeopardy.

The genetic composition of all our bison has been and is being changed by six processes: 1) continental and local founder effects limiting genetic diversity to that of very few animals; 2) crossbreeding with domestic cattle genes; 3) inbreeding; 4) genetic drift; 5) artificial, human-determined selection; and 6) natural selection. Together, the first five of these weaken or replace natural selection, especially in commercial bison herds where domestication is well under way. But all five processes exist in our conservation herds of bison as well. 

Founder Effects: After the slaughter culminating in the 1880s, there were only about 100 bison left in the Great Plains and Yellowstone Park.  Genetic diversity from the eastern United States, most of the Rocky Mountains and the far West was lost.  Worse, each of today’s conservation herds of bison has been founded with very few individuals, small samples from the remaining genetic diversity of plains bison.

Cattle Genes: Early on, bison were crossbred with domestic cattle.  Today, almost all plains bison have small to modest amounts of cattle genes.  It is likely that only four conservation herds in the country are free of cattle genes.  Each cattle gene replaces a wild bison gene.

Inbreeding: Breeding of closely-related individuals occurs in small herds.  Probably, there are behavioral tendencies in bison to avoid inbreeding when possible.  But we know little of this, and nothing about the effectiveness of outbreeding behavior in small herds.  Negative effects of inbreeding are likely in at least 20 of our 45 conservation herds with no more than 100 bison, and may occur in many larger herds as well. 

Genetic drift: Rare genes are lost over time when random effects determine which genes survive across generations of bison.  Random effects occur with each production of ova or sperm and with accidents of mortality and reproduction that treat the most- and least-fit bison equally.  Research suggests that a herd of 2000-3000 bison will lose 5 percent of its genes each century.  Genes associated with disease resistance appear to be especially vulnerable.  Only 1 of our 45 conservation herds in the United States exceeds this threshold.

Artificial selection occurs when human decisions affect which bison survive and reproduce.  It is abundant in private, commercial herds; but also occurs in all our conservation herds.  Artificial selection results from: strict confinement and pasture rotation; roundups, handling and selective culling; supplemental feeding and watering; vaccinations; running unnaturally young herds with skewed sex ratios; maintaining stable herd sizes; and preventing natural predation.  Most conservation herds are subjected to several of these processes. 

Natural selection has given us wild bison.  It is necessary to retain their unique and valuable characteristics.  Weakening and replacing natural selection is turning wild bison into a domestic species.  Worldwide, several native species are abundant in domestication and rare or gone from the wild; and in the United States, bison are being forced in this direction.  

As a result of bison domestication, most states recognize bison only as livestock.  Their laws do not allow for wild bison.

So, the future of wild bison depends upon how we measure the quality as well as quantity of animals.  Wild bison must be defined as animals subjected to a preponderance of natural selection.  Their wild characteristics will persist only in large, diverse habitats where most may live and die naturally.  Politically comfortable efforts for bison restoration may give us more small conservation herds on small refuges — but we should not be misled.  Anything less than 1000 bison, and preferably many more, on less than 100 square miles of diverse habitat will accomplish little to restore wild bison.  We already have many small, intensively managed herds of plains bison.  We need larger herds on larger landscapes.

Opportunities to restore wild plains bison exist in several areas where public land is abundant and relatively contiguous.  Over time, grassland reserves could be assembled by purchasing inholdings from willing sellers.  South of Canada, this is the only strategy for preserving wild plains bison.  It will require difficult, long-term political commitments.  These commitments will not begin without a broad public understanding of the meaning and values of “wildness."

James Bailey, retired professor of wildlife biology at Colorado State University, considers the past, present and future of bison in his forthcoming book, American Plains Bison: Rewilding an Icon.


Photo courtesy USFWS