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Bison were once so many that the estimate for North America before the 1800s runs to the tens of millions, and by 1880 the number was down to hundreds. Today the Park Service says the herd at Wind Cave National Park is among the most genetically diverse and pure in the world, even though it started with only 14 animals sent from the New York Zoological Society in 1913.
Bison at Wind Cave. NPS photo.
Two threats to bison genes
Restoring bison from a few public and private herds has saved the species from extinction, but two things now hinder conservation: cattle introgression and inbreeding depression. Both can erode genetic diversity.
Cattle genes. When bison numbers collapsed, private owners bred bison and cattle together to build up herds, so cattle genes entered the bison genome, and moving privately owned animals onto public land spread them. A 2006 study sampled 11 U.S. federal herds, including Wind Cave, taking tissue, hair and blood and analysing the DNA of the mitochondria and nucleus. Initial tests found evidence of cattle introgression only in the National Bison Range herd, and further testing found it in seven of the 11 herds. Researchers sampled every bison where possible, 3301 samples in all. Because herds tend to be small, only the Wind Cave and Yellowstone herds were large enough for firm results, and neither shows evidence of cattle introgression.
Inbreeding. The Wind Cave National Game Preserve was created in 1912 to restore bison, and its herd grew to 200 by 1930. The park keeps it between 350 and 500 animals, a size its resources can sustain, which from a genetic standpoint is small and isolated. A 2017 study tested for inbreeding depression in small, isolated national park herds. Simulations of growth and reproduction matched the recorded populations, and a test with lethal equivalents matched the recorded herd only when they were zero, so the study found no sign of inbreeding depression at Wind Cave.
Wind Cave's bison and Custer State Park's are genetically different and managed differently, so they are kept apart. NPS photo.
How herds are managed
A 2010 workshop of Interior Department specialists grouped herds by cattle introgression and by size.
| Group | Recommendation |
|---|---|
| No molecular evidence of cattle introgression (only Yellowstone and Wind Cave) | Protect from unintentional introgression; no mixing of herds |
| Molecular evidence of introgression | Do not mix with herds that have none, and manage to preserve variation |
| More than 1,000 animals | Only Yellowstone; monitor the effects of culling |
| 500 to 1,000 animals | Manage for no introgression or inbreeding depression and grow toward 1,000 |
| Fewer than 500 animals | Immediate action: keep the population size, maximise the number of breeding males, increase diversity and monitor genetics |
The department's Bison Conservation Initiative, begun in 2008, lists four actions: a bison metapopulation strategy, a stewardship plan, better support for ecocultural restoration with live bison, and low-stress capture and handling. The Park Service sums it up: bison numbers have grown a great deal, but most herds remain small and isolated, so management will keep adapting.
Bison are rounded up into corrals every few years as part of Wind Cave's management plan. NPS photo.
Sources
- National Park Service, "Genetic Diversity of Wind Cave's Bison Herd" (https://www.nps.gov/articles/000/wind-cave-bison-genetics.htm), drawing on studies by Natalie D. Halbert and James N. Derr (2006), by Halbert and colleagues (2007) and by Daniel S. Licht (2017), and on the 2010 Bison Genetics Workshop report; rewritten in hubnx's own words.
Dans ces publicationsWind Cave National Park
Licence : CC0 1.0 (domaine public) · Adapté de www.nps.gov
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