In bison recovery, scientists start small
High Country News
Last Nov. 1, about 400 spectators watched in delight as 10 huge, shaggy bison rumbled out of a holding corral onto 1,000 acres of windy shortgrass prairie, 30 miles north of Fort Collins, Colorado. The fenced grassland here is part of some 32,000 acres of city and county natural areas stretching from the foothills of the Rocky Mountains to the Great Plains. Local managers plan to gradually expand the herd’s range to 2,500 acres as it grows through a combination of natural reproduction and more reintroductions. The herd already has its own Facebook page and, of course, a limited-release commemorative microbrew, Prairie Thunder Imperial Brown Ale.
The release restores the bison to the merest sliver of the species’ vast historic range, and yet it represents a major conservation success. These animals are descended from the bison in Yellowstone National Park, the only population to survive wholesale slaughter by settlers during the late 19th century, and the last major reservoir of bison genes that have not been polluted by cattle DNA from cross-breeding. Yet using them in restoration efforts outside the park has been difficult because many Yellowstone bison carry brucellosis, a disease that can cause cattle to abort or prematurely give birth. The Laramie Foothills herd, however, is brucellosis-free, thanks to novel assisted-reproduction technologies. That makes these bison an early test case for efforts to expand the species’ gene pool outside Yellowstone.
Up to 60 million bison once wandered the plains. The largest land mammal in North America, the bison is now recognized as a keystone species that helps maintain the ecology of grasslands. Their grazing habits influence the diversity of forbs and grasses, and their hooves help aerate the soil. Even their dirt wallows create seasonal habitat for birds and affect how fire moves through grasslands.
Today, there are an estimated 500,000 scattered across the plains but nearly all are managed as livestock, destined to become buffalo burger. Fewer than 21,000 are part of 62 “conservation herds” that are managed for environmental purposes with limited human intervention, and many of those have cattle genes. Even fewer genetically pure animals are considered truly free-roaming and “wild.” Many scientists consider the species to be ecologically extinct, meaning that its functional role in the landscape has been eliminated.
So while the reintroduction of 10 bison in the Laramie Foothills may not sound like that big a deal, genetically pure conservation herds like this one are a crucial step toward restoring wild bison to the Western landscape. They could help calm ranchers’ longstanding worries about disease, and over time new herds have the potential to become self-sustaining populations that more closely resemble historic herds — if, that is, state and local managers are willing to give them room to grow.
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Today, Yellowstone is home to 4,900 bison. It’s the largest of four wild populations in North America, and contains 75 percent of the species’ genetic diversity. Every winter, state and federal officials round up most bison that wander outside the park’s borders. Up to 900 are removed annually through hunting or slaughter, largely to prevent the possible spread of brucellosis. They are either hazed back into the park, or quarantined and tested for brucellosis; any infected animals are killed.
These heavy-handed tactics have come under increasing attack in recent years. For one thing, there have been no documented cases of brucellosis transmission from bison to cattle, and even though elk also have been known to spread the disease, they aren’t as aggressively managed as bison. “The (disease) myth has been pretty much debunked now, but the stigma has spread far and wide,” says Steve Forrest, an ecologist for Defenders of Wildlife.
Assisted reproduction may help solve the problem. Jennifer Barfield of Colorado State University adapted techniques traditionally used for livestock, which involve cleaning semen from bison from the Yellowstone bloodline in the lab, and then using it to impregnate disease-free females. She has also collected eggs from brucellosis-infected Yellowstone bison destined for slaughter, cleaned them and fertilized them with clean sperm in the lab. The embryos are then implanted in surrogate bison cows.
This is one way conservationists can draw from the Yellowstone gene pool, while ensuring that newborn calves don’t carry the disease, Barfield says. Already, biologists plan to use male calves from the Laramie Foothills herd to build up other conservation populations, while minimizing inbreeding in the local population.
These steps are important because it’s risky to have so much of the species’ genetic material banked in one wild population. An unexpected disease outbreak or other catastrophic event in Yellowstone could be a significant setback for the entire species. But for conservation herds to become viable populations on their own, scientists estimate they must grow to at least 1,000 animals. And because a herd of that size probably needs at least 100,000 acres to roam, getting there will likely entail letting them mingle with cattle and roam across jurisdictional boundaries.
For now, the only genetically pure and brucellosis-free conservation herds allowed to roam freely on federal public lands in the U.S. are in South Dakota’s Wind Cave National Park and Utah’s Henry Mountains. Both herds are descended from Yellowstone bison, but managers have limited the populations to 400 or fewer animals.
“Every little herd is important in its own way,” says Montana State University wildlife ecologist Dustin Ranglack, who has studied the Henry Mountains herd. He has found that, due to eating and grazing habits, the bison rarely compete with cattle for forage, another common misperception. “But for an ecological future for bison, we need relatively large herds on large landscapes,” he says.
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