Don't Blame Humans For Bison Loss

Big game hunters may be off the hook in the latest twist of a prehistoric whodunit that tries to explain why bison populations sharply crashed thousands of years ago.

Proponents of the overkill theory blamed the first Americans to cross an ice-free corridor — connecting what's now Alaska and Siberia — for hunting bison within a whisper of disappearance. Those super hunters are also faulted for pushing massive mammals, like woolly mammoths, short-faced bears and North American lions into extinction.

A team of 27 scientists used ancient DNA to track the hulking herbivore's boom-and-bust population patterns, adding to growing evidence that climate change was to blame.

"The interesting thing that we say about the extinctions, is that whatever happened, it wasn't due to humans," said the paper's lead author, Beth Shapiro, a Wellcome Trust Research Fellow at Oxford University. By the time people arrived, "these populations are already significantly in decline and on the brink of whatever was going to happen to them in the future."

The story written into the bison's DNA is one of an exponential increase in diversity with herd sizes doubling every 10,200 years. Then, 32,000 to 42,000 years ago, the last glacial cycle kicked in, beginning a lengthy cooling trend. Bison genetic diversity plummeted. A significant wave of humans didn't appear in the archaeological record at eastern Beringia until more than 15,000 years later, the authors write in Friday's Science. Beringia is the region surrounding the land bridge that connected North America and Asia.

The Science paper refers to dates in radiocarbon years, a dating technique that doesn't match up precisely with conventional calendars. For instance, 12,000 years before the present in radiocarbon years equates to 14,000 years ago according to calendars, Shapiro said.

Cold "and arid conditions increasingly dominated, and some component of these ecological changes may have been sufficient to stress bison populations across Beringia," the authors noted.

About the same time, brown bears and a type of horse went extinct in Alaska.

The results "offer the first evidence of the initial decline of a population, rather than simply the resulting extinction event," the authors wrote.

Tapping genetic information gives scientists the means to assess the health of bison over thousands of years, said Russell Graham, director of the Earth & Mineral Sciences Museum at Penn State University.

At given points in the distant past, the researchers could tell if bison were thriving or decimated and attach a firm date to that health check.

"The real importance of the paper, at least from my perspective, is it provides us a way of measuring what is happening to a population of animals through time," Graham said.

John Alroy, a University of California-Santa Barbara research biologist and overkill proponent, remains unconvinced. The near-extinction of bison would not have happened without the handiwork of human hunters, he said, adding: "I think the interpretation is off-base and inappropriate, and I'm not persuaded at all by their claims."

Researchers have looked at modern animals to flip back in time to better understand how their ancestors fared during the peak of the last Ice Age. Because of severe populations crashes, though, modern bison lost much of the genetic diversity locked up in the bones of their Ice Age ancestors.

Only two subspecies remain in North America, the plains and the wood bison.

"If you just used modern bison populations and tried to figure out what was going on in the past, you wouldn't get the right answer," Shapiro said.

The team collected 442 bison fossils from Alaska, the lower 48 states, Canada, China and Siberia.

Bones stored in the attic by such institutions as New York's American Museum of Natural History were the toughest to decode, since blistering summer heat and frigid winters sapped much of their DNA. The best fossils at yielding genetic secrets were plucked straight from the permafrost in Alaska and Siberia, sometimes with the help of gold miners, she said.

The miners use high-pressure hoses to slice through permafrost.

"When they're doing that, they get the gold and they get these troublesome bones that they just pile up," she said. "We went out one year and saw it and then thought 'Hey that's a fantastic resource, would they mind if we took some big chunks out of them?'

"They're like, whatever, get rid of 'em."

By Diedtra Henderson