Paying ranchers to let bison roam in areas typically used for cattle grazing — rather than killing the giant animals — could reduce the risk that the bison will transmit a bacterial disease to cows, ecologists say.

Some 1,600 Yellowstone National Park bison were killed last winter to control the spread of brucellosis, a disease that causes miscarriages, weight loss and reduced milk production in cattle that inhale infected bison afterbirth or aborted fetuses. Brucellosis was widespread in cows in the 1800s, but cattle in most states are now free of the disease. No cases of bison-to-cattle transmission of brucellosis outside of captivity have ever been documented, according to the National Park Service, but they are periodically slaughtered as a precaution.

Yellowstone's 4,000 bison typically stay in the park's higher elevations, but they sometimes graze at lower levels near cattle-grazing areas in the Montana section of the park if heavy snow or ice makes food scarce. (The park spans three states: Wyoming, Montana and Idaho.) Limited numbers are allowed to roam in those areas as part of the Interagency Bison Management Plan devised by government agencies including the National Park Service, U.S. Department of Agriculture, Montana Department of Livestock and the state's Fish Wildlife & Parks department. If too many bison leave the park and are found to have antibodies to brucellosis, they're killed.

But mathematical modeling shows that even as the bison population grows, the chances are low that the animals will migrate and give birth to infected offspring in areas that cattle graze in, zoologist Marm Kilpatrick reports in today's Journal of Applied Ecology. Compensating ranchers for the use of their land and allowing more bison to roam there, then, would cost less than killing the animals, he writes. "If you could work out something with those ranches, you could let a lot more of those bison roam without increasing the risk," Kilpatrick, an assistant professor of population biology at the University of California, Santa Cruz, told the Associated Press.

The other option is reclassifying the brucellosis infection-status of the area surrounding Yellowstone so that if bison were to transmit disease to cattle, "the cost of testing all the animals and the economic loss will be relatively small" for ranchers, Kilpatrick tells

In 1902, hunting had reduced Yellowstone's bison population to just 23. Preservation efforts restored the population, and there are now about 150,000 bison in public and private U.S. herds, 4,000 of them in Yellowstone, the Park Service says. Still, the majestic bison is beloved by conservationists who consider it a symbol of the American West.

"The risk of disease transmission from bison or elk — the two wild hosts to cattle — is not uniform" across the park's 2 million acres, Glenn Plumb, Yellowstone's chief of aquatic and wildlife management, tells us. "Estimating the risk of transmission in a spatial context will be helpful to understanding where to prioritize risk management."

A DOI veterinarian, Jack Rhyan, told the AP that the study's modeling looked accurate, but that buying grazing rights wasn’t necessarily the best alternative to killing the bison. "It's a temporary way to take the heat off of the problem," Rhyan told the newswire, "but that just allows the problem to get that much bigger."

Third paragraph updated on Jan. 13 at 2:15 p.m. to add geographical clarity.

Image of bison by U.S. Department of Agriculture via WikiMedia Commons