Montana shoots bison that migrate in from Yellowstone National Park because the population carries brucellosis, a disease that can cause abortion in livestock. Because cattle could potentially catch brucellosis by eating infected birth membranes, it would be a good idea to keep them away from the bison population during its birthing period. One pitfall would be if brucellosis-infected bison gave birth over a period longer than the normal two months. However, this is not the case, say Joel Berger of the University of Nevada in Reno and Steven Cain of Grand Teton National Park in Moose, Wyoming.
While millions of bison once lived in North America, by the turn of the century there were fewer than 1000 left. Since then bison have been protected in reserves and ranches and the population has grown to 150,000. But over the last 10 years nearly 3000 bison have been killed as they leave Yellowstone and enter Montana. "This is the greatest public slaughter of bison probably since the heyday some 125 years or more ago," says Berger.
Berger and Cain compared the length of birthing periods in a brucellosis-exposed bison population (in Jackson Hole, Wyoming) with three non-brucellosis-exposed populations (in National Bison Range in Montana and in Wind Cave and Badlands National Parks in South Dakota). The researchers found that the brucellosis-exposed bison did not give birth over a longer period than the three non-brucellosis-exposed populations.
This finding is important because ranchers can predict when it will be safe to put cattle on public lands where bison live. Because 95% of the bison in the Yellowstone area gave birth during May and June, Berger and Cain suggest that ranchers delay putting cattle in the area until July.
Although ranchers are concerned that their cattle might catch brucellosis from bison, this has never happened in the wild. Cattle have caught the disease from bison in experiments--but only at bison densities 1000 times higher than those in Yellowstone. These high densities are unlikely to occur in the wild, says Berger.
Journal
Conservation Biology