Yellowstone National Park has one of the largest purebred wild bison herds in the world, but according to officials, at 5,000 animals it might be getting too big. In a controversial agreement signed in the year 2000, if the bison stray too far into Montana and threaten to spread the infectious bacterial disease brucellosis to cattle – which they are predicted to do this year – then the animals can be culled. Because of this, Yellowstone officials are considering killing a thousand of the animals this winter.

Last year the authorities culled over 700 of the animals, despite having a target of 900, but it had a limited effect on their overall numbers. That is why this year it has been proposed that they should target mostly calves and females. This, they argue, will reduce the reproductive rate of the animals and have the biggest impact on keeping the bison numbers at the level supported by the ecosystem, meaning that they are less likely to migrate across the state border.

“Through the legal agreement the National Park Service has to do this,” said Yellowstone spokeswoman Sandy Snell-Dobert to Associated Press. “If there was more tolerance north of the park in Montana for wildlife, particularly bison as well as other wildlife, to travel outside the park boundaries, it wouldn't be an issue.”

Once roaming the American prairie from Texas in the south to Montana in the north, bison used to number around 60 million and migrated in enormous herds. One account from 1871 recorded how it took six days to pass through a single herd of bison which was “impossible to approximate the millions that composed it.” After extensive and unmitigated hunting, by the end of the 19th century these numbers had dwindled to just 325 animals.

Since then the recovery of the American bison has been slow, as despite there being approximately 500,000 bison in commercial herds across the country, many of these are hybrids with cattle which makes them more docile and easier to control. According to the International Union for the Conservation of Nature, there are around 19,000 plains bison in 54 conservation herds, although only five of these herds exceed 1,000 individuals and so classify as viable, which includes the one at Yellowstone.

Attempts to relocate the animals rather than kill them have had limited success in the past, and are met with opposition from some ranchers who are afraid that they might also spread disease to cattle as well as increase competition for grazing. The decision now rests with Montana governor Steve Bullock, who is not adverse to having the bison roam the state, as long as there are fewer than 3,500 of the animals.