At one time, there were as many as 60 million buffalo roaming North America, so many that they became the symbolic representation of our westward expansion. Unfortunately for them, they were also a popular food source for a rapidly expanding army of settlers, profitable for traders, and relatively easy to capture and kill. By 1884, there were only around 325 of the beasts left in the wild, with just 25 of them in Yellowstone National Park. 

Now Yellowstone’s scruffy flock of horned ungulates runs at around 5,000, at least as of last weekend. After yesterday, the number could be dramatically lower.

On Monday, members of the National Park Service, Forest Service, Montana Department of Fish and Wildlife, and other agencies rounded up between six and nine hundred buffalo from a closed area, and will soon send them to be systematically slaughtered.

It’s an annual program that has been going on for years, part of an agreement between the park and the state of Montana to keep bison populations in the park “manageable” in order to prevent the spread of brucellosis between bison and cattle. It’s also, though this is less talked about, to help prevent overgrazing—that is, to stop the wild buffalo from eating vegetation on public land that private ranchers feel should be saved for their for-profit cattle herds.