Park officials say the bison have altered vegetation patterns, damaged archaeologically valuable sites and even threatened the water supply for animals and humans in the Grand Canyon.
“You’ll see them wading in the pools, munching grass next to the pool, just having a great time,” Ms. Thomas said. “They don’t generally go somewhere else to go to the bathroom, so they’ll just go right there,” leading to E. coli contamination, she said.
There were an estimated 30 million to 60 million bison in North America in the 1500s, according to the Park Service. By the 1800s, the figure had dropped to about 400. Conservation efforts helped restore the numbers, and there are currently an estimated 500,000 bison throughout North America, according to the Park Service.
The herd in Grand Canyon National Park is believed to be made up of direct descendants of the bison introduced to the area by Charles Jones, known as Buffalo Jones, in the 1900s “as a ranching experiment to crossbreed bison and cattle,” according to the Park Service. The venture failed, and the bison migrated to the south end of the House Rock Valley area. In 1927, the herd was sold to the Arizona Game and Fish Department.
By the 1990s, hunting and fires in the area had led some in the herd to migrate back into Grand Canyon National Park, officials said. For a while, the herd wandered between the park and the area now known as the House Rock Wildlife Area. Since 2009, most of the bison have settled down within the park.
“The bison, they’ve become relatively aware of the fact that when they go beyond the park boundary, they are at risk from the state-led hunt,” Ms. Thomas said. “So that’s why you often see them almost exclusively ranging inside that small area.”