I was careful to stay near trees where I could dodge if a suspicious bison charged, but when another vehicle came along the dirt track in the valley below, the small herd spooked and ran south along the hilltop. Bison are extremely destructive to delicate habitat, and the park is struggling to find a solution.The massive animals have reduced vegetation in meadows to nubs, traveled into Mexican spotted owl habitat, knocked over walls at American Indian cliff dwellings below the North Rim, defecated in lakes and left ruts in wetlands. They can weigh more than 2,000 pounds and run three times faster than humans.