Before European settlement, 20-30 million American Plains bison lived in the tallgrass and shortgrass prairies of central North America from Canada to Mexico, moving in nomadic herds, grazing mostly on grasses, migrating seasonally, and creating paths later followed by American Indians, explorers, and pioneers. Rather than harming the prairie ecosystem, bison grazing kept grasses short, their hooves stirred the soil, and sunlight warmed the ground, helping new native plants and more kinds of wildflowers grow while deep-rooted prairie plants restored seasonal balance. Bison were a vital resource for American Indian tribes, who hunted only what they needed and used nearly every part of the animal, but by 1900 only a few hundred remained because of unregulated commercial hunting and habitat loss. Today bison are ecologically extinct from most of their historic range, though Blue Mounds State Park has one of the few genetically pure herds remaining in the United States, and conservation efforts continue to preserve such herds. In the tallgrass prairie, many birds and mammals live close to the ground or in burrows that protect them from weather, wildfire, and predators, and rare species such as the burrowing owl are still being studied. Tallgrass prairie, named for grasses that can grow more than six feet tall and rooted more than 15 feet deep, supports more than 900 recorded plant species, changes with the seasons, resists non-native plants when natural processes such as fire remain active, and is one of North America's most endangered and fragmented ecosystems. Once stretching from Saskatchewan to Mexico and from east of the Mississippi River west to the Central Great Plains, and once covering one-third of Minnesota, this ecosystem now survives as only one percent of the state's original tallgrass prairie.