from pyrolusite (). Also created were arches, natural bridges, walls, and windows. Hoodoos are composed of soft sedimentary rock and are topped by a piece of harder, less easily eroded stone that protects the column from the elements. Bryce Canyon has one of the highest concentrations of hoodoos of any place on Earth. The formations exposed in the area of the park are part of the Grand Staircase. The oldest members of this supersequence of rock units are exposed in the Grand Canyon, the intermediate ones in Zion National Park, and its youngest parts are laid bare in Bryce Canyon area. A small amount of overlap occurs in and around each park. Ecology More than 400 native plant species live in the park. There are three life zones in the park based on elevation: the lowest areas of the park are dominated by dwarf forests of pinyon pine and juniper with manzanita, serviceberry, and antelope bitterbrush in between. Aspen, cottonwood, water birch, and willow grow along streams. Ponderosa pine forests cover the mid-elevations with blue spruce and Douglas fir in water-rich areas and manzanita and bitterbrush as underbrush. Douglas fir and white fir, along with aspen and Engelmann spruce, make up the forests on the Paunsaugunt Plateau. The harshest areas have limber pine and ancient Great Basin bristlecone pine, some more than 1,600 years old, holding on. The forests and meadows of Bryce Canyon provide the habitat to support diverse animal life including foxes, badgers, porcupines, elk, skunks, black bears, bobcats, and woodpeckers. Mule deer are the most common large mammals in the park. Elk and pronghorn, which have been reintroduced nearby, sometimes venture into the park. Bryce Canyon National Park forms part of the habitat of three wildlife species that are listed under the Endangered Species Act: the Utah prairie dog, the California condor, and the southwestern willow flycatcher. The Utah prairie dog is a threatened species that was reintroduced to the park for conservation, and the largest protected population is found within the park's boundaries. About 170 species of birds visit the park each year, including swifts and swallows. Most species migrate to warmer regions in winter, although jays, ravens, nuthatches, eagles, and owls stay. In winter, the mule deer, cougars, and coyotes migrate to lower elevations. Ground squirrels and marmots pass the winter in hibernation. Eleven species of reptiles and four species of amphibians have been found