Wildlife in Big Bend. Wide-ranging ecosystems within the Big Bend provide habitat for more than 450 species of birds, 75 species of mammals, 56 species of reptiles, and 11 species of amphibians. Included are more than 100 miles of low-elevation river corridor, thousands of acres of Chihuahuan desert with a scattering of desert oases, a transition zone of upland shrubs, grasses, and junipers, and the higher (and cooler) elevations of the Chisos Mountains, a sky island wholly contained within Big Bend National Park.