The Madrean sky islands are probably the most studied sky islands in the world. Found in the U.S. states of New Mexico and Arizona and the Mexican states of Chihuahua and Sonora, these numerous mountains form links in a chain connecting the northern end of the Sierra Madre Occidental and the southern Colorado Plateau. Sky islands of the central and northern mountains in the United States are often called island ranges, especially by populations within view of such islands of mountains surrounded by plains such as those found within the Wichita Mountains of southwestern Oklahoma.