Located in northwest Mexico, southwest New Mexico and southeast Arizona, the Madrean Archipelago is a 70,000 square-mile ecoregion of sky island mountain ranges surrounded by an ocean of desert scrub and grasslands. The region, which straddles the US-Mexico borderlands is a blend of tropical and temperate climates and home to a biological diversity that exceeds any other region of the United States.

The Madrean Archipelago Ecoregion, also known as the Madrean Sky Islands or Sky Islands, covers an area of aproximately 30,000 square miles in the southwest US and about 40,000 square miles in northwest Mexico.

This ecoregion is bounded on the west by the Sonoran Desert ecoregion (2S), on the east by the Chihuahuan Deserts ecoregion (2C), on the north by the Arizona/New Mexico Mountains ecoregion (2N), and on the south by the Sierra Madre Occidental ecoregion (13W) in Mexico.

This area of basin-and-range topography is one of the most biologically diverse in the world. Although the mountains in this ecoregion bridge the Rocky Mountains to the north and the Sierra Madre Occidental to the south, the lower elevations act as a barrier to species dispersal. Nevertheless, the geographic convergence of these two major continental mountain ranges, as well as of the Chihuahuan Desert to the east and the Sonoran Desert to the west, forms the foundation for ecological interactions found nowhere else on Earth.

The Madrean Sky Islands are enclaves of Madrean pine–oak woodlands, found at higher elevations in a complex of small mountain ranges in southern and southeastern Arizona, southwestern New Mexico, and northwestern Mexico. The sky islands are surrounded at lower elevations by the Sonoran and Chihuahuan deserts. The northern west–east perimeter of the sky island region merges into the higher elevation eastern Mogollon Rim and the White Mountains of eastern Arizona (southern Anasazi region).

The sky islands are the northernmost of the Madrean pine–oak woodlands, and are classified as part of the Sierra Madre Occidental pine–oak forests ecoregion, of the tropical and subtropical coniferous forests biome. The sky islands were isolated from one another and from the pine–oak woodlands of the Sierra Madre Occidental to the south by the warming and drying of the climate since the ice ages.

There are approximately 27 Madrean sky islands in the United States, and 15 in northern Mexico. The major Madrean sky island ranges in Arizona are the Baboquivari Mountains, Chiricahua Mountains, Huachuca Mountains, Pinaleño Mountains, Santa Catalina Mountains, Santa Rita Mountains and Whetstone Mountains. Similar sky island ranges include the Animas Mountains in New Mexico and the Guadalupe Mountains, Davis Mountains and Chisos Mountains in west Texas.

Sierra San Luis

Huachuca Mountains, Sierra del Tigre, Sierra La Esmeralda, Sierra San Antonio, Sierra San Jose, and Sierra San Luis.