The Madrean Archipelago (Sky Island Region)

The terms Madrean Archipelago and Sky Island Region are often used interchangeably for the region between the Sierra Madre Occidental in Sonora and the Mogollon Rim in Central Arizona. On the second United States-Mexico Boundary Survey in 1892-93, Lieutenant David Gaillard took detailed notes on the natural history of the borderlands including the Sierra San Luis and Cajón Bonito (Mearns 1907). He described the region as “bare, jagged mountains rising out of the plains like islands from the sea”. Weldon Heald, a resident of the Chiricahua Mountains, named the ranges crowned with woodlands and forests in southeastern Arizona sky islands (Heald 1951), evoking the image of continental islands emergent from inland seas (lowlands).

Fred Gehlbach’s 1981 book Mountain Islands and Desert Seas. A Natural History of the U.S.-Mexican Borderlands is an important regional overview and source of information. In the north in southeastern Arizona, southwestern New Mexico, and northernSonora, the ‘desert seas’ are desert grassland or Chihuahuan desertscrub in limestone areas. To the west at lower elevations, Sonoran desertscrub surrounds mountains. Farther south in Sonora closer to the New World tropics, foothills thornscrub is present below oak woodland.

Biotic communities (Vegetation types) are zoned in bands on mountain, reflecting increased rainfall and decreased temperatures and areas at higher elevations. More mesic (wetter) vegetation occurs above drier vegetation in all mountainous areas, not just in the sky islands. Considering that species richness generally increases at higher elevations, oak woodlands and pine-oak forest in Sky Island ranges have more species than lowland areas. The analogy to oceanic islands is limited because Sky Islands differ from true oceanic islands in high species diversity and relatively few endemic and non-native species (McLaughlin 1955).

The term Madrean comes from the Sierra Madre. The Mexican Plateau is a vast, 1,800 km long area of grasslands and in the double rain shadow formed by the Sierra Madre Oriental on the east and the Sierra Madre Occidental on the west. The Sierra Madre Occidental extends up western Mexico from Zacatecas and Jalisco north to Chihuahua and Sonora, Mexico. Highest elevations for much of this cordillera exceed 2800 m, and its continuity provides an important route for fauna and flora dispersing between tropical and temperate pine forests, and between tropical forests and northern grasslands. The Continental Divide follows the Sierra Madre Occidental northward along the Chihuahua-Sonora border, and  through the Sierra San Luis in Sonora to the Ánimas Mountains in New Mexico. Warshall (1995) estimated that there were about 40 Sky Island ranges in the Madrean Archipelago. We now recognize 55 Sky Island ranges and complexes (separate ranges joined by topography and oak woodland), with 35 of them in Sonora.  
Literature Cited
Gehlbach, F. H. 1981. Mountain islands and desert seas: A natural history of the US‑Mexican borderlands. Texas A & M University Press, College Station, TX.

Heald, W. F. 1951. Sky Islands of Arizona. Natural History 60: 56-63, 95-96.

McLaughlin, S. P. 1995. An overview of the flora of the Sky Islands, southeastern Arizona: Diversity, affinities, and insularity. Pp. 60-70 in L. F. DeBano, P. F. Ffolliott, A. Ortega‑Rubio, G. J. Gottfried, R. H. Hamre, and C. B. Edminster, technical coordinators. 1995. Biodiversity and management of the Madrean archipelago: The sky islands of southwestern United States and northwestern Mexico. USDA Forest Service, General Technical Report RM‑GTR‑264.

Mearns, E. A. 1907.Mammals of the Mexican boundary of the United States. Part 1. Smithsonian Institution, Bulletin 56, Government Printing Office.

Warshall, P. 1995. The Madrean Sky Island Archipelago: a planetary overview. Pp. 6-18 in G Gottfried, R. H. Hamre, C. B. Edminster, P. F. Ffolliott, and A. Ortega-Rubio (tech. coords.). Biodiversity and Management of the Madrean Archipelago; The sky islands of the southwestern United States and northwestern Mexico, USDA Forest Service, Gen. Tech. Rept. RM-GTR-264, Ft. Collins, CO.