Biodiversity in the Madrean Archipelago of Sonora, Mexico

Flowery rhetoric often gives birth to new terms that convey images and concepts, lead to inspiration and initiative. On the 1892-1894 expedition to resurvey the United States-Mexico boundary, Lieutenant David Dubose Gaillard described the Arizona-Sonora borderlands as “bare, jagged mountains rising out of the plains like islands from the sea” (Mearns 1907; Hunt and Anderson 2002). Later Galliard was the lead engineer on the Panama Canal construction project.

In 1951, Weldon Heald, a resident of the Chiricahua Mountains, coined the term ‘Sky Islands’ for the ranges in southeastern Arizona (Heald 1951). Frederick H. Gehlbach’s 1981 book, Mountain Islands and Desert Seas: A Natural History of the US-Mexican Borderlands, provided an overview of the natural history of the Sky Islands in the southwestern United States. The ‘desert seas’ in the valleys are desert grassland and Chihuahuan desertscrub in the north, Sonoran desertscrub to the west, and foothills thornscrub in the south. Variants of the term include Sky Island region (McLaughlin 1995; Gottfried and others 2005; Skroch 2008), sky-island ranges (Felger and Wilson 1995, Fishbein and others 1995), and sky island bioregion (Fishbein and others 1995; Skroch 2008). Today we mostly use ‘Sky Island Region’ with capital letters. The best name in Spanish is Islas Serranas, rather than the literal translation Islas del Cielo, because cielo means both ‘sky’ and ‘heaven.’ The analogy to oceanic islands (Warshall 1995) is limited because Sky Islands differ from true insular areas in high species diversity, low local and regional endemism, and low percent- ages of non-native species (McLaughlin 1995). While any isolated area is a potential area for speciation in small populations, there are relatively few species restricted to the Sky Island Mountains.

The term ‘Madrean’ comes from the Sierra Madre. The Mexican Plateau is a vast area of grasslands and desertscrub between the Rocky Mountains in New Mexico and ca. 1,300 km to the south the Trans-Mexican Volcanic Belt in south-central Mexico. The Plateau is open to incursions of frigid Arctic air from the north, and the Sierra Madres Oriental and Occidental create a double rain shadow and the Chihuahuan Desert. Madrean is a general term used to describe things related to the Sierra Madres. In a biogeographical analysis of the herpetofauna of Saguaro National Monument, University of Arizona herpetologist and ecologist Charles H. Lowe was probably the first to use the term ‘Madrean Archipelago’ to describe the Sky Island ranges between the Sierra Madre Occidental in Sonora and Chihuahua and the Mogollon Rim of central Arizona (Lowe, 1992). Warshall (1995) and McLaughlin (1995) expanded and defined the area and concept.