Sky island

Sky islands are isolated mountains surrounded by radically different lowland environments. The term originally referred to those found on the Mexican Plateau and has extended to similarly isolated high-elevation forests. The isolation has significant implications for these natural habitats. Endemism, altitudinal migration, and relict populations are some of the natural phenomena to be found on sky islands.

The complex dynamics of species richness on sky islands draws attention from the discipline of biogeography, and likewise the biodiversity is of concern to conservation biology. One of the key elements of a sky island is separation by physical distance from the other mountain ranges, resulting in a habitat island, such as a forest surrounded by desert.

Some sky islands serve as refugia for boreal species stranded by warming climates since the last glacial period. In other cases, localized populations of plants and animals tend towards speciation, similar to oceanic islands such as the Galápagos Islands of Ecuador.

Herpetologist Edward H. Taylor presented the concept of "Islands" on the Mexican Plateau in 1940 at the 8th American Scientific Congress in Washington, D.C. His abstract on the topic was published in 1942.

The sky island concept was later applied in 1943 when Natt N. Dodge, in an article in Arizona Highways magazine, referred to the Chiricahua Mountains in southeastern Arizona as a "mountain island in a desert sea".

In about the same era, the term was used to refer to high alpine, unglaciated, ancient topographic landform surfaces on the crest of the Sierra Nevada, California.

The term was popularized by nature writer Weldon Heald, a resident of southeastern Arizona. In his 1967 book Sky Island, he demonstrated the concept by describing a drive from the town of Rodeo, New Mexico, in the western Chihuahuan desert, to a peak in the Chiricahua Mountains, 56 km (35 miles) away and 1,700 m (5,600 ft) higher in elevation, ascending from the hot, arid desert, to grasslands, then to oak–pine woodland, pine forest, and finally to spruce-fir-aspen forest. His book mentions the concept of a biome but prefers the terminology of life zones and makes reference to the work of Clinton Hart Merriam. The book also describes the wildlife and living conditions of the Chiricahuas.

Around the same time, the idea of mountains as islands of habitat took hold with scientists and has been used by such popular writers as David Quammen and John McPhee. This concept falls within the study of island biogeography. It is not limited to mountains in southwestern North America but can be applied to mountains, highlands, and massifs around the world.