Sky islands are mountains that are isolated by surrounding lowlands of a dramatically different environment, a situation which, in combination with the altitudinal zonation of ecosystems, has significant implications for natural habitats. Endemism, vertical 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 sheer 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 ice age. In other cases, localized populations of plants and animals tend towards speciation, the same process that happens on oceanic islands such as the Galápagos Islands.
Origin of the term
The sky island concept originated 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 very 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, 35 mi (56 km) away and 5,600 ft (1,707 m) higher in elevation. Ascending from the hot, arid desert, the environment transitions to grassland, then to oak-pine woodland, pine forest, and finally to spruce-fir-aspen forest. The book mentions the concept of 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.
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 Rocky Mountains in the United States are often called island ranges, especially by populations within view of such islands of mountains surrounded by plains.
Some more northerly examples are the Crazy Mountains, Castle Mountains, Bears Paw Mountains, Highwood Mountains, and Little Rocky Mountains, all in the US state of Montana. Each of these ranges is forested and has tundra and snowpack above treeline, but is not connected to any other range by forested ridges; the ranges are completely surrounded by treeless prairie and/or semi-arid scrubland below. Other well-known sky islands of North America are the Great Basin montane forests, such as the White Mountains in California, and the Spring Mountains near Las Vegas, Nevada. One of the unique aspects of the sky islands of the U.S.-Mexico border region is the mix of floristic affinities, that is, the trees and plants of higher elevations are more characteristic of northern latitudes, while the flora of the lower elevations has ties to the desert and the mountains further south. Some unique plants and animals are found in these sky islands, such as the mountain yucca, Mount Graham Red Squirrel, Huachuca springsnail, and Jemez Mountains Salamander.
Some montane species apparently evolved within their current range, adapting to their local environment, such as the Mount Lyell Shrew. However, it has also been noted that some isolated mountain ecosystems have a tendency to lose species over time, perhaps because small, insularized populations are vulnerable to the forces of extinction, and the isolation of the habitat reduces the possibility of colonization by new species. Furthermore, some species, such as the Grizzly bear, require a range of habitats. These bears historically made use of the forests and meadows found in the Madrean sky islands, as well as lower-elevation habitats such as riparian zones. (Grizzlies were extirpated from the region in the 20th century.) Seasonal movements between highland and lowland habitats can be a kind of migration, such as that undertaken by the Mountain Quail of the Great Basin mountains. These birds live in high elevations when free of snow, and instead of migrating south for the winter, they migrate down.
Confusing the matter somewhat is the potential for an archipelago of sky islands or even the valleys between them to act not only as a barrier to biological dispersal, but also as a path for migration. Examples of birds and mammals making use of the Madrean archipelago to extend their ranges northward are the Elegant Trogon and White-nosed Coati.
List of sky islands of the world by terrestrial ecozone
- Cal Madow
- Cameroonian Highlands forests
- Ethiopian Highlands
- Highlands of southern Africa
- Mount Kilimanjaro
- Rwenzori Mountains
- Fan Si Pan
- Jade Mountain
- Mount Kinabalu
- Nat Ma Taung
- Central Highlands (Vietnam)
- Titiwangsa Mountains
- Western Ghats
- Madrean sky islands
- Great Basin montane forests
- Guadalupe Mountains
- Colorado Plateau's San Francisco Peaks
- Davis Mountains
- Baja California
- Cordillera of Central America
- Guiana Shield
- Serra do Mar
- Sierra de Tamaulipas
- Venezuelan Coastal Range
- Altitudinal zonation
- Life zone
- Holdridge life zones
- List of life zones by region
- Monadnock or Inselberg
- Refugium (population biology)
- Dodge, Natt (March 1943). "Monument in the Mountain". Arizona Highways (Phoenix, Arizona: Arizona Highway Department) 19 (3): 20–28.
- Howell, John Thomas (May 1947). "Mono Mesa - Sierra Sky Island". Sierra Club Bulletin (San Francisco, California: Sierra Club) 32 (5): 15–18.
- Heald, Weldon (1967). Sky Island. Princeton, New Jersey: Van Nostrand. pp. 114–126.
- Quammen, David (2004). The Song of the Dodo: Island Biogeography in An Age of Extinctions. New York: Scribner. pp. 436–447. ISBN 978-0-684-82712-4.
- McPhee, John (1981). Basin and Range. New York: Farrar, Straus and Giroux. p. 46. ISBN 0-374-10914-1.
- Warshall, Peter (19 September 1994). "The Madrean Sky Island Archipelago: A Planetary Overview". Biodiversity and Management of the Madrean Archipelago .The Sky Islands of Southwestern United States and Northwestern Mexico. Fort Collins, Colorado: United States Forest Service.
- McLaughlin, Steven P. (19 September 1994), An Overview of the Flora of the Sky Islands, Southeastern Arizona: Diversity, Affinities, and Insularity, Biodiversity and Management of the Madrean Archipelago .The Sky Islands of Southwestern United States and Northwestern Mexico, Fort Collins, Colorado: United States Forest Service
- Wilson, Don; Ruff, Sue (1999). The Smithsonian Book of North American Mammals. Washington, D.C.: Smithsonian Institution Press. p. 26. ISBN 1-56098-845-2.
- Brown, David E. (1985). The Grizzly in the Southwest. Norman: University of Oklahoma Press. pp. 77–95, 136–159. ISBN 0-8061-1930-6.
- "Mountain Quail fact sheet". Nevada Department of Wildlife. Retrieved 8 May 2011.
- Tweit, Susan J. (1992). The Great Southwest Nature Factbook. Alaska Northwest Books. pp. 209–210. ISBN 0-88240-434-2.