Characteristics
Probably the most studied sky islands are in North America, namely the Madrean sky islands 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 to 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.
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