A sky island is a mountain that is not connected to a range. It is typically forested and
raises dramatically out of the surrounding landscape. While a sky island may be classified as part of a mountain range, it is not physically connected and is separated by a ‘sea’ of either desert, grassland, or in some cases scrubland. Because a sky island is an isolated ecosystem surrounded by another, dramatically different one, it includes species and/or subspecies not found anywhere else. Due to the steady change in altitude characteristic of a mountain, the range of any given species is even smaller. The biodiversity of an entire sky island is therefore very high. The biodiversity contribution of sky island ecosystems to biodiversity in general is invaluable. Predictably, these unique environments have attracted the attention of researchers in all areas of natural science.
While the interest of biologists, botanists and ecologists is predictable, there is another group of scientists whose interest is less intuitive and unfortunately often in direct conflict to those of the aforementioned natural scientists.
It didn’t take astronomers long to figure out what a prime location for observatories these isolated peaks, in the middle desert land, away from city lights and other pollutants were for telescopes. Unfortunately the goals of the sky gazers and those of the ground preservers don’t tend to mix. Clearing a space for observatories on top of sky islands has sparked more than one bitter battle.1
Sky islands have the layered ecosystems associated with a rapid change in elevation. The lowest levels contain semi-arid vegetation which progress to cool fir or spruce forested peaks. An example of a typical sky island progression in the Southwestern United States is as follows:
Sonoran Desertscrub Under 3,500ft this vegetation is an odd mix of cacti and non-succulent shrubs. Suguaros stand among mesquite and palo verde along with the infamous teddy bear cactus.
Chaparral begins at about 4,000 feet and consists of dense shrubs and small trees. Scrub oak, manzanita and some cacti such as prickly pear some commonly found species in chaparral.
Pinyon Pine-Juniper begins at about 5,500 feet and is dominated by small juniper and pine species. While it gets a reasonable amount of rainfall (12 to 20 inches in some places) this vegetation is drought resistant and prefers a somewhat arid climate. Unlike chaparral, this community consists of widely spaced trees and intermittent shrubs, grasses and wildflowers between.
Ponderosa Pine Forest begins at about 6000 to 6500 feet. It is dominated by tall shady Ponderosa with next to no undergrowth.
Fir Forest, or Mixed Fir Forest dominates many of the peaks of Southwestern Sky Islands. It occurs at between 8,000 and 10,000 feet. Unlike the Ponderosa forest this ecosystem contains many different species including several types of fir, spruce and a diverse undercover. Rainfall in this area can be up to 30 inches a year.
1The controversial history of Mount Graham is the subject of another node I am working on. It is one of the more dramatic examples of competing interests in a sky island.
Warshall, Peter “Southwestern Sky Island Ecosystems” accessed April 10, 2003
"Sky Island Region" Arizona Roadside Environments accesssed April 10, 2003