Islands in the sky. To someone with a basic understand of geography, this sounds like an oxymoronic statement. The geographic definition of an island being a landmass surrounded by water on all sides. Islands cannot be found in the sky. Not unless those islands were just floating in the clouds.
What are Sky Islands?
Sky islands are not a type of island. “Sky island” refers to a type of mountain range. The root word for isolated is “insula”. Insula means “island” in Latin”.
Isolated is a fitting term to describe sky islands. Sky island is a type of mountain range that is characterized by physical isolation. Sky islands stand out from the surrounding landscape. They are small, but high mountains that are separated from other mountain ranges and tower over the much flatter, lower elevations.
Sky Islands and Biogeography
The relative isolation of sky islands shapes their biogeography. Sky islands stand out immensely from the surrounding landscapes in terms of appearance. The biogeography of the sky islands differs acutely from surrounding regions. Sharp changes in elevation are instrumental in shaping their environs.
As the elevation rises, the temperatures become cooler. Due to the drop in temperatures, the dew point is more easily reached. The dew point is the temperature where condensation takes place. This resulting condensation results in the formation of clouds and a higher amount of precipitation often takes place at higher elevations as a result.
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With temperatures and precipitation contrasting vastly from the surrounding areas, the plant and animal life within the sky islands differs from the outer lying areas at much lower elevations. Plants and animals that would otherwise not easily survive in hotter regions find it easier to live among higher altitudes (and thus, cooler, more moist conditions) of sky islands.
The Madrean Sky Islands
Many mountains that could be classified as “sky islands” can be found in desert regions. One example of this is the Madrean Sky Islands. The Madrean Sky Islands are isolated mountains and mountain ranges flanked by the Sonoran and Chihuahuan deserts.
They are found in the southwestern USA and northwestern Mexico. Within the USA, these sky islands are located within southeastern Arizona and southwestern New Mexico, known as “the Bootheel of New Mexico”. Sky islands can also be found as far north as northwestern New Mexico. Within Mexico, they are located within the states of Chihuahua and Sonora.
Home to the Madrean Pine-oak Woodlands
The Madrean Sky Islands are home to the northernmost part of the Madrean pine-oak woodlands. There are at least 7,000 species of flora and fauna found within the mountain ranges. This is an ecoregion that consists of North American subtropical and tropical conifers. Conifers are cone-bearing trees such as pines and firs. Subtropical coniferous and oak forests thrive in these sky islands. These forests are the northernmost extension of the Sierra Madre Occidental pine-oak forests.
Pine trees are a major part of the Madrean sky island ecosystem. In fact, one of the sky islands is known as the Pinaleno Mountains. “Pinaleno” is Spanish for “full of pines”, referring to the pine trees growing in the mountains.
Another mountain range, the Pinal Mountains, gets its name due to its pine trees. “Pinal” means “stand of pines” in Spanish. Within Mexico, there are at least 44 species of pine trees, which can be found within the Madrean Sky Islands.
Fir trees can be found among these isolated mountain ranges. The Douglas fir is often associated with the northern Rocky Mountains and the Pacific Northwest. Within the mountain ranges of northern Mexico and the southwestern USA, the Rocky Mountain Douglas fir thrives. In fact, the Madrean Sky Island ranges mark the southernmost point where Douglas firs grow.
Islands of Distinct Biogeography
The Madrean Sky Islands serve as pockets of cooler, more moist environs. They serve as mountain oases among the deserts. As an oasis is an enclave of lush vegetation among the desert, so serves the Madrean Sky Islands. Pine and oak forests thrive in these sky islands because of the effect elevation has. In the higher elevations, away from the lower, hotter areas, there is more precipitation, more rain and snow.
Such biodiversity is not limited to the flora. This is also represented with its fauna. The North American jaguar hunted to the brink of extinction. Within the southwestern United States of America, it was considered locally extinct. However, a few North American jaguars have been spotted in the state of Arizona since the mid-1990s. Of the jaguar that have been spotting in Arizona, all have been found in southeastern Arizona, within the Madrean Sky Islands.
A few jaguars being spotting is just the tip of the proverbial iceberg in terms of fauna. Ocelots live among the Madrean Sky Islands and are found as far north as Arizona. Mountain lions and bobcats are found in the region.
Several species of hummingbirds, flycatchers, woodpeckers, and warblers make their homes in the sky islands. The white-tailed deer and black bears call some of the sky islands home.
The Sonoran and Chihuahuan deserts receive less than 10 inches of rain per annum. Within the desert regions, the Madrean Sky Islands stand apart from the lower, drier, hotter environs. These mountains and mountain ranges are essentially islands rising into the sky. Within the sky islands themselves, vegetation varies based on elevation.
Among the lower elevations of the sky islands, grasslands and scrub are plentiful. At higher the elevations, the more cone-bearing trees (firs, pines, etc) are far more plentiful. There is also a high percentage of endemism within the sky islands. About three-quarters of the species that live in the sky islands are not found anywhere else in the world. The unique environments of these sky islands (in contrast to the surrounding desert) provide optimal environments for many plants and animals.
Sky islands are like islands of unique ecosystems surrounded by contrasting environments. Isolation is what shapes them. The Madrean Sky Islands serve flora, fauna, and people alike.
Watch: Islands in the Sky
References and Further Resources
McLaughlin, S. P. (1995). 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, 60-70. https://www.fs.fed.us/rm/pubs_rm/rm_gtr264/rm_gtr264_060_070.pdf
Sky islands. (2018, December 7). NPS.gov (U.S. National Park Service). https://www.nps.gov/chir/learn/nature/sky-islands.htm
The sky islands. (2020, July 13). Sky Island Alliance. https://skyislandalliance.org/the-sky-islands/
Warshall, P. (1995). The Madrean sky island archipelago: a planetary overview (Vol. 1995, pp. 6-18). Fort Collins, CO: US Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station.
