Sky islands are a natural oddity | ECOVIEWS

If asked, most people would define an island as a terrestrial habitat surrounded by water. Some might add that the plant communities and animal inhabitants of water and land are ecologically distinct from each other. Fair enough. But not all islands are surrounded by water. Some, called sky islands, are surrounded by land.

Last summer, I drove with my son and grandson through deserts around Tucson. We passed creosote bushes and saguaro cactuses on our way to a different world.

In two hours, we had left the triple-digit temperatures behind and found ourselves alongside a mountain stream bordered by oak trees and lush grasses. We had ascended from the desert to a sky island.

The Sky Islands of the Coronado National Forest in southern Arizona and New Mexico are quite distinct from the encircling deserts. The cooler air was a blessing. Trying to catch our breath at nearly 7,000 feet was not.

Daily temperatures and the amount of water lost to evaporation are two major environmental differences between montane and desert habitats, resulting in greater tree canopy, shadier conditions and more luxuriant ground cover on the mountain.

In “Amphibians of the Sky Islands” (ECO Natural History Series, 2023) Gordon W. Schuett, Charles F. Smith and William Wells address the biodiversity and dramatically different habitats of sky islands.

Interspersed with photographs of frogs, toads and the spectacular western tiger salamander are pictures of lush mountain canyons and aspen forests with an understory of bracken ferns. The habitat contrast between these cool, wet mountain ecosystems and the hot, dry desert surrounding them cannot be overstated.

Sky islands are a worldwide ecological phenomenon. In the broadest sense, sky islands occur in South America (the Andes), Africa (Kilimanjaro) and Asia (the Western Ghats of India). The common denominator is high elevation habitats (islands) separated from others in the region by desert or grasslands.

Another common feature is that the biological composition of each sky island differs not only from the surrounding “sea of flat land” but also from its nearest mountainous neighbors in all directions. As is true for oceanic islands, sky islands differ from each other in size, ecology and bio-personality.

The term "sky island" was first used in the mid-20th century in reference to the Madrean Sky Islands along the U.S. and Mexican borders.

Some species may be found on all or most sky islands in a region, although they are isolated from one another by uninhabitable terrestrial habitat, much like species on islands separated by water.

More: Wind power comes with environmental costs, hazards | ECOVIEWS

The Chiricahua leopard frog, which the authors describe as a “handsome frog,” was found historically to inhabit several sky islands in Arizona and New Mexico. It is now protected by the U.S. Endangered Species Act. Like plant and animal populations anywhere, those isolated for eons may evolve into new species or subspecies. For example, the Mount Graham red squirrel in Arizona is distinctive enough from other red squirrels, which are found throughout most of the western states and Canada, to warrant designation as a subspecies. It also is protected by the Endangered Species Act.

Legislative protection is vital for many wildlife species today. Unfortunately, several of the sky island species are declining due to habitat disruption and the introduction of nonnative predators, including game fish. The disappearance of sky islands is especially disquieting because some species are found on only a few mountains or even a single one and have little chance of repopulating on another.

Anyone who has an opportunity to visit a sky island will appreciate the remarkable change from arid desert to montane habitat. Temperatures drop as altitude increases; flora and fauna change dramatically. Imagine moving with astonishing speed up the East Coast from a southern clime to a northern one.

I recommend that outdoor enthusiasts who visit sky-island country in southern Arizona and New Mexico have a copy of "Amphibians of the Sky Islands" in their backpack.

Whit Gibbons is professor of zoology and senior biologist at the University of Georgia’s Savannah River Ecology Laboratory. If you have an environmental question or comment, email ecoviews@gmail.com.

This article originally appeared on The Tuscaloosa News: Sky islands are a natural oddity | ECOVIEWS