Sky Islands Life on the Mountaintops That Float Above the Plains

In southeastern Arizona, you can start a summer morning among saguaro cactus and creosote, in heat that makes the pavement shimmer. By early afternoon you can be standing in a forest of fir and aspen, breathing air cool enough to need a jacket. The trip takes a few hours on a paved road. To get the same change in scenery by traveling north, you would have to drive well into Canada.

Ecologists call the mountains that make this possible sky islands. The definition is simple. A sky island is an isolated mountain surrounded by radically different lowland environments. The sea in this metaphor is dry. Between the peaks lie valleys of desert scrub and grassland, hot and parched, and for most mountain creatures they are as impassable as open ocean is for a land snail.

The term first referred to the mountains of the Mexican Plateau and the borderlands of the American Southwest. More than fifty ranges in Arizona, Sonora, and a small corner of New Mexico qualify, rising above the Sonoran and Chihuahuan deserts. Many of them top ten thousand feet. Their upper slopes catch more rain and stay cooler than the plains, so they hold pine, fir, and oak woodland, along with the animals that depend on them.

The idea is flexible, and some people stretch it a long way. In North Texas, the Medicine Mounds of Hardeman County rise only about three hundred feet above the plains. Even that modest bump has been described as a kind of sky island, a small patch of different habitat lifted slightly above its surroundings. At the other extreme are the table-top mountains of Venezuela and the volcanic highlands of Ethiopia, where the isolation is far more dramatic and has lasted far longer.

This episode is about what happens to life when it gets stranded on a peak.