When I first heard the term sky islands it conjured up images of a cloud city in a galaxy far, far away. However, the real meaning has more of a terrestrial basis on Earth. In their essence, sky islands are mountainous environments that are extremely different than the surrounding low-lying habitat; think a forested mountaintop surrounded by a desert. The term was first popularized by nature writer Weldon Heald in his 1967 book entitled “Sky Island” and appeared in the scientific literature from the same decade in work by ecology heavy weights such as Robert MacArthur and E.O. Wilson’s theory of island biogeography and Robert Whittaker’s work on gradient analysis.
I first became acquainted with sky islands when I moved to western Texas for graduate school at Texas Tech University. Our lab did a lot of work in the Guadalupe, Chisos, and Davis Mountains, all of which are sky islands. I had the distinct pleasure of helping my lab mates with their projects in these awe-inspiring landscapes. After days of hiking, camping, and star watching it was impossible not to be enamored by these places or as one southwest writer Natt N. Dodge aptly wrote “mountain island in a desert sea”.
The southwest has many examples of sky islands, especially in states like Arizona and New Mexico. In fact, this is where the term and concept was first born. So what does a sky island look like ecologically? The image below is a nice schematic of how the changes in elevation on mountains can be layered very much like the changes you would see in habitats on a latitudinal basis. The community layers are very similar with many of the same species inhabiting those communities. Much of this has to do with the latitude of the mountain itself as well as the changes of temperature and precipitation due to the elevation and aspect of the mountain.
So as an ecologist why do we care about sky islands? It is not only the fact that you have topographic differences in the terrain but that these mountains or mountain ranges are isolated from each other by basins or valleys of desert or grasslands. The isolation and differences in topography act as barriers to the movement of species. In many cases you can find species that you cannot find anywhere else, known as endemic species. This idea of these ecologically unique areas is steeped in the ecological theory of island biogeography. The theory of island biogeography is at its base a balance between the rates of colonization by new species to the island versus the rate at which populations of established species on the island become extinct (helpful images below). This is highly influenced by the distance between islands and the size of an island or in another words how far a species has to go to get there (migration) and how much room there is for a species to establish (niche). In many cases sky islands represent areas with higher biodiversity than the surrounding community and are often seen as refugia for species that might have been lost due to climatic changes like those that occurred after the end of an ice age.
Examples of isolated sky islands.
Examples of the theory of island biogeography.
Ironically, the difference in temperature that once made these places safe havens for certain species may also be the force that undoes this sensitive ecosystem. With the projected changes in temperature and precipitation expected with climate change these starkly beautiful, species rich environments may quickly disappear. Just as species may be lost to latitudinal range shifts associated with warming, the same is true with these mountain habitats. The thing that makes them unique, being an isolated spot of refugia, is also the same thing that will cause displacement of many species, as the physical barrier for species to move to suitable habitats may be too great to overcome or simply there may be no suitable habitat to move to. Over the last 50 years many ecologists and concerned citizens have lent their talents to explore, protect, and understand sky islands. There is still a lot of research that needs to be done and questions that have yet to be explored in these lonely mountains. Unfortunately we may lose the opportunity to enjoy and explore our mountain islands with the looming presence of climate change threating this delicately balanced ecosystem.