sky island

stefan mostert

11 may 2025

800 words

8 minutes

From Saguaro to Snow in Arizona’s Sky Islands

We always imagine a holiday as something that should happen far away. We want to cross the ocean to a place with different trees, different people, different things.  All that wonderful change and transformation that pulls our attention back to the world around us, before everything becomes too much the same. Yet the world is very big, so when a large part of it can be experienced in a single journey, that becomes a very special place.

From my home in Tucson, Arizona, in the heart of the Sonoran Desert, there is just such a place. Locally, it’s called a Sky Island, and this particular one is part of the Santa Catalina Mountain Range. From here, I can experience almost the entire North American landscape in a single journey.

Sky Island describes these mountains perfectly. They are massive islands scattered across the desert floor. Each island is an isolated habitat with its own microclimate, separated from the next by kilometers of cacti and a roaring sun. Therefore, many species occur only on one Sky Island, because the next suitable habitat for that particular species, lies beyond a sea of desert.

Therefore, to travel through the North American landscape, I could change my latitude and go there physically, or I could simply change my altitude and stay right here in Arizona. Scientists call this the Altitude–Latitude Equivalence—or the Biogeographical Rule of Thumb.

It says that for every 1,000 feet you climb, you’ll see the same transformation in the landscape as travelling 300 miles north. In metric terms, that is roughly 300 meters up for every 500 kilometers north.

I visit these mountains often—especially on my bicycle. On an early Saturday morning, I’ll leave the city and pedal slowly through Tucson’s extensive cycle network, which includes a 200-kilometre-long bike path encircling the entire city. I’ll begin my ride with a broad sweep around the southeast toward the summit of Mount Lemmon, more than 9,000 feet in the air. With me I have a few things to eat, plenty of water and an audiobook.

The road that takes me to the top is the Catalina Highway—everything that already makes America great, in one: a wide and smooth ribbon of asphalt stretching from the desert floor up into the forest. Thirty miles that lift you almost seven thousand feet—the equivalent of travelling from Tucson to Banff, Canada. Scattered on both sides of the road lie seven campgrounds, several picnic spots, and many hiking trails: The Great American Outdoors.

The campgrounds are spread across the distinct habitats created by the change in elevation. Through the noisy city appears the Lower Sonoran Desert. Then follows the Upper Sonoran Desert Shrublands, the Desert Grasslands, the Chaparral and Oak Woodlands, and finally—past the Pinyon–Junipers and Ponderosa Pines—the Mixed Conifer Forest. Just like in Canada, complete with winter snow and a few roaming bears.

To witness this natural transformation firsthand, the best way is by bicycle. That way, I can personally greet the last saguaro cactus and appreciate every habitat in detail. I can also stop wherever I want to, taking in the views while my body catches its breath.

To round off the Catalina Highway, there is a little wooden house at the very top that serves an enormous, freshly baked cookie and a hot cup of coffee (you’ll feel you’ve earned it). Outside, the wind whistles through the conifer needles, and the entire experience feels like a world away from what’s happening down in the desert.

But the best reason to take on such a trip by bike is the long downhill you get at the end of the day—all 30 miles of it! Then you can experience the transformation again—this time in reverse, and a little bit faster: the green trees turning into brown trees, then shrubs, then grasslands, and later the first saguaro cactus waving its arms in the air. Then it’s back to the city, another two hours of backstreets, and finally home.

Arizona’s Sky Islands offer ideal conditions to experience this natural phenomenon, but it’s not the only place. It can be observed all over the world—also in the Southern Hemisphere, where climbing 1,000 feet provides the same transformation as travelling 300 miles south. Thus, in certain aspects, we can compare the climate of South Africa’s Drakensberg at a particular elevation, to that of Cape Town.

What binds it all together is the incredible biodiversity our planet offers—not only when we set off on grand journeys, but sometimes right in our own backyard.