SKY ISLANDS: A Recipe for Biodiversity

Capstone Project written by PCMN Carrie Barcom

I came across a brochure in the Sabino Canyon Gift shop titled, “Map of Sky Island Scenic Byway”. I know a lot has changed in the 16 years I’ve been away from Tucson, but when did we get Sky Islands? And more importantly, what are they?

Sky islands are a type of continental or inland terrain made up of a sequence of alternating valleys and mountain ranges. Isolated mountains are separated from one another by physical distance resulting in a mountain “island” surrounded by “oceans” of desert or grassland. The slopes and summit have a dramatically different ecosystem from the base and distinctly different biospheres occur all the way up the mountain with various types of life existing at each level. Sky islands have a stack of biotic communities that allow seasonal vertical migration between highland and lowland habitats but the valleys between them act as a barrier preventing species from crossing from one mountain range to another. There are about twenty groups of sky islands on the planet, all continents with the exception of Australia harbor sky island complexes.

One of the most renowned chains of sky islands can be found in an exspansive cluster in Arizona, New Mexico and northern Mexico, known collectively as the “Sky Islands”, “Madrean Sky Islands” or “Madrean Archipelago”. The descriptions of this area read like a giant cosmic recipe for biodiversity: Begin by connecting the massive continental back- bones of the Rocky Mountains with the Sierra Madre Occidental. Blend in temperate, sub-tropical, and tropical latitudes. Add a dash of elevation, sometimes up to a 6,000 ft. gradient from valley to mountain peak. Flank these peaks with the Chihuahuan and Sonoran deserts in an east-west overlap. Finally, mix it all together with a bi-seasonal rainfall pattern, with frontal winter precipitation and convective summer thunder storms arriving from different regions of the Pacific Ocean and providing life-sustaining rainfall. This unique and intricate blend of topography, location, and weather create an explosion of life found nowhere else on the planet.

This concept originated in 1943 when Natt N. Dodge referred to the Chiricahua Mountains as “a mountain island in a desert sea” in an Arizona Highways article. Nature writer Weldon Heald, who lived in the Chiricahua Mountains of southeastern Arizona, popularized the term. In his 1967 book, Sky Island, he demonstrated the concept by describing a drive from the town of Rodeo, New Mexico, in the western Chihuahuan desert, to a peak in the Chiricahua Mountains, 56 kilometers (35 miles) away and 1,700 meters (5,600 feet) higher in elevation. Ascending from the hot, arid desert, to grasslands, then to oak- pine woodland, pine forest, and finally to spruce-fir-aspen forest.

This region spans four states and two countries, covering a patchwork of protected and unprotected public and private lands. The majority of the Sky Island mountain ranges in the U.S. are part of the Coronado National Forest. In Mexico, the mountains and low desert valleys are a mixture of private ranches, ejitidos (communal farm plots) and Reserva Forestal National. The Sky Island region has approximately 65 isolated mountain ranges covering roughly 70,000 square miles overall. The U.S. Mexico borders splits this region nearly in half with 20,000 square miles in the United States.


Valley floors within the Sky Island region vary between 2,500 and 4,500 feet while the isolated mountain peaks rise 6,000 to 11,000 feet with temperatures dropping about 4 degrees Fahrenheit every 1000 feet.

Eight distinct biotic communities are recognized on the highest Sky Islands of Arizona: Desert Scrub, Desert Grassland, Oak-Grassland, Oak-Woodland, Chaparral, Pine-Oak Woodland, Pine Forest and Mixed Conifer Forest. These 8 communities encompass five of the world’s great biomes: Desert, Grassland, Mediterranean Woodland and Shrub land, Temperate Broad-leaf Forest, and Coniferous Forest. Not every Sky Island is high enough to sustain them all; many are lacking pine and mixed coniferous forest.

Moist air moves up from the Gulf of California in the summer and the North Pacific in the winter. When these air masses are forced up and over the mountains, water vapor con-denses, forming clouds and rain. The mountains pull the water from the sky and bring it back to the landscape where it creates washes that recharge the aquifers and supply riparian areas, bringing life to plants, animals and people as it moves to the valleys below. One of the unique aspects of this region is the mix of floristic affinities, the trees and plants of higher elevations are more characteristic of northern latitudes while the flora of the lower elevation has ties to the desert and mountains further south. It is the only group of sky islands on the planet straddling two major floristic and two faunal realms as well as three major climatic zones: Tropical, Subtropical, and Temperate.


The Madrean Sky Islands contain some of the most rugged and remote lands in the southwest and feature some of the highest levels of biodiversity in the world. The Sonoran portion of the Sky Islands is not as well known biologically speaking but the Arizona portion is amazingly diverse hosting species that draw from all directions, elevations and latitudes. Although some species show little genetic variation between mountain ranges, other species have evolved on these mountain islands. Like a bunch of separated terrariums, they serve as natural laboratories for the study of evolution because conditions differ from island to island.

More than half the bird species in North America are found here as well as, 29 bat species, over 3,500 species of plant, 150,000 invertebrate species and 104 species of mammals, the highest concentration of mammals in the United States.

Plants and animals of every shape, size, and specialty reside here. From familiar favorites like the giant Saguaro Cactus towering 60 feet above the earth “holding it’s breath” all day to conserve water, to the many species of tiny Talus Snail, found deep intalus crevices and leaf litter, only surfacing after it rains. Mexican Gray Wolf, Thick Billed Parrot, New Mexican Ridge Nosed Rattlesnake and Apache Trout are a few of the endangered species that live here and in recent years the Jaguar and Ocelot have returned. There are many other rare creatures found here, with names that capture our imagination like Tropical Soldier Fly, Santa Catalina Geometer Moth, Sonoran Tiger Salamander, and Elegant Trogon just to name a few. Several new species discoveries have been made such as the colorful Fire -Tailed Ground Snake, the Black Tailed Ringtail Dragonfly and a yet unnamed species of Bog Orchid. Bee, reptile, and ant diversity is unparalleled here, speaking volumes about the sub-tropical influences withinthe region.


The bi-national Madrean Archipelago or Sky Island region of the southwestern UnitedStates and northwestern Mexico is recognized for its unique biological diversity, naturalbeauty, and cultural heritage.

Today the most significant threats to the Sky Island Region are climate change, over-grazing, fire suppression, loss of predators, energy development, mining, and borderissues. Groups such as The Sky Island Alliance are working on both sides of the border to protect and restore the biodiversity and natural heritage of the Sky Islands. They work with volunteers, scientists, landowners, public officials, government agencies and over a hundred partner organizations to establish protected areas, restore healthy landscapes, and promote public appreciation of the region’s unique biological diversity. Their Madrean Archipelago Biodiversity Assessment (MABA) project is a groundbreaking effort to extend knowledge of animals and plants in the Sky Islands of Arizona and New Mexico southward into Sonora and Chihuahua, Mexico. The biodiversity in the Madrean Archipelago is recognized globally, but information about species distributions is critical to understanding and protecting biodiversity.

The Biodiversity and Management of the Madrean Archipelago Conference (known as the “Madrean Conference” or “MADCON”) is a regional event for sharing science and building collaboration and is part of a series that still shapes our understanding of the region through presentations given in 1994, 2004 and 2012. The conference is a dynamic opportunity for researchers, managers, and conservation practitioners to learn the latest science and translate it intopractical management approaches.

There is still so much to discover and learn about this region and with the very real threat of climate change there is no time to waste. Just as our magnificent Sky Islands connects mountain ranges, deserts, states, and even countries, successful conservation also requires connections. Not just across the physical and political landscape but most importantly conservation requires connection between people and the place they live. It’s good to know that there are so many working to foster these connections in order to restore and preserve the truly unique biodiversity of this region that is our home.


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The Madrean Sky Islands of the United States and Mexico map by Sky JacobsCatalina Mountains photo by Carrie Barcom

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