When you hear the word “island,” you probably think of a body of land surrounded by water. However, some islands can be surrounded by land. Mountains that are isolated by valleys are called “sky islands.”

There are approximately 20 sky island complexes in the world. One example is the Madrean Archipelago, also called the Southwestern Sky Islands, located in southern Arizona and New Mexico in the United States and in northwestern Sonora and Chihuahua in Mexico. An archipelago is a group of islands. According to the U.S. Geological Survey (USGS), the Madrean Archipelago is one of the most biologically diverse areas in the world.

Spanning 181,300 square kilometers (70,000 square miles), the Madrean Archipelago includes approximately 25 islands/mountains in the U.S. and about 15 in Mexico. Here, mountains with pine-oak woodlands are surrounded by “seas” of desert scrub and grasslands.

“Since the last glaciation, these forested mountain ranges have become relatively isolated from each other,” scientist Peter Warshall says in an article for the USGS. The expanding scrub and grasslands have “limited genetic interchange between populations.”

This bioregion is unique on the planet. It is the only group of sky islands that extends between two climactic zones: temperate to the north and subtropical to the south. It sits at the crossroad of two mountain ranges (the Rocky Mountains and the western Sierra Madre) and two deserts (Sonoran and Chihuahuan). Although the climate is arid, rivers, streams, and ponds help sustain life. The area’s topography, variations in elevation, and unique location help account for its biological richness.

The Sky Island Alliance (SIA) is a grass-roots organization dedicated to the protection and restoration of native species and habitats in the Southwestern Sky Islands. More than 3,000 species of plants grow in this region, according to the SIA. Many of these are rare, such as the lemon lily and the amsonia. Some, such as certain species of fleabane, are found nowhere else on Earth.

Animals in the Archipelago

Like flora (plant life), fauna (animal life) is abundant in the area. SIA uses field data reports, wildlife tracking, photographs, and video to assess the presence of animals and to monitor activity in various areas. The organization states that the region harbors well over half the bird species of North America, as well as many species of fish, amphibians, reptiles, and insects. In addition, there are 104 species of mammals—the greatest diversity of animal species in North America, according to the U.S. Fish & Wildlife Service.

For some of the animals, the region is their permanent home. For others, it is in a migratory route or part of their hunting grounds. The animals regularly move across the Sonoran and Chihuahuan deserts between the United States and Mexico.

“The borders between states and countries are political. They do not mean anything to the animals or plants that live in or travel through the region,” says Sergio Avila, a wildlife biologist with SIA.

Many of the animals in the Madrean Archipelago are classified as threatened or endangered by the United States, Arizona, or New Mexico governments. Species that are threatened are vulnerable to extinction in the near future and include Chiricahua leopard frogs, gray hawks, and grizzly bears. Endangered species are in danger of becoming extinct and include Mexican spotted owls, thick-billed parrots, ocelots, and jaguars.

Biodiversity Hotspot

The biodiversity of plants and animals in the Madrean Archipelago is among the most threatened in North America. In fact, the region is one of 34 around the globe that is recognized as a “biodiversity hotspot” by the nonprofit organization Conservation International.

A biodiversity hotspot is an area that contains at least 1,500 species of native, vascular plants—about 0.5 percent of the species on the planet—and has lost at least 70 percent of its original habitat. The designation is meant to highlight a region as a priority for conservation efforts.

Claude Gascon of Conservation International says the organization’s work “deals with key issues ranging from protecting our supplies of food and fresh water to preventing the spread of diseases and reducing climate change. At the heart of all of these is biodiversity. It is what sustains us all.”

Scientists estimate that about 86 percent of the Madrean Archipelago’s vegetation has already disappeared. While climate change is a contributing factor, most of the destruction has been directly caused by humans. This includes land development, extraction of resources, poor livestock grazing practices, fire suppression, and off-road vehicle use.

The presence of non-native plants and animals can also impact the region. For SIA, conservation and restoration efforts include the removal of some non-native species. “If the presence of an exotic, or non-native, plant or animal species has negative effects on the native species, we will remove it from the area,” says wildlife biologist Avila.

About 80 percent of the land on the U.S. side is publicly held, either by the federal government (such as the U.S. Forest Service) or the Arizona or New Mexico state governments. In Mexico, most of the land is privately owned by ranchers.

SIA works with volunteers, scientists, land owners, public officials, and government agencies on various protection and restoration projects on both sides of the border. These include identifying at-risk wildlife corridors, securing protections from the government, and restoring riparian (river-related) habitats.

In addition, SIA recently launched the Madrean Archipelago Biodiversity Assessment Project. A multinational scientific research and conservation project (with Mexican, French, and American collaborators), it will establish data on various forms of vegetation and wildlife, and provide community education and outreach activities. SIA expects its visibility and applicability to “reach around the globe.”

Desert Islands
The Bavispe River flows through the so-called Southwestern Sky Islands.

International Year of Biodiversity
The United Nations declared 2010 the International Year of Biodiversity. It is "a celebration of life on Earth and of the value of biodiversity for our lives."

Objectives include:

  • Raising awareness of the importance of conserving biodiversity for human well-being and promoting the understanding of the economic value of biodiversity.
  • Enhancing public knowledge of the threats to biodiversity and the means to conserve it.
  • Encouraging organizations and individuals to participate in biodiversity conservation activities.

person who strongly and actively supports an issue or point of view.


an animal able to live both on land and in water.


a group of closely scattered islands in a large body of water.




area with specific ecological characteristics, including living and nonliving things. A bioregion is larger than an ecosystem.


management of a natural resource to prevent exploitation, destruction, or neglect.


to meet or come together.


area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.


community and interactions of living and nonliving things in an area.


to put at risk.

endemic species

species that naturally occurs in only one area or region.


process of complete disappearance of a species from Earth.


to eliminate or remove from an area.


animals associated with an area or time period.


plants associated with an area or time period.


process of a glacier carving out a landscape.

grass roots

made up of people who are not socially or economically elite and do not represent the government.


environment where an organism lives throughout the year or for shorter periods of time.


large spotted cat native to the Americas.


industry engaged in cutting down trees and moving the wood to sawmills.


animal with hair that gives birth to live offspring. Female mammals produce milk to feed their offspring.


to move from one place or activity to another.

nonprofit organization

business that uses surplus funds to pursue its goals, not to make money.


spotted American wildcat.


large farm on which livestock are raised.


animal that breathes air and usually has scales.


having to do with a river or stream.

sky island

mountainous ecosystem where a single mountain is so isolated from nearby mountains that it is ecologically distinct.


bordering the tropics, just north of the Tropic of Cancer and south of the Tropic of Capricorn.




to scare or be a source of danger.

vascular plant

group of plants which have specific tissues for transporting water and minerals throughout the plant.