Sky island

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View of the Santa Rita Mountains across the Tucson valley from the Santa Catalina Mountains-the Santa Ritas are among the most prominent of the "sky islands" in southern Arizona. SkyIslands from SantaCatalinaMtns.JPG
View of the Santa Rita Mountains across the Tucson valley from the Santa Catalina Mountains-the Santa Ritas are among the most prominent of the "sky islands" in southern Arizona.
2,700 m (9,000 ft) Chiricahua Mountains above the desert Chiricahua mtns-kmf.JPG
2,700 m (9,000 ft) Chiricahua Mountains above the desert
The 2,400 m (8,000 ft) Portal Peak in the Chiricahua Mountains surrounded by clouds Skyislandschiricahuamountains.JPG
The 2,400 m (8,000 ft) Portal Peak in the Chiricahua Mountains surrounded by clouds

Sky islands are isolated mountains surrounded by radically different lowland environments. The term originally referred to those found near the southern borders of Arizona and New Mexico, and has extended to similarly isolated high-altitude forests. The isolation has significant implications for these natural habitats. The American Southwest region began warming up between ∼20,000–10,000 years BP and atmospheric temperatures increased substantially, resulting in the formation of vast deserts that isolated the Sky Islands. [1] Endemism, altitudinal migration, and relict populations are some of the natural phenomena to be found on sky islands.

Endemism Ecological state of being unique to a defined geographic location or habitat

Endemism is the ecological state of a species being unique to a defined geographic location, such as an island, nation, country or other defined zone, or habitat type; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. The extreme opposite of endemism is cosmopolitan distribution. An alternative term for a species that is endemic is precinctive, which applies to species that are restricted to a defined geographical area.

Altitudinal migration

Altitudinal migration is a short-distance animal migration from lower altitudes to higher altitudes and back. It is commonly thought to happen in response to climate and food availability changes as well as increasingly due to anthropogenic influence. These migrations can occur both during reproductive and non-reproductive seasons. Altitudinal avian migration is common, and can also be found in other vertebrates, and can be seen in some invertebrates.

A relict is a surviving remnant of a natural phenomenon.


The complex dynamics of species richness on sky islands draws attention from the discipline of biogeography, and likewise the biodiversity is of concern to conservation biology. One of the key elements of a sky island is separation by physical distance from the other mountain ranges, resulting in a habitat island, such as a forest surrounded by desert.

Species richness is the number of different species represented in an ecological community, landscape or region. Species richness is simply a count of species, and it does not take into account the abundances of the species or their relative abundance distributions. Species diversity takes into account both species richness and species evenness.

Biogeography The study of the distribution of species and ecosystems in geographic space and through geological time

Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevation, isolation and habitat area. Phytogeography is the branch of biogeography that studies the distribution of plants. Zoogeography is the branch that studies distribution of animals.

Biodiversity Variety and variability of life forms

Biodiversity refers to the variety and variability of life on Earth. Biodiversity is typically a measure of variation at the genetic, species, and ecosystem level. Terrestrial biodiversity is usually greater near the equator, which is the result of the warm climate and high primary productivity. Biodiversity is not distributed evenly on Earth, and is richest in the tropics. These tropical forest ecosystems cover less than 10 percent of earth's surface, and contain about 90 percent of the world's species. Marine biodiversity is usually highest along coasts in the Western Pacific, where sea surface temperature is highest, and in the mid-latitudinal band in all oceans. There are latitudinal gradients in species diversity. Biodiversity generally tends to cluster in hotspots, and has been increasing through time, but will be likely to slow in the future.

Some sky islands serve as refugia for boreal species stranded by warming climates since the last glacial period. In other cases, localized populations of plants and animals tend towards speciation, similar to oceanic islands such as the Galápagos Islands of Ecuador.

Refugium (population biology)

In biology, a refugium is a location which supports an isolated or relict population of a once more widespread species. This isolation (allopatry) can be due to climatic changes, geography, or human activities such as deforestation and overhunting.

Speciation The evolutionary process by which populations evolve to become distinct species

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species. He also identified sexual selection as a likely mechanism, but found it problematic.

Galápagos Islands archipelago in the Pacific Ocean

The Galápagos Islands, part of the Republic of Ecuador, are an archipelago of volcanic islands distributed on either side of the equator in the Pacific Ocean surrounding the centre of the Western Hemisphere, 906 km (563 mi) west of continental Ecuador. The islands are known for their large number of endemic species and were studied by Charles Darwin during the second voyage of HMS Beagle. His observations and collections contributed to the inception of Darwin's theory of evolution by means of natural selection.

Origin of the term

The sky island concept originated in 1943 when Natt N. Dodge, in an article in Arizona Highways magazine, referred to the Chiricahua Mountains in southeastern Arizona as a "mountain island in a desert sea". [2]

Chiricahua Mountains mountain range in Arizona, United States

The Chiricahua Mountains massif is a large mountain range in southeastern Arizona which is part of the Basin and Range province of the west and southwest USA and northwest Mexico; the range is part of the Coronado National Forest. The highest point, Chiricahua Peak, rises 9,759 feet (2,975 m) above sea level, approximately 6,000 feet (1,800 m) above the surrounding valleys.

In about the same era, the term was used to refer to high alpine, unglaciated, ancient topographic landform surfaces on the crest of the Sierra Nevada, California. [3]

The term was popularized by nature writer Weldon Heald, a resident of southeastern Arizona. 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 kilometres (35 miles) away and 1,700 metres (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. His book mentions the concept of biome, but prefers the terminology of life zones, and makes reference to the work of Clinton Hart Merriam. The book also describes the wildlife and living conditions of the Chiricahuas. [4]

Rodeo, New Mexico Census-designated place in New Mexico, United States

Rodeo is an unincorporated community and census-designated place (CDP) in Hidalgo County in the southwestern corner of New Mexico at 31°50′13″N109°01′54″W. It lies less than one mile from the border with Arizona on New Mexico State Highway 80. As of the 2010 census, the population of Rodeo was 101.

Biome Distinct biological communities that have formed in response to a shared physical climate

A biome is a community of plants and animals that have common characteristics for the environment they exist in. They can be found over a range of continents. Biomes are distinct biological communities that have formed in response to a shared physical climate. Biome is a broader term than habitat; any biome can comprise a variety of habitats.

The life zone concept was developed by C. Hart Merriam in 1889 as a means of describing areas with similar plant and animal communities. Merriam observed that the changes in these communities with an increase in latitude at a constant elevation are similar to the changes seen with an increase in elevation at a constant latitude.

Around the same time, the idea of mountains as islands of habitat took hold with scientists and has been used by such popular writers as David Quammen [5] and John McPhee. [6] This concept falls within the study of island biogeography. It is not limited to mountains in southwestern North America but can be applied to mountains, highlands, and massifs around the world. [7]


The Madrean sky islands are probably the most studied sky islands in the world. Found in the U.S. states of New Mexico and Arizona and the Mexican states of Chihuahua and Sonora, these numerous mountains form links in a chain connecting the northern end of the Sierra Madre Occidental and the southern Colorado Plateau. Sky Islands of the central and northern mountains in the United States are often called island ranges, especially by populations within view of such islands of mountains surrounded by plains such as those found within the Wichita Mountains of southwestern Oklahoma.

View from above 2,100 m (7,000 ft) in the Santa Catalina Mountains, showing pines and snow in the foreground and desert beyond Catalina Mountains elev7000.JPG
View from above 2,100 m (7,000 ft) in the Santa Catalina Mountains, showing pines and snow in the foreground and desert beyond

Some more northerly examples are the Crazy Mountains, Castle Mountains, Bears Paw Mountains, Highwood Mountains, and Little Rocky Mountains, all in the US state of Montana. Each of these ranges is forested and has tundra and snowpack above treeline, but is not connected to any other range by forested ridges; the ranges are completely surrounded by treeless prairie and/or semi-arid scrubland below. Other well-known sky islands of North America are the Great Basin montane forests, such as the White Mountains in California, and the Spring Mountains near Las Vegas, Nevada. One of the unique aspects of the sky islands of the U.S.-Mexico border region is the mix of floristic affinities, that is, the trees and plants of higher elevations are more characteristic of northern latitudes, while the flora of the lower elevations has ties to the desert and the mountains further south. [8] Some unique plants and animals are found in these sky islands, such as the mountain yucca, Mount Graham red squirrel, Huachuca springsnail, and Jemez Mountains salamander.

Some montane species apparently evolved within their current range, adapting to their local environment, such as the Mount Lyell shrew. [9] However, it has also been noted that some isolated mountain ecosystems have a tendency to lose species over time, perhaps because small, insularized populations are vulnerable to the forces of extinction, and the isolation of the habitat reduces the possibility of colonization by new species. [5] Furthermore, some species, such as the grizzly bear, require a range of habitats. These bears historically made use of the forests and meadows found in the Madrean sky islands, as well as lower-elevation habitats such as riparian zones. (Grizzlies were extirpated from the region in the 20th century.) [10] Seasonal movements between highland and lowland habitats can be a kind of migration, such as that undertaken by the mountain quail of the Great Basin mountains. These birds live in high elevations when free of snow, and instead of migrating south for the winter, they migrate down. [11]

Confusing the matter somewhat is the potential for an archipelago of sky islands or even the valleys between them to act not only as a barrier to biological dispersal, but also as a path for migration. Examples of birds and mammals making use of the Madrean archipelago to extend their ranges northward are the elegant trogon and white-nosed coati. [12]

List by terrestrial ecozone






Palearctic ecozone

See also

Related Research Articles

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Mount Graham mountain

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Madrean pine-oak woodlands

The Madrean pine-oak woodlands is an ecoregion of the Tropical and subtropical coniferous forests biome, located in North America. They are subtropical woodlands found in the mountains of Mexico and the southwestern United States.

Madrean Sky Islands

The Madrean Sky Islands are enclaves of Madrean pine-oak woodlands, found at higher elevations in a complex of small mountain ranges in southern and southeastern Arizona, southwestern New Mexico, and northwestern Mexico. The sky islands are surrounded at lower elevations by the Sonoran and Chihuahuan deserts. The northern west–east perimeter of the sky island region merges into the higher elevation eastern Mogollon Rim and the White Mountains of eastern Arizona.

Sierra Madre Occidental pine-oak forests subtropical coniferous forest ecoregion of the Sierra Madre Occidental range

The Sierra Madre Occidental pine-oak forests are a subtropical coniferous forest ecoregion of the Sierra Madre Occidental range from the southwest USA region to the western part of Mexico. They are home to a large number of endemic plants and important habitat for wildlife.

Dragoon Mountains mountain range in Arizona, USA

The Dragoon Mountains are a range of mountains located in Cochise County, Arizona. The range is about 25 mi (40 km) long, running on an axis extending south-south east through Willcox.

Afromontane concept

The Afromontane regions are subregions of the Afrotropical realm, one of the Earth's eight biogeographic realms, covering the plant and animal species found in the mountains of Africa and the southern Arabian Peninsula. The Afromontane regions of Africa are discontinuous, separated from each other by lower-lying areas, and are sometimes referred to as the Afromontane archipelago, as their distribution is analogous to a series of sky islands.

Mexican fox squirrel species of mammal

The Mexican fox squirrel(Sciurus nayaritensis) is a species of tree squirrel found throughout the Sierra Madre Occidental of Mexico as far south as Jalisco — and northward into the Chiricahua Mountains of southeastern Arizona, U.S.

Madrean Region

The Madrean Region is a floristic region within the Holarctic Kingdom in North America, as delineated by Armen Takhtajan and Robert F. Thorne. It occupies arid or semiarid areas in the southwestern United States and northwestern Mexico and is bordered by the Rocky Mountain Region and North American Atlantic Region of the Holarctic Kingdom in the north and in the east, Caribbean Region of the Neotropical Kingdom in the south.

Peloncillo Mountains (Hidalgo County)

The Peloncillo Mountains of Hidalgo County,, is a major 35-mile (56 km) long mountain range southwest of New Mexico's Hidalgo County, and also part of the New Mexico Bootheel region. The range continues to the northwest into Arizona as the Peloncillo Mountains of Cochise County, Arizona. The extreme southwest corner of the range also lies in Arizona. It is a linear range bordering the linear San Bernardino Valley of southeast Cochise County, Arizona.

Sierra del Tigre is a mountain range in northeastern Sonora, Mexico at the northern region of the Sierra Madre Occidental. The region contains sky island mountain ranges, called the Madrean Sky Islands, some separated from the Sierra Madre Occidental proper, and occurring in the northeastern Sonoran Desert, and extreme west-northwestern Chihuahuan Desert. Many of the ranges occur in southeast Arizona.

Patagonia Mountains

The Patagonia Mountains are a 15-mile-long (24 km) mountain range within the Coronado National Forest, and in Santa Cruz County, Arizona.

The Sierra San Antonio is a mountain range in southernmost Arizona state (U.S.) and northern Sonora state (México).

<i>Yucca</i> × <i>schottii</i> species of plant

Yucca × schottii is a plant species in the genus Yucca, native to southern Arizona, southwestern New Mexico, and the northern parts of Sonora and Chihuahua. The common names are Schott's yucca, hoary yucca, and mountain yucca. The "×" in the name indicates that this is a nothospecies, regarded as being a natural hybrid between two other species. In this case, Yucca × schottii is believed to have originated as a hybrid between Y. baccata and Y. madrensis. Yucca × schottii is firmly established and does reproduce freely in the wild.

Sulphur Springs Valley valley in the eastern half of Cochise County, Arizona

The Sulphur Springs Valley is a valley in the eastern half of Cochise County, Arizona. The valley covers an approximated vertical rectangle west of the Chiricahua Mountains–Dos Cabezas Mountains complex. The Sulphur Springs Valley is the large flatland to the west.

Pajarito Mountains, Arizona

The Pajarito Mountains are a small mountain range of western Santa Cruz County, Arizona. The range is adjacent the Atascosa Mountains at its north, with both ranges in the center of a north-south sequence of ranges called the Tumacacori Highlands. The Highlands have the Tumacacori Mountains at the north, and south of the U.S.-Mexico border, the Sierra La Esmeralda range,. The Tumacacori Highlands are part of a regional conservancy study of 'travel corridors' for cats, called Cuatro Gatos, Four Cats, for mountain lions, ocelot, bobcat, and jaguar.

Montane ecosystems ecosystems found in mountains

Montane ecosystems refers to any ecosystem found in mountains. These ecosystems are strongly affected by climate, which gets colder as elevation increases. They are stratified according to elevation. Dense forests are common at moderate elevations. However, as the elevation increases, the climate becomes harsher, and the plant community transitions to grasslands or tundra.


  1. Favé, M., Johnson (2015). "Past climate change on Sky Islands drives novelty in a core developmental gene network and its phenotype". BMC Evolutionary Biology. 15: 183. doi:10.1186/s12862-015-0448-4. PMC   4560157 . PMID   26338531.
  2. Dodge, Natt (March 1943). "Monument in the Mountain". Arizona Highways. Phoenix, Arizona: Arizona Highway Department. 19 (3): 20–28.
  3. Howell, John Thomas (May 1947). "Mono Mesa - Sierra Sky Island". Sierra Club Bulletin. San Francisco, California: Sierra Club. 32 (5): 15–18.
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  8. McLaughlin, Steven P. (19 September 1994), An Overview of the Flora of the Sky Islands, Southeastern Arizona: Diversity, Affinities, and Insularity, Biodiversity and Management of the Madrean Archipelago .The Sky Islands of Southwestern United States and Northwestern Mexico, Fort Collins, Colorado: United States Forest Service
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