Sky island
Encyclopedia
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...
s that are isolated by surrounding lowlands of a dramatically different environment, a situation which, in combination with the altitudinal zonation
Altitudinal zonation
Altitudinal zonation in mountainous regions describes the natural layering of ecosystems that occurs at distinct altitudes due to varying environmental conditions. Temperature, humidity, soil composition, and solar radiation are important factors in determining altitudinal zones, which consequently...
of ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s, has significant implications for natural habitats. Endemism, vertical migration, and relict
Relict
A relict is a surviving remnant of a natural phenomenon.* In biology a relict is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas....
populations are some of the natural phenomena to be found on sky islands. The complex dynamics of species richness
Species richness
Species richness is the number of different species in a given area. It is represented in equation form as S.Species richness is the fundamental unit in which to assess the homogeneity of an environment. Typically, species richness is used in conservation studies to determine the sensitivity of...
on sky islands draws attention from the discipline of biogeography
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
, and likewise the biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
is of concern to conservation biology
Conservation biology
Conservation biology is the scientific study of the nature and status of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction...
. One of the key elements of a sky island is separation by sheer physical distance from the other mountain range
Mountain range
A mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...
s, resulting in a habitat island
Island
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...
, such as a forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
surrounded by desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
. Some sky islands serve as refugia for boreal species stranded by warming climates since the last ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
. In other cases, localized populations of plants and animals tend towards speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
, the same process that happens on oceanic islands such as the Galápagos Islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
.
Origin of the term
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 MexicoRodeo, New Mexico
Rodeo, is an unincorporated town in Hidalgo County in the southwestern corner of New Mexico at . It lies less than one mile from the border with Arizona on New Mexico State Highway 80....
, in the lowland Sonora Desert, to a peak in the Chiricahua Mountains
Chiricahua Mountains
The Chiricahua Mountains are a mountain range in southeastern Arizona which are part of the Basin and Range province of the southwest, and part of the Coronado National Forest...
, 35 miles away and 5600 ft (1,707 m) higher in elevation. Ascending from the hot, arid desert, the environment transitions to grassland, then to oak-pine woodland, pine forest, and finally to spruce-fir-aspen forest. The book mentions the concept of biome
Biome
Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems. Some parts of the earth have more or less the same kind of abiotic and biotic factors spread over a...
, but prefers the terminology of life zone
Life zone
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...
s, and makes reference to the work of Clinton Hart Merriam
Clinton Hart Merriam
Clinton Hart Merriam was an American zoologist, ornithologist, entomologist and ethnographer.Known as "Hart" to his friends, Dr. Clinton Hart Merriam was born in New York City in 1855. His father, Clinton Levi Merriam, was a U.S. congressman. He studied biology and anatomy at Yale University and...
. The book also describes the wildlife and living conditions of the Chiricahuas.
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
David Quammen
David Quammen is a science, nature and travel writer whose work has appeared in publications such as National Geographic, Outside, Harper's, Rolling Stone, and The New York Times Book Review....
and John McPhee
John McPhee
John Angus McPhee is an American Pulitzer Prize-winning writer, widely considered one of the pioneers of creative nonfiction....
. This concept falls within the study of island biogeography
Island biogeography
Island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness of natural communities. The theory was developed to explain species richness of actual islands...
. It is not limited to mountains in southwestern North America but can be applied to mountains, highlands, and massifs around the world.
However, the sky island concept was mentioned much earlier in 1943 by Natt N. Dodge in an article in Arizona Highways when he referred to the Chiricahua Mountains in southeastern Arizona as a ". . . mountain island in a desert sea."
Characteristics
Probably the most studied sky islands are in North America, namely the Madrean sky islandsMadrean 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...
in the U.S. states of New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
and Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
and the Mexican states of Chihuahua and Sonora
Sonora
Sonora officially Estado Libre y Soberano de Sonora is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided into 72 municipalities; the capital city is Hermosillo....
. These numerous mountains form links in a chain connecting the northern end of the Sierra Madre Occidental
Sierra Madre Occidental
The Sierra Madre Occidental is a mountain range in western Mexico.-Setting:The range runs north to south, from just south of the Sonora–Arizona border southeast through eastern Sonora, western Chihuahua, Sinaloa, Durango, Zacatecas, Nayarit, Jalisco, Aguascalientes to Guanajuato, where it joins...
to the southern Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
. Sky Islands of the central and northern Rocky Mountains in the United States are often called island range
Island range
An island range is a term used to describe a mountain range that exists in total or almost total isolation from a larger chain of ranges and sub-ranges. From a distance on the plains, these ranges appear as "islands" of higher ground...
s, especially by populations within view of such islands of mountains surrounded by plains. Some examples are the Crazy Mountains
Crazy Mountains
The Crazy Mountains, often called the Crazies, are a mountain range in the northern Rocky Mountains in the U.S. state of Montana.-Geography:...
, Castle Mountains
Castle Mountains
for the continuation of the range in NevadaThe Castle Mountains are located south and east of the New York Mountains in California and west of Cal-Nev-Ari, Nevada, USA....
, Bears Paw Mountains, Highwood Mountains
Highwood Mountains
The Highwood Mountains cover approximately 4,659 km² in north central Montana in the U.S., east of Great Falls and Benton Lake National Wildlife Refuge, at the northern end of the Lewis and Clark National Forest...
, and Little Rocky Mountains
Little Rocky Mountains
The Little Rocky Mountains, also known as the Little Rockies, are a group of buttes, roughly 765 km2 in area, located towards the southern end of the Fort Belknap Agency in Blaine County and Phillips County in north-central Montana...
, all in the US state of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
. Each of these ranges is a true sky island / island range, in that the range 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
Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...
montane forests, such as the White Mountains
White Mountains (California)
The White Mountains of California are a triangular fault block mountain range facing the Sierra Nevada across the upper Owens Valley. They extend for approximately as a greatly elevated plateau about wide on the south, narrowing to a point at the north, with elevations generally increasing...
, Mt. San Jacinto in California, and the Spring Mountains
Spring Mountains
The Spring Mountains are a mountain range of southern Nevada in the United States, running generally northwest-southeast along the west side of Las Vegas and down to the border with California...
near Las Vegas, Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
. 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. Some unique plants and animals are found in these sky islands, such as the mountain yucca
Yucca schottii
Yucca schottii is an evergreen shrub of the genus Yucca. The common names are Schott's Yucca, Hoary Yucca, and Mountain Yucca....
, Mount Graham Red Squirrel
Mount Graham Red Squirrel
The Mount Graham Red Squirrel is an endangered subspecies of the American Red Squirrel native to the Pinaleño Mountains of Arizona. It is smaller than most other subspecies of Red Squirrel, and also does not have the white-fringed tail that is common to the species...
, Huachuca springsnail
Huachuca springsnail
The Huachuca springsnail scientific name Pyrgulopsis thompsoni, is a species of very small freshwater snail with an operculum, an aquatic gastropod mollusk in the family Hydrobiidae....
, and Jemez Mountains Salamander
Jemez Mountains Salamander
The Jemez Mountains Salamander is a species of salamander in the Plethodontidae family.It is endemic to New Mexico in the United States.Its natural habitat is temperate forests.It is threatened by habitat loss.-Source:...
.
Some montane species apparently evolved within their current range, adapting to their local environment, such as the Mount Lyell Shrew
Mount Lyell Shrew
The Mount Lyell Shrew is a species of mammal in the family Soricidae. It is endemic to a small area in the Sierra Nevada Mountains of California in the United States....
. 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
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
, and the isolation of the habitat reduces the possibility of colonization by new species. Furthermore, some species, such as the Grizzly bear
Grizzly Bear
The grizzly bear , also known as the silvertip bear, the grizzly, or the North American brown bear, is a subspecies of brown bear that generally lives in the uplands of western North America...
, 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 zone
Riparian zone
A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the fifteen terrestrial biomes of the earth. Plant habitats and communities along the river margins and banks are called riparian vegetation, characterized by...
s. (Grizzlies were extirpated from the region in the 20th century.) Seasonal movements between highland and lowland habitats can be a kind of migration, such as that undertaken by the Mountain Quail
Mountain Quail
The Mountain Quail, , is a small ground-dwelling bird in the New World quail family. This species is the only one in the genus Oreortyx, which is sometimes included in Callipepla...
of the Great Basin mountains. These birds live in high elevations when free of snow, and instead of migrating south for the winter, migrate down.
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
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
, 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
Elegant Trogon
The Elegant Trogon, Trogon elegans , is a near passerine bird in the trogon family. It breeds from southeasternmost Arizona in the United States to northwestern Costa Rica. It occasionally is found as a vagrant in southeasternmost and western Texas.It is a resident of the lower levels of semi-arid...
and White-nosed Coati.
Afrotropic ecozone
- Cameroonian Highlands forestsCameroonian Highlands forestsThe Cameroonian Highlands forests are a montane tropical moist broadleaf forest ecoregion located on the range of mountains that runs inland from the Gulf of Guinea and forms the border between Cameroon and Nigeria...
- Ethiopian HighlandsEthiopian HighlandsThe Ethiopian Highlands are a rugged mass of mountains in Ethiopia, Eritrea , and northern Somalia in the Horn of Africa...
- Highlands of southern Africa
- Mount KilimanjaroMount KilimanjaroKilimanjaro, with its three volcanic cones, Kibo, Mawenzi, and Shira, is a dormant volcano in Kilimanjaro National Park, Tanzania and the highest mountain in Africa at above sea level .-Geology:...
- Rwenzori Mountains
Indomalaya ecozone
- Fan Si PanFansipanFansipan is a mountain in Vietnam, the highest in Indochina, at . It is located in the Lào Cai province of Northwest Vietnam, 9 km southwest of Sa Pa Township in the Hoang Lien Son mountain range....
- Nat Ma TaungNat Ma TaungNat Ma Taung , is the highest mountain in the Chin State of western Burma. Located in Kanpalet Township, Mindat District, Mount Victoria is part of the Chin Hills range, and rises to above sea level....
- Tây NguyênTây NguyênTây Nguyên, translated as Western Highlands and sometimes also called Central Highlands, is one of the regions of Vietnam. It contains the provinces of Đắk Lắk, Đắk Nông, Gia Lai, Kon Tum, Lâm Đồng....
- Titiwangsa MountainsTitiwangsa MountainsThe Titiwangsa Mountains are the main mountain range that forms the backbone of the Malay Peninsula.-Geology:...
- Western GhatsWestern GhatsThe Western Ghats, Western Ghauts or the Sahyādri is a mountain range along the western side of India. It runs north to south along the western edge of the Deccan Plateau, and separates the plateau from a narrow coastal plain along the Arabian Sea. The Western Ghats block rainfall to the Deccan...
- Jade MountainJade MountainYushan in Taiwan is the highest mountain in East Asia and the fourth highest mountain on an island. In the past, Yushan was known among English-speaking expats and missionaries as Mt. Morrison, thought to have been named in honor of the 19th century missionary Robert Morrison...
Neotropic ecozone
- Baja CaliforniaBaja CaliforniaBaja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...
- Cordillera of Central America
- Guiana Shield
- Serra do MarSerra do MarSerra do Mar is a 1,500 km long system of mountain ranges and escarpments in Southeastern Brazil, which runs in parallel to the Atlantic Ocean coast from the state of Espírito Santo to southern Santa Catarina, although some include Serra Geral in the Serra do Mar, in which case this range...
- Venezuelan Coastal RangeVenezuelan Coastal RangeThe Venezuelan Coastal Range is a mountain range that runs along the central and eastern portions of Venezuela's northern coast...
Palearctic ecozone
- Aïr MountainsAïr MountainsThe Aïr Mountains is a triangular massif, located in northern Niger, within the Sahara desert...
- Altai Mountains
- Baikal MountainsBaikal Mountainsthumb|right|300px|The mountains and lake in the summer, as seen from [[Bolshiye Koty]] on the southwest shoreThe Baikal Mountains or Baikal Range rise steeply over the northwestern shore of Lake Baikal in southern Siberia, Russia...
- CaucasusCaucasusThe Caucasus, also Caucas or Caucasia , is a geopolitical region at the border of Europe and Asia, and situated between the Black and the Caspian sea...
- Tibesti MountainsTibesti MountainsThe Tibesti Mountains are a range of inactive volcanoes located on the northern edge of the Chad Basin in the Borkou- and Tibesti Region of northern Chad. The massif is one of the most prominent features of the Central-Sahara desert and covers an area of approximately 100,000 km². The northern...
- Tien Shan
See also
- Altitudinal zonationAltitudinal zonationAltitudinal zonation in mountainous regions describes the natural layering of ecosystems that occurs at distinct altitudes due to varying environmental conditions. Temperature, humidity, soil composition, and solar radiation are important factors in determining altitudinal zones, which consequently...
- Life zoneLife zoneThe 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...
- Holdridge life zonesHoldridge life zonesThe Holdridge life zones system is a global bioclimatic scheme for the classification of land areas. It was first published by Leslie Holdridge in 1947, and updated in 1967. It is a relatively simple system based on few empirical data, giving objective mapping criteria...
- List of life zones by region
- Refugium (population biology)
- Tepuis