Isolated from the rest of the world by vast plains of open water, Madagascar and the Galapagos Islands have a reputation for churning out some of the world's rarest endemic wildlife and plant species. But you don't have to be a remote hunk of land flanked by ocean to boast this kind of distinguished biodiversity. Many of the world's most majestic mountains produce similar results, by virtue of the drastic changes in their elevation. When the slopes and summit of a mountain exhibit a dramatically different ecosystem from the lowlands, it's called a "sky island."
One of the most reknowned chains of sky islands can be found in an expansive cluster in Arizona, New Mexico and northern Mexico. Known collectively as the Madrean sky islands, these elevated pine-oak woodland peaks are surrounded by the lower-lying Sonoran and Chihuahuan deserts. The Chiricahua Mountains (seen above surrounded by thousands of rhyolite tuff hoodoos) are just one example of the approximately 42 sky islands contained within the Madrean chain.
The elevated terrain of Chiricahua and similar mountains are home to many arboreal species that quite literally "ran for the hills" when the last Ice Age came to a close. The United Nations Environment Programme explained in its 2006 "Global Deserts Outlook" report:
"As they ascended into the isolated desert mountains, the communities of the desert 'sky islands' became separated from other mountains by harsh desert plains. [...] Because they have been reproducing in isolation for 15,000-20,000 years, many of their populations have developed unique genetic traits and have evolved into new species. Thus, in a similar fashion to evolution in remote oceanic islands, the biota of the desert sky islands is composed by a large number of endemic species and has immense value for biological conservation."
Deserts are the only places you'll find sky islands. If you travel halfway across the world to the island of Borneo, you'll find Mount Kinabolu, a sky island surrounded by tropical lowland rain forest:
The area that surrounds and contains Kinabolu boasts between 5,000 and 6,000 different plant species — that's more varieties than are found in Europe and North America combined. From the surrounding tropical rain forest to the lush alpine meadows to the scrubby shrubbery found at the summit (pictured below), Kinabolu is without a doubt one of the most important biological sites on Earth.
Continue below for a brief tour of other sky islands that grace the planet's most nosebleed-prone altitudes: