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Ranking the world's most endangered places isn't easy. Humans have encroached upon nearly every habitat on the planet, and climate change threatens places from coastlines to glaciers worldwide.
But some threatened spots would be a particularly devastating loss to the planet's biodiversity. Here are eight places that could disappear, taking with them thousands of species of plants and animals:
MadagascarSlide 2 of 17
This island off the east coast of Africa is home to 21 million people. According to the nonprofit Conservation International, it's also home to eight unique plant families, four unique bird families, and five unique primate families, including 50 species of lemur found nowhere else on the planet.
Thanks to cattle grazing, logging and slash-and-burn agriculture, only 17 percent of Madagascar's original vegetation remains. In addition, invasive species have devastated local flora and fauna. As recently as May, the conservation agency BirdLife International declared the Alaotra Grebe, a black-and-yellow water bird, extinct. The species succumbed to the combined threats of fishermen's nets and non-native carnivorous fish.Slide 3 of 17
BorneoSlide 4 of 17
This lush rainforest island, divided between Indonesia, Brunei and Malaysia, is home to the endangered Bornean orangutan, the critically endangered Sumatran rhinoceros, and about 1,000 pygmy elephants that are unique to the island.
The area's biodiversity is enormous: Between July 2005 and September 2006 alone, according to the World Wide Fund for Nature (WWF), 52 new species of plants and animals were discovered in the Borneo rainforest.
Unfortunately, the rainforest itself is under threat. According to a 2005 WWF report, Indonesian Borneo lost more than 1.21 million hectares of rainforest per year between 1997 and 2000. (One hectare is about 2.5 acres.) Illegal logging, forest fires and the development of palm oil plantations are to blame.
Meanwhile, according to the same report, the illegal trade of protected wildlife is a billion-Euro-a-year business in Indonesia. The orangutan is particularly prized: A 2003 survey by wildlife trade monitoring group TRAFFIC found that in just one month, Indonesian officials confiscated 30 orangutans from would-be wildlife dealers.Slide 5 of 17
Micronesia and PolynesiaSlide 6 of 17
Micronesia and Polynesia
Called the "epicenter of the current global extinction," by Conservation International, this smattering of more than 4,000 South Pacific islands is at risk from both local human activity and global climate change.
Humans settled on these islands between 2,000 and 3,000 years ago. Since then, thousands of bird species have gone extinct, according to a 1989 review in the Journal of Archaeological Science. Hunting and agriculture helped usher these species into extinction, but invasive species played a major role. One of the worst offenders, according to a 1992 study in the journal Oryx, is the common rat, which preys on birds and reptiles alike.
While invasive species ravage the islands from the inside, global warming threatens from without. According to the Intergovernmental Panel on Climate Change, a one-meter rise in sea level would submerge more than 4 square miles (10 square kilometers) of the 100-square mile (257-square kilometer) island of Tongatapu, Tonga. The surging ocean waters from a typical tropical storm would swamp an additional 27 square kilometers. Other low-lying islands face similar fates.Slide 7 of 17
Arizona, New Mexico and the Chihuahuan desert: Sky IslandsSlide 8 of 17