New Caledonia, a French territory in the South Pacific, is the hottest spot on Earth for biodiversity, says a study in the latest issue of the Proceedings of the National Academy of Sciences USA. That island boasts a rich array of unique plant and animal species, which are increasingly under threat from humans.

In the study, ecologist Holger Kreft of the University of California, San Diego and his colleagues looked at what they termed "endemism richness," or the number of unique species combined with the overall number of species around the world.

The researchers were surprised to find that islands from Micronesia to the Atlantic—just 3.6 percent of the surface of the planet—boasted plant endemism richness 9.5 times higher than the continents and more than 8 times higher for animals with backbones.

"Islands should have their place in global conservation strategies because islands harbor such unique flora and fauna, which cannot be protected elsewhere," Kreft says.

The 70,000 unique species of island plants (and the animals that depend on them) may be particularly threatened. In addition to having less land set aside in protected areas or parks, habitat loss through human activity is accelerating on islands—continuing a legacy that extends from the extinct elephant bird of Madagascar (see picture right) to the dodo of Mauritius.

As it stands, the kagu, a small groundbird found only in the forested highlands of New Caledonia, and Amborella trichopoda, an evergreen shrub that is the most ancient living example of a flowering plant, and other unique denizens of the southwest Pacific island refuge are in danger of disappearing for good.

Map: This world map shows the species richness and rarity of plants for 90 regions in a combined index. Accordingly, oceanic islands are especially valuable regions. Among continental regions with high index values are tropical mountains and regions with Mediterranean-type climates, such as California or southwest Australia. Credit: Courtesy of Holger Kreft

Image 1: An example for the threat of biodiversity on islands: a fossil egg of the elephant bird. Until a few hundred years ago, elephant birds were living on Madagascar. The 3 meter tall and 400 kilogram heavy birds were hunted to extinction after the colonization of the island by humans but survived as the giant bird Roc in Arabian myths such as Sinbad the Sailor. Credit: Wilhelm Barthlott.

Image 2: Amborella trichopoda is the most primordial of all flowering plants. It only occurs on the island of New Caledonia which is a relict of the ancient continent Gondwana located in the Pacific. Credit: Wilhelm Barthlott.