New Caledonia's rich biodiversity and high endemism are due to its long-isolated evolution, as well as the variety of precipitation levels and very particular ultrabasic soils that cover more than one third of the country.
Endemism in the hotspot is especially high among vascular plants. There are about 3,270 plant species recorded on the islands, about 74 percent of which are endemic. There are also 108 endemic genera, and a remarkable five endemic plant families: Amborellaceae, Paracryphiaceae, Strasburgeriaceae, Oncothecaceae and Phellinaceae. Among countries, only Australia, South Africa and Madagascar have more endemic plant families; these nations are also vastly larger than New Caledonia. Given this remarkable floristic endemism, New Caledonia is often considered a distinct floristic subkingdom.
Out of more than 100 birds found in New Caledonia, more than 20 are endemic. There are three endemic genera, two of which are monotypic, including the Endangered kagu (Rhynochetos jubatus), the only living member of the endemic family, Rhynochetidae. Other distinctive bird species include the endemic New Caledonia imperial-pigeon (Ducula goliath), which is the world's largest arboreal pigeon, the endemic cloven-feathered dove (Drepanoptila holosericea), and the Critically Endangered New Caledonian owlet-nightjar (Aegotheles savesi), known from only a few specimens.
All of New Caledonia's nine land mammal species are bats, six of which are endemic, including the Critically Endangered New Caledonia long-eared bat (Nyctophilus nebulosus), which was discovered in 1991 around Nouméa. Bats have been poorly studied and the potential for discovering new species is high.
There is an extremely high level of reptile endemism in this hotspot. More than 60 of about 70 terrestrial reptiles are endemic, as are 11 of 23 genera. Nearly all of these species are lizards in two families of geckos and one family of skinks. The best known among these are the giant geckos of the endemic genus Rhacodactylus. The New Caledonia giant gecko (R. leachianus), the largest gecko in the world, can grow up to 28 centimeters long, and weigh up to 700 grams. The Endangered Bocourt's eyelid skin (Phoboscincus bocourti), a 50-centimeter-long lizard, was previously known from a single specimen captured around 1870 and was long presumed extinct. In 2003, on a tiny islet, the species was rediscovered.
There are no native amphibians on New Caledonia.
Aquatic diversity on the islands is high given the size of the hotspot, with about 85 species of freshwater fish, although less than 10 are endemic. The most notable species is perhaps the Endangered, endemic galaxiid (Galaxias neocaledonicus), the northernmost representative of a group that is mostly restricted to the southern tips of New Zealand, Australia, South America and Africa. A single genus, Protogobius, is endemic.
Among invertebrates, the hotspot supports a rich endemic diversity of land snails. The largest of these, Placostylus fibratus, can grow to more than 15 centimeters long and weigh up to 100 grams.
The hotspot has an estimated 37 species of macro-crustaceans, of which 40 percent are endemic. Grande Terre island is one of the most biodiverse islands for this taxonomic group, with species found only in ultrabasic substrata and others only in some short, oxygenated rivers of the north.
About 4,000 insect species have been catalogued to date, showing high endemism at the species and genus levels. The total insect fauna in the hotspot is projected to be between 8,000 and 20,000 species, including more than 70 native species of butterfly, more than 300 species of moth, and 16 tiger beetles.
Nearly 200 spiders have been recognized thus far, including Bradystichidae, the only family of spiders endemic to a single island.