New Caledonia has the largest known deposits of nickel in the world. Nickel mining generates about 90 percent of the region's foreign exchange at present and produces fully half of the world's nickel. However, this asset is also a threat to the island's ecosystems. The impacts of the nickel mining industry have been devastating. Open-cast mining has led to large expanses of deforestation and habitat destruction, resulting in bare slopes and waste heaps. The erosion of mining devastated areas has caused the siltation and destruction of streams and offshore coral reef areas and the pollution of water supplies. Today, open-cast mining is better managed, and more localized than in the past. However, two new large-scale nickel mining and processing sites are presently threatening the adjacent coral reefs and very localized plant and animal species. Even though environmental impact studies have been undertaken and mitigation measures implemented, some very highly adapted species will probably disappear.
Other pressures on New Caledonia's biodiversity now include bush fires, logging, hunting and selective illegal collection of animals. The most valuable timber species are already gone, but even limited logging threatens the habitats of species with highly restricted ranges. Like other islands in the Pacific, hunting and habitat modification since the arrival of the first humans has led to a number of extinctions, including 11 species of non-passerine birds, and continues to pose a threat to the New Caledonian imperial-pigeon and to flying foxes. International demand for rare species of birds and marine animals, such as the Uvéa parakeet (Eunymphicus uvaeensis, EN), the horned parakeet (Eunymphicus cornutus, EN), and an endemic "living fossil" cephalopod (Nautilus macromphalus), threatens the already sparse populations of these species.
The intentional and accidental introduction of alien species for food or recreational purposes has been devastating. There are nearly 800 alien plant species, more than 400 alien invertebrates, and some 35 alien vertebrates established on the islands (including an introduced amphibian), out-competing and replacing much of the original vegetation and faunal species. Among the most problematic of these introductions have probably been the ship rat (Rattus rattus), the Indonesian deer (Cervus timorensis) and the fire ant (Wasmannia auropunctata). Pigs, cats, deer and rats occur from sea level to the highest elevations, and are present throughout the main island. The combined effect of these four species is dramatic and is rapidly destroying species and ecosystems. Controlling these species is the top conservation priority in the hotspot, including inside protected areas. This very demanding task is the only way to conserve fragile and localized species. The palm tree Lavoixia macrocarpa (CR), only known from 30 trees, is highly threatened due to seed predation by rats. Ouvea island is currently free of ship rats; if the species is introduced, it will likely wipe out the entire population of the Uvéa parakeet (Eunymphicus uvaeensis, EN). Feral dogs are now breeding in the wild and could be the reason why the kagu (Rhynochetos jubatus, EN) have disappeared from the north of Province Nord within the last 30 years.
Today, of the 5,050 km² of relatively pristine habitat left in New Caledonia, 4,000 km² are rain forests and 1,000 km² are low- to mid-altitude maquis. Sclerophyllous forest once covered around 23 percent of New Caledonia, but now only exists in a patch of around 45 km². This area itself is generally very degraded and fragmented into smaller patches of 20-30 hectares or less (though there are a few blocks that exceed 100 hectares), which are surrounded by agricultural land. A recent study suggests that New Caledonia's sclerophyll forest is the most threatened tropical dry forest in the world.