As a result of their isolation and small size, the island ecosystems of the Polynesia-Micronesia hotspot are exceedingly vulnerable to habitat degradation and the introduction of invasive species. Due mainly to these threats, species in this hotspot are some of the most endangered in the world. Species extinction rates are among the highest in the world, especially for birds and reptiles.
Human activities have threatened the unique biodiversity in Polynesia-Micronesia for at least 3,000 years, since the first Polynesian people began to migrate east from Southeast Asia and New Guinea. Micronesians are linguistically different from the Polynesians and show a clear migration from the Philippines, both prehistorically and contemporarily. These earliest settlers introduced several plants and animals, which they used for food, medicine, building materials, and ornamentation. They converted land for agriculture and hunted many birds and reptiles to extinction. The fossil record for the Pacific Islands reveals that as many as 2,000 bird species may have disappeared since humans colonized the islands.
Over the years, human-induced disturbance has resulted in the establishment of savannas, grasslands, and secondary forests throughout the region. Small islands, in particular, are heavily impacted by conversion of natural vegetation to anthropogenic landscapes. In many Pacific islands, there is no natural lowland vegetation left, because the land is under such demand from human populations, which are growing by one to three percent a year. In Fiji, for example, logging has affected nearly all the lowland forests. Overall, the combined effects of agriculture, logging and development have left only 10,015 km², or 21 percent of the region's original vegetation, in more or less pristine condition throughout the Polynesia-Micronesia hotspot. Increasing urbanization and commercialization has also led to the loss of traditional knowledge and resource management techniques in many areas, leading to further environmental degradation.
Isolation has left the island biota extremely vulnerable to invasive alien species, especially introduced predators and browsers. The native biota of the islands evolved for millions of years in the absence of mammalian predators, and the introduction of rats, pigs, goats, feral cats, and mongooses has had a devastating effect on small vertebrate populations on many islands. In some areas, birds also are threatened with extinction from avian malaria that was introduced with alien mosquito species. Some non-native species of ants and snails have also been highly destructive when introduced.
Two classic case studies serve to highlight the severity of the threat of invasive alien species on the endemic native fauna of the Pacific. The first example concerns the impact of the brown tree snake (Boiga irregularis) in Guam. The snake was inadvertently introduced in the early 1950s to the island by military ships and has already caused the extinction of nine native bird species and all endemic lizard species on the island. This mainly arboreal and nocturnal tree snake, which has reached densities of 4,600 per km² on Guam, has now been found on neighboring islands (it has even been found on cargo ships docked in Hawai'i on several occasions) as well and may be established on Saipan. The second classic example concerns the introduction of the predatory snail Euglandina rosea from the southeastern United States to the Society Islands for purposes of biological control. This species single-handedly resulted in the loss of 57 of the archipelago's 61 endemic partulid land snails.
The impacts of non-native plants can be just as damaging, as invasive species out-compete and replace native ones. Habitat fragmentation and degradation increases the destructiveness of invasive plant and animal species, many of which favor secondary forest and edge areas. Sadly, in present-day Hawaii there are more non-native plant species than native ones. One fast-growing South American tree species, Miconia calvescens, has crowded out much of the native vegetation in Tahiti, and is now estimated to cover 65 percent of the island.
Other pressures on biodiversity include the hunting and trapping of bats, birds, and other species. Species at risk include the coconut crab (Birgus latro) and pigeons (especially Ducula and Ptilinopus). The overharvesting of birds and bats can be particularly harmful, since these species often act as pollinators and seed dispersers for native plant species. Another threat is fire, which is often used to clear land. Sea level rise due to global warming is a major concern, as low-lying atolls, including Tokelau, Tuvalu, Kiribati, and the Marshall Islands, are at risk of disappearing completely under rising waters. Natural phenomena such as cyclones and floods can also extirpate isolated populations.