DIVERSITY & ENDEMISM
The wide geographic range and thousands of isolated island ecosystems have led to extremely high levels of endemism in the Polynesia-Micronesia hotspot.
There are roughly 5,330 species of vascular plants native to Polynesia-Micronesia, of which more than 3,070 (58 percent) are endemic. One family, the Degeneriaceae, is endemic to Fiji, and includes a single tree species, Degeneria vitiensis, which is of considerable scientific interest because of its unique primitive floral characters. In some isolated parts of the hotspot, like Hawai'i, endemism is quite marked; indeed, 87 percent of the vascular plants on Hawai'i are endemic, such that it is sometimes treated as a single floristic region.
Among the noteworthy plants found in the hotspot are the Hawaiian silverswords (Argyroxiphium spp.), a group of five endemic species found only on the slopes of Hawai'i's highest volcanoes. When the plants are about 10-15 years old, they grow two-meter high stalks that briefly display magnificent blossoms before dying.
Unfortunately, many of these plants are highly threatened. Alarmingly, there are currently more invasive than native plant species in Hawai'i. More than half of the remaining native flora on Hawaii have fewer than 5,000 individuals each in wild populations. At least 14 species are down to only a single remaining individual in the wild, and an additional 46 species have only 2-10 individuals remaining in the wild.
Birds are the dominant terrestrial vertebrates in the Pacific, and the Polynesia-Micronesia hotspot is home to about 290 regularly occurring species, roughly 160 of which are endemic (an additional 25 species that were endemic became extinct after the arrival of Europeans). In general, avian endemism increases with the isolation and topographic diversity of the islands, with most endemic species being found in the larger and higher islands. The high level of endemism seen at the species level is also manifested at the genus level, with around one-quarter of the genera represented found only in this hotspot. Perhaps not surprisingly then, BirdLife International has identified no less than 15 Endemic Bird Areas confined entirely to the islands in Polynesia-Micronesia.
The Hawai'i honeycreepers, belonging to the finch subfamily Drepanididae, are one of the best-known bird groups in the region. They represent one of the world's most splendid examples of adaptive radiation. Eighteen genera and 34 extant and recently extinct species are believed to have evolved from a single North American finch ancestor. Bill morphology is vastly different among these species, ranging from massive, conical, seed-cracking beaks, to slender, recurved, nectivorous beaks, and even a spectacular wood-pecking variation in the 'Akia pola'au (Hemignathus lucidus). Unfortunately, in addition to eight Critically Endangered species, six more are considered Endangered and four Vulnerable. Thirteen honeycreepers have already gone extinct, and another, the Po'o-uli (Melamprosops phaeosoma), currently listed as Critically Endangered, was declared by scientists in Hawai'i to be extinct after the last individual died in captivity in November, 2004.
The dispersal of birds to the islands in the middle of the vast Pacific Ocean is just as remarkable. Fruit doves, imperial pigeons, lorikeets, reed warblers, monarch flycatchers, kingfishers, and, strangest of all, rails and ground doves, have repeatedly crossed vast expanses of ocean to colonize islands in Polynesia and Micronesia.
The only terrestrial mammals native to Polynesia-Micronesia are 15 species of bats, of which 11 are endemic. Two other endemic bat species have gone extinct. Most bat species are restricted to the high islands (Mariana Islands, Palau, Chuuk (or Truk), Kosrae, Pohnpei, Yap, Samoa, Fiji, and the Hawaiian Islands). The Fijian monkey-faced flying fox (Pteralopex acrodonta, CR), one of the most primitive species of fruit bats in the world, is the only mammal endemic to Fiji.
A single pinniped species, the Hawaiian monk seal (Monachus schauinslandi, EN) is an endemic breeding species in this hotspot and brings the total number of mammals to 16. Breeds mainly on the northwestern Hawaiian islands, they are thought to number fewer than 1,500 individuals.
There are more than 60 species of native terrestrial reptiles in Polynesia-Micronesia, including seven snakes, the saltwater crocodile (Crocodylus porosus) and more than 50 lizards. Over 30 reptile species are endemic. This reptile fauna is almost entirely Indo-Pacific in origin, with the exception of two species of iguana, the Fiji banded iguana (Brachylophus fasciatus, EN) and Fiji crested iguana (Brachylophus vitiensis, CR), which are endemic to the Fiji-Tonga area and the only species with close relatives from the Americas. The sub-fossil remains of a giant iguana species have been found in Tonga and Fiji.
A fascinating phenomenon among reptiles has been the spread of Lepidodactylus geckos across the region. These geckos are parthenogenic, meaning that they do not need males to reproduce, and can colonize new islands more easily than other geckos that rely on finding other individuals to reproduce sexually.
Besides the 30-odd species endemic to this hotspot, two reptile genera are endemic: the genus Brachylophus (which comprises the two species of iguanas), and the genus Ogmodon, represented by a single species, the venomous 'bola' or Fiji snake (O. vitianus, VU) which, as its name implies, occurs only on Fiji and is a member of the cobra family. A third endemic genus, Tachygia, which included a single skink species formerly restricted to Tonga, is extinct.
Only three amphibians are native to the hotspot, and all are ranid frogs of the genus Platymantis. Two species are endemic to Fiji, the Fiji tree frog (Platymantis vitiensis) and Fiji ground frog (P. vitiana, EN), and one, the Palau frog (P. pelewensis), is endemic to Palau. All three species are related to other Platymantis species in the Solomon Islands and in New Guinea.
Although there are no truly freshwater fishes in Polynesia-Micronesia, other than introduced species, nearly 100 native species are found in freshwater as adults (having pelagic marine larval stages); about 20 of these are endemic. Goboid fishes of the families Gobiidae and Eleotridae comprise the largest element of the freshwater fish fauna in the hotspot, and many of these are restricted to a single island or island group. Hawai'i has five species of gobioid fishes (four endemic), while Guam in western Micronesia has four times as many.
Invertebrate diversity in Polynesia-Micronesia is high for certain groups, particularly land snails, which are a conspicuous feature of Pacific Island ecosystems. Of the 13 major indigenous pulmonate land snail families on the Pacific Islands, four are endemic to the central Pacific. The Hawaiian Islands have more than 763 species, of which a staggering 748 are endemic. The Samoan Islands have 99 native species, of which 64 are endemics. The land snails of the subfamily Achatinellinae are among the most remarkable: in some parts of Hawai'i, these brilliantly colored snails may number as many as 2,000 individual snails per tree.