Central Polynesian tropical moist forests
This ecoregion covers eight inhabited and nine uninhabited atolls and sand islands straddling the equator. Climate of the islands ranges from continually wet to chronically drought-stricken depending on location in relation to the equator and the trade wind belt. The majority of these islands have been heavily disturbed by military activities and phosphate mining though the area does includes some of the largest seabird colonies in the world, including millions of sooty terns and wedge-tailed shearwaters.
Location and General Description
There are a number of archipelagos in this region, including the northern Line Islands, southern Line Islands, and northern Cook Islands. Isolated Johnston is also included in this ecoregion. All islands are atolls with open or closed lagoons or are raised reef platforms with most land area less than 4 meters (m) above sea level. Much of the land area would have been submerged during a period of higher sea level 4,500 to 1,500 years ago. Typical of the trade wind latitudes, tropical islands in the north and south of the region have moderately wet, unseasonal climates with rainfalls between 1,500 to 3,000 mm. However, islands within 5° latitude of the equator, such as Johnson Atoll, are outside the trade wind belt and often experience extreme droughts with annual rainfall typically less than 1,000 mm. This has prevented many of them from being settled permanently.
The plant communities of these islands are extremely depauperate with floras ranging from 2 to 40 species and with almost no endemism. The wetter islands once supported large areas of moist forest although coconut (Cocos nucifera) plantations now cover most of the larger inhabited atolls. Most forest species are common throughout the coastal Indo-Pacific and include Pisonia grandis, Calophyllum inophyllum, heliotrope (Tournefortia argentea), Pandanus tectorius, Cordia subcordata, Guettarda speciosa, and the shrubs Morinda citrifolia, Scaevola taccada, Suriana maritime, and Pemphis acidula. The driest and lowest islands have an extremely simple flora, usually dominated by grass (Lepturus repens) and Tribulus cistoides or Portulaca lutea, sometimes with areas of T. argentea, P. acidula, and S. taccada scrub.
There are few endemic plant species on central Polynesian islands and the only passerine bird is the endemic bokikokiko (Acrocephalus aequinoctialis), a small reed-warbler found on Teraina, Tabueran, and Kiritimati. Kuhl’s lorikeet (Vini kuhlii) is an endangered parrot, extinct in the Cook Islands, and now only found on Rimatara, Tubuai Islands, and on Kiritimati and Teraina Islands, Kiribati. Teraina Island supports the only substantial population, with more than 1,000 birds. Interestingly, much of the former and current distribution of this species is the result of a prehistoric trade in red-feathered birds by Polynesians.
Despite low endemism and biodiversity, these islands protect a number of outstanding biological resources. These include the freshwater lagoons and bogs on Teraina (Washington) and Flint Islands that support freshwater plants and populations of migratory ducks and shorebirds. The islands also support some of the larger seabird populations in the world. This includes Starbuck Island with 3 million to 6 million sooty terns, Palmyra Island with 10,000 brown noddies (Anous stolidus) and 20,000 black noddies (A. minutus), and Jarvis Island with 1 million sooty terns (Sterna fuscata). Kiritimati Island is home to 18 breeding species, including 8,500 red-tailed tropicbirds (Phaethon rubricauda), 48,000 Phoenix petrels (Pterodroma alba), 24,000 Christmas shearwaters (Puffinus nativitatus), 2 million wedge-tailed shearwaters (P. pacificus), and the world’s largest colony of sooty terns with up to 15 million birds. Pacific atolls are also important wintering areas and stopover habitats for migratory shorebirds throughout the region. Flint Island and Caroline Island also hold some of the world’s largest populations of the enormous, terrestrial coconut crab (Birgus latro) some of which weigh 4 kg.
These islands have experienced extensive disturbances in the past, including attempts to establish copra (coconut) plantations and phosphate mines on most of them, military occupation on Palmyra and Kiritimati, nuclear weapons testing on Kiritimati, and construction of a massive airport and chemical weapons incinerator on Johnston Atoll. However, the minuscule land areas and frequent droughts have made long-term occupation unsustainable, and many islands are now uninhabited and have been set aside as nature reserves with varying degrees of protection. This includes Jarvis and now Palmyra Atoll under the protection of the United States, Malden, Starbuck, and Vostok in Kiribati, and Suwarrow in the Cook Islands. Rats and cats have been eradicated from Jarvis.
Among unprotected areas, Caroline Island and the wetlands on Teraina (Washington) Island are priorities for protection. The extensive forests of heliotrope, Cordia subcordata forest, and Pisonia grandis on Caroline Island are probably the most intact and largest forests of these species remaining in the Pacific. The island also supports some of the most spectacular concentrations of seabirds in the world and should be protected from development that might result in the introduction of Norway rats or black rats or other introduced species. Unfortunately, attempts by the Nature Conservancy to purchase and protect the island fell through, and the island is currently leased to a land developer.
Types and Severity of Threats
Introduced predators pose the greatest current threat to existing seabird colonies. Many islands, such as Kiritimati (Christmas), have feral cat populations that have extirpated ground-nesting seabirds from areas they can reach. Polynesian rats are present on most islands but can co-exist with most seabird species, unlike the ship rats introduced to Tabueran Atoll in the early 1920’s and probably responsible for the extirpation of Bokikokiko and the decline of Kuhl’s Lorikeet there. The fate of fauna here will be depend on the control of introduced species and the amount of land needed to support growing human populations. Land area will also be affected by global warming. If sea level rises significantly in the twenty-first century, most of these islands will be completely submerged and their ecosystems lost forever.
Justification of Ecoregion Delineation
This ecoregion consists of the relatively well-defined Northern and Southern Line Islands groups as well as the ill-defined Northern Cook Islands (Pukapuka, Rakahanga, Manihiki, Penrhyn, and Suwarrow Atolls). Mueller-Dombois & Fosberg consider the Northern Cooks to be part of Central Polynesia and treats the Southern Cook Islands separately. Van Balgooy groups the low coral islands of the Marshalls, Gilberts, Tuvalu, Tokelau, Kiribati, and Line Islands based on floristic affinities. Birdlife International highlights the Northern Line Islands as a Secondary Endemic Bird Area due to the presence of one endemic bird and several restricted-range bird species. The Northern Line Islands have been lumped with the dispersed Southern Line Islands and the Northern Cook Islands to form one ecoregion. The Northern Cook Islands are not a proper archipelago but a scattering of atolls under the jurisdiction of the Cook Islands – they are generally associated with Central Polynesia, while the Southern Cook Islands are generally considered part of Eastern Polynesia. The delineation of this ecoregion is under review.
Additional information on this ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
- Dahl, A.L. 1980. Regional ecosystems survey of the South Pacific area. South Pacific Commission, Noumea, New Caledonia.
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- Kepler, A.K. and C.B. Kepler. 1994. Natural history of Caroline Island. Part I. History, physiography, botany, and islet descriptions. Atoll Research Bulletin 397:1-132.
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- Van Balgooy, P.H. Hovenkamp, and P.C. Van Welzen. 1996. Phytogeography of the Pacific – floristic and historical distribution patterns in plants. Pages 191-213 in Keast, A. and S.E. Miller, editors, The origin and evolution of Pacific island biotas, New Guinea to Eastern Polynesia: Patterns and processes. SPB Academic Publishing, Amsterdam. ISBN: 905103136X
- Watling, D. 1995. Notes on the status of Kuhl’s Lorikeet (Vini kuhlii) in the Northern Line Islands, Kiribati. Bird Conservation International 5:481-489.
- Wester, L., J.O. Juvik, and P. Holthus. 1992. Vegetation history of Washington Island (Teraina), Northern Line Islands. Atoll Research Bulletin 358:1-50.
- Woodroffe, C.D. and R.F. McLean. 1998. Pleistocene morphology and Holocene emergence of Christmas (Kiritimati) Island, Pacific Ocean. Coral Reefs 17:235-248.
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