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 m above sea level (Mueller-Dombois & Fosberg 1998). Much of the land area would have been submerged during a period of higher sea level 4,500 to 1,500 years ago (Gray et al. 1992, Woodroffe & McLean 1998). 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 (Mueller-Dombois & Fosberg 1998).
The plant communities of these islands are extremely depauperate with floras ranging from 2 to 40 species and with almost no endemism (Mueller-Dombois & Fosberg 1998). 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 (Wester et al.1992, Kepler & Kepler 1994, Mueller-Dombois & Fosberg 1998). 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 (Watling 1995). Teraina Island supports the only substantial population, with more than 1,000 birds (Perry 1980). Interestingly, much of the former and current distribution of this species is the result of a prehistoric trade in red-feathered birds by Polynesians (Watling 1995).
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 (Dahl 1980). 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 (Perry 1980). 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 (Kepler et al. 1994).
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 (Dahl 1980). Rats and cats have been eradicated from Jarvis (Rauzon 1985).
Among unprotected areas, Caroline Island and the wetlands on Teraina (Washington) Island are priorities for protection (Dahl 1980). 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 (Kepler & Kepler 1994). 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 (Kepler et al 1994). Unfortunately, attempts by the Nature Conservancy to purchase and protect the island fell through, and the island is currently leased to a land developer (Kepler et al. 1994).
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 (Perry 1980). 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 (Watling 1995). 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 (1998) consider the Northern Cooks to be part of Central Polynesia and treats the Southern Cook Islands separately. Van Balgooy (1996) groups the low coral islands of the Marshalls, Gilberts, Tuvalu, Tokelau, Kiribati, and Line Islands based on floristic affinities. Birdlife International (Stattersfield et al. 1998) 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 (Mueller-Dumbois & Fosberg 1998). The delineation of this ecoregion is under review.
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Prepared by: Tim Male
Reviewed by: In process