Location and General Description
These low atolls and limestone platforms are clustered in three archipelagos with the 8 atolls in the Phoenix Islands ranging from 2o to 5oS latitude and 171º to 175oW longitude, the 4 in Tokelau ranging from 8º to 12oS latitude and 170º to 173oW longitude, and the 9 in Tuvalu ranging from 6º to 9oS latitude and 176º to 180o E longitude. Howland and Baker Islands are isolated. All were formed as coraline limestone accumulated atop subsiding volcanoes over a period of millions of years. Subsequent development of lagoons and dunes has occurred during sea level changes during the Holocene (Schofield 1977, Mueller-Dombois and Fosberg 1998). The climate here is tropical, but rainfall differs dramatically. Howland Island, Baker Island, and most of the Phoenix Islands receive less than 1,000 mm of rain annually with a dry period from March through June. In addition, they are frequently exposed to long droughts during El Nino periods and extremely high rainfalls from cyclones (Mueller-Dombois and Fosberg 1998). The atolls of Tokelau and Tuvalu receive annual rainfall of between 1,500 to 3,500 mm, and their rainfall is also more consistent within and between years (Rodgers 1991).
In accordance with patterns of rainfall, the vegetation on wetter islands supports moist forest in protected areas as well as shrubby and herbaceous communities in salt-exposed and rocky areas. Moist forest is dominated by single or mixed species stands of Pisonia grandis up to 25 m high, Cordia subcordata, or Heliotrope (Tournefortia argentea) with areas of Scaevola taccada and Morinda citrifolia scrub, usually near forest edges. Other common tree species include Calophyllum inophyllum, Pandanus tectorius, Hernandia nymphaeifolia, Ficus tinctoria, Guettarda speciosa, the shrubs Suriana maritime, and Pemphis acidula, the fern Asplenium nidus, and the vine Ipomoea tuba (Parham 1970, Mueller-Dombois and Fosberg 1998). The drier islands support predominantly herbaceous vegetation. Herbaceous vegetation includes sparse grassland (Lepturus repens) mixed with patches of Portulaca spp., Sida fallax, Sesuvium portulacastrum, Eragrostis whitneyi and some scattered shrubs of C. subcordata, Abutilon asiaticum, Suriana maritime, Pemphis acidula, and Tribulus cistoides (Mueller-Dombois and Fosberg 1998).
There are few endemic plants or animals in Western Polynesia, and communities are dominated by species found throughout the Pacific. The reptile and mammal fauna is depauperate, cosmopolitan, and dominated by introduced species (Rodgers 1991). There are no forest passerines and the Pacific pigeon (Ducula pacifica) and migratory long-tailed cuckoo (Eudynamis taitensis) are the only forest birds present anywhere in the islands although banded rails (Rallus philippensis) have recently colonized Nuilakita, Tuvalu from Fiji (Wodzicki and Laird 1970, McQuarrie 1991). The Tokelau Islands support 38 indigenous plants, at least 150 insect species, and 10 land crab species that are present throughout forests (Yaldwyn and Wodzicki 1979).
The Phoenix Islands are unique in the vast numbers of seabirds they support. McKean Island has the world’s largest nesting population of lesser frigatebird (Fregata ariel) with up to 85,000 birds. There are at least 16 other species of seabirds present with more than 1 million total birds in the islands (Garnett 1983). These islands have also been uninhabited for most of their history so plant communities are intact and few introduced species except Polynesian rats (Rattus exulans) are present. Baker and Howard Islands also support large populations of seabirds.
Mining for phosphate occurred on Howland, Baker, McKean, Enderbury, Phoenix, and Sydney Islands during the last century, and disturbance and sparse settlement has occurred on others of the Phoenix Islands. However, all are now uninhabited, have relatively intact vegetation, and support huge numbers of breeding seabirds. The Phoenix Islands have been declared wildlife sanctuaries protected under Kiribati law, but are not patrolled (Dahl 1980). In addition, Howland and Baker Islands have been part of a wildlife refuge since 1974, and cats have been eradicated from both (Dahl 1980). The original vegetation on most of the Tuvalu and Tokelau Islands has been replaced by coconut plantation (Cocos nucifera) although in some places this has been abandoned and scrubby forest is present.
Types and Severity of Threats
Introduced mammalian predators pose the greatest threat to remaining ecosystems in Western Polynesia. Polynesian rats are found in the Tokelau Islands and these probably contribute to the relatively low density (1 per 10 m2) of land crabs (Yaldwyn and Wodzicki 1979). Introduced cats and rats pose the greatest threats to nesting seabirds, consuming both adults and young. Where pigs are present, they also have a substantial impact (Wodzicki and Laird 1970). The greatest future threat is that the low atolls and islands in this area will disappear beneath a rising warmer ocean in this century (Roy and Connell 1997). Even if islands do not disappear, a slight rise in water level, along with rapidly growing human populations, will reduce the size of the freshwater supply and agricultural lands possibly resulting in a shift in population to areas and atolls that currently support natural vegetation and fauna (Roy and Connell 1997).
Justification of Ecoregion Delineation
This ecoregion consists of Tuvalu, Tokelau, Kiribati (Phoenix Islands), and Howland and Baker Islands. Van Balgooy (1996) groups the low coral islands of the Tuvalu and Kiribati with the Marshalls, Gilberts, and Line Islands based on floristic affinities. Mueller-Dumbois & Fosberg (1998) treats Tuvalu, Tokelau, the Northern Cooks, and the Southern Line islands as a unit based on vegetation and climate (moist-zone atolls). This last group are climatically moist low coral islands located in the southeast trade belt. Kiribati, and Howland and Baker Islands, on the other hand, are located in the Equatorial Dry Zone and are treated as a unit (Pacific Equitorial Sporades) with the Southern Line Islands by Mueller-Dumbois & Fosberg. All of these islands are depauperate of native landbirds; Kiribati and Tuvalu share Pacific pigeon (Ducula pacifica) while the Line Islands do not (but contain 2 restricted-range bird species). The Northern Cooks and Southern Line Islands have been split off from the larger groups and joined with the Northern Line Islands to form the Central Polynesia ecoregion, leaving the remaining West-Central Polynesian islands to form the Western Polynesian ecoregion. This arrangement is under review.
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Prepared by: Tim Male
Reviewed by: In process