Location and General Description
This ecoregion encompasses an immense area of the Pacific Ocean between 13º to 25oS latitude and 124º to 149oW longitude. Ducie and Oeno Atolls in the Pitcairn Islands, and all of the Tuamotus, with the exception of Makatea Island, are coral atolls less than 6 m in elevation, some of which surround enormous lagoons. The coralline base of the Tuamotus is more than 400 m thick and sits atop eroded volcanic basalt that is between 4 and 18 million years old (Pirazzoli et al. 1988). Makatea Island and pristine Henderson Island in the Pitcairn Group are both limestone plateaus, raised 100 m or more above sea level by seismic activity below the Pacific Plate. Henderson Island has been above the sea surface for at least 380,000 years (Blake 1995). Pitcairn Island and most of the Gambier Islands are volcanic high islands, Pitcairn reaching 350 m and Mangareva in the Gambier Islands reaching 435 m (Fosberg 1998). Pitcairn is about 900,000 years old and the Gambier Islands are probably the remnants of a more ancient island that would have once encompassed much of the area inside the huge fringing reef and atoll (Thibault & Bretagnolle 1999). The climate varies from tropical in the northern Tuamotus with a mean annual temperature of 27oC and 80 percent humidity to subtropical in the Pitcairn Islands with a mean annual temperature of 23oC. Annual rainfall averages 1,500 to 2,000 mm throughout the islands, although the high volcanic islands receive slightly more (Fosberg 1998).
Undisturbed atolls and lowlands of other islands support a mixed broadleaf strand forest with a zonation in species dominance from foreshore toward the island interior (Waldron et al. 1995). Suriana maritime or Pemphis acidula are found on sandy areas above the beach, next to a zone of Scaevola spp. and Guettarda speciosa surrounding taller Tournefortia argentea forest. Inside this is a more diverse forest that includes the former species as well as Pisonia grandis, Pandanus tectorius, Pipturus argenteus, Sesbania coccinea, Cordia subcordata, Morinda citrifolia, and Calophyllum inophyllum. The endemic Myrsine niauensis is common on Niau Atoll (Fosberg 1998).
Uplifted limestone areas support a dense, tall forest dominated by Pisonia grandis, Pandanus tectorius, Ficus prolixa, Homalium moua, Guettarda speciosa, and the endemic palm Pritchardia vuyltekeana on Makatea Island (Fosberg 1998). Henderson Island plateau forest also contains Thespesia populnea and the shrub Bidens hendersonensis, Celtis sp., Nesoluma st-johnianum, and Geniostoma hendersonensis (Waldron et al. 1995). Forests at higher altitudes on Pitcairn (and perhaps once in the Gambier Islands) are dominated by H. mouo, Metrosideros collina, F. prolixa, P. tectorius, and T. populnea (Williams 1960).
While the smaller of these islands support very few vascular plant species, such as Ducie Island, which claims only two, the endemism rate is significant in some cases. Both Henderson and Pitcairn Islands have an approximate 14 percent rate of endemism for vascular plants (Florence et al. 1995). A great diversity of seabirds breed here, including 22 species in the Tuamotus and 14 species in the Gambier and Pitcairn Islands (Pratt et al. 1987, Thibault and Bretagnolle 1999). In an incredibly small area, the Pitcairn and Henderson Islands support 5 endemic land birds (Stattersfield et al. 1998). The Tuamotus are the smallest area in the world with an endemic sandpiper, the Tuamotu sandpiper (Prosobonia cancellata), which is now restricted to uninhabited, rat-free islands. The Tuamotus are also important wintering habitat for the Bristle-thighed curlew (Numenius tahitensis)(Stattersfield et al. 1998).
Unlike much of the Pacific, this ecoregion still possesses a number of fairly pristine islands, although for differing reasons. Henderson Island provides a glimpse into what the forests of atoll and limestone islands would have once looked like throughout the Pacific. Its natural vegetation has been intact since the 1600’s and Polynesian rat (Rattus exulans) is the only introduced species (Wragg and Weisler 1994). As such, it supports an extremely large number of endemic species for an island its size. This includes 2 species of bird, 9 endemic land snails, and at least 450 arthropod species, including 50 endemics. (Pratt et al. 1987, Vickery 1994, Preece 1995). Henderson Island is also interesting because its ecosystem is simple. The endemic parrot, the Henderson lory (Vini stepheni), feeds largely on the nectar of two plant species, a degree of specialization unknown in nectarivorous birds elsewhere (Trevelyan 1995). The Henderson Island fruit-dove (Ptilinopus insularis) has a narrower fruit diet than all other Ptilinopus species so far studied, feeding on all 14 fruiting species available on the island and dispersing seeds of those plants (Brooke and Jones 1995).
Atolls in the southeast of the Tuamotu archipelago are some of the most devastated and most pristine in the Pacific. Those atolls, used for above-ground and below-ground testing of nuclear weapons by the French government, are highly impacted, but surrounding atolls are uninhabited and experience little disturbance. There is little published information readily available on the flora and fauna of these atolls (Fosberg 1998).
Henderson Island is currently one of the most few relatively undisturbed makatea (raised limestone) islands in the world (Stattersfield et al. 1998). Archaeological evidence revealed that Henderson Island once supported a number of other endemic birds including an endemic genus of pigeon, two other pigeons, and a sandpiper (Wragg and Weisler 1994). These were extirpated by Polynesians during a period of occupation between 1,200 to 400 years ago, along with 4 other still extant species (Wragg and Weisler 1994). Extrapolation from all archaeological evidence in the eastern Pacific suggests that prehistoric Polynesian settlement probably resulted in the extinction of at least 8 to 15 pigeon species as well as dozens of other endemic birds from the Tuamotu ecoregion (Steadman 1989, 1997). These extinctions were caused directly by hunting and land clearing, but also by introduced species traveling with Polynesians. Historically, this occurred in the Tuamotus during the late nineteenth and early twentieth centuries as rats (Rattus rattus and R. norwegicus) were introduced accidentally when coconut plantations were established across many of atolls, and cats (Felis catus) were introduced to control them. This chain of events led to the extinction of many seabird and land bird populations (Thibault and Guyot 1987).
In the Tuamotus, there are still substantial areas of forest left on Makatea Island where the Polynesian pigeon (Ducula aurorae) survives in small numbers as well as the Makatea fruit-dove (Ptilinopus chalcurus) and Tuamotu reed-warbler (Acrocephalus atypha). There are also forests remaining on Niau and atolls in the southeast of the archipelago (Dahl 1980). Polynesians arrived in the Gambier Islands by the 1100s, and since then humans and introduced species including three rat species, cats, goats, and rabbits have resulted in the elimination of 98 percent of the native vegetation in the Gambier Islands (Thibault and Bretagnolle 1999). The only area of native vegetation remaining in the Gambier Islands is on the steep south slope of Mt. Mokota, Mangareva Island. This area should be set aside in a reserve (Dahl 1980).
Between 1966 and 1996, France conducted 193 nuclear tests on the Moruroa and Fangataufa atolls. The first tests were atmospheric but, following protests from other countries in the region, in particular Australia, subsequent tests were conducted underground. Nuclear testing holds many consequences for a small ecosystem, including physical damage to reefs; possible triggering of landslides, tsunamis, and earthquakes; and immediate as well as long-term contamination by volatile fission products.
Typesand Severity of Threats
Henderson Island was occupied by Polynesians from 800 to 1600 but not since, and it has now been set aside as a World Heritage Site by UNESCO. Conservation organizations are working with Pitcairn Islanders to establish a management plan that will protect the flora and fauna of Henderson, Oeno, and Ducie Islands from introduced species and unsustainable use (Vickery 1994). In addition, some of Pitcairn’s 22 species of land snail and plant species occupy less than 1 hectare of habitat, and forest conservation and control of the invasive tree Syzygium jambos is important for the survival of both snail and plant species (Preece 1995). Henderson, Oeno, and Ducie Islands together support more than 600,000 breeding petrels (Brooke 1995). It is hoped that Polynesian rats can be eradicated from Oeno Island to improve nesting success of breeding seabirds (Vickery 1994). Similar efforts would benefit breeding seabirds in the Tuamotu archipelago and the Gambier Islands. Predicted rises in sea level from global warming will inundate most low-lying atolls of this ecoregion, likely causing the extinction of several threatened species such as the Polynesian ground-dove.
Justification of Ecoregion Delineation
This ecoregion includes the widely dispersed islands of the Tuamoto archipelago and Pitcairn group, stretching from Manihi and Rangiroa to Pitcairn, Henderson, and Ducie Islands. Allison treats the Cooks, Societies, Tuamotus, and Marquesas as a unit herpetologically as they share a similar reptile assemblage. Van Balgooy similarly lumps the Cooks, Niue, Societies, Tuamotus, Tubaui, and Marquesas based on floristic affinities. However, Birdlife International (Stattersfield et al. 1998) distinguishes the Tuamotu archipelago as an Endemic Bird Area based on the presence of 6 endemic birds. In addition, Birdlife has identified a Henderson Island Endemic Bird Area, and Pitcairn as a Secondary Endemic Bird Area as a result of the presence of 4 and 1 endemic bird species, respectively. We have lumped Pitcairn and Henderson together with the Tuamotus due to their relative geographic proximity. The possibility of splitting Pitcairn and Henderson into a separate ecoregional unit is under review. Recent discussions with invertebrate specialist (e.g., Dan Polhemus) suggest that the Pitcairn and Henderson Island group should be distinct from the Tuamotus and Gambier Islands in future ecoregionalizations.
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Prepared by: Tim Male
Reviewed by: In process