Spread over 3,000 km2 of the Pacific Ocean, this ecoregion encompasses the 76 atolls and islands of Tuamotu, which stretch 1,800 kilometers (km) to the Gambier Islands and the Pitcairn Islands 1,000 km to the east.
The biodiversity present here highlights the effects of extreme isolation and challenging conditions of atolls on flora and fauna. The Tuamotus and Gambier Islands are largely atolls, while the Pitcairn group consists of raised limestone islands and high islands. The Tuamotus contain some of the most intact and damaged atolls in the Pacific. Natural communities are highly threatened by land clearing, introduced predators, and nuclear testing.
Location and General Description
This ecoregion encompasses an immense area of the Pacific Ocean between 13º to 25 °S latitude and 124º to 149 °W 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 meters (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. 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. 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. 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. The climate varies from tropical in the northern Tuamotus with a mean annual temperature of 27 °C and 80 percent humidity to subtropical in the Pitcairn Islands with a mean annual temperature of 23 °C. Annual rainfall averages 1,500 to 2,000 millimeters (mm) throughout the islands, although the high volcanic islands receive slightly more.
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. 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.
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. Henderson Island plateau forest also contains Thespesia populnea and the shrub Bidens hendersonensis, Celtis sp., Nesoluma st-johnianum', and Geniostoma hendersonensis. 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.
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. A great diversity of seabirds breed here, including 22 species in the Tuamotus and 14 species in the Gambier and Pitcairn Islands. In an incredibly small area, the Pitcairn and Henderson Islands support 5 endemic land birds. 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).
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 1600s and Polynesian rat (Rattus exulans) is the only introduced species. 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. 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. 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.
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.
Henderson Island is currently one of the most few relatively undisturbed makatea (raised limestone) islands in the world. 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. These were extirpated by Polynesians during a period of occupation between 1,200 to 400 years ago, along with 4 other still extant species. 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. 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.
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. 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. 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.
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, possibly triggering landslides, tsunami, and earthquakes, and immediate as well as long-term contamination by volatile fission products.
Types and 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 United Nations Educational, Scientific and Cultural Organization (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. 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. Henderson, Oeno, and Ducie Islands together support more than 600,000 breeding petrels. It is hoped that Polynesian rats can be eradicated from Oeno Island to improve nesting success of breeding seabirds. 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 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. WWF has 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 specialists (e.g., Dan Polhemus) suggest that the Pitcairn and Henderson Island group should be distinct from the Tuamotus and Gambier Islands in future ecoregionalizations.
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
- Allison, A. 1996. Zoogeography of amphibians and reptiles of New Guinea and the Pacific region. Pages 407-436 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
- Blake, S.G. 1995. Late quaternary history of Henderson Island, Pitcairn group. Biological Journal of the Linnean Society, 56:43-62.
- Brooke, M. De L. 1995. The breeding biology of the gadfly petrels Pterodroma spp. of the Pitcairn Islands: characteristics, population sizes, and controls. Biological Journal of the Linnean Society, 56:213-231.
- Brooke, M. De L. and P.J. Jones. 1995. The diet of the Henderson Fruit-dove Ptilinopus insularis. I. Field observations of fruit choice. Biological Journal of the Linnean Society, 56:149-165.
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- Wragg, G.M. and M.I. Weisler. 1994. Extinctions and new records of birds from Henderson Island, Pitcairn group, south Pacific Ocean. Notornis, 41:60-70.
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