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Henderson Island is, without question, one of the world's best remaining examples of an uplifted coral atoll. Compared to other such raised atolls, it has suffered limited disturbance. Thus, the original ecosystem is largely intact, and the number of introduced species is low. Uplift occurred via lithospheric flexure when the oceanic floor became loaded under Pitcairn Island. The island now rises 33 m above sea level , and is believed to have been emergent for approximately 380,000 years. Thus, the island plateau is what was formerly the lagoon floor. It is cloaked in dense vegetation, growing on poor limestone soil and coral rubble. There is no permanent water and the rocks are often savagely fissured. The fact that the island is uplifted, and therefore, unlike a low atoll, secure from the devastating effects of periodic inundation during tropical storms, has been important in permitting the evolution of the wide range of endemic species and a generally higher terrestrial species diversity.There are fringing reefs along the east, north and north-west coasts. Behind each of these reefs is a beach, with beach-back vegetation, before steep slopes or cliffs rise to the plateau. Where there is no reef, the sea beats directly against sheer 30m cliffs. The tidal range is small, about 1 m.Weather records have only been maintained during 1991/92 when the average daily maximum varied from 29.6oC (Feb. 1991) to 24.2oC (June 1991): the comparable minima in those two months were 22.2oC and 15.7oC respectively. Total rainfall from February 1991 to January 1992 was 1623 mm.Archaeological studies have revealed that Henderson was occupied by Polynesians from early in the eight century AD for at least the next 600 years. While the human population may have reached 100, it is uncertain whether there was seasonal ebb and flow between Henderson and Pitcairn when, for example, people travelled to Henderson to exploit turtles during their nesting season. Certainly the Polynesians ate birds in large numbers. They introduced Pacific rats Rattus exulans. They also burned the northern and eastern margins of the plateau inland from their habitation sites, at least partly to make way for cultivated species. Whether these several impacts - bird harvesting, rats, horticulture - had any great lasting impact on the plant communities is uncertain. However, at least six of 22 land snail species disappeared because of the Polynesian impact. The impact on birds was also profound. Five landbird species, of which four were endemic to Henderson, disappeared. In addition, two or three seabird species were locally extirpated, while the gadfly petrels which continue to breed (see Birds) are probably present in numbers much lower than formerly. Notwithstanding this undeniable impact, the crux of the conservation perspective, that Henderson is the Pacific's most pristine raised coral island, is not undermined.Henderson was not discovered by European seafarers until Pedro Fernandez de Quiros passed without landing in 1606. The island received its present name when next visited in 1819 by the Hercules under the command of Capt. Henderson. Pitcairners started regularly to use two woods, toa Cordia subcordata and miro Thespesia populnea, collected from the island, shortly after World War I when they were taught artisanal carving techniques by an Austrian named Edward Laeffler. The exploitation of these two species, both of which were probably introduced by the Polynesians, has continued to this day, with the Pitcairners visiting the island once every 1-2 years. Provided due care is taken, exploitation can continue indefinitely on a sustainable basis. Because crossing the reef is potentially dangerous, passing yachtsmen rarely set foot on Henderson. However, cruise ships land their passengers once or twice a year, either on the North or the North-west Beaches.In the early 1980s, an American strip-mining millionaire proposed building a home and airstrip on Henderson. After intense lobbying by the conservation community, the proposal was rejected by the British Government. In 1988, the island was inscribed as a World Heritage Site because it is such a remarkable example of a raised coral atoll. At the time of the inscription, it was widely realised that scientific knowledge of the island was limited. This was partly rectified in 1991/92 when the Sir Peter Scott Commemorative Expedition to the Pitcairn Islands took place. Concentrating its multi-disciplinary scientific efforts on Henderson, the Expedition involved 34 people from seven countries in the field over a 15 month period. Since then, only brief scientific visits have taken place, the last in 2003, to finalise a Management Plan for the island (2004 - 2009). Monitoring changes in the island's fauna and flora will remain difficult because of the island's exceptional remoteness and ruggedness, the very features that have contributed to preserving it thus far.
Henderson Island is of great ornithological importance, both for its landbirds and its seabirds. All four breeding landbird species are endemic to the island.Of pre-eminent interest is the flightless Henderson Crake, one of only seven species of flightless rail extant on Pacific islands. Population estimates in 1987 and, using perhaps a more reliable technique, in 1991/92 were 3240 and 6200 individuals respectively. While some eggs may be lost to the introduced Pacific rats Rattus exulans, the crakes are very aggressive towards the rats, and have co-existed with them for some 800 years. There is no immediate concern for the crake, provided other predators do not reach Henderson. The Henderson Fruit-dove is an endemic representative of a widespread Pacific genus. Its diet includes most fruit species available on the island, but the watery Procris pedunculata is especially important. Population estimates in 1987 and 1991/92 were 3420 and 3140 individuals respectively.The Henderson Reed-warbler, formerly considered conspecific with the Pitcairn Reed-warbler, has been the subject of a detailed single-season breeding study which established that about one-third of breeding territories were occupied not by pairs but by trios. Such trios, either two male/one female or one male/two females, were of birds unrelated to each other. Population estimates in 1987 and in 1991/92 were 10,800 and 9,500 individuals respectively.The scarcest of the landbirds is the Henderson Lorikeet, which feeds on nectar, pollen fruit and also arthropods. No nest has ever been found. It is the only species of Vini living in habitats relatively little altered by man. A population estimate in 1987 was 720-1820 individuals.A two-day, and therefore necessarily superficial inspection visit to Henderson in November 2000 and a 6 week visit in 2003 confirmed that all four species remained present in numbers that did not appear to have altered substantially from those suggested by the prior surveys.Henderson Island also qualifies as an IBA on account of substantial populations of three surface-nesting gadfly petrel species. The most important of these is that of the endangered Henderson Petrel. While the situation of this species at French Polynesian islands to the north-west of the Pitcairn Islands requires clarification, it seems likely that the majority, perhaps the overwhelming majority of this taxon, breeds on Henderson, where there are 16,000 pairs. The populations of Herald and Kermadec Petrel are 11,100 and 10,000 pairs respectively, in both cases about 20 percent of the species' world populations.There are also about 2500 pairs of Murphy's Petrel, a small number in comparison with those on Ducie and Oeno. Up to 50 Bristle-thighed Curlews winter regularly on the island.The Fairy Tern population on Henderson numbers thousands and could be as high as 10,000 pairs. There are also small populations of other, widespread tropical seabirds. A study of the four petrel species in 1991/92 found heavy predation of their chicks by Pacific rats. If such predation occurs every year, then it is probable that either the petrel populations are in long-term decline, or are sustained by immigration. To check whether the 1991/92 situation was typical, a return visit was made in July/August 2003. Predation of Murphy's Petrel chicks was as heavy in 1991. Study of the other three species is made more difficult by their habit of nesting scattered at low density across the entire island and breeding more or less throughout the year.
Non-bird biodiversity: Thanks to the work of the Sir Peter Scott Commemorative Expedition, the land flora and fauna of Henderson Island are reasonably well-documented. Among the better-studied groups in which dispersal is limited, endemicity is high. Thus, seven of 16 species of land snail and nine of 63 native vascular plant species are endemic. As with the land birds, most of the endemic vascular plants are widespread across the island, and not in immediate danger. Endemicity is also likely to be comparably high in the insect fauna, where about 180 species are known to date. Because Henderson is so remote, this total is below what might be expected on the basis of the island's area. Endemicity is lower in some other groups, either because natural dispersal is high (e.g. lichens) or because the species on Henderson include a considerable proportion that 'hitchhiked' to the island in prehistoric times, probably during the Polynesian occupation (e.g. lizards).During a breeding season that lasts from about December - April, approximately 10 green turtles Chelonia mydas (EN) lay annually on Henderson. While this number is trivial globally, it represents about one percent of the French Polynesian total.
BirdLife International (2019) Important Bird Areas factsheet: Henderson Island. Downloaded from http://www.birdlife.org on 15/09/2019.