|UNESCO World Heritage Site|
Map of Henderson Island
|Criteria||Natural: vii, x|
|Inscription||1988 (12th Session)|
Henderson Island (formerly also San Juan Bautista and Elizabeth Island) is an uninhabited member of the Pitcairn Islands archipelago in the south Pacific Ocean. Its remote location and unsuitability for human habitation have made it the subject of scientific study. Ten of its 51 flowering plants, all four of its land birds and about a third of the identified insects and gastropods are endemic – a remarkable diversity given the island's size. It was previously considered one of the world's last two raised coral atolls whose ecosystems remained relatively unaffected by human contact. However, research in 2017 showed that a large amount of plastic debris has accumulated on the island.
Measuring 9.6 kilometres (6.0 mi) by 5.1 kilometres (3.2 mi), it has an area of 37.3 square kilometres (14.4 sq mi) and is located 193 kilometres (120 mi) northeast of Pitcairn Island. It has poor soil and little fresh water, and is unsuitable for agriculture. There are three beaches on the northern end and the remaining coast comprises steep (mostly undercut) cliffs up to 15 metres (49 ft) in height.
Archaeological evidence suggests that a small permanent Polynesian settlement existed on Henderson at some time between the twelfth and fifteenth centuries. The reasons for the group's disappearance remain unknown, but may relate to the similar disappearance of the Polynesians on Pitcairn Island, on whom the Hendersonians would have depended for many of the basics of life, especially stone for making tools. The Pitcairn Polynesians may in turn have disappeared because of the decline of nearby Mangareva; thus, Henderson was at the end of a chain of small, dependent colonies of Mangareva.
On 29 January 1606, Portuguese captain Pedro Fernandes de Queirós, leading a Spanish expedition, was the first European to see the island, and named it San Juan Bautista. Captain Henderson of the British East India Company ship Hercules rediscovered the island on 17 January 1819 and named it Henderson Island. On 2 March 1819, Captain Henry King, in the Elizabeth, landed on the island to find the king's colours already flying. His crew scratched the name of their ship into a tree.
A sperm whale rammed and sank the Nantucket whaleship Essex on 20 November 1820 (a report of which inspired Herman Melville to write Moby-Dick), and the crew arrived at Henderson on 20 December in three small whaleboats. They found the island's only known drinkable water-source – a brackish spring on the north shore, exposed at half tide – and ate fish, birds, eggs, crabs and peppergrass, but they had largely exhausted the available food within a week, and on 27 December the three boats set sail for South America, leaving behind Thomas Chappel, Seth Weeks, and William Wright, who chose to stay. They survived until their rescue on 9 April 1821. In his account of the ordeal, Chappel reported having seen human skeletons in a cave.
In August 1851, visitors from Pitcairn Island also found skeletons in a cave and wreckage on the adjacent beach. After a party of Pitcairn Islanders collecting miro wood rediscovered the skeletons in March 1958, a medical examination determined that the bones were of Caucasian origin, and they were then buried in a shallow grave inside the cave. An American survey team examined the bones in 1966 and buried them in five coffins in the left-hand corner of the cave, tightly jamming a large cross between the ceiling and the rock floor at the entrance. They concluded the remains were of five or six people, one of whom was between three and five years of age. It was presumed they were the survivors of a shipwreck who died of dehydration. A final examination in the context of a scientific expedition in 1991 concluded that the human remains on Henderson Island were prehistoric Polynesians.
In 1902, Henderson Island, along with Oeno and Ducie islands, was formally annexed to the British Empire by Captain G. F. Jones, who visited the islands in a cutter with a crew of Pitcairn Islanders. In August 1937, HMS Leander, on a journey from Europe to New Zealand, carried out an aerial survey of Henderson, Oeno and Ducie, and on each island a British flag was planted and an inscription was nailed up proclaiming: "This island belongs to H.B.M. King George VI." 
In 1957, a 27-year-old American, Robert Tomarchin, lived the life of a castaway on the island for approximately 2 months, accompanied by a pet chimpanzee, apparently as a publicity stunt, until people from Pitcairn rescued him in two longboats.
In the early 1980s, conservative American businessman Arthur "Smiley" Ratliff expressed interest in establishing a mansion for himself on the island, with an airstrip. The Pitcairn Island Council approved his plans but, after environmentalists had lobbied to protect the natural ecology and environment of the island, the British Foreign and Commonwealth Office overrode the decision and vetoed the proposed development. Henderson Island was listed as a World Heritage Site in 1988.
Since the introduction of aluminium-hulled long-boats in the 20th century, Pitcairners have made regular trips to Henderson to harvest the wood of miro and tou trees. Usually they venture to Henderson once per year, but they may make up to three trips if the weather is favourable. Pitcairners carve the wood into curios for tourists, from which they derive much of their income.
Henderson Island is a raised coral atoll that, with Pitcairn, Ducie and Oeno Islands, forms the Pitcairn Island Group. The nearest major landmass is more than 5,000 kilometres (3,100 mi) away. This coral limestone island sits atop a conical (presumed volcanic) mound, rising from a depth of roughly 3,500 metres (11,500 ft). Its surface is mostly reef-rubble and dissected limestone – an extremely rugged mixture of steep, jagged pinnacles and shallow sink holes – and, except for the north end, the island is encircled by steep limestone cliffs up to 15 metres (49 ft) high.
There are three main beaches, on the northwest, north, and northeast sides, and the north and northwest sides are fringed by reefs. The depression at the island's centre is thought to be a raised lagoon. There is only one known potable water source, a brackish spring on the north shore exposed at half tide, rising from a crevice in flat rock, large surfaces of which compose the face of the beach. The surrounding ocean tidal range is about one metre at spring tide.
Apart from five species bordering the beaches, including coconut palms, the vegetation is undisturbed. Henderson Island is covered by 5–10 m tall tangled scrub forest, more thinly covered in the central depression. It has 51 native species of flowering plants, ten of which are unique to the island (endemic). Dominant tree species include coconut palms, Pandanus tectorius, Thespesia populnea (miro), Tournefortia argentea, Cordia subcordata (tou), Guettarda speciosa, Pisonia grandis, Geniostoma hendersonense, Nesoluma st.-johnianum, Hernandia stokesii, Myrsine hosakae, and Celtis sp.
The island is presently home to four endemic land bird species – the Henderson fruit dove, Henderson lorikeet, Henderson reed warbler and the flightless Henderson crake. Three species of the family Columbidae – the Henderson ground dove, the Henderson imperial pigeon and the Henderson archaic pigeon – were formerly endemic to the island, but became extinct when the Polynesians arrived around 1000 CE. Of the fifteen non-endemic seabird species found, nine or more are believed to breed on the island. Breeding colonies of the globally endangered Henderson petrel formerly existed on Ducie, but were wiped out by invasive rats by 1922. It is believed to now nest uniquely on Henderson island.
Bones associated with prehistoric Polynesian settlement sites dating to somewhere between 500 and 800 years ago include those of the Polynesian storm petrel, Marquesan imperial pigeon, and Polynesian or Pacific imperial pigeon which are no longer found on the island, and two others – Christmas shearwater and red-footed booby – that still visit but no longer nest. It is hypothesized that the Polynesian settlers may have driven these bird species, along with six terrestrial snail species, to local extinction, and this loss of a ready and regular food supply may have contributed to the Polynesians' subsequent disappearance. The island has been identified by BirdLife International as an Important Bird Area for its endemic landbirds and breeding seabirds.
The invertebrate species are largely unknown but a third of the island's known non-marine gastropods and insects are endemic. There are no native mammals but the Pacific rat, introduced by Polynesians 800 years ago, abounds. A skink (Emoia cyanura), and the green sea turtle have been identified, and an unidentified gecko has been reported. There are also crabs.
Land bird populations appear to be relatively stable but there is high risk of introduction to the island of predators, disease vectors and diseases by unauthorised landings of yachts. Introduction of the Eurasian black rat or the domestic cat would be likely to cause almost immediate extinction of the ground-dwelling Henderson crake and possibly other species. The endemic birds may have no immunity to the fatal avian pox which is transmitted by biting flies such as hippoboscidae.
Between July and November 2011 a partnership of the Pitcairn Islands Government and the Royal Society for the Protection of Birds implemented a poison baiting programme aimed at eradicating the Pacific rat. Mortality was massive but of the 50,000 to 100,000 population, 60 to 80 individuals survived and the population has now fully recovered.
Research published in April 2017 looked at debris on several beaches, and reported "the highest density of plastic rubbish anywhere in the world" as a result of the South Pacific Gyre. The beaches contain an estimated 37.7 million items of debris together weighing 17.6 tonnes. In a study transect on North Beach, each day 17 to 268 new items washed up on a 10-metre section. The study noted that purple hermit crabs (Coenobita spinosus) make their homes in plastic containers washed up on beaches.
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