Category: Henderson.

Henderson Island

Located around 170 km northeast from Pitcairn Island, the 9.6 km by 5.1 km island of Henderson takes the form of a raised coral plateau with an area of 37 km² — making it by far the largest island of the Pitcairn Group. The island was last inhabited by Polynesians in around 1600, having been settled since 800 AD.

The island is surrounded by steep, but low, limestone cliffs of around 15 m in height — much of which has been under cut by wave action, except in the north, forming overhangs and caves. Although largely cliff-lined, the island has 3 large sandy beaches that have formed around the northern coastline — nesting sites for Green Turtles (Chelonia mydas). Offshore there is a fringing reef of at least 200 m in width that surrounds the island on the north, northwest and northeast sides. Reefs off the north and northeast beaches are seaward sloping reef platforms without reef crests, and are not typical fringing reefs. There are two narrow channels through the reef on the north and northwestern coasts. The interior plateau of the island has an elevation of up to 30 m and is largely composed of treacherous, dissected limestone formations and reef-rubble. Much of the interior forms a central depression that is thought to be all that remains of its uplifted lagoon.

Henderson Island is a classic example of a makatea formation — a large coral limestone mass that has been geologically uplifted high above sea level. Originally formed as a volcanic structure rising from the seabed, the volcano has long since disappeared and is now capped by aeons of coral growth. It is these layers of fossilised reef that forms the structure of the island to a great depth. Local seismic activity and global changes in sea levels have raised the island out of the water so that it is now largely surrounded by limestone cliffs. The island is thought to have been above the sea surface for the last 380,000 years — its rocks have been used as a record of climate change with some of its ancient reefs having being dated to around 600,000 years ago.

Most of Henderson Island is densely vegetated with forest and thick scrub that has been undisturbed for the last 400 years — providing a valuable insight in to the original vegetation of remote Polynesian islands. There are very few non-native plant species on the island. The dense forests are dominated by species such as Pisonia grandis, Celtis pacifica, the endemic Nesoluma st-johnianum and numerous other species such as the endemics Santalum hendersonense, Myrsine hoskae, Celtis paniculata var. viridis, and two endemic varieties of Bidens hendersonensis. The canopy is generally low in height being between 5 and 10 m — the tallest trees are the Screw-pine Pandanus tectorius. Hardwoods such as Cordia subcordata are also found on the island. The shrub layer can be sparse in certain locations, but is generally dominated by Psydrax odoratum and the endemics Ixora fragrans and Geniostoma hendersonense. The ground layer is dominated by ferns and the endemic Peperomia hendersonensis. In the exposed southern parts of the island mainly stunted, salt-tolerant species are found, while in the central depression vegetation cover is less dense.

All four of the island’s land birds are endemic: the Henderson Fruit Dove (Ptilinopus insularis), the flightless Henderson Rail (Nesophylax ater), Henderson Warbler (Acrocephalus vaughani taiti) and Stephen’s Lorikeet (Vini stepheni). Seabird species are numerous on the island, with ten species breeding on the island: Blue-grey Noddy (Procelsterna caerulea), Brown Noddy (Anous stolida), Fairy Tern (Gygis alba), Herald Petrel (Pterodroma arminjoniana), Kermadec Petrel (P. neglecta), Murphy’s Petrel (P. ultima), Phoenix Petrel (P. alba), Masked Booby (Sula dactylatra), Red-tailed Tropicbird (Phaethon rubicauda) and the Shearwater (Puffinus pacificus). The island has no native mammalian species — the only mammal being the introduced Polynesian Rat (Rattus exulans).

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Category: Henderson
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