Ecosystem of Henderson Island

Makatea

Henderson Island consists of limestone – it is elevated coralline island (makatea). Island is a top of a reef-capped extinct volcano, rising 3.5 km from the depth of the Pacific.

As the island is raised higher above the sea and very remote, it contains unique species of animals and plants.

Lonely island

There are smaller islands a few hundred kilometers from Henderson Island but no major landmass in 5000 km distance. Island is surrounded by 15 – 30 m tall, undercut limestone cliffs. Only in some places, mostly in the north, there are beaches.

Freshwater also is very scarce on the island. Water seeps into some caves. There have been found also several springs on the northern side of the island which for most time are under sea level – only at very low tide, it is possible to get brackish water from these springs.

Ancient Polynesians

Up to 100 Polynesians lived on the island between the 12th and 15th centuries. They left few traces – including an eerie group of six skeletons found in one cave in 1958. Unfortunately, they left also a negative impact on the ecosystem of the island – it is considered that several species of birds were exterminated by them. Forest though was left intact, just smaller fields were made.

Since then humans have not lived on this island except for several short-term occasions.

Inaccessible forest

Most of the island is covered with tangled scrub and 5 – 10 m tall forest.

Researchers have recognized that walking on Henderson island is dangerous to life – one wrong step might end with a fall on sharp stones. Whole island is covered with thicket and there are countless deep pits and sharp limestone pikes hidden under this thicket.

This inaccessibility is one of the main reasons why this island has been preserved that well.

Unique animals and plants

Henderson Island is poorly researched and it is possible to find new species unknown to science here. Thus far here have been found more than 60 species of organisms unique to this island.

Endemic plants

Out of 63 known native plants the following 9 are endemic:

• Alyxia fosbergii – up to 8 m tall shrub with orange fragrant flowers;
• Bidens hendersonensis var. hendersonensis and Bidens hendersonensis var. spathulata – shrub or tree. Other plants of this genera are more grass-like, this is the only tree in this species rich genera.
• Geniostoma hendersonense;
• Ixora fragrans – fragrant ixora;
• Myrsine hosakae – a tree;
• Nesoluma st-johnianum;
• Peperomia hendersonensis (Peperomia rhomboidea);
• Santalum insulare var. hendersonense – tree with beautiful wood, used for decorative woodcarving.
• Xylosma suaveolens var. haroldii – a tree.

A tree Hernandia stokesii is also very rare – it is found only on Henderson Island and 1600 km distant Rapa Island.

Endemic birds

All four land birds of Henderson Island are found only on this island. These birds are:

• Henderson crake (Porzana atra, earlier Nesophylax ater) – small, black, flightless bird.
• Henderson fruit dove (Ptilinopus insularis) – small, pale-blue and green pigeon.
• Henderson warbler (Acrocephalus taiti, earlier Acrocephalus vaughani var. taiti) – large, nearly all white warbler.
• Stephen’s lorikeet (Henderson lorikeet, Vini stepheni) – bright green, red and blue parakeet.

Other animals

Thus far there have been found 16 species of land snails, 8 are endemics – including Diastole glaucina, Georissa hendersoni, Tubuaia hendersoni and Minidonta hendersoni. There have been studied also some 180 species of insects – approximately 35 – 40 are endemics, including a distinct, orange colored species of hawk-moth Hippotion hateleyi.

References

1. Henderson Island World Heritage Site, Management Plan 2004 – 2009. 2004. Accessed in 22.04.11.
2. World Heritage Nomination – IUCN Summary. 487: Henderson Island (United Kingdom). Accessed in 22.04.11.
3. Steven Waldren, Henderson Island Flora. Accessed in 22.04.11.