Henderson Island Michael Brooke
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Important Bird Areas Henderson Island Michael Brooke Abstract Subtropical Henderson Island is one of the Pitcairn Islands in the central South Pacific. Although over 600 years of Polynesian occupation have left an ecological mark, not least the continuing presence of Polynesian Rats Rattus exulans, the 37-km2 island is the Pacific’s best example of an ecologically intact raised coral island. For this reason it was designated a World Heritage Site in 1988. Four landbirds occur, all endemic to the island, of which the most distinctive is the flightless Henderson Crake Porzana atra. The others are a lorikeet, a fruit dove and a warbler. Seabirds abound. These comprise a mix of widespread tropical species such as boobies and noddies, and species with more restricted Pacific distributions, notably Pterodroma petrels. For one of these, the Henderson Petrel P. atrata, Henderson is the only known nesting station. However, fieldwork has established that the rats are devastating predators of petrel chicks. To benefit petrels and to achieve wider ecosystem restoration, rat eradication is now being actively planned. If successful, it would be easily the largest rat eradication achieved on British territory. enderson Island is a name that is Remote does not imply unwelcoming. perversely uninformative. It neither Mean monthly temperatures vary from Hgenerates an instant association with around 23°C (February) to 16°C (June), one of the giants of exploration, like the Ross while annual rainfall, about 1,600mm, is Sea, nor smacks of fire and brimstone, like plentiful but not excessive (Spencer 1995). Tierra del Fuego, nor smells of frangipani Trade winds from an easterly or southeasterly wafting across the South Seas, like Tahiti. But direction predominate. Benign indeed, espe- prosaically named Henderson, one of the cially in the absence of mosquitoes and other UK’s two World Heritage Sites designated for invertebrate irritants. its natural history, as opposed to cultural, From afar, Henderson appears as a low interest, is in fact a South Pacific conserva- slab barely showing above the horizon. But tion jewel. increasing numbers of gadfly petrels hint to Lying just south of the Tropic of Capri- the approaching seafarer that land is not far corn at 24°20’S 128°20’W, Henderson is one off. Most of that land is a pan-flat plateau, of the Pitcairn Islands. The other islands of about 30 m above sea level. Two-thirds of the this UK Overseas Territory are Pitcairn itself, perimeter is surrounded by vertical cliffs the only inhabited island and famously the dropping into blue waters that are supremely hideaway of the Bounty mutineers, and the clear because ocean productivity in the low atolls of Oeno and Ducie (all three of region is low. The remaining third of the them also Important Bird Areas). To a first island is girt with coral reefs, a narrow lagoon approximation, the islands are halfway and white beaches. On one of those beaches, between New Zealand’s North Island and the East Beach, Green Turtles Chelonia mydas Ecuador. Some 4,500 km from the nearest nest. Behind the beaches are pockets of continental land, 37-km2 Henderson is one of woodland which give way to fern-clad slopes the world’s remotest islands (fig. 1). rising towards the plateau woodland. 428 © British Birds 103 • August 2010 • 428–444 Henderson Island The plateau woodland rises to about 6 m and is thick, so thick in fact that it is difficult and sometimes impossible to walk except where a path has been cut by machete. Also impeding progress are the rough condi- tions underfoot: brittle slabs of dead coral, giant clam shells, limestone dissected by 2-m ‘crevasses’. These provide the clue to Henderson’s origin and ultimately its biological interest. The volcanic base of Henderson, emerging from the abyssal plain 2 km below, is about 13 million years old (Spencer 1995). When this emerged from the sea, an island surrounded by reefs was presumably formed and, Art © Fluke in time, this developed into a Fig. 1. Henderson Island and the Pitcairn group. low atoll. Then, around one million years ago, 170 km to the southwest, History Pitcairn erupted. As a result, the earth’s crust In 1988, Henderson’s natural history was not under Pitcairn was depressed while, by a well known, since visits had been few and far process of lithospheric flexure, the crust between. European discovery is attributed to under Henderson started to rise. So too did the Portuguese sailor Pedro Fernandez de the overlying Henderson Island, and it con- Quiros, who passed without landing in 1606 tinues to rise at about 1 mm per decade. The (but named it San João Baptista). The island result is that the coral of the plateau has now received its current name when visited in been high and dry and very dead for about 1819 by the British East India Company ship 380,000 years (Blake 1995). Hercules under the command of Captain Most of the Pacific’s raised coral or Henderson. The following year witnessed one makatea islands of any size have been sub- of the most remarkable episodes in Hen- stantially damaged by human settlement. Not derson’s recorded history. Three small whale- so Henderson, which currently has no boats arrived carrying the surviving crew of permanent human inhabitants. What makes the whaler Essex, wrecked 2,700 km to the this particularly significant is that the island’s north by an irate Sperm Whale Physeter gradual continuing uplift has enhanced catodon, the inspiration for Herman Henderson’s biological interest. No longer do Melville’s Moby Dick. After a hungry, week- tropical storms wash over the island, setting long sojourn on Henderson, the 17 seamen the evolutionary clock back to zero. Instead, set forth in their boats for Chile. The voyage terrestrial plants and animals reaching the saw intense hardship, the disappearance of island have the opportunity to evolve and one boat, and cannibalism. Only five men ultimately speciate into endemic taxa. It was survived. Meanwhile, the three sailors who this combination of high endemism and an elected to remain on Henderson were rescued intact, distinctive ecosystem relatively unbur- after a stay of four months (Philbrick 2000). dened with introduced species that led to Not surprisingly, this episode added little to Henderson’s designation as a World Heritage the stock of scientific knowledge. This stock Site in 1988. was, however, increased a century or so later British Birds 103 • August 2010 • 428–444 429 Brooke Michael Brooke 242. While apparently pristine and used by nesting Green Turtles Chelonia mydas, Henderson’s East Beach is depressingly cluttered with such jetsam as fishing buoys and plastic bottles. Michael Brooke 243. Climb a tree in the centre of Henderson and the view across the canopy cloaking the flat plateau is the same in every direction. A compass is essential! All photographs in this article were taken during a visit to Henderson in August/September 2009. 430 British Birds 103 • August 2010 • 428–444 Henderson Island by a phosphate-prospecting party which evidence of this occupation, which is obvious spent about five months on Henderson in even to a non-specialist, for example human 1912 under D. R. Tait, without finding com- bones and crafted clam shells. A large mercially useful deposits. midden crammed with fish and bird bones Specifically scientific visits followed from extends over some 300 m by 30 m under the the Whitney South Seas Expedition (1922), woodland behind the North Beach, in the the Mangarevan Expedition (1934) and the very spot opposite the reef passage used as a Westward Expedition (1971). In the early campsite by modern expeditions. The ebb 1980s, an American strip-mining millionaire, and flow of the Polynesian occupation was A. M. ‘Smiley’ Ratliff, proposed building a unravelled by Marshall Weisler of the holiday home and airstrip. This proposal 1991/92 Expedition and brought to wider drew howls of anguish from the conservation notice by Jared Diamond (2005) in his book community, indirectly contributed to the Collapse. World Heritage designation, and prompted a In the early centuries of the occupation, realisation that Henderson was inadequately those few tens of people dwelling on Hen- documented. A small Smithsonian Exped - derson maintained contact with Pitcairn, 170 ition in 1987 might have been the precursor km to the southwest, and Mangareva in the to a larger venture. However, this did not Gambier group of southeast French Poly- eventuate and the Sir Peter Scott Commem - nesia, a further 450 km to the northwest. The orative Expedition to the Pitcairn Islands of evidence of contact with those two islands 1991/92 filled the gap. The expedition comes from archaeological items such as vol- involved 34 people from seven countries, canic glass tools and oyster-shell fish-hooks with expertise in most branches of natural respectively, derived from materials not avail- history. Their efforts, spread over 15 months, able on Henderson itself. While the bones of documented Henderson’s biota, geology and children suggest that the Henderson popula- archaeology. Since then, there have been tion was resident, it was very possibly supple- several shorter follow-up visits, but the sheer mented by visitors from Pitcairn at key times difficulty of reaching isolated Henderson and of year, for example during the turtle nesting the inaccessibility of most parts of the island season, which runs from January to March. still curtail scientific progress. However, around 1450, inter-island contact It was this isolation that shaped the period ceased. Instead of Pitcairn’s volcanic stone, of Polynesian occupation, which began the inhabitants turned to locally sourced around AD 900 (Weisler 1995). To this day, giant clam shells for their stone tools, Hen- nearly all of Henderson’s larger caves show derson’s limestone being wholly unsuitable.