St. Helena—one of the most isolated inhabited islands in the world—lies at latitude 15°58' S and longitude 5°43' W, about 800 km east of the Mid–Atlantic Ridge in the southern Atlantic Ocean. The island played a significant part in the development of biological concepts such as endemism, extinction, and the origins of insular biota. Less than a decade after publication of Charles Darwin's The Origin of Species, the botanist Joseph Hooker used his firsthand knowledge of St. Helena in a seminal lecture on insular floras to the British Association for the Advancement of Science, and the chapter on St. Helena in Alfred Russel Wallace's Island Life (I892) includes an elegant analysis of key factors in the colonization of oceanic islands and the origin and evolution of their distinctive endemic species. In relation to current thinking, the isolated islands of St. Helena and its distant neighbor Ascension provide insights into evolutionary processes distinct from those offered by archipelagoes such as the Galápagos and Hawaiian Islands.
Africa is the nearest continent to St. Helena, with the coast of Angola lying I800 km to the east and South America resting 3260 km to the west. Ascension Island, the closest land, lies 1300 km to the northwest. The island lies in a region of low marine productivity, although it is influenced by the Benguela current flowing to the northwest from the coast of southern Africa. The richer waters close inshore probably once supported seals, and there are still numerous dolphins and moderately diverse marine life, although coral reefs are absent. St. Helena has a subtropical oceanic climate with relatively slight seasonal change. The southeast tradewinds, dominant throughout the year, generate much condensation on the mountains. Annual rainfall is about I m on the central ridge, but less than a fifth of this in the driest areas.
St. Helena is about I7 km long and I0 km wide, with an area of 121.7 km2. It is the eroded summit of a 5000–m conical volcanic pile rising from the floor of the deep ocean. The island emerged into the air some 12–14 million years ago, eventually forming a mountain much higher than the modern central ridge, which still reaches 820 m above sea level. The rocks were laid down by successive activity from two volcanic centers producing long series of eruptions, mainly of relatively liquid basaltic lavas from fissures now preserved as striking dike swarms. The northeastern volcano was active during the emergence of the island and for up to another 3 million years. Much of it has now disappeared, but the north of the island is made up of rocks generated during this phase, comprising basal breccias and the predominant mass of subaerial basaltic lavas and pyroclastics.
The southwestern volcano created the greater part of the modern island between 11 and 7 million years ago. Its initial phase produced the lower shield, composed mainly of pyroclastics, the erosion of which formed the great amphitheater of Sandy Bay, open to the southeast.
The main shield, made up primarily of basalt and trachy–basalt lava flows, forms most of the southern and western parts of the island. Recent analysis by Ian Baker indicates that about 9 million years ago a huge landslide removed several cubic kilometers of the eastern flank of the older (northeastern) volcano. The resultant depression was rapidly infilled by thick trachybasalt and trachyandesite flows erupting from near the modern peaks. These eastern flows (previously termed the upper shield) formed most of the relatively level areas in the northeast of St. Helena.
A late intrusive phase, which occurred about 7.5 million years ago, injected massive intrusions of trachyte and phonolite into the southwestern volcano; some of these are now exposed, forming a series of landmarks. There has been no later volcanic activity, so marine and terrestrial erosion has shaped the modern island. It is ringed by sea cliffs up to 400 m high, interrupted by steep–sided valleys, and is surrounded by a broad and irregular shelf created by wave action. This was largely exposed during glacial episodes in the Pleistocene, sometimes doubling the size of the island.
St. Helena was untouched by humans until its discovery by Joao de Nova in 1502. For one and a half centuries it remained free of resident people but was frequently visited, initially by Portuguese mariners and later also by the Dutch and English. Settlement was organized in 1659 by the English East India Company, and the island became an important staging point on return voyages from Asia. St. Helena came to international prominence in I815 when Napoleon Bonaparte was exiled there, where he remained until his death in I821. Later in the nineteenth century, the island was used as a base by the British navy for suppression of the slave trade, and since that time it has been governed from Britain. The resident population had origins in England, West Africa, Malaysia, China, the Maldive Islands, and many other places. Natural resources are few, and the economy of the island is heavily subsidized. Access is only by sea, although an airport is now planned.
The extreme isolation of St. Helena has ensured that few kinds of land plants and animals reach it naturally; it prevents colonization by poor dispersers such as freshwater fish, amphibians, and terrestrial reptiles. However, the considerable age of the island has led to high rates of endemism among successfully colonizing taxa and to some splitting of lineages after arrival. The affinities of the indigenous biota are overwhelmingly with southern Africa.
Of the 37 endemic flowering plant species (six ofwhich are now extinct), the four species of Commidendrum and the one species of Melanodendron (both Asteraceae) probably arose from a single dispersal event, and the three modern species of Trochetiopsis (Sterculiaceae) derive from one other colonization. Additional stocks are represented by the seven monotypic genera Trimeris (Campanulaceae); Lachanodes, Petrobium, and Pladaroxylon (all Asteraceae); Nesiota (Rhamnaceae); Nesohedyotis (Rubiaceae); and Mellissia (Solanaceae) and by several other groups with one or more endemic species. Half a dozen non–endemic flowering plants are coastal halophytes and are probably native, but almost all of the roughly 250 additional species have been introduced, from many parts of the world. St. Helena also has 30 species of native ferns (about 13 of them endemic) and a rich bryophyte flora.
Fossil evidence indicates that ancestors of the modern Trochetiopsis and Lachanodes species, and also the tree fern Dicksonia and other fern genera, were already present some 9 million years ago. Also represented among the fossils are plant groups not found in the modern flora, including two lineages of palms (including Voamniola)and the genus Gunnera (Haloragaceae), which probably became extinct as a result of major volcanism more than 7 million years ago.
The only land vertebrates that reached St. Helena naturally were birds. Deposits of fossil bird bones show that the island once supported large colonies of seabirds, which are now reduced to remnants. At least eight species of seabird have been lost to the island, and three of them are now extinct: Bulweria bifax, Pterodroma rupinarum, and Puffinus pacificoides (earlier extinction). Endemic landbirds known only from fossils are the two rails Aphanocrex (ex Atlantisia) podarces and Porzana astrictocarpus, a cuckoo Nannococcyx psix, a hoopoe Upupa antaios, and a dove Dysmoropelia dek–archiskos. Songbirds are not represented in the fossil record. The only surviving endemic bird is the wirebird Charadrius sanctaehelenae. Introduced landbirds comprise two game–birds, two pigeons, and five passerines.
Much of the zoological interest of the island stems from its diverse endemic invertebrates. Around 80 genera and 400 species (~38% of the fauna) of invertebrates are currently recognized as endemic and many species–as in the plants—are highly distinctive descendants from ancient colonizations. Beetles are by far the most diverse group, with about 150 endemic species (~58%) and 32 endemic genera. There have been spectacular radiations in the weevil family Curculionidae (77 endemic species) and the related Anthribidae (27 endemics). Speciation has also been striking in the Lepidoptera, with some 50 endemic moth species including at least 20 in the genus Opogona (Tine–idae). Spiders also have about 50 endemic species (~48%), but these are spread among many families. Other groups showing significant radiations include snails of the family Charopidae, in which the only survivor is the ammonite snail Helenoconcha relicta; bugs of the subfamily Phylinae (family Miridae) with at least ten species; and the homop–teran family Cicadellidae with 13 endemic species probably derived from only a few colonizing stocks. The best known endemic invertebrate is the St. Helena giant earwig Labidura herculeana (Labiduridae). This is the world's largest dermapteran, with a maximum length of over 80 mm, but it has not been seen alive for half a century.
The original habitats of St. Helena were diverse. The fringes of the island were either semi–desert or were occupied by scrubwood formed by several endemic shrubs. Inland from these were ebony gumwood thicket and dry gumwood woodland, with the latter grading upward into moist gumwood woodland and then into cabbage tree woodland containing half a dozen tree species. The highest part of the central ridge (above 700 m) was once covered by a cloud forest of tree fern thicket with the most drought–intolerant endemic trees and shrubs. The moist mountain habitats still support a high proportion of the rich fern and bryophyte flora of the island.
Invertebrate diversity is also highest in the forests on the central ridge, but surviving fragments of dry gum–wood woodland and the semi–desert regions also have many endemic species. Of particular interest is an arid area in the east comprising Prosperous Bay Plain and its immediate surroundings, which unfortunately is also the most appropriate site for an airport. The area has six genera and around 40 species of invertebrates that have been recorded only in this part of the island and nowhere else in the world. The fauna of the plain includes a remarkable array of endemic nocturnal wolf spiders (Lycosidae).
Discovery of St. Helena in 1502 was followed by introduction of a range of herbivores and predators. Pigs, dogs, cats, and rats decimated seabird colonies and drove vulnerable endemic land birds to extinction. Goats, released within a decade or so of 1502, were the prime destroyers of the native vegetation, although rabbits, doubtless, also played a part. Tree felling for timber and firewood, along with burning of wood in the making of lime, became major factors after settlement in 1659. By the early nineteenth century, much of the island had been denuded of native trees. Near the end of the same century, cultivation of the New Zealand flax Phormium tenax was started and soon led to massive destruction of surviving native forest on the central ridge. Invasion of forest remnants by introduced plants (especially flax) continued through the second half of the twentieth century. Drier parts of the island also have many invasive plants, including the creeper Carpobrotus edulis,the prickly pear Opuntia spp., and Lantana camara.
Within a few decades of settlement, some enlightened administrators of St. Helena instituted measures to prevent extinction of an endangered tree species (St. Helena redwood Trochetiopsis erythroxylon) and to conserve the native forests. Failure to ensure continuity of effort resulted in reduction of the original forests that once covered the greater part of the island to about I ha of gumwood woodland and 16 ha of cabbage tree woodland and tree fern thicket along the central ridge. The ecological devastation of St. Helena through human agency, recorded in the correspondence of the East India Company, was one of the examples that influenced the development—during the late seventeenth to early nineteenth centuries—of western philosophical and scientific ideas about the relationship of humans with the natural world.
In recent years, serious efforts have been made to halt the loss of native forest and to care for relict stands of endemic plants. Establishment of the Diana's Peak National Park in 1996 focused attention on a key area. Educational efforts have raised awareness of endemic species, and the endangered St. Helena wirebird is the subject of a special conservation initiative. There have been important conservation gains, including rediscovery and protection of the St. Helena ebony Trochetiopsis ebenus and boxwood Mellissia begoniifolia (Fig. 1), both of which had been thought to be extinct. However, the recent death of the last specimen of the St. Helena olive Nesiota ellip–tica (Fig. 2), a generic endemic, is a poignant reminder that in attempting to preserve extremely rare species, success can only be assured in the short term, whereas failure is forever.
Recent plans for construction of an airport and the development of tourism will bring new threats to the native habitats and species on the island. Conservationists on the island are continually overstretched, and resources are limited. Nonetheless, some initiatives in ecological restoration have been undertaken. These aim to re–create large areas of native forest and have sometimes involved participation by a large proportion of the population. Experience suggests that the future of the natural environment and biodiversity of St. Helena will depend on the presence of individuals who care and have relevant expertise, on funding and support by the authorities even in the face of commercial pressures, and on the maintenance of protection and restoration measures over the long term.
Ascension / Deforestation / Fossil Birds / Insect Radiations / Introduced Species / Wallace, Alfred Russel
- St Helena and Ascension Island: a natural history. Oswestry, UK: Anthony Nelson (current distributor: www.kidstonmill.org.uk). , and . 2000.
- St Helena–one man's island. Windsor, UK: Wilton 65. 2004.
- The endemic flora of St Helena. Oswestry, UK: Anthony Nelson. 2000.
- Fish and fisheries of Saint Helena Island. Newcastle upon Tyne, UK: Government of Saint Helena and the University of Newcastle upon Tyne. 1990.
- Green imperialism: colonial expansion, tropical island edens and the origins of environmentalism, 1600–1860. Cambridge: Cambridge University Press. 1995.
- The birds of St Helena. Tring, UK: British Ornithologists' Union Checklist Number 16. British Ornithologists' Union. , , , , and . 1998.
- A guide to the geology of Ascension Island and St Helena. School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA. 1999.
Saint Helena is a remote island in the South Atlantic, 1,920 km [1,190 mi] from the coast of west Africa. It is volcanic, though...
This island was discovered, uninhabited, on 21 May 1502 by the Portuguese Admiral Joao da Nova, who named it St Helena. First settled by the...
Rocky island in the S Atlantic, 1,920km (1,190mi) from the coast of W Africa; the capital is Jamestown. Discovered by the Portuguese in 1502, it...