Ascension scrub and grasslands

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Ascension Island, United Kingdom. (Photograph by Tony Baczynski)

Introduction

Located just west of the mid-Atlantic ridge, Ascension Island, with its surrounding islets and seastacks, is one of the most important breeding places for seabirds in the tropical Atlantic. It has a relatively dry, barren landscape marked by distinctive lava fields. Only about one million years old, it is one of the Atlantic’s youngest islands and therefore presents a fauna quite different from those of older islands. Ascension has been inhabited by humans since 1815, and has suffered serious degradation as a result of exploitation and the introduction of destructive animal species.

Location and General Description

caption WWF

Ascension Island (7°56'S, 14°22'W) lies in the equatorial Atlantic Ocean, 1,200 kilometers (km) northwest of the Island of St. Helena and 1,700 km from the nearest mainland in Africa. The island covers 97 square kilometers (km2), with a high peak of 859 km at Green Mountain. Ascension is a relatively young volcanic island with 44 distinct craters, all considered "dormant" rather than extinct. The last major volcanic eruption occurred approximately 600 years ago. Much of the island remains covered by basalt lava fields and cinder cones, and steep cliffs mark the south and southeast coasts. There is a sloping plain to the northwest and sandy beaches. Ascension Island’s climate is subtropical with temperatures of 10-32°C. Rainfall is relatively low, with an annual mean of 709 millimeters (mm), and showers occur throughout the year with slightly heavier rain January-April.

caption WWF

Ascension Island was discovered in 1501 by the Portuguese sailor Joao da Nova Castelia, and was then visited two years later on Ascension Day by Alphonse d'Albuquerque, who named the island. The dry, barren nature of Ascension made it of little use to the East Indies fleets, and it remained uninhabited until Napoleon was incarcerated on St. Helena in 1815 and a small British naval garrison was established. The island served use as a resting stop for ships engaged in the suppression of the slave trade around the West African coast during this period. Ascension was managed by the Royal Marines from 1823 until it was made a Dependency of St. Helena in 1922, and was then managed by the Eastern Telegraph Company, later named Cable and Wireless. In 1964, radio broadcast stations were established to facilitate transmissions to southern Africa, and an Administrator was appointed.

Forests were never present on Ascension Island. Much of the island, particularly to the north and the west, is marked by barren desert areas with some grass interspersed with the endemic Euphorbia origanoides, while a thick scrub of introduced species and Opuntia occurs at higher elevations. The high peak of Green Mountain is covered with lush natural vegetation that is increasingly spreading throughout the island. Twenty-five native vascular plants are recorded for Ascension Island, with 6 endemic ferns and 5 endemic flowering plants. One of these, Oldenlandia adscensionis (a rubiaceous shrub) has not been seen since 1889, and is probably extinct. The following endemic plants are endangered: two grasses, Sporobolus durus (possibly extinct) and Sporobolus caespitosus (few tufts remaining on Green Mountain at 730 m); Dryopteris ascensionis (a single plant was seen in 1976, occurs in moist ravines), Pteris adscensionis, and Anogramma ascensionis. Rare endemics include Euphorbia origanoides, Asplenium ascensionis, Marattia purpurascens (a fern), and Xiphopteris ascensionense. Endemic lichens and bryophytes are also known from Ascension Island. The majority of plants occurring on the island are introduced species.

Biodiversity Features

Because it is such a young island, Ascension’s indigenous fauna provides a view of an early stage in the processes of colonization, adaptive evolution, and radiation. Older islands, such as Ascension's closest neighbor, St. Helena Island a separate ecoregion, present richer and more distinctive faunas where, for example, a few invertebrate taxa have undergone repetitive speciation and some adaptive radiation. At least 311 species of land animals (a few of which are now extinct) have established themselves on Ascension Island, while only about 95 species are considered to be native. This includes two marine turtles, twelve seabirds, and two extinct landbirds. Natural colonists arrive primarily by air, though some apparently crossed the ocean attached to birds or other animals or on floating objects. The natural colonization of Ascension Island was almost entirely from Africa. In 1997, Philip and Myrtle Ashmole published a reconstructed picture of prehistoric fauna on Ascension:

caption Ascension Island, United Kingdom. (Photograph by Tony Baczynski)

"Before the arrival of humans, Ascension had an early successional ecosystem. The fauna in the lava and cinder deserts of the lowlands-both on the surface and in subterranean cracks and caves-was dominated by taxonomically varied scavengers and mainly arachnid predators. The scattered angiosperms here and in the foothills supported some host-specific herbivores with associated predators; they were also exploited (especially after exceptional rains) by a number of Orthoptera, Hemiptera, and Lepidoptera derived from migratory African populations and perhaps reinforced at intervals by additional groups of colonists. The more extensive and largely cryptogamic vegetation on the central peak had a poor fauna probably composed mainly of micro-arthropods. Along coasts, on islets and in the extensive seabird colonies there were additional arthropod species and also a flightless rail and a night heron (both now extinct). Invertebrate stocks that colonized Ascension underwent a variety of evolutionary changes including phyletic evolution leading to endemic status, adaptation to subterranean life (Araneae, Pseudoscorpiones, Collembola, and Psocoptera), character release (phorid Diptera), and probably splitting of lineages (speciation) within the island (Isopoda, Collembola, and gryllid Orthoptera). The relatively high diversity of Pseudoscorpiones (five species in five genera) and their 100% apparent endemicity is notable."

While at one time there were millions of sea birds on Ascension Island, the introduction of feral cats in the early 1800’s eradicated the majority of them. The sooty, or "wideawake", tern (Sterna fuscata) continues to breed on the island, with large numbers using an area called the "fairs" on the southwest coast on a ten-month breeding cycle. All other species nest on inaccessible cliffs and offshore islands where they are safe from predators. Boatswain-Bird Island is a flat-topped rock some 365 m long and 90 m high just off the east coast of the main island. The endemic Ascension frigatebird (Fregata aquila), which subsists on food stolen from other birds, lives on Boatswain Island exclusively now. Other seabirds include white booby (Sula dactylatra), brown booby (Sula leucogasta), fairy tern (Gygis alba), Madeiran petrel (Oceanodroma castra), and black noddy (Arious tenuirestris). Two prominent birds are the red-billed boatswain (Phaethon aethereus) and the yellow-billed boatswain (Phaethon lepturus), both of which have showy tail feathers that are over half the length of their body. Some introduced resident landbirds are waxbill (Estrilda astrild), common myna (Acridotheres tristis), and canary (Serinus flaviventris).

Ascension is also known for its green turtles (Chelonia mydas), which nest on the island’s beaches from January to May. Tagging studies of the turtles have shown that they "shuttle" back-and-forth between this remote island and their feeding grounds on the Brazilian coast, a distance of at least 2,300 km. A detailed survey of green turtles conducted in 1998-1999 suggested that two to three times more turtles were nesting on Ascension than when previous surveys were made in the 1970's. Hawksbill turtles (Eretomochelys imbricata) are also present and a native land crab, Gecarcinus lagostoma, lives on the island. There are no native terrestrial mammals on Ascension Island, though the island is home to feral donkeys, sheep, cats, rabbits, rats, and mice. Law protects all wildlife, with the exception of feral cats, rabbits, rats, and mice.

Current Status

Due to human exploitation and introduced predators, the populations of most of Ascension Island’s seabirds have seen significant declines since the island's discovery by humans in 1501. Many of the persisting seabirds have been exiled to the offshore islets by introduced predators. The fragile flora ecosystem has suffered as well, with many endemic species extinct or endangered.

Types and Severity of Threats

The introduced Mexican thorn bush (Prosopis juliflora) has become a significant threat to marine turtles. Land development continues to be a conservation issue on Ascension.

A particular challenge for conservation of Ascension Island’s biodiversity is the need to eradicate feral cats and to control rats, donkeys, and sheep. Initiatives to restore Ascension's seabird colonies have included proposals to exterminate introduced rats with an island-wide application of the anticoagulant rodenticide, brodifacoum. Pain et al. conducted a study of the effects of brodifacoum on the island’s native crab, Gecarcinus lagostoma, and found that no crabs died and no residues were present in the crabs’ body tissues one month after exposure. Further investigations will surely be needed to assess the feasibility and safety of this action. A beetle may be introduced with the aim of controlling the salt-tolerant Mexican thorn.

Justification of Ecoregion Delineation

Ascension is an isolated oceanic island, 1,200 km northwest of it nearest neighbor, St. Helena Island and 1,700 km from the nearest mainland in Africa. Its isolation and relatively young age (about one million years) make Ascension biogeographically unique. Ascension’s indigenous fauna provides a view of an early stage in the processes of colonization, adaptive evolution, and radiation. This is markedly different from native fauna on the older island of St. Helena.

Additional Information on this Ecoregion

Further Reading

  • Ashmole, N. P. and Ashmole, M. J. 1997. The land fauna of Ascension Island: New data from caves and lava flows, and a reconstruction of the prehistoric ecosystem. Journal of Biogeography 24:549-589.
  • Godley, B. B. A. H. G. 2001. Nesting of green turtles Chelonia mydas at Ascension Island, South Atlantic. Biological Conservation 97:151-158.
  • Luschi, P., Hays, G. C., Del Seppia, C., Marsh, R., and Papi, F. 1998. The navigational feats of green sea turtles migrating from Ascension Island investigated by satellite telemetry. Proceedings of the Royal Society of London Series B Biological Sciences 265:2279-2284.
  • Pain, D. J., de L. Brooke, M., Finnie, J. K., and Jackson, A. 2000. Effects of brodifacoum on the land crab of Ascension island. Journal of Wildlife Management 64:380-387.
  • UKOTCF. 1999. UK Overseas Territories Conservation Forum. Retrieved (2001).
  • UNEP. 1990. Islands of Ascension Island (United Kingdom), (UNEP) United Nations Environment Programme, Geneva, Switzerland. Retrieved (2001).

 

 

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

 

 

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Citation

Fund, W. (2014). Ascension scrub and grasslands. Retrieved from http://www.eoearth.org/view/article/150256

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