Aldabra

Introduction

Aldabra is 1150 km south of Victoria, the capital of the Seychelles Islands, on the island of Mahé. It is closer to Africa (640 km) and Madagascar (400 km) than to its own capital.

Aldabra is of volcanic origin and is estimated to have risen out of the sea only 50 000 years ago, a mere moment in geological terms, and all life has arrived or evolved here since. The atoll rises from ocean depths of more than 4000 metres suddenly, as if from nowhere. Aldabra was probably discovered by Portuguese navigators early in the 16th century. It was first mentioned on maps in 1511 and referred to as Ilha Dara and Alhadra, the name Aldabra seems to have been a corruption of the Arabic word 'Al Khadra' meaning 'the green'. The first recorded sighting of Aldabra was by Captain Lazare Picault and Captain Jean Grossin in 1742 (during the voyage which discovered the Seychelles). They reported that its large lagoon produced a green reflection in the sky above the atoll that they could see for many miles out to sea.

Its geological youth, remoteness and the unique fauna have earned it the name 'Galapagos of the Indian Ocean'. In fact Darwin, who made the Galapagos tortoise famous, also played a role in the early conservation of the Indian Ocean tortoise so prolific on Aldabra. Others, notably Sir Julian Huxley have called Aldabra "one of nature's treasures". He felt it should be preserved for the whole world. When, in the early 1960's, it became known that Aldabra was to be surveyed for a major BBC station and a military airfield, the Royal Society, the Smithsonian Institution and the National Academy of Sciences in Washington raised vigorous opposition to the plans and support for the conservation of Aldabra's unique environment. Since then the formal threats to Aldabra have passed and the United Nations has proclaimed Aldabra a World Heritage Site. It is managed by the Seychelles Islands Foundation (SIF), which is sponsored by the Royal Society and the Smithsonian amongst others. The atoll is virtually off limits to visitors unless special permission is granted through the SIF.

The island manager and his small staff make up the only inhabitants of the island, there are no permanent residents as such. They grow a meagre supply of vegetables on the island but much is brought in by irregular steamer from Mahé.

Aldabra is visited by a few scientists for some months of the year. On West Island there are disused fishing settlement huts, a camp of wooden and palm-thatched cottages, and a small disused hospital complex that may be used to house visiting scientists in relative comfort.

The Aldabra atoll is the world's largest raised coral atoll and is made up of a chain of more than a dozen islands. Together they are about 35 km wide and enclose a huge tidal lagoon. Aldabra (155 m²⁾is larger in land area than Mahé and makes up a third of the Seychelles total land area. Its shoreline is mainly fossilised coral limestone undercut by the relentless wave action. At low tide more than 80% of the area of the lagoon is completely dry. Water covering almost 100 square kilometres has to drain totally every low tide and re-enter with every high tide.

The islands are separated by deep narrow channels which form the only way that water can enter or exit the lagoon with the tides. Within these channels lie many mushroom-shaped coral outcrops undercut and shaped by the constant erosion of the tides rushing in and out twice a day.

The uplifted reefs that make up terrestrial Aldabra consist of solid and partially fossilised corals which have been raised several metres above sea level. Primarily, the ground is hard coralline limestone eroded by the sea and rain to form sharp-edged holes. There is little or no soil. What soil there is is derived from guano, coral and other marine detritus. It supports a unique flora and fauna that includes more than a hundred thousand giant tortoises (Geochelone gigantea).

The terrestrial flora includes some 178 species of indigenous flowering plants, of which about 20% are thought to be endemic. Many of these plants are considered to be threatened. Mangrove swamp grows around the edge of the lagoon and inshore waters support sea-grass meadows. Much of the fretted limestone terrain is covered with dense Pemphis acidula thicket. On flat limestone, there is a mixed growth of low trees, shrubs, herbs and grasses. On the west coast, there are a few limited areas where coconut groves have been planted.

The exposed surface of Aldabra is reminiscent of the wild volcanic surfaces of the Comores Islands, a rocky mixture of holes, pinnacles and sharp edges. Some of the holes are rather large and tortoises have been known to fall into them and die of starvation. The skeletal remains are grim evidence of their fate. Some of these holes link up to form vast networks and labyrinths that link the open sea with several small inland lakes.

Climate

The climate is tropical with an average annual temperature of 27°C. Average rainfall is about 1200mm, although the annual rainfall varies greatly between years. There is a pronounced wet season from November to April and a drier season from May to November. The area is occasionally affected by cyclones.

References

Collar, N.J. (1994). The conservation status in 1982 of the Aldabra white-throated rail Dryolimnas cuvieri aldabranus. Bird Conservation International 3(4): 299-305.

Directorate of Overseas Surveys Print Laydown (1969). 1:25,000. West sheetand East sheet DOS (PL SEY) 3099A and 3099B.

Feare, C.J. (1984). Seabird Status and Conservation in the Tropical Indian Ocean. In: Croxhall, J.P., Evans, P.G.H. and Schreiber, R.W. (Eds) Status and Conservation of the World's seabirds. ICBP, Cambridge.

Grulke, W.E. (1993). Aldabra - The Jewel and the Crown of the Indian Ocean. Africa - Environment and Wildlife (November/December 1993)

Prys-Jones, R.P. (1979). The ecology and conservation of the Aldabra brush warbler Nesillas aldabranus. Philosophical Transactions- Royal Society of London Series B Biological Sciences B 286: 211-224.

Seabrook, W. (1990). The impact of the feral cat Felis catus on the native fauna of Aldabra Atoll Seychelles, Indian Ocean. Revue d'Ecologie la Terre et la Vie 45(2): 135-146.

Stoddart, D.R. (1971). 'Settlement, development and conservation of Aldabra'. Philosophical Transactions- Royal Society of London Series B Biological Sciences B 260: 611-628.

Stoddart, D.R. (1976). Publications resulting from the Royal Society Research Programme at Aldabra and nearby islands, 1967-1976. Aldabra Research Committee, the Royal Society ALD/13(76). 10 pp. (List of over 140 references).

Stoddart, D.R. (1995). Bibliography of Aldabra Atoll (Third Edition). Seychelles Islands Foundation. 93pp.

Phelsuma species

Phelsuma abbotti abbotti

Localities

Aldabra