Aldabra Atoll - Ideal Natural Laboratory

The Aldabra atoll comprise four large coral island which enclose a sallow lagoon;the group of island is itself surrounded by a coral reef. Due to difficulties of access and the atoll’s isolation, Aldabra has been protected from human infuluance and thus retains some 152,000 giant tortoises, the world’s largest population of reptile.
Aldabra Atoll (924’ S, 4620’ E) is a large (34 Km long, maximum 14.5 km wide , area 155 km2 ) raised atoll located in the Western Indian Ocean. It is situated 1150 km southwest of Victoria ( the capital of the Seychelles on the island of Mahe ) and 420 km north of Madagascar. Aldabra has been described as “One of the Wonders of the world” by Sir David Attenborough a isolation in a remote area of the Indian Ocean, combined with an inhospitable terrestrial environment, has helped preserve in a relatively natural state.
Increasing level of stress from human activities are contributing to the decline of the world’s coral reefs, Aldabra has so far escaped the worst of these stresses and provides an ideal natural laboratory for studying tropical marine ecosystems and related environments (Such as Sea grass and mangroves)

THE LIMESTONE

Aldabra if formed from late Quaternary raised reef limestone, averaging 2 km in width and up to 8m above sea level, and rimming a shallow central lagoon. The limestone has been eroded over the years to from a dangerous terrain of sharp spiky rocks and numerous pits, making walking off established tracks unadvisable.
Many of the pits contain fresh or brackish water that sits on top of surrounding seawater as a lense and rises and falls with tides. Aldabra has monthly mean maximum (December) and minimum (August) temperatures of 31C and 22C respectively. Average rainfall, with Aldabra located relatively dry zone of the southwest Indian Ocean, is 1100mm per year. Climate is heavily influenced by the NW monsoon winds from November to March bringing the heaviest rainfall, with SE trades blowing throughout the remainder of the year. The lagoon At Aldabra is linked to the ocean by two major and smaller channels and by several smaller reef passages. Tidal range is 2 to 3 m and results in large exchange of water between the lagoon and open ocean through the channels. The main cannel alone drains approximately 60% of the lagoon.
The scientific history of Aldabra encompasses almost 100 years of the both terrestrial and marine based investigations. Early contributions regarding the flora and fauna, and indeed geomorphological structure, of Aldabra made it in 1910 one of the better known Indian Ocean reef island. In the mid 1960s Aldabra was thrust into the international spotlight, being considered by the British Government as a possible air-staging outpost, with the threat of the construction of an airstrip and support facility.

PHYSICAL FEATURES

Aldabra is classic coral atoll, 34 km long by maximum of 14.5 km wide, which has been built up from a seabed. It comprises four main islands of coral limestone separated by narrow passes and encloses a large shallow lagoon. Most of the land surface comprises ancient coral reef (about 125,000 years old) now raised above sea level, the rest being even older reef limestones. The lagoon, which covers some 14,000 ha, contains many smaller islands and the entire atoll is surrounded by an outer reef. Geomorphological processes have produced a varied and generally rugged topography. Weathering has eroded the limestone into hole and pits over much of the surface of the island, although the surface at the eastern end comprises raised lagoonal sediments. The limestone cliffs along the cost are undercut, and there is a perched beach and sand dunes on the southern (windward) cost. Tidal range is more than 3 m which can lead to strong channel currents.

CLIMATE

Tropical with an average annual temperature of 27 C. Average rainfall is about 1,200 mm, although the annual rainfall varies greatly between years. There is a pronounced wet season from November to April drier season from May to November. The area is occasionally affected by cyclones.

VEGETATION

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 water support sea-grass meadows. Much of the fretted limestone terrain is covered with dense Pemphis acidulla thicket. On the flat limestone, there is a mixed growth of low trees, shrubs and grasses. On the west coast, there are few limited areas where coconut groves have been palnted.

FAUNA

This island group is one of the few areas in the world where reptiles dominate the terrestrial Fauna, with the largest world population (152,000) of giant tortoise Geochelone gigintea, which arrears to be self-sustaining. Green turtle Chelonia mydas and Hawksbill turtle Eretmocheyls imbricate breed here. There 13 species of terrestrial birds including the last representative of the wetern Indian Ocean flightless birds- the endemic Aldabran white-throated rail Dryolimnas cuvieri aldabranus (about 5,000 individuals). Aldabran drongo Dicrurus aldabranus (approximately 1,500 individuals) is also endemic species which inhabits scrub, mangrove and casuarinas areas. Aldabra warbler Nesillas aldabranus has not been seen for several years and might be naturally extinct.
Previously restricted to 10 ha of costal tall scrub, This was considered possible the most endangered bird in the world, as only five birds have been seen since its discovery in 1968.Aldabra is the main breeding site in the Indian Ocean for red-tailed tropic bird Phaethon rubricauda, red-footed boody sula sula, greater frigatebird Fregata minor and lasser frigatebird F. ariel. There are also large numbers of white-tailed tropic bird P. lepturus, masked boody sula dactylatra, Abbott’s boody s. leucogaster.