Aldabra, Seychelles : Giant tortoises, marine & terrestrial ecology
One of the largest tortoises in the world, Geochelone gigantea, or the Aldabra Giant Tortoise, Lives in the islands of the Aldabra Atoll in the Seychelles. The males average around 250 kg in weight and 120 cm long.
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 as its isolation in a remote area of the Indian Ocean, combined with an inhospitable terrestrial environment, has helped preserve it in a relatively natural state. Increasing levels of stress from human activities are contributing to the decline of the worlds 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 seagrass and mangroves).
Aldabra is formed from late Quaternary raised reef limestones, averaging 2km in width and up to 8m above sea level, and rimming a shallow central lagoon. The limestone has been eroded over the years to form an 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 lens and rises and falls with the tides. Aldabra has monthly mean maximum (December) and minmum (August) temperatures of 31ºC and 22ºC respectively. Average rainfall, with Aldabra located in the 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 one smaller channels and by several smaller reef passages. Tidal range is 2 to 3 m and results in large exchanges of water between the lagoon and open ocean through the channels. The main channel alone drains approximately 60% of the lagoon.
The scientific history of Aldabra encompasses almost 100 years of 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 islands. 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.