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Aldabra is the world's second largest coral atoll. It is situated in the Aldabra Group of islands in the Indian Ocean that form part of the Seychelles. Uninhabited and extremely isolated, Aldabra is virtually untouched by humans. It has distinctive island fauna including the Aldabra Giant Tortoise (Aldabrachelys gigantea). It consists of four islets around a large shallow lagoon, encircled by fringing coral reef. The atoll reflects both fossil and geomorphological features, the former is the source of the live biodiversity. The atoll has about two thirds of the world population of Giant tortoises, about 100,000 out of a reported 150,000. Sir David Attenborough called it Aldabra "one of the wonders of the world", and it is also known as one of “Crown Jewels of our Ocean” in the Indian Ocean. Aside from itts vast population of tortoises, it is also the largest raised coral reef in the world with an elevation of 26 ft; the second largest atoll in the world after Kiritimati Atoll; a habitat for the biggest crab, the coconut crab; and habitat for the Indian Ocean’s Aldabra rail, the only surviving flightless bird species of its kind in the world.

Designated by UNESCO as a World Heritage Site, it is one of the two UNESCO World Heritage Sites in the Seychelles inscribed in 1982; both are administered by the Independent Seychelles Islands Foundation (SIF). BirdLife International declared Aldabra as an endemic bird area in 2001 on account of its large seabird colonies.

History
Historically, the name Aldabra, was Al-Hadra (with several variants) which was first given by the Arabs for "the atoll’s harsh, sun-baked environment"; also seen under this name in the Portuguese maps of the 16th century.; they had named the Indian Ocean as Bahr-el zanj. It was visited by Portuguese navigators in 1511. The islands were already known to the Persians and Arabs, from whom they got their name. In the middle of the 18th century, they became dependencies of the French colony of Réunion, from where expeditions were made for the capture of the giant tortoises. As there no surface freshwater sources on Aldabra, the interests of the explorers (no proof of any explorer’s visit prior to 1742) was only to exploit the species of tortoise, turtle and fish, and not to inhabit the atoll. After the Concession was granted by the Seychelles authorities in 1888, a few small settlements were established in Ile Picard. Around the rim of the lagoon are Ile Picard (West Island), Polymnie, Malabar Island (middle island) and Grand terre (South island). Goats were introduced in the late 19th century only as a food source for the few inhabitants (about 200) living there. Pied crow was then native to the atoll but ship rats were recorded before 1870, and house geckos were noted from the 1970s.

In 1810, with Mauritius, Réunion, the Seychelles and other islands, Aldabra passed into the possession of Great Britain. Réunion was later returned to France, and Mauritius gained possession of Aldabra as well as the rest of the Seychelles. The previous inhabitants were emigrants from the Seychelles. Sailors landed on the atoll in the 19th century and attempted to raid the island for tortoises as food; in 1842, two ships were reported to have taken 1200 of them. By 1900, the tortoises were nearly extinct, and a crew would often have to hunt for three days to find one.

In the early 1800s, concessions given to individuals almost destroyed the forests and tortoise habitats in many islands in Seychelles but in Aldabara Atoll, in view of its remoteness and rugged topography only small area of forests were cleared for agricultural operations (mostly coconut plantations were possible) but the tortoises were intensely captured for meat and trade. However, James Spurs, who had the concession of the atoll, was responsible initially for saving the tortoise on the atoll when he banned killing them in 1891.

The Aldabra Atoll, along with Des Roches and Farquhar, was part of the British Indian Ocean Territory from 1965 until Seychelles's independence in 1976. In the 1960s, as a part of their 'Ocean Island Policy', the British government considered establishing a RAF base on the island and they invited the United States to help fund the project in return for shared use of the facility and a settlement of 11 million dollars. Simultaneously (mid-1960s), the British Broadcasting Corporation became interested in Aldabra as a possible site to locate transmitters with which to rebroadcast the BBC Overseas Service (BBC) into the African mainland. The BBC mounted a fact finding expedition (Expedition Turtle) to assess its suitability for this purpose. The BBC were dependent upon the RAF for developing the atoll and without this their own ambitions would not have been feasible. After an international protest by ecologists, however, the military plans were abandoned and the wildlife habitat instead received full protection. The "Environmental lobbyists" under the leadership of Julian Huxley got the British venture torpedoed. In 1966, the Minister of Defense Dennis Healy of the British Government had observed that: "As I understand it, the island of Aldabra is inhabited - like Her Majesty's Opposition Front bench - by giant turtles, frigate birds and boobies.

Subsequent to the thwarting of plans to establish a military station at Aldabra, the Royal Society of London restarted their scientific study of the flora and fauna of the atoll (only terrestrial fauna of avian, giant tortoise and invertebrate species) with the efforts of Professor David Stoddart as the leader. The Royal Society was involved in the studies from 1967 to 1979 in the Atoll. In 1979, after completion of the assigned work, the Royal Society handed over the management of the environmental aspects of the Atoll to the Seychelles Island Foundation (SIF) of the Government of the Seychelles.

Fossil studies have revealed that the atoll was submerged about 175,000 years ago, destroying all the animals and plants, and that such events occurred many times and only the tortoise survived as they were camouflaged by the bushes of this atoll and also in view of its remoteness. While intensive scientific research work was done during the period 1965-80, there is now a lull in this activity. In 1971, the first research station was established with reputed institutions like the Smithsonian, the United Nations, and World Wide Fund for Nature providing the financial support to do research, which ensured that the pristine ecology of the atoll with its precious tortoise species (129,000 was reported in 1973) and nesting birds were left untouched by developmental activities.

The most easterly location on the Aldbara is named after Jean-François Hodoul, a pirate (a corsair) turned justice of the peace. The abandoned settlement Picard on the southwestern tip of West Island is now home to the research officer, the island manager and their rangers and staff. There is no other permanent population.

Geography
Aldabra is a raised coral atoll, located at -9.4°N, 46.36667°W and belongs to the Aldabra Group, one of the island groups of the Outer Islands of the Seychelles, which includes the island of Assumption and the atolls of Astove and Cosmoledo. At more than 700 mi from Mahé, the principal island of the Seychelles, it is closer to the coast of Africa, and is in the most southwesterly part of the Seychelles. It is 265 mi northwest of Madagascar and a similar distance northeast from the Comoro Islands. The reef limestones (with irregular beds called "champignon" ) extend over an average width of 2 km rising to a height of 8 m above sea level, and forming the rim line of the shallow central lagoon are of late Quaternary period. The atoll is the second largest in the world by land area, after Kiritimati. It is 34 km long (in east-west direction) and 14.5 km wide. It is up to 8 m above sea level, and has a land area of 155.4 km2 (145 sqkm is also mentioned).). The lagoon measures 2244 km2 in area, of which roughly two thirds fall dry during low tide. The atoll consists of a ring of four larger islands: counterclockwise, they are South Island (Grand Terre, 116.1 km2), Malabar or Middle Island (26.8 km2), Polymnieli or Polymnie Island (4.75 km2) and Picard or West Island (9.4 km2).

The atoll consists of several old corals in the upper rim of the lagoon. This rim has three passages which connect to the lagoon, which is 6 - 10 km in width as it opens to the sea. The depth of water in the lagoon is about 5 m; however, the passages that open to the sea are 20 m deep and affected by tidal currents. The coral formations are of Pleistocene age and rise to a height of 7.5 m and are made up of two layers of varying stages of crystallization. They form low cliffs with "deep notches, preceded by jagged pinnacles". Aldabra is one of the 40 granite islands of Seychelles (apart from small islets within the lagoon) formed of cays, beaches, sand dunes, reef limestone islets, and also the remnants of a fringed coral reef. Additionally, there are some 40 smaller islands and rocks, all inside the lagoon, as well as a few very small islets at the West Channels between South and Polymnie Islands, the largest of those being Îlot Magnan. The largest islands inside the lagoon are Île Michel (Michael Island), in the east (0.34 km2) and Île Esprit (Spirit Island), in the west (0.41 km2). Geologically the bed of the limestone beds have been subjected to striation, sink holes and pits with prominent and continuous limestone bed on the eastern side above the sediment deposits. The coastline has undercut limestone cliffs above a perched beach; it is in two clear terraces of 8 m and 6 m height above the sea level. Sand dunes dominate the windward south coast.

While the terrestrial topography (spread over an elevation range of 0 - 8 m ) is rugged and dictated by the geomorphic conditions, the land surface comprises coral reef of about 125,000 years age, which has uplifted many times above the sea level. Because of this setting, the biota that has evolved is mostly of endemic nature. The surface conditions are criss-crossed and riddled with pot holes and pits. In the eastern zone of the lagoon, though the surface is continuous, sediment beds are also seen. The coastline is formed of limestone cliff above the high rise beaches. The windward southern coast is made up of sand dunes. The undercut limestone is of mushroom shape known as champignon.

In the Aldebra complex of islands, the Assumption Island and Astove islets, east of Aldabra, were also studied since February 2002 by establishing permanent monitoring sites in the Aldabra group, but inhabited locations, which will further improve the protected condition of the Aldabra. There is also a small settlement established since 1874, housing labourers for commercial purposes, on specific occasions for tasks of fishing, mining of guano and extraction of copra.

A small scientific research station of the SIF is operated here by 10 assigned scientists to study its biodiversity and preserve the reserve in its pristine condition and to make it a "bellwringer" for all other coral islands in the Indian Ocean. The Seychelles Island Foundation (SIF) in charge of the island is based in Victoria. The nearest airport on Assumption Island is about 27 km south of Aldabra and cruises are operated by the Indian Ocean Explorer. Tourism is not an option here. However, cruise ships or dive boats may visit the atoll on expedition tours. Supply ships operating from Mahe provide food and other essentials once every three months to the scientists stationed at the research station.

List of smaller islands in the lagoon

 * Île aux Cendres
 * Île Michel (Coconut Island)
 * Île Esprit (Euphrates Island)
 * Île Moustiques
 * Îlot Parc
 * Îlot Emile
 * Îlot Yangue
 * Îlot Dubois
 * Île Lanier
 * Champignon des Os
 * Euphrate
 * Grand Mentor
 * Grand ÎIot
 * Gros ÎIot Gionnet
 * Gros ÎIot Sésame


 * Heron Rock
 * Hide Island
 * Île aux Aigrettes
 * Île aux Cèdres
 * Îles Chalands
 * Île Fangame
 * Île Héron
 * Île Michel
 * Île Suacco
 * Île Sylvestre
 * Île Verte
 * Îlot Déder
 * Îlot du Sud
 * Îlot du Milieu


 * Îlot du Nord
 * Îlot Macoa
 * Îlot Marquoix
 * Îlots Niçois
 * Îlot Salade
 * Middle Row Island
 * Nobby Rock
 * North Row Island
 * Petit Mentor
 * Petit Mentor Endans
 * Petits ÎIots
 * Pink Rock
 * South Row Island
 * Table Ronde

Climate
The Aldabra atoll is situated in the dry zone of the southwest Indian Ocean. The north-westerly monsoon winds from November to March constitutes the rainy season when rainfall is heavy. In the remaining months, the south-easterly trade winds are recorded. It receives, on an average, 960 mm rainfall per year (1100 mm is also reported ). Cyclones are rare in the Seychelles on account of its nearness to the Equator but the atoll experiences them occasionally when heavy storms have been recorded. Tides in the coastal zone rise to more than 3 m height causing channel currents, and huge influx of water; the main channel drains 60% flow into the lagoon. The reported monthly mean maximum temperature recorded in December is 31 C. The minimum temperature recorded in August is 22 C. Access from the Indian Ocean to the lagoon is through two major channels and one smaller channel and also by many smaller reef passages.

Wildlife
The earliest study of the flora and fauna, and also the geomorphological structure is dated to 1910. While the terrestrial fauna studies have been fully studied, the marine fauna has so far been covered to the extent of 25% only. There are 307 species of animals and plants in the atoll. The richness in endemic flora and fauna is attributed to the size and morphological diversity of the atoll. The only mammal reported is flying fox. There is also a record of about 1,000 species of insects (both new and endemic), sea birds. Reptiles are the prominent terrestrial fauna.

Flora
Aldabra has a number of endemic plant species. The higher areas are covered in species of pemphis, thick coastal shrubs, while the lower areas which are home to the giant tortoises, are a mixture of trees, shrubs, herbs and grasses. The terrestrial flora recorded is of 273 species of flowering plant, shrubs, and ferns with dense thickets of Pemohis acidula, and low grasses in 15% area called the "tortoise turf";  these include 19 endemic species and 22 species common to neighbouring islands also, and several of them are in the threatened list. The lagoon has mangrove forests, and in inland seagrass meadows are large. Tropicbird orchid Angraecum Seychellarum is the national flower of Seychelles found in the dry craggy limestone champignon of Aldebra. Specific species reported are endemic plants such as Pandanus aldabrensis, the Aldabra Lily, Lomatophyllum aldebrense and sub species of tropicbird orchid Angraecum eburneum.

Fauna
The atoll is home to the world's largest population of Giant Tortoises, the Aldabra Giant Tortoise (Dipsochelys dussumieri), numbering some 100,000 individuals according to a recent count, living mainly on Granda Terre. Young hawksbill sea turtle (Eretmochelys imbricata) and green turtles (Chelonia mydas) are also found in large numbers. The adult Grand Tortoise typically has a caraspace length of about 105 cm and can weigh about 350 kg. They are noisy creatures, and feed on the forage of trees, using their very strong hind legs to kick and fell trees to get to the foliage. Their proliferation in large numbers in the atoll is attributed to the breeding characteristics. They mate between February to May, nestling from June to September in areas with suitable soil layers. Females breeding characteristics is to lay eggs (of the size of golf balls ) in a clutch of 3-5 eggs every few years in high density areas; They lay 14-16 eggs in low density areas, and in several clutches in a year; the incubation period is of 73–160 days and the juveniles have to survive the attack of coconut crabs, land crabs, rats and birds. They have been relocated to many other islands in Seychelles and also to Victoria Botanical Garden in Mahe and in front of many hotels in Mahe.) The atoll is also the habitat for the Coconut crab (Birgus latro), the world's largest land crab, and hammerhead sharks, manta rays, barracuda and is a breeding ground for green turtles and hawksbill turtles; in older times, its dominant land predator was the crocodillian Aldabrachampsus. Cryptoblepharus boutonii is widely distributed on the atoll. Flying fox is the mammal species found in the atoll. There are 1,000 species of insects both new and endemic. Many species of butterflies also flutter around the Aldabra. Birds, which are endemic species of particular note, are the Aldabra Brush Warbler (Nesillas aldabrana) and Aldabra Drongo (Dicrurus aldabranus) (black with a forked tail). There are also other eleven bird species with subspecies such as the White-throated Rail, a flightless bird of the Western Indian Ocean, which is now found only here. Waterbird colonies are in large numbers, as also frigatebird colonies of greater and lesser frigate birds (Fregata minor and Fregata ariel) (reported to the second largest number left in the world) and two oceanic flamingo populations, which are also found now only here. Birds of Aldabra also include the Aldabra rail, the last surviving flightless bird of the Indian Ocean region, and the endangered Malagasy Sacred Ibis as well as large numbers of nesting seabirds. Two species of bats, Paratriaenops pauliani and Pteropus aldabrensis, are known only from Aldabra. Aldabra's uniqueness lies in the fact that it does not have a single introduced bird species. The Aldabra warbler (Nesillas aldabrana), discovered in 1968, restricted to an area of 10 ha in the Aldabara, is probably extinct or critically endangered, as not more than 5 birds have been sighted in all the years after its discovery. Sub species of Aldabran drongo are also sighted in the atoll. These are of least concern, as more than 8,000 of these birds are found here. Frigate birds and red-footed boobies are found in large numbers in the foliage of the mangrove forests on the coast.

The IBA listed triggered species of birds in the order of their IUCN Categorization, apart from those mentioned earlier, are: Madagascar Pond-heron (Ardeola idae) (EN), Little Egret (Egretta garzetta) (LC), Red-tailed Tropicbird (Phaethon rubricauda) (LC), White-tailed Tropicbird (Phaethon lepturus) (LC), Red-footed Booby (Sula sula) (LC), Crab Plover (Dromas ardeola) (LC), Ruddy Turnstone (Arenaria interpres) (LC), Black-naped Tern (Sterna sumatrana) (LC),Comoro Blue-pigeon (Alectroenas sganzini) (LC), and Red-headed Fody (Foudia eminentissima) (LC). Land birds in the atoll are descendants from Madagascar or Comoros and these are: Madagascar Sacred ibis (threskiornis berniere, Madagascar Coucal (centropus toulou), Madagascar Nightjar (caprimulgus madagascariensis), Madagascar Bulbul (Hypsipetus madagascariensis), Souimanga Sunbird (Nectarina souimanga).

Threats and conservation
The earliest threat posed in the 1960s was when, as part of the British Indian Ocean Territory, the British made plans to set up a military establishment on the atoll which was aborted in 1967. Invasive species of rats, cats and goats introduced in the past need to be completely eradicated from the island to preserve its ecology; while cats are removed from Picard, but Dryolimnas cuvieri reintroduced to the island is a threat. Exotic bird species, including Pycnonotus jocosus and Foudia madagascariensis, residents of Assumption Island, may find their way to Aldabra, which needs to be stopped. Aldabran Giant Tortoise species is also now under threat, in addition to other fauna, due to commercial trading permitted with specific license, as it has been included in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) (also known as Washington Convention Listed under Appendix II which connotes commercial trade of the taxon with a permit to export from the country of its origin; animals from the atoll have been traced in United States of America, United Kingdom, Germany, South Africa, North Africa, Canada, India and Hongkong (tortoise is reported to fetch prize of $ 6000 each). This situation has to be rectified by changing it to Appendix I list of CITES. Due to the limited space of its habitat, extreme weather conditions, epidemic and limited range could also pose serious threats to the entire ecology of the atoll. Feral goats introduced on the atoll also pose problems on the atoll as they inhibit growth of natural vegetation.

Protection
Aldabra, listed as "Aldabra Atoll", was designated a UNESCO World Heritage Site on 19 November 1982, and is administered from Mahé, the capital city by the Seychelles Island Foundation. The seaward boundary limits of the protected area extend 20 km into the sea to ensure preservation of marine fauna. Eco tourism is controlled and introduction of invasive species is strictly monitored. Based on the evaluation process, UNESCO inscribed the site, a legally protected special reserve of 35000 ha, on the list of World heritage Sites under three criteria: Criterion (vii) for its rugged topography forming many types of habitats with a large stock of biota, which are mostly endemic with marine habitats of coral reefs, seagrass and mudflats with mangrove forests; Criterion (ix): The habitats created by the geomorphic conditions have resulted in a large biota, mostly of endemic nature, with marine habitats dominated by coral reefs and beds of seagrass, mudflats with mangrove forests. The herbivorous tortoise graze on grasses, shrubs and plants; and Criterion (x) because of its 400 endemic species and its sub species which are not only vertebrates and invertebrates but also plants which are of great interest for scientific research. The hundred thousand or more Great Tortoises (largest such concentration in the world) found here are reported to be "their only remaining habitat"; these also include the endangered green turtles and critically endangered hawksbill turtles which breed here. Water birds are also found in very large numbers.

BirdLife International declared Aldabra as an endemic bird area in 2001 on account of its large seabird colonies under categories A1, A2, A4i, A4ii and A4iii, covering an area of 33180 ha overlapping with the special reserve area of 35000 ha of Aldabra Atoll.

Assigned scientists only stay here during scientific investigations. Visitors are allowed only to Picard, Polymnie and Camp Frégate (Malabar) on the atoll, after the scientists leave the locations. Though poaching and illegal fishing is limited in the atoll due to its remoteness, management plans have been drawn up to address this situation as and when it arises.