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Aldabra, the world's second-largest coral atoll (the largest is Kiritimati), is located east of the continent of Africa. It is part of the Aldabra Group of in the Indian Ocean that are part of the Outer Islands of the Seychelles, with a distance of 1,120 km (700 mi) southwest of the capital, Victoria on Mahé Island. Initially named by Arab seafarers for its harsh environment, Aldabra became a French colony dependency in the 18th century, leading to the exploitation of its natural resources, particularly . After passing through British hands, Aldabra faced potential military use in the 1960s, but international protests resulted in its protection. The atoll boasts unique geography, featuring the world's largest raised coral reef and a large shallow lagoon. Aldabra's history involves human impact, including failed agricultural ventures.
Post-World War II, conservation efforts increased, leading to its inclusion in the British Indian Ocean Territory and eventual independence for Seychelles. Following an international protest by scientists known as the 'Aldabra Affair,' the Royal Society and later the Seychelles Islands Foundation conducted research, leading to Aldabra's declaration as a UNESCO World Heritage Site in 1982. The atoll's flora and fauna, notably the giant tortoises, comprise an 'ecological wonder', with Aldabra serving as a critical breeding ground for various species. Conservation challenges, such as invasive species and plastic pollution, persist, but protective measures, including UNESCO designation, underscore Aldabra's significance as a natural vital scientific research site. Aldabra receives limited tourism to safeguard its delicate ecosystem and is accessible primarily through guided tours.
As there are no surface freshwater sources on Aldabra, the interests of the explorers (no proof of any European explorer's visit prior to 1742) was only to exploit the species of tortoise, turtle and fish, and not to inhabit the atoll. 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.
Admiral W. J. L. Wharton of the British Navy landed in Aldabra in 1878 to conduct hydrographic surveys of the islands. Aldabra from Ministry of environment website In 1888, the first settlement was established after the Concession was granted by the Seychelles authorities. A small settlement was established on Picard Island facing west near the beach. The intention was to exploit and export the natural resources of the islands. The villagers built a chapel, in the middle of the badamier trees, using timber and steel; the chapel was considered an essential addition to the plantation houses and office buildings. As Aldabra had no freshwater resources, large rectangular-shaped water storage structures were built adjoining each building. A two-roomed jail was also built in the village, a remnant of which is extant.
The exploitation of tortoises for commercial purposes at that time is illustrated by the remnants of a crushing mill at Picard Island, which was used to crush bones of tortoises, which were also brought in from other islands in the atoll. Efforts made to grow plantation crops of coconuts, cotton, and sisal failed due to inadequate water sources on the atoll; relics of these plantations are still found on some of the islands. In the late 19th century, were introduced as a food source for the villagers (about 200) living there. were inadvertently introduced and recorded before 1870, and were noted from the 1970s. Sailors landed on the atoll in the 19th century and captured 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.Carpin, Sarah,(1998) Seychelles, Odyssey Guides, p.162, The Guidebook Company Ltd., Accessed 22 June 2008
In the early 1800s, concessions given to individuals almost destroyed the forests and tortoise habitats in many islands in Seychelles; on Aldabra Atoll, in view of its remoteness and rugged topography, only small areas of forests were cleared for agricultural operations (mostly coconut plantations) 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 tortoises on the atoll when he banned killing them in 1891.
Following World War II, exploitation of Aldabra for commercial use came to an end and restrictions were even imposed on the number of people who could stay on the islands; this number was fixed at 200 at a time. Introduction of invasive species was banned, faunal species were protected under law, and active research on the ecology and biodiversity of the atoll was undertaken by the Royal Society from the mid-1970s.
Aldabra, along with Desroches and Farquhar Group, was part of the British Indian Ocean Territory from 1965 until Seychelles' independence in 1976. In the 1960s, as a part of their 'Ocean Island Policy', and to support East of Suez commitments, the British government considered establishing an RAF base on the island and 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 transmitter site, to broadcast 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 relied on the RAF for developing the atoll as without this their project would not have been feasible.
After an international protest by scientists (known as 'the Aldabra Affair'), however, the military plans were abandoned and the atoll instead received full protection. The "Environmental lobbyists" under the leadership of Julian Huxley, with the support of MPs Tam Dalyell and Robin Cook, got the British venture torpedoed. In 1966, British Defence Minister Denis Healey had observed: "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 (which instead focused on Diego Garcia in the Chagos Islands), the Royal Society of London resumed their scientific study of the flora and fauna of the atoll with Professor David Stoddart as the leader. The Royal Society bought the lease of the atoll in 1970 and their research station became functional from 1970. After completion of their assigned work, the Royal Society left and the Seychelles Islands Foundation (SIF), a public trust of Seychelles, assumed management and protection of the atoll in 1979. SIF functions under the patronage of the President of Seychelles and Aldabra was declared a Special Nature Reserve in 1981; it became a UNESCO World Heritage Site on 19 November 1982. A brass plaque inscribed with the citation "Aldabra, wonder of nature given to humanity by the people of the Republic of Seychelles" was erected on the atoll. This appreciation befits the atoll which is truly one of the greatest ecologically undisturbed raised coral atolls in the world.
Around the rim of the lagoon are the larger islands of the atoll. The total land area of the atoll is . The size including the lagoon is . The outside rim of the atoll has three passages which connect to the lagoon, which is in width where it opens to the sea. The water depth in the lagoon averages about ; however, the passages that open to the sea are up to deep and strongly affected by tidal currents.
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The coastline has undercut limestone cliffs above a perched beach; it is in two clear terraces of and height above sea level. While the terrestrial topography (spread over an elevation range of ) is rugged and dictated by the geomorphic conditions, the land surface comprises limestone of about 125,000 years age, which has uplifted many times above the sea level. 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 windward southern coast is made up of sand dunes.
The hydrodynamics of Aldabra's lagoon are dominated by the tides. The maximum tidal range at Aldabra is around 2.7m, which is exceptionally large for an atoll. Flow between the lagoon and open ocean is only possible through a small number of narrow channels, generating tidal currents of over 3 m/s at Passe Gionnet, and over 1 m/s at most other channels. Due to the restriction of water exchange through these channels, tides within the lagoon are lagged compared to the open ocean, and the tidal range varies across the lagoon.
Aldabra sits in the path of the Indian Ocean's westward-flowing South Equatorial Current. Eddies are generated in the wake of Aldabra as the South Equatorial Current passes it, driving upwelling of deep waters that bring nutrients to the surface, and drive phytoplankton growth.
The lagoon is bordered by mangrove forests, and has large inland as well as areas of coral reef and sand flats. The mangroves, which thrive in tidal mudflat areas and saline conditions, are seen on the shores of the lagoon and are integral to the coastal ecosystem. There are seven species of mangrove on Aldabra, three of which are rarely occurring species. These include 'Mangliye blan' or white mangrove ( Avicennia marina) which grows to , 'Mangliye lat' or black mangrove (Bruguiera gymnorhiza) which grows to in a conical shape, 'Mangliye zonn' (Ceriops tagal) which grows to with a buttressed trunk, and 'Mangliye rouz' or red mangrove ( Rhizophora mucronata) which is the tallest species up to in height.
Aldabra is a breeding ground for the hawksbill sea turtle ( Eretmochelys imbricata) and green sea turtle ( Chelonia mydas). Aldabra has one of the largest populations of nesting green turtles in the Western Indian Ocean. Aldabra has a large population of the world's largest terrestrial arthropod, the coconut crab ( Birgus latro); and hosts the white-throated rail, the only surviving flightless rail species in the Indian Ocean. , , and barracuda populate the seas surrounding the island. During the Pleistocene the dominant land predator was the crocodilian Aldabrachampsus which is now extinct. Three extant species of lizards occur, the skink Cryptoblepharus and the geckos Phelsuma abbotti and Hemidactylus mercatorius. Pleistocene fossils also indicate the former presence of an Oplurus iguana and other skink and gecko species. There are three endemic species of bat from Aldabra: Paratriaenops pauliani, Chaerephon pusilla and the Aldabra flying fox ( Pteropus aldabrensis), as well as the more widely distributed Mauritian tomb bat ( Taphozous mauritianus). There are 1,000 species of insects, many of them endemic. Many species of butterflies also flutter around Aldabra.
Endemic birds include the Aldabra drongo ( Dicrurus aldabranus), the Aldabran subspecies of the white-throated rail ( Dryolimnas cuvieri aldabranus), the last surviving flightless bird of the Indian Ocean region, and the endemic Aldabra fody ( Foudia aldabrana), The islands are important breeding grounds for thousands of seabirds, including several species of tern, red-tailed tropicbirds, white-tailed tropicbirds, red-footed boobies, and the world's second largest breeding populations of great and lesser frigate birds. The bird fauna is most similar to Madagascar or Comoros and other birds found here include , the Malagasy pond heron, Comoros blue pigeon, Malagasy kestrel, Malagasy coucal, Madagascar nightjar, Malagasy bulbul and souimanga sunbird. Seychelles Bird Records Committee, Accessed 26 June 2014 Bird Checklist
At least 13 species of , including , , and especially , Seychelles' atoll of Aldabra sees decline in humpback whale count; El Niño blamedSeychelles Islands Foundation (SIF). 2017. Aldabra Humpback Whales 2017. YouTube. Accessed 26 September 2017 have been identified in the waters. Atoll Research Bulletin No. 564Burt A. 2017. Aldabra Humpback Whales on Vimeo. Accessed 4 September 2017 , thought to be regionally extinct in the 18th century, Are Dugons extinct on Mauritius? . Accessed 4 September 2017 have been confirmed multiple times in recent years. Returning home? Sightings of dugongs increase in Seychelles’ Aldabra atollRothauscher's Dugong Page. 2003. Dugongs seen in Aldabra. Accessed 4 September 2017
Invasive alien species such as , and that were introduced in the past threaten the native biodiversity of the atoll. Goats were eradicated from the atoll in 2012 after a long-term eradication program. Cats have been removed from all of the islands except Grande Terre Island, which allowed for the reintroduction of the Aldabra rail to Picard Island. Research into a feasibility study to eradicate rats from the atoll has been undertaken.
Until recently, Aldabra had remained free of invasive birds. However, the Red fody ( Foudia madagascariensis) has spread to Aldabra after being introduced to Assumption Island. An eradication program for this bird on both Assumption and Aldabra is almost complete. 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.
Aldabra also accumulates large amounts of plastic pollution on its coastline and, despite a major clean-up effort in 2019, over 500 tonnes is thought to remain on the atoll, with significant impacts on wildlife. Most of this pollution originates from the regional fishing industry, although a significant proportion is also transported from southeast Asia through ocean currents.
The coral reefs at Aldabra were severely affected by the 2016 El Niño event. Although thermal stress was not exceptionally high (less than 4 Coral bleaching), hard coral cover on Aldabra's seaward-facing reefs was reduced by over 50% (35% in the lagoon), and soft coral cover fell by over 90%. Fortunately, there are some signs of recovery, as the abundance of juvenile corals has since increased significantly, particularly within the lagoon, although Aldabra's reefs of course remain critically threatened by climate change.
Based on the evaluation process, UNESCO inscribed the site, a legally protected special reserve of , on the list of World Heritage Sites under three criteria: Criterion (vii): Aldabra Atoll encompasses a large expanse of relatively untouched natural beauty where a number of important animal species and some plant species thrive, along with remarkable land formations, and its process provides a unique spectacle of natural phenomena; Criterion (ix): The atoll is a superlative example of an oceanic island ecosystem in which evolutionary processes are active within a rich biota. The size and morphological diversity of the atoll has permitted the development of a variety of discrete insular communities with a high incidence of endemicity among the constituent species that are typical of island ecosystems. The natural processes take place with minimal human interference and can be clearly demonstrated in their full complexity; and Criterion (x): Aldabra provides a natural laboratory for the study of the process of evolutionary ecology and is a platform for key scientific discovery. The atoll constitutes a refuge harboring viable populations of a range of rare and endangered species of plants and animals, including the last giant tortoise and flightless bird populations of the Western Indian Ocean, a substantial marine turtle breeding population, and large seabird colonies which number in the tens of thousands. The substantial tortoise population is self-sustaining and all the elements of its inter-relationship with the terrestrial environment are evident.
BirdLife International declared Aldabra as an Important Endemic Bird Area (IBA) in 2001 due to its large seabird colonies under categories A1, A2, A4i, A4ii and A4iii, covering an area of overlapping with the special reserve area of of Aldabra Atoll.
Aldabra became a Ramsar Convention Wetland Site of International Importance in 2010. Covering 25,100 ha (over half the area of the whole atoll) the wetland ecosystem of Aldabra includes the extensive shallow lagoon inside the atoll, which is carpeted with lush seagrass beds and patchy coral reefs, the intertidal mud flats, the coral reefs outside the lagoon, freshwater pools, beaches, and 2000 ha of mangrove stands. These wetlands support several endangered species including the increasing number of turtles at the atoll, dugongs and many other bird, fish and invertebrate species.
Aldabra was designated as a site under the Indian Ocean South East Asia (IOSEA) turtle network, in their 2014 convention.
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