Aldabra Atoll

Aldabra is made of coral.

Long ago it was a reef under the sea. Then it ended up above the water. Now the rock is rough and rugged.

Aldabra is a raised coral atoll. Most of its land is ancient coral reef, about 125,000 years old, that has been lifted above sea level more than once. Over time, natural processes shaped the coral limestone into a rugged landscape, famous for its limestone "champignon" terrain.

Inside the ring of islands is the lagoon. It is the atoll's tidal heartbeat: twice every day, millions of tonnes of seawater rush in and out through the channels between the islands.

UNESCO describes Aldabra as an outstanding raised coral atoll whose land surface is mostly ancient coral reef, about 125,000 years old, that has been repeatedly raised above sea level. Geomorphological processes have produced a rugged topography, including the limestone "champignon" terrain for which SIF says the atoll is famous, and four main islands of coral limestone separated by narrow passes enclosing a large, shallow lagoon with many smaller islands, all ringed by a fringing reef.

SIF calls the lagoon the tidal heartbeat of the atoll, with millions of tonnes of water rushing in and out twice a day. The lagoon is large enough, SIF notes, that Mahé, the largest inhabited island of Seychelles, would fit inside it.

Aldabra is home to giant tortoises.

There are more than 100,000 of them! They munch grass and bushes. On Aldabra, the boss of the food chain is a plant-eater: the tortoise!

More than 100,000 Aldabra giant tortoises live on the atoll, the largest population of giant tortoises in the world. Tortoises like these once lived on other islands in the Indian Ocean, but UNESCO says Aldabra is now their only remaining home.

On Aldabra, unusually, the animal at the top of the food chain on land is a plant-eater: the giant tortoise. The tortoises graze on grasses and shrubs, and some plants on the atoll have changed over time in response to the way the tortoises feed.

UNESCO's statement records a population of over 100,000 Aldabra giant tortoises, the largest giant tortoise population in the world and entirely self-sustaining. They are the last survivors of a life form once found on other Indian Ocean islands, and Aldabra is now their only remaining habitat.

Unusually, the top of the terrestrial food chain is occupied by a herbivore. The tortoises feed on grasses and shrubbery, including plants that have evolved in response to their grazing, so UNESCO describes their intricate relationship with the environment as fully evident. Because human interference has been minimal, these ecological processes can be observed in their full complexity.

Lots of birds nest on Aldabra.

There are flamingos and big seabirds called frigatebirds. Sea turtles come there to breed. One little bird, a rail, cannot fly at all!

Aldabra is a natural laboratory, UNESCO says, with more than 400 species and subspecies found nowhere else. Birds found only here include the Aldabra drongo and the Aldabra fody, which the Seychelles Islands Foundation says genetic tests confirmed as a species of its own. The white-throated rail on Aldabra cannot fly, and it is the last flightless bird left in the western Indian Ocean.

Huge colonies of waterbirds nest here, including the world's second-largest frigatebird colonies and a rare population of flamingos that live on an ocean island. Endangered green turtles breed on Aldabra, and hawksbill turtles are found there too. UNESCO describes the coral reefs around it as being in excellent health.

UNESCO describes Aldabra as an outstanding example of an oceanic island ecosystem in which evolutionary processes remain active, with a refuge for over 400 endemic species and subspecies of vertebrates, invertebrates and plants. UNESCO records two endemic bird species (the Aldabra brush warbler and the Aldabra drongo; SIF adds that the Aldabra fody has since been genetically confirmed as a distinct species), and eleven more have distinct subspecies, among them the white-throated rail, the last remaining flightless bird of the western Indian Ocean.

There are vast waterbird colonies, including the second-largest frigatebird colonies in the world and one of only two oceanic flamingo populations, globally important breeding populations of endangered green turtles, and critically endangered hawksbill turtles. Marine habitats range from coral reefs to seagrass beds and mangrove mudflats, and the fringing reef is, in UNESCO's words, in excellent health.

No towns are built on Aldabra.

Scientists work at a research station there. They study the reefs and the wildlife. Schoolchildren from Seychelles visit to learn about it.

The earliest recorded visit to Aldabra was by a French captain, Picault, in 1742. Later settlers came to cut mangrove wood and to catch turtles and tortoises, which greatly reduced their numbers.

In 1971 members of the Royal Society, a British science academy, set up a research station on Picard Island. Since 1979 the Seychelles Islands Foundation has managed and protected the atoll. Its scientists monitor the wildlife, and the foundation brings Seychellois schoolchildren to visit.

SIF's history records the earliest recorded visit in 1742, by the French captain Picault, and a first settlement on Île Magnan in 1888, where lessees exploited mangrove timber, green turtles and fish and harvested giant tortoises for meat, dramatically reducing their numbers; settlers also released domestic animals such as pigs and goats. SIF reports that the name Aldabra may come from the Arabic "Al-Khadra", "the green", after the lagoon's green reflection on the clouds.

In 1971 the Royal Society established a research station near the old settlement on Picard Island, and in 1979 SIF was created as a public trust to manage and conserve the atoll. The station runs on a 25 kWp hybrid solar-diesel system, a marine monitoring program has tracked corals and fish since 2013, and SIF brings Seychellois schoolchildren to the atoll.

Aldabra is very hard to reach.

Only a few visitors are allowed, so the animals stay safe. Visitors should never bring new plants or animals. They leave the tortoises and birds in peace.

Aldabra is so remote that very few people go there, and UNESCO says tourism is limited and carefully controlled. That helps keep the atoll wild.

The constant threats to Aldabra, according to UNESCO, are invasive species (plants and animals brought from elsewhere), climate change and oil spills. Anyone who visits should take great care not to bring seeds, insects or other animals by accident, and should keep a respectful distance from tortoises, nesting birds and turtles.

UNESCO notes that tourism is limited and carefully controlled, and that the atoll's remoteness, while it limits human interference, poses tremendous logistical challenges for management. Constant threats are invasive alien species, climate change and oil spills, particularly if oil exploration in the wider region increases; UNESCO also suggests that extending the seaward boundary some 20 km would give marine animals extra protection.

Responsible visitors take strict care not to carry seeds or animals between islands and avoid disturbing tortoises, nesting seabirds or turtles.