A Reptilian Kingdom
The Aldabra Atoll supports the world's largest population of giant tortoises (Aldabrachelys gigantea), with a fluctuating population of around 100,000 individuals. These reptiles are the last survivors of a dozen giant tortoise species that once inhabited the islands of the Indian Ocean before being hunted to extinction by sailors. The tortoises are ecosystem engineers; their intense grazing on a variety of grasses and herbs creates a unique, species-rich habitat known as "tortoise turf". Males can weigh up to 250 kilograms and measure 1.2 meters in length, with lifespans that often exceed a century.
The atoll is also an important habitat for the Aldabra rail (Dryolimnas cuvieri aldabranus), the last flightless bird in the Indian Ocean. Fossil records show that the bird's ancestors colonized the atoll over 130,000 years ago and evolved to be flightless. When a major sea-level rise completely submerged the atoll, that population went extinct. After the atoll re-emerged thousands of years later, the same rail species recolonized it and, in a rare case of iterative evolution, evolved to be flightless for a second time.
An Isolated Coral World
Aldabra is the world's second-largest raised coral atoll, located over 1,000 kilometers southwest of Mahé, the main island of Seychelles. Its extreme isolation and lack of fresh water have protected it from significant human disturbance. The atoll consists of four main coral limestone islands—Grand Terre, Malabar, Picard, and Polymnie—that encircle a large, shallow lagoon. The land area is about 155 square kilometers, but the lagoon is so vast at 196 square kilometers that the entire island of Mahé could fit inside it.
This remote location was inscribed as a UNESCO World Heritage Site in 1982. The atoll's ecosystem is heavily influenced by the tides, which rush in and out of the lagoon through several channels, creating powerful currents. The terrain is rugged, made of sharp, weathered limestone known as "champignon". The atoll is also a breeding site for green and hawksbill turtles and hosts large seabird colonies, including the world's second-largest breeding population of frigatebirds.