The remote Aldabra Atoll in the Indian Ocean is home to the world’s largest population of giant tortoises.
It is so remote and difficult to access that the beautiful chain of islands in the outer Seychelles has remained largely unspoilt by humans. It’s one of the most important places in the world for scientists studying evolutionary processes.
More than 400 endemic species live there, including more than 100,000 giant tortoises and the white-throated rail, the last surviving flightless bird in the Indian Ocean.
However, the UNESCO biological Word Heritage site is under threat from climate change, ecotourism and introduced species including rats, cats, and goats.
These threats to life on the islands are nothing new. Aldabra has always been prone to catastrophe and at threat from climate change.
A new study led by palaeo-ornithologist Dr Julian Hume, a Research Associate based at the Museum, has examined the history of the area over the last 136,000 years. The results show that the area is sensitive to changing sea levels, and animals there have been killed by rising water several times in the past.
Rising and falling sea levels
Aldabra is made up of 22 islands around a blue lagoon. Its maximum height is eight metres above sea level.
Falling sea levels during an ice age about 136,000 years ago considerably increased Aldabra’s size. But rising sea levels during warmer periods may have inundated the island, killing its flora and fauna completely.
When sea levels fell again, a long and slow process of rebirth began. Aldabra has now probably been completely above sea level for only about 118,000 years. During that time it has been recolonised by giant tortoises, bats, flightless white-throated rails, pigeons, songbirds and a large number of seabirds.
Some of these animals are endemic to Aldabra, and we do not know from where many of them originated. Perhaps some the original populations on mainland Africa or Asia have since become extinct.
Old fossils are examined
A large set of fossil bones were collected from Aldabra in the 1980s and sent to the Smithsonian Institution in Washington, DC.
The fossils went unstudied for more than three decades after the collector moved on. But Dr Hume came across them while examining the collection during a study of recently extinct birds on Indian Ocean islands.
The material was shipped over to the palaeontology department at the University of Portsmouth. Under the guidance of Prof David Martill, the rocks containing fossils were dissolved in acid, revealing exquisitely preserved bird bones inside. This included the preservation of adult and juveniles, skulls, beaks and other fragile and rare bones.
These bones reveal previously unrecorded information about the original diversity of birds on the island, before high sea levels affected populations.
Dr Hume says, ‘Among them were the remains of several birds, some not previously recorded. The fossils showed that Aldabra was once inhabited by an endemic gadfly petrel and endemic duck, herons, large petrels and shearwaters, tropicbirds, a gull and a flightless white-throated rail.’
Harrier hawks and barn owls preyed on those smaller birds. Living alongside the birds were giant tortoises, an endemic horned crocodile, an iguana and a number of skinks and geckos.
All of these species disappeared during a period of high sea levels between 136,000 and 118,000 years ago, with some becoming totally extinct, such as the gadfly petrel, duck and crocodile.
Dr Hume added, ‘Our research has shown that low islands and atolls and associated fauna have always been vulnerable to climate change, and that complete turnovers of fauna can occur.
‘This natural process has been going on for millennia, but it is extremely rare to see it in the fossil record.
‘Unfortunately, due to human activity, climate change is now happening at an unprecedented rate, and many low islands around the world may be subject to similar inundation events that occurred on Aldabra over 118,000 years ago.’