Africa

The last survivors

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The giant tortoise has for centuries been outcompeted, chased and poached towards extinction. But on a remote atoll outside the Seychelles, a large colony is prospering.

 

The story of the giant tortoise is often a sad one. Since the 17th century, settlers and explorers have raided islands in the Indian Ocean, putting tortoises on their ships as food. The reptiles can go six months without food or water, providing starved sailors with fresh meat. Where humans weren’t involved, rival species snared its food and attacked its eggs; the giant tortoise, after all, was never designed for combat or getaways. By the late 1800s, the global population was estimated at below 1,000.

The narrative is quite different at the Aldabra Atoll, an isolated ecosystem controlled by the Seychelles. Here, a colony of around 100,000 giant tortoises flourish in a largely self-sustainable environment. The only other surviving colony, though much smaller, lives on the Galápagos Islands, 600 kilometres west of Ecuador. The difference between Aldabra and other places, experts say, is its remoteness; it lies 1,100 kilometres south-west of Mahé, the largest of the Seychelles’ 115 islands. Explorers have been unable – or seen no reason – to visit it, leaving the habitat largely undisturbed. Today access is strictly regulated: the only current settlement is a small research station for accredited scientists.

There is much to study. The atoll’s four main islands – 125,000-year-old coral reeves elevated above sea level – enclose a large, shallow lagoon of endemic and endangered species; large seabird colonies, green turtles, hawksbill turtles, and the white-throated rail – the last flightless bird of the Western Indian Ocean. UNESCO, listing it in 1982, labelled the atoll ‘an outstanding natural laboratory for scientific research and discovery’.

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Unusually, at Aldabra, the giant tortoises dominate the ecosystem. After introduced feral goats were eradicated from the atoll in August last year, their competition for food and shade was gone. The tortoises are central to the ecological composition, feeding on grass to keep it short and dispersing native seeds. “They are absolutely essential,” says Rich Baxter, project officer for the Zürich Aldabra Research Platform, a scientific collaboration between the University of Zürich and the Seychelles Islands Foundation, which manages the atoll. “Without them it would be a completely different habitat.”

The giant tortoise is one of the world’s longest living creatures. Besides being large – males can weigh up to 250kg – they often live to more than 100 years old. Their maximum age, though, remains unclear. One giant tortoise was supposedly 176 years old when it died in 2006 in Australia Zoo, Queensland. Another reputedly hit 255 years when passing away in Calcutta, India, the same year. But there is no scientific documentation for either. The fact is that for many tortoises, proper records have never been maintained; they have simply outlived the scientists who study them.

Unlike at the Galápagos, there is less need for active conservation management of the tortoises. “The tortoises are so isolated out there that they are protected anyway,” says Baxter. ”If you left them alone for five years they would be absolutely fine.” Rather, it’s about potential dangers. “We are trying to understand the system, in case there are threats to Aldabra, like climate change,” he says. “We need to know as much about Aldabra as possible so that we can counter these threats if they arrive.”

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A central concept in preserving the species is to find alternative locations, should the atoll get in trouble. Aldabran Giant Tortoises have already been shipped to America, Prague, Zürich and elsewhere to form ‘reserve colonies’. At other places, such as Mauritius, giant tortoises have been dispatched to recreate the biological role of other extinct tortoise species. Whether that will work, says Baxter, depends on the studies at the atoll: “If we can understand what they do in an original wild habitat, we can maybe take what we learn from here and better manage the species at other islands.”

That said, even the Aldabra Atoll isn’t completely safe. “Aldabra has been under water several times in its history,” Baxter says. “You can find old tortoise fossils embedded in the limestone coral. So it’s been underwater, and then re-colonised again by other tortoises. We believe the current tortoises may originate from Madagascar.”

The tortoises living at Madagascar, of course, are long extinct. And so this time, should the Aldabra Atoll drown, no alternative colony would exist. For the Aldabran species, that scenario is a realistic danger. “The main threat would be global warming and climate change because the Aldabra is incredibly low-lying,” Baxter says. “It’s only eight meters above sea level, so even the smallest rise is going to make a huge difference.” He also adds, however, that while that is certain to happen at some point, it may take 100,000 years. Beyond anything unexpected, with the exception of a sea level rise, one can conclude that the giant tortoise is probably safe for now.

Photos: Rich Baxter.

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