In the middle of the Indian Ocean, a sliver of coral is home to more shelled residents than there are people in the entire country it belongs to.
Some 183,000 giant tortoises (Aldabrachelys gigantea) live on the Aldabra Atoll, according to a scientific census conducted by the University of Exeter and the Seychelles Islands Foundation.
That’s far more than the roughly 120,000 people who live in the Republic of Seychelles, and it cements Aldabra‘s status as the species’ largest wild stronghold.
Today’s population is the result of four decades of strict isolation and environmental protection. In the late 20th century, the population was estimated at roughly 100,000 animals, so the new count confirms that the ecosystem has recovered from its decline.
Counting Tortoises Hidden in the Scrub
Surveying giant tortoises on Aldabra means crossing razor-sharp coral covered in dense scrub and mangroves.
During the hottest hours of the day, the tortoises seek shelter in caves or under bushes — a habit that skewed the direct visual counts used in earlier decades.
To correct this bias, the research team analyzed field data collected between 2018 and 2022 using a hierarchical Bayesian distance sampling model. This technique calculates the probability of spotting each animal in a given habitat, then uses that to estimate how many tortoises went unseen.
The analysis showed that tortoise density remained stable throughout the five years of sampling. This suggests the atoll’s ecosystem is naturally balanced, with no sign of a collapse caused by food shortages or overpopulation.
Hunting, Decline, and Recovery
Aldabra’s thriving tortoise population stands in stark contrast to its situation two centuries ago.
During the 18th and 19th centuries, crews from whaling, merchant, and pirate ships landed on the region’s islands to capture tortoises by the thousands. Because they can survive for months without food or water, the animals were prized as a source of fresh meat on long sea voyages.
That unchecked exploitation drove giant tortoise populations to extinction on several Indian Ocean islands and reduced Aldabra‘s tortoise population to fewer than 1,000 animals by the late 19th century.
The species remained on the brink of extinction until the Seychelles government and the international community stepped in to protect the atoll.
Aldabra was declared a Strict Nature Reserve in 1981 and became a UNESCO World Heritage Site the following year.
Authorities banned commercial activity and restricted visitor access. In 2012, conservationists completed the eradication of the feral goats that had been destroying the grasslands. Free from introduced herbivores and human pressure, the native vegetation bounced back, allowing the tortoise population to grow steadily.
A New Threat: Climate Change
Despite the success of the conservation plan, the study’s authors warned that the population now faces threats tied to the climate crisis.
Increasingly frequent droughts are thinning the vegetation and depleting the freshwater sources the tortoises rely on to keep cool in a landscape with little shade.
Because Aldabra is a low-lying coral atoll, rising seas threaten to push salt water into coastal areas and wipe out the grasslands the tortoises feed on.
Meanwhile, feral rats and cats that remain on the atoll prey on eggs and hatchlings, reducing nest survival.
Today, conservation on Aldabra is no longer about defending the atoll from passing ships, but about protecting the species in the face of a warming planet.
