A survey of Aldabra Atoll has recorded approximately 180,000 giant tortoises, the highest figure ever counted for the species and a number that far exceeds the human population of Seychelles.
The count of the world’s largest free-roaming giant tortoise population was led by the Seychelles Islands Foundation (SIF) and the University of Exeter. Researchers analysed five years of tortoise counts conducted between 2018 and 2022.
It is the first update to the widely cited figure of 100,000 tortoises recorded in the late 1990s.
The study, published in the journal Ecology and Evolution, found the population was relatively stable over the survey period, with a best estimate of almost 183,000 adult and sub-adult tortoises.
Aldabra Atoll is one of only two places where giant tortoises live in the wild. The other is the Galápagos, where the population is much smaller.
Dr Frauke Fleischer-Dogley, chief executive of SIF, said more than 180,000 tortoises roaming freely on the atoll was “proof of what long-term commitment to conservation can achieve”. She called it wonderful news for Seychelles, for the world and for everyone who had worked to protect Aldabra.
The species was pushed to the brink in the 19th century, when exploitation left fewer than 1,000 Aldabra tortoises on the island. A subsequent recovery programme led to the atoll being designated a strict nature reserve in 1981 and a Unesco site in 1982.
Researchers describe the recovery as one of the greatest conservation success stories in the Indian Ocean, and say the new study confirms it.
Dr Nancy Bunbury, a co-author of the study, said Aldabra’s monitoring programme is among the longest running in the region, but the atoll’s dense vegetation makes counting tortoises difficult in places. “By modelling what we didn’t see, as well as what we did, we’ve produced the most reliable baseline yet for this iconic Seychelles species, establishing a benchmark we can use to track the population for decades to come,” she said.
The study’s authors warn that vigilance remains necessary. Aldabra’s climate is changing, and longer, more frequent droughts are predicted to reduce the shade and food available to tortoises. Rising sea levels also threaten low-lying parts of the atoll.
Invasive mammals remain a further threat, according to the authors, including feral cats and black rats that prey on tortoise eggs and hatchlings. They say planned eradication of these invasive mammals could remove the last barriers to population growth and help tortoises withstand future climate pressures.
Dr Dave Hudson, a postdoctoral research associate at the University of Exeter’s Cornwall campus, said the results were great news for the tortoises and a testament to the work done in the field. But he cautioned that the numbers provide a baseline, not an endpoint.
“Hopefully we have provided the conservation managers the evidence they need to track future change and act early as the tortoises face increasing pressures from climate change, drought and invasive species,” he said.
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