A landmark survey of the world's largest free-roaming giant tortoise population has revealed the once endangered species is thriving and stable in the Seychelles.
The Aldabra giant tortoise (Aldabrachelys gigantea) population on Aldabra Atoll stands at approximately 180,000 individuals - the highest figure ever recorded for the species - following the survey led by the Seychelles Islands Foundation (SIF) and the University of Exeter.
The new figure was obtained by the team, who analysed five years of tortoise counts collected by SIF research staff between 2018 and 2022.
It is the first rigorous, atoll-wide estimate of the population and the first update since the widely cited figure of 100,000 tortoises from the late 1990s. The findings have now been published in the scientific journal Ecology and Evolution.
Aldabra Atoll is one of only two places in the world where giant tortoises can be found in the wild, with a much smaller population resident in the Galapagos.
Dr Frauke Fleischer-Dogley, CEO of SIF, said: "The monitoring data in this study were collected by SIF staff living and working on the atoll, year after year. More than 180,000 giant tortoises roaming freely on our atoll is proof of what long-term commitment to conservation can achieve. This is wonderful news for Seychelles, for the world, and for everyone who has worked to protect Aldabra."
The study found that the population was relatively stable for this period, with a best estimate of almost 183,000 adult and sub-adult tortoises. This figure exceeds the human population of the Seychelles and is a far cry from the species' darkest hour in the 19th century, when heavy exploitation reduced Aldabra's tortoises to fewer than 1,000 individuals. Their recovery, aided by protection since Aldabra became a strict nature reserve in 1981 and a UNESCO site in 1982, is one of the greatest conservation success stories of the Indian Ocean, and the new study confirms that legacy.
"Aldabra's giant tortoise monitoring programme is one of the longest running in the region, but the atoll's dense vegetation makes counting tortoises genuinely difficult in places," said Dr Nancy Bunbury, one of the study's co-authors. "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."
The researchers caution that the new headline figure does not mean the population has nearly doubled since 1999. Earlier surveys did not account for tortoises that were present but not seen by surveyors, hidden in dense scrub or shade. The new study used a sophisticated statistical technique, known as Bayesian hierarchical distance sampling, which corrects for missed animals and for differences in visibility across Aldabra's habitats. Because earlier methods likely missed tortoises, the study suggests previous figures were probably underestimates - while still pointing to a population that has genuinely grown and now appears healthy and stable.
The study also found that tortoises occur at highest densities in Aldabra's grasslands, or 'tortoise turf', followed by open exposed surfaces and open mixed scrub. Tortoises were harder to detect in dense scrub, mangrove and Pemphis habitats, where the new modelling helps correct for reduced visibility.
Despite the good news, the study's authors warn that vigilance is essential. Aldabra's climate is changing, with longer and more frequent droughts predicted to reduce the shade and food available to tortoises, while rising sea levels threaten low-lying parts of the atoll.
Invasive mammals also remain a key threat: feral goats, which devastated the atoll's vegetation before their successful eradication in 2012, are gone, but feral cats and black rats still prey on tortoise eggs and hatchlings. The planned eradication of these remaining invasive mammals, the authors say, could lift the last barriers to further growth of the population and help it withstand future climate pressures.
"These results are great news for the tortoises and testament to the fantastic work done in the field," said Dr Dave Hudson, Postdoctoral Research Associate at the University of Exeter, Cornwall, who supported the development of the statistical modelling framework used to analyse the long-term monitoring data and estimate the population. "But it's important to remember 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."
