Just decades after facing mass extinction, the giant tortoise is now thriving as a species after a significant baby boom.
The hard-shelled, slow-moving animal now outnumbers humans in the Seychelles, the archipelago that houses the world’s largest free-roaming population of giant tortoises.
Researchers estimate that about 180,000 Aldabra giant tortoises (Aldabrachelys gigantea) live on Aldabra Atoll, the highest population estimate ever recorded.
The study, led by the Seychelles Islands Foundation (SIF) and the University of Exeter, analysed five years of tortoise counts collected by SIF research staff between 2018 and 2022. It is the first comprehensive estimate covering the entire atoll and the first major update to the widely cited figure of 100,000 tortoises from the late 1990s. The findings have been published in the scientific journal Ecology and Evolution.
Aldabra Atoll is one of only two places in the world where giant tortoises live in the wild. The other is the Galapagos Islands, which support a much smaller population.
Dr Frauke Fleischer-Dogley, chief executive of SIF, said the findings demonstrated the value of sustained conservation efforts.
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‘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 researchers’ best estimate put the number of adult and sub-adult tortoises at almost 183,000, with the population remaining relatively stable over the study period.
The figure exceeds the human population of the Seychelles and marks a dramatic recovery from the 19th Century, when heavy exploitation reduced the tortoise population on Aldabra to fewer than 1,000.
Conservation measures helped protect the species after Aldabra became a strict nature reserve in 1981 and a Unesco World Heritage Site in 1982. The recovery is regarded as one of the Indian Ocean’s major conservation success stories.
However, the researchers cautioned that the new estimate did not mean the population had nearly doubled since 1999.
Previous surveys did not fully account for tortoises that were present but hidden from view in dense vegetation or shaded areas. The latest study used a statistical method known as Bayesian hierarchical distance sampling to account for animals missed by surveyors and differences in visibility across the atoll’s habitats.
The findings suggest earlier estimates probably underestimated the true population, although the researchers said the evidence also indicated that the species had genuinely increased in number and was now in a healthy, stable position.
Dr Nancy Bunbury, a co-author of the study, said the atoll’s vegetation made accurate counts challenging.
‘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,’
She added: ‘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 study found that tortoises were most densely concentrated in grasslands, known as ‘tortoise turf’, followed by open, exposed areas and open mixed scrub.
They were harder to detect in dense scrub, mangroves and areas dominated by Pemphis, a type of coastal shrub. The researchers’ modelling helped account for the reduced visibility in these habitats.
Despite the encouraging findings, the study’s authors warned that the population continued to face environmental threats.
Climate change is expected to bring longer and more frequent droughts, potentially reducing the availability of food and shade. Rising sea levels also pose a risk to the atoll’s low-lying areas.
Invasive species remain another concern. Feral goats, which severely damaged Aldabra’s vegetation, were successfully eradicated in 2012. However, feral cats and black rats continue to prey on tortoise eggs and hatchlings.
The researchers said eradicating the remaining invasive mammals could remove some of the barriers to further population growth and help the tortoises withstand future climate pressures.
Dr Dave Hudson, a postdoctoral research associate at the University of Exeter’s Cornwall campus, who helped develop the statistical modelling framework, said the findings were encouraging but should provide a starting point for continued monitoring.
‘These results are great news for the tortoises and testament to the fantastic work done in the field,’ he added. ‘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.’
The research paper was published in Ecology and Evolution.
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