Aldabra’s tortoise estimate tops 180,000, setting a new conservation baseline

Aldabra Atoll in Seychelles has approximately 180,000 giant tortoises, according to research reported on 8 October by South West News Service in the Guardian. The estimate gives conservation managers a new benchmark for the world’s largest wild giant tortoise population, but does not establish a recent surge in numbers.

Led by Seychelles Islands Foundation and the University of Exeter, the research draws on survey seasons labelled 2018–2022. The study appeared in Ecology and Evolution on 4 September; the figures are estimates from earlier fieldwork, rather than a count of animals living on the atoll this October.

What Aldabra’s higher tortoise estimate means

The researchers accounted for animals that observers could not see, modelling how detection varied with habitat. Dense vegetation makes counting difficult in parts of Aldabra, co-author Nancy Bunbury explained in the SWNS report. The work was therefore an estimate of abundance, rather than an exhaustive census of individual tortoises.

The estimate primarily covers adults and sub-adults. Concealed juveniles were effectively unavailable for detection, limiting what the surveys could establish about younger animals. That distinction matters when interpreting the headline total: it does not represent a direct count of every tortoise across all life stages.

The study’s results and discussion give slightly different exact totals, both close to 183,000. Approximately 180,000 captures their shared finding without treating either precise figure as definitive. The researchers describe the estimate as the highest recorded for this population.

The Guardian report compares the new estimate with a widely cited figure of 100,000 from the late 1990s. But earlier estimates used different methods without explicitly modelling detection. Subtracting the old figure from the new one would not establish how many animals the population has gained, or how quickly it has grown.

The original study sets out the sampling approach and its limitations in Ecology and Evolution.

Why giant tortoises matter to Aldabra’s ecosystem

Aldabra consists of four large coral islands enclosing a shallow lagoon, surrounded by a coral reef. UNESCO identifies it as the home of the world’s largest giant tortoise population and records its inscription on the World Heritage List in 1982.

The tortoises also help shape the habitat they occupy. UNESCO describes them as dominant herbivores feeding on grasses and shrubbery, including vegetation that has evolved in response to their grazing. Their abundance is consequently relevant to the wider island ecosystem as well as the conservation of the species.

UNESCO’s World Heritage assessment credits Aldabra’s remoteness and difficult access with limiting human interference. That isolation also creates logistical challenges for management. Seychelles Islands Foundation manages the legally protected property, where tourism is limited and carefully controlled.

A long recovery, with limits on what the survey proves

According to the Guardian report, nineteenth-century exploitation left fewer than 1,000 Aldabra tortoises on the atoll. Aldabra became a strict nature reserve in 1981, before its World Heritage inscription the following year. Those protections long predate the fieldwork behind the latest estimate.

Seychelles Islands Foundation chief executive Frauke Fleischer-Dogley welcomed the finding, telling SWNS: “More than 180,000 giant tortoises roaming freely on our atoll is proof of what long-term commitment to conservation can achieve.”

That is the foundation’s interpretation of conservation success. The population study does not isolate the contribution of each protective intervention. Its more immediate use is as a benchmark: Bunbury said modelling unseen animals alongside observations would help managers track the population for decades.

Drought and invasive mammals remain threats

The researchers warn that longer and more frequent droughts could reduce food and shade, while rising sea levels threaten low-lying areas. UNESCO separately identifies climate change and invasive species as continuing management threats; its assessment provides context rather than validation of the new statistical estimate.

Feral cats and black rats prey on tortoise eggs and hatchlings, according to the authors’ warning reported by the Guardian. They say planned eradication could remove barriers to further population growth and improve resilience. The report does not establish that eradication has been completed or that those benefits have occurred.

The paper recommends representative wet-season monitoring and targeted studies of hatchling and juvenile survival. Further observations will be needed to establish future trends, including changes that a higher headline estimate alone cannot reveal.

University of Exeter researcher Dave Hudson put that distinction plainly in the SWNS report: “But it’s important to remember that the numbers provide a baseline – not an endpoint.” He said he hoped the evidence would help managers track change and respond early to mounting pressures.