Aldabra giant tortoises in Seychelles reach record population of 180,000

Researchers surveying the remote Aldabra Atoll in Seychelles have found that the population of giant tortoises has reached approximately 180,000. It is the highest number ever recorded for the species, confirming one of the world's most successful conservation turnarounds.

The figure is the result of a five-year study by the Seychelles Islands Foundation (SIF) and the University of Exeter, who analysed tortoise counts collected between 2018 and 2022. Published in the scientific journal Ecology and Evolution, the study estimates there are almost 183,000 adult and sub-adult tortoises roaming the island.

This is the first major update on the population since the late 1990s, when the widely cited estimate stood at 100,000. The new total means the giant tortoises on Aldabra now outnumber the human population of the entire Seychelles archipelago.

Only two places on Earth are home to wild giant tortoises: the Galápagos Islands in the Pacific, and Aldabra Atoll in the Indian Ocean. The Aldabra population is by far the larger of the two.

It is a stark contrast to the late 19th century, when heavy exploitation by humans left fewer than 1,000 of the creatures alive on the atoll. The recovery began in earnest when Aldabra was declared a strict nature reserve in 1981, followed by its designation as a Unesco World Heritage site in 1982.

"More than 180,000 giant tortoises roaming freely on our atoll is proof of what long-term commitment to conservation can achieve," said Dr Frauke Fleischer-Dogley, the chief executive of SIF.

Counting animals in the wild is notoriously difficult, particularly on Aldabra, where the terrain is covered in dense, tangled vegetation. To get an accurate picture, researchers used statistical modelling to account for the tortoises they could not see during physical counts.

"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," said Dr Nancy Bunbury, a co-author of the study.

However, conservationists warn that the tortoises are not out of danger. The climate on Aldabra is shifting. Longer and more frequent droughts are expected to reduce both the food supply and the shade tortoises rely on to regulate their body temperature. Rising sea levels also threaten the low-lying parts of the atoll.

Invasive predators pose another immediate threat. Feral cats and black rats on the island routinely prey on tortoise eggs and newly hatched young. SIF researchers say that planned eradication programmes for these invasive mammals could lift the remaining barriers to population growth and help the tortoises withstand future environmental pressures.

Dr Dave Hudson, a researcher at the University of Exeter, said the new figures represent a baseline rather than 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.