Giant Tortoise Population is Thriving–Survey Finds 180,000 on One Island Chain.
A survey led by the Seychelles Islands Foundation and the University of Exeter estimated about 180,000 Aldabra giant tortoises on Aldabra Atoll, the highest figure recorded for the species, and found the population stable.
Researchers from the Seychelles Islands Foundation (SIF) and the University of Exeter report that Aldabra Atoll in the Seychelles holds about 180,000 Aldabra giant tortoises. The university describes it as the world's largest free-roaming giant tortoise population and the highest figure ever recorded for the species. The work is published in the journal Ecology and Evolution.
The team analyzed five years of tortoise counts gathered by SIF research staff on the atoll between 2018 and 2022. According to the university, this is the first rigorous, atoll-wide estimate and the first update since a widely cited figure of 100,000 from the late 1990s. The best estimate was almost 183,000 adult and sub-adult tortoises, and the population appeared relatively stable over the period.
The researchers caution that the new number does not mean the population has nearly doubled since 1999. Earlier surveys did not account for tortoises that were present but unseen in dense scrub or shade. The new study used a statistical method called Bayesian hierarchical distance sampling, which corrects for missed animals and for differences in visibility across habitats. The study suggests earlier figures were probably underestimates, while still pointing to a population that has grown and appears healthy.
Tortoise densities were highest in grasslands known as tortoise turf, followed by open exposed surfaces and open mixed scrub. Animals were harder to detect in dense scrub, mangrove and Pemphis habitats.
The authors say continued vigilance is needed. Longer and more frequent droughts are predicted to reduce shade and food, and rising sea levels threaten low-lying areas. Feral goats were eradicated in 2012, but feral cats and black rats still prey on tortoise eggs and hatchlings. The authors say a planned eradication of these remaining invasive mammals could allow further growth and help the population withstand climate pressures.
Dr Dave Hudson of the University of Exeter said the numbers provide a baseline and not an endpoint. The page notes that the tortoises were reduced to fewer than 1,000 individuals in the 19th century through heavy exploitation, and that Aldabra became a strict nature reserve in 1981 and a UNESCO site in 1982.
Note: The headline implies the population is thriving at 180,000, but the researchers say the figure does not mean the population nearly doubled since the late 1990s, because earlier surveys likely undercounted. The page also describes the figure as an estimate for one atoll, Aldabra.
At a Glance
- The Aldabra Atoll population is estimated at about 180,000 giant tortoises, based on SIF counts from 2018 to 2022.
- The previous widely cited figure was 100,000, from the late 1990s, but researchers say it was probably an underestimate.
- Heavy exploitation in the 19th century reduced Aldabra's tortoises to fewer than 1,000 individuals.
- Feral goats were eradicated from Aldabra in 2012; feral cats and black rats still prey on eggs and hatchlings.
Sources
This brief was written by Brightside Report from the reporting linked above. Spot an error? Request a correction.