Full Breakdown
Aldabra Atoll Giant Tortoise Population Surpasses 180,000 in New Survey
By Drooid · · How we work
Core Findings: Record-High Tortoise Numbers
A joint survey by the Seychelles Islands Foundation (SIF) and the University of Exeter estimates that roughly 180,000 Aldabra giant tortoises now roam Aldabra Atoll. The study, based on five years of counts (2018-2022) and published in *Ecology and Evolution*, provides the most reliable baseline for the species to date. Researchers describe the population as stable over the survey period, with a best-estimate of almost 183,000 adult and sub-adult individuals.
Background & Context
The Aldabra giant tortoise (*Aldabrachelys gigantea*) was driven to fewer than 1,000 individuals in the 19th century due to intensive exploitation. Protection began when Aldabra became a strict nature reserve in 1981 and a UNESCO World Heritage site in 1982. A previous widely cited estimate from the late 1990s placed the population at about 100,000 tortoises. The new figure therefore marks a substantial increase and is the first atoll-wide, rigorously modelled count.
Data & Statistics
- ? 180,000 tortoises overall (survey estimate)
- ? 183,000 adult/sub-adult tortoises (modelled best estimate)
- > 100,000 previous estimate (late 1990s)
- < 1,000 individuals in the 19th century
- Population exceeds the human population of the Seychelles (no exact figure given)
The study employed Bayesian hierarchical distance sampling to correct for animals hidden in dense vegetation, a method that explains why earlier counts likely under-reported numbers.
Official Statements & Responses
Dr Frauke Fleischer-Dogley, CEO of SIF, said the new count demonstrates the results of long-term conservation commitment and celebrates the work of all involved in protecting Aldabra. Dr Nancy Bunbury, a co-author, highlighted the difficulty of counting tortoises in dense scrub and noted that the modelling approach now offers a robust benchmark for future monitoring. Both officials stressed that the baseline figure is not an endpoint and that continued vigilance is required.
Challenges and Future Threats
Researchers warn that climate change is lengthening drought periods and reducing shade and food resources, while rising sea levels threaten low-lying parts of the atoll. Invasive mammals—particularly feral cats and black rats—continue to prey on eggs and hatchlings, posing the most immediate biological threat. Although feral goats were eradicated in 2012, the remaining invasive species still limit potential population growth.
What’s Next
The study’s authors recommend a targeted eradication program for the remaining invasive mammals. Successful removal is expected to remove the last major barrier to further population expansion and improve the species’ resilience to climate-related stresses.
Verbatim Quotes
- “These results are great news for the tortoises and testament to the fantastic work done in the field,” — Dr Dave Hudson, a postdoctoral research associate at the University of Exeter’s Cornwall campus
- “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,” — Dr Nancy Bunbury, one of the study’s co-authors