Aldabra survey sets a new baseline for giant tortoise numbers
The story at a glance
A study estimates a mean of 183,274 Aldabra giant tortoises on Aldabra Atoll between 2018 and 2021. The researchers describe the population as apparently relatively stable over that period.1
The estimate accounts for animals that surveys may have missed, so it is not evidence that numbers have nearly doubled since the earlier estimate of 100,000. Small juveniles may also be under-represented.12
The story in brief
What you need to know
- Researchers analysed transect counts gathered from 2018 to 2022 using Bayesian hierarchical distance sampling. The model allowed for differences in how visible tortoises are in different habitats and for animals that were not detected.1
- For 2018–2021, the study gives a mean estimate of 183,274 tortoises, with a standard deviation of 12,172. The authors characterise the population as apparently relatively stable during those years.1
- The 2022 estimate was left out of the period mean because survey effort was considerably lower. The researchers considered that estimate potentially biased upwards.1
- The newer estimate should not be read as a direct increase from the previously cited figure of 100,000. The updated method accounts for missed animals, making the two estimates difficult to compare as a trend.12
- The study identifies drought and sea-level rise, alongside predation on eggs and hatchlings by feral cats and black rats, as ongoing pressures. It recommends maintaining and expanding invasive-species eradication efforts.1
Conservation context
Why this matters
A detection-corrected estimate provides a baseline for tracking this atoll’s tortoise population, but future comparisons will be more informative if survey effort and methods remain comparable.1
The developing story
What happens next
The study points to continued monitoring and invasive-species eradication. Future surveys could help clarify population change, while complementary monitoring may be needed to assess juvenile recruitment.1
Established evidence
What we know
- The study reports the highest estimate to date for the Aldabra giant tortoise population: about 180,000 animals on Aldabra Atoll.123
- The analysis uses transect counts collected between 2018 and 2022 and models differences in detectability across habitats.1
- The 2018–2021 mean is 183,274, with a standard deviation of 12,172; the researchers describe the population as apparently relatively stable during that period.1
Open questions
What we don't know
- It remains uncertain how well the established, purposively placed transects represent all habitat areas across Aldabra when used to produce atoll-wide estimates.1
- The study indicates that small juveniles are likely under-represented in transect surveys; how best to measure their recruitment remains an open question.1
- Future surveys will need comparable effort and methods to distinguish population change more confidently from changes in detectability.1
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