Tortoises clustered at the highest densities in Aldabra's grasslands, dubbed "tortoise turf," followed by open exposed surfaces and open mixed scrub. Dense scrub, mangrove, and Pemphis habitats held lower detectable numbers, but the modelling accounts for that reduced visibility.

What drove the recovery?

In the 19th century, exploitation reduced Aldabra's tortoises to fewer than 1,000 individuals. The atoll became a strict nature reserve in 1981 and a UNESCO World Heritage Site in 1982, and four decades of protection followed.

Dr. Frauke Fleischer-Dogley, CEO of SIF, said: "More than 180,000 giant tortoises roaming freely on our atoll is proof of what long-term commitment to conservation can achieve. This is wonderful news for Seychelles, for the world, and for everyone who has worked to protect Aldabra."

The study also documents the impact of removing feral goats, which had stripped the atoll's vegetation. SIF eradicated those goats in 2012. Recovery of plant cover has likely supported the tortoise boom.

What still threatens the population?

The authors warn that the good news is not the end of the story. Aldabra's climate is shifting: longer and more frequent droughts will reduce shade and forage, while rising seas threaten low-lying parts of the atoll.

Invasive predators also remain. Feral cats and black rats still raid tortoise nests and prey on eggs and hatchlings. The planned removal of those remaining invasive mammals could remove the last major barrier to further growth.

Dr. Dave Hudson, a postdoctoral research associate at the University of Exeter in Cornwall who helped develop the statistical framework, said: "These results are great news for the tortoises and testament to the fantastic work done in the field. But it's important to remember that the numbers provide a baseline — not an endpoint."

"Hopefully we have provided the conservation managers the evidence they need to track future change and act early," Hudson added, "as the tortoises face increasing pressures from climate change, drought, and invasive species."

What does the research look like in practice?

Counting tortoises on Aldabra is harder than it looks. The atoll covers about 155 square kilometres of mixed mangrove, scrub, and grassland, and surveyors cannot see every animal. SIF staff walked transects repeatedly across the five-year window, recording the distance to each tortoise they spotted. Bayesian hierarchical distance sampling then estimates the density of animals that were present but missed, while accounting for habitat-specific differences in how easy tortoises are to detect.

Lead author Lara B. Hillyard and colleagues report the method and the resulting baseline in the paper "Inferring the Abundance of an Island Megaherbivore From Semi‐Structured Distance Sampling," published in Ecology and Evolution (DOI: 10.1002/ece3.74282).

The team expects the baseline to serve as a reference point for monitoring how the population responds as climate stress intensifies and as invasive predators are removed.