A study of the world’s largest population of free-ranging giant tortoises has revealed that this once-threatened species is thriving.
A population of Aldabra giant tortoises lives on an island in the Seychelles called Aldabra Atoll.
The most recent survey, carried out by the Seychelles Islands Foundation (SIF) and the University of Exeter, found that around 180,000 turtles lived on the island. This is the highest figure ever recorded for the species and far exceeds the human population in Seychelles.
The new figure was arrived at by a team that analyzed five years of turtle counts between 2018 and 2022. It is the first update since a widely cited figure of 100,000 turtles from the late 1990s.
Aldabra Atoll is one of only two places in the world where giant tortoises are found in the wild, with a much smaller population living in the Galapagos.
Dr Frauke Fleischer-Dogley, CEO of SIF, said: “More than 180,000 giant tortoises roaming freely on our atoll are proof of what a 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, published in the scientific journal Ecology and Evolution, found that the population was relatively stable for this period, with a best estimate of nearly 183,000 adult and subadult turtles.
In the 19th century, exploitation of the animals meant that fewer than 1,000 Aldabra tortoises remained on the island, but a recovery program led to it becoming a strict nature reserve in 1981 and a UNESCO site in 1982.
Researchers now say it is one of the Indian Ocean’s greatest conservation success stories, with a new study confirming it. “The Aldabra giant tortoise monitoring program is one of the oldest in the region, but the atoll’s dense vegetation makes counting tortoises really difficult in places,” said Dr. Nancy Bunbury, one of the study’s co-authors.
“By modeling what we didn’t see, as well as what we did, we produced the most reliable baseline to date for this iconic Seychelles species, establishing a baseline we can use to track the population for decades to come.”
Despite this good news, the authors of the study say that vigilance is required. Aldabra’s climate is changing, with longer and more frequent droughts expected to reduce the shade and food available to turtles, while rising sea levels threaten low-lying parts of the atoll.
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The study authors say invasive mammals remain a threat, including feral cats and black rats that prey on turtle eggs and hatchlings. According to the authors, the planned eradication of these last invasive mammals could remove the last obstacles to population growth and help it resist future climatic pressures.
“These results are great news for turtles and demonstrate the fantastic work being done in this area,” said Dr Dave Hudson, a postdoctoral researcher at the University of Exeter’s Cornwall campus.
“But it’s important to remember that the numbers provide a baseline, not an end point.
“I hope we have provided conservation managers with the evidence they need to track future changes and act quickly as turtles face increasing pressures from climate change, drought and invasive species.”
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