A survey of the Aldabra Atoll in the Seychelles has identified approximately 183,000 Aldabra giant tortoises, a figure that surpasses the human population of the entire country. The research, led by the Seychelles Islands Foundation (SIF) and the University of Exeter, confirms the species is thriving and marks a significant recovery from the 19th century, when exploitation left fewer than 1,000 individuals remaining.
Population Estimates and Survey Methodology
The new population estimate of 183,000 adult and sub-adult tortoises, published in the journal Ecology and Evolution, is the highest ever recorded for the species. This figure provides a more accurate baseline than the widely cited estimate of 100,000 tortoises from the late 1990s.
Researchers utilized a statistical method known as Bayesian hierarchical distance sampling to account for the dense vegetation on the atoll, which previously made accurate counts difficult. Instead of relying only on visible tortoises, the team modeled data from five years of counts collected between 2018 and 2022 to account for animals hidden in shade or thick brush. This approach suggests that previous surveys likely underestimated the population size rather than indicating a sudden doubling of numbers. Dr. Nancy Bunbury, a study co-author, noted that while the monitoring programme is among the region’s longest running, the thick vegetation makes counting “genuinely difficult in places.” By modeling both seen and unseen animals, the team established a benchmark to track the population for coming decades.
Conservation Success and Historical Recovery
The recovery of the Aldabra giant tortoise is considered one of the most successful conservation initiatives in the Indian Ocean. Following the near-extinction of the population in the 1800s, the atoll was designated a strict nature reserve in 1981 and gained UNESCO World Heritage status in 1982.
Dr. Frauke Fleischer-Dogley, CEO of the SIF, stated that the current population density serves as evidence of the results achievable through long-term conservation commitment. She called the presence of more than 180,000 tortoises “wonderful news for Seychelles, for the world, and for everyone who has worked to protect Aldabra.” While the population is currently stable, researchers are viewing this data as a benchmark for future monitoring rather than an endpoint for conservation efforts. Dr.
Carrying Capacity
The findings challenge previous scientific assumptions about the island. Researchers stated their results refute the old notion that the population had already reached its carrying capacity in the late 1900s. The study suggests the ecological limit of the atoll may change as invasive species are removed and vegetation recovers.
Climate Change and Invasive Species Threaten Tortoise Population
Despite the positive findings, the study authors emphasize that the tortoises remain vulnerable to environmental and biological threats. Climate change poses a persistent risk, with predicted increases in the frequency and duration of droughts expected to limit access to essential food and shade. Rising sea levels also threaten the low-lying areas of the atoll.
Invasive species continue to impact the ecosystem. Feral cats and black rats remain a concern, as they prey on tortoise eggs and hatchlings. According to the research team, the planned eradication of these invasive mammals could remove significant barriers to further population growth and increase the species’ resilience against shifting climate conditions.
Survey Methods Reveal Highest Tortoise Population Ever Recorded
Why is this the highest population count ever recorded?
This is the first comprehensive survey to use Bayesian hierarchical distance sampling on the atoll. The method accounts for tortoises that were present but hidden by dense vegetation, providing a more reliable total than older, less sophisticated counting techniques.
What is the primary threat to the tortoises moving forward?
Researchers identified climate change and invasive species as the two main challenges. Longer droughts may reduce the availability of food and shade, while feral cats and black rats continue to pose a threat to the survival of eggs and hatchlings.
Are there other wild populations of these giant tortoises?
Aldabra Atoll is one of only two places in the world where giant tortoises are found in the wild. The only other wild population exists on the Galápagos Islands, though that group is significantly smaller than the one found in the Seychelles.
Did you know?
The Aldabra giant tortoise population is now large enough that it outnumbers the human population of the Seychelles.
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