We take a look at the parallel histories of Aldabra and Galapagos giant tortoises, and the ways in which both are now helping to restore island ecosystems.
Rich Baxter is Director of the Indian Ocean Tortoise Alliance and an island ecologist with over a decade of experience in the Western Indian Ocean. In this time, he has developed a deep appreciation and fascination for the region’s biodiversity, leading him to become a country member of the IUCN SSC Tortoise and Freshwater Turtle Specialist Group.
Aldabra Atoll, a UNESCO World Heritage site in the Indian Ocean, is home to the Aldabra banded snail, the Aldabra fody, the Aldabra rail and the remarkable Aldabra giant tortoise. These tortoises form the largest giant tortoise population in the world, with four times as many individuals as there are in all of Galapagos.
Despite their geographical separation, both the Aldabra giant tortoises of the Seychelles and the various species of Galapagos tortoise followed similar evolutionary trajectories, evolving to be large herbivores on isolated islands. This convergent evolution shows how similar pressures can lead to comparable adaptations in distinct species. However, while the Galapagos tortoises diversified into several species across the Archipelago, the Aldabra tortoises remained a single population, thriving in the relatively uniform conditions of the Aldabra Atoll. Their rounder faces and ability to suck water up from shallow pools of water through their nostrils are key to their survival.
Human exploitation has led to the extinction of giant tortoises from all the world’s large landmasses, and they only survived on islands. Early explorers, sailors and whalers saw tortoises as an abundant and convenient food source, and tens of thousands were taken from their islands, loaded onto ships and slaughtered en route to new lands and museums. By the late 19th century, giant tortoises on islands across the globe had gone extinct, including several species of Galapagos tortoise, while Aldabra tortoises narrowly avoided a similar fate. The Mascarene islands of Mauritius, Réunion and Rodrigues all lost their endemic giants, as did Madagascar, the ancestral home of the Aldabra giant tortoise.
Today, threats to the last giants have shifted but are still significant. In the Seychelles, habitat degradation, poaching and introduced predators pressure their survival, while rising sea levels and longer droughts threaten low-lying islands like Aldabra. In Galapagos, encroachment and introduced species, such as rats and goats, continue to threaten giant tortoises.
Human exploitation has led to the extinction of giant tortoises from all the world’s large landmasses, and they only survived on islands.
Conservationists are using tortoises and their ecosystem-engineering status to rejuvenate and restore ecosystem functions to degraded islands. While the Aldabra population remains stable, the tortoises have been introduced to higher-elevation islands in the Seychelles, where they will be more secure from rising sea levels. Some of these islands now hold large, self-sustaining tortoise populations and could even serve as a genetic back-up stock for the Aldabra population.
Further south in the Indian Ocean, Aldabra tortoises are being used as ecological replacements for extinct species on islands like Round Island and Rodrigues in Mauritius, where they play a critical role in herbivory, seed dispersal, nutrient cycling and habitat formation. Reintroducing giant tortoises to areas they once inhabited helps rebuild ecological functionality, which in turn supports many other species.
In Galapagos, conservationists are reintroducing Floreana tortoises – a species thought extinct – by using individuals from Isabela island with Floreana genes. This genetic rescue aims to restore a missing ecosystem engineer species and preserve genetic diversity. Galapagos has long been a focal point of global conservation, thanks to Darwin, while conservation efforts in the Seychelles have received less global limelight.
Organisations like the Indian Ocean Tortoise Alliance are working to promote the Aldabra tortoise as a flagship species for Indian Ocean conservation, and there are increasing numbers of tortoises in protected areas in Seychelles, along with successful rewilding projects in Madagascar and Mauritius. The survival of both emblematic giants appears to be heading in a slow and steady positive direction. How very tortoise-like.
The remarkable story of the Floreana giant tortoise, a species that the world thought was lost forever.
