A population's genetic scar
The Aldabra Atoll, a raised coral reef in the Indian Ocean, hosts the world's largest population of giant tortoises, Aldabrachelys gigantea. Over 100,000 of these animals navigate the rugged limestone landscape. Their collective DNA, however, tells a story of a near-total collapse. Genetic studies of the tortoise population show remarkably low diversity, particularly in their mitochondrial DNA. This uniformity points to a severe population bottleneck in the recent past. The cause was not a natural disaster, but human activity.
Throughout the 18th and 19th centuries, sailors, whalers, and settlers harvested giant tortoises across the Indian Ocean for food. Their ability to survive for months without food or water made them a convenient fresh meat supply for long voyages. This intense exploitation drove every other wild population of Indian Ocean giant tortoises to extinction. By the late 1800s, the Aldabra population had also been reduced to a few thousand individuals, and possibly as low as 1,000 by 1892. This catastrophic decline squeezed the entire species' gene pool through a tiny opening. Every one of the 100,000 tortoises on the atoll today is descended from this small, depleted group of survivors. The event left a permanent signature of low genetic variability, a scar still visible to scientists sequencing their genomes.
Engineering an ecosystem's recovery
The population's recovery from the brink of extinction began in the 20th century after exploitation ceased. By the early 1970s, their numbers had rebounded to an estimated 129,000. Today, the Aldabra Atoll is a UNESCO World Heritage Site, largely because of this unique tortoise population. The tortoises are the dominant herbivore and primary ecosystem engineer on the atoll. They create open pathways through dense scrub, disperse seeds, and maintain "tortoise turf," lawn-like areas of mixed grasses and herbs.
While the sheer number of tortoises suggests a healthy population, their low genetic diversity presents a long-term conservation challenge. A genetically uniform population can be more susceptible to new diseases or environmental changes. To mitigate this risk, and to restore ecosystems where tortoises once lived, Aldabra tortoises are used in rewilding projects. Small groups have been introduced to other islands in the Seychelles and Mauritius. These relocated populations act as ecological replacements for extinct tortoise species and serve as an important genetic backup for the main Aldabra population, ensuring the species' survival is not tied to a single, low-lying atoll vulnerable to sea-level rise. The complete, chromosome-level genome of the Aldabra giant tortoise was sequenced in 2022, helps manage the genetic health of both the wild and rewilded populations.