Thousands of Giant Tortoises Sustain a Flourishing Ecosystem in Aldabra, an Isolated Atoll of the Indian Ocean

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The animals graze the vegetation into a picturesque turf, fertilize the soil with their dung and disperse seeds over large distances

Kevin Gepford

April 15, 2025 8:00 a.m.

An Aldabra giant tortoise stands tall. Males can weigh more than 500 pounds.

Rich Baxter

Life on Aldabra atoll, located on the western edge of the Indian Ocean, operates at the rhythm set by its most ancient residents. During my morning stroll from the research facility on Picard Island to the lagoon, I have to sidestep a few giant tortoises along the sandy path. My attention is captured by a rather unassuming yet fascinating sight—a sizable mound of tortoise dung resembling a pile of yams. A group of hermit crabs is busily dismantling this heap. Crouching closer, I see them freeze, uncertain of my next move. Just yesterday, the dung was fresh, dense and alive with tiny flies. In about a week, this will transform into a dry, half-digested blend of grass and leaves mixed with seeds—nutrient-rich contributions that have nourished Aldabra for millennia.

The presence of giant tortoises, akin to the role of elephants, plays a critical part in community ecology. The tortoises not only fertilize the sparse soil with their dung but also disseminate seeds across extensive areas. They aerate and turn over the soil through their slow but steady movements, fostering a specialized grassland ecosystem known as “tortoise turf” that serves numerous other species.

Historically, giant tortoises thrived in abundance across many islands throughout the western Indian Ocean until the Age of Exploration (1400s-1600s) led to their near extinction due to human exploitation. Tortoises were an ideal food source—it was easy to keep them alive during long voyages, and they were considered delicious. Presently, only the tortoises of Aldabra remain.

Today, Aldabra is home to solely a small group of researchers and support staff who operate from a modest research station nestled around a red-and-white lighthouse. In 1982, Aldabra became UNESCO World Heritage-listed, and it is currently managed by the Seychelles Islands Foundation, which also oversees the Vallée de Mai Nature Reserve on Praslin Island, notable for the unique coco de mer palm.

Aldabra, the world’s second-largest atoll, belongs to the Seychelles, an island nation off the eastern African coast. According to Nancy Bunbury, the Seychelles Islands Foundation’s director of research and conservation, “It feels like a true wilderness, which is a rare feeling for a comparatively small tropical island.” Approximately 150,000 giant tortoises (Aldabrachelys gigantea) inhabit Aldabra, boasting numbers significantly greater than their famed Galápagos relatives. In fact, there are enough tortoises on this island to fill Yankee Stadium three times over.

The hermit crabs initially hesitate at my approach but soon abandon their meal, scrambling away as I dig around to investigate further. Besides fibrous remnants, I find a few seeds among the dung, though fewer than anticipated. This single dung pile is part of a more extensive ecosystem network that wildlife biologists, including Wilfredo Falcón, have been studying for years. Falcón, now affiliated with the U.S. Forest Service, has mapped over ten animals, including birds, bats, and crabs, correlating them with the seeds they consume. Tortoises are known to consume 20 species of plants, coming in second only to the blue pigeon, which eats 26 species.

Dennis Hansen from the University of Zurich has dedicated parts of several years to studying Aldabra. He highlights the tortoises’ efficiency as seed dispersers, noting, “Tortoises are brilliant seed dispersers because they don’t have teeth. They don’t masticate. They don’t chew.” Any food item fitting into their mouths is swallowed whole—unlike fruit bats, which consume figs on the spot and discard seeds. Some seeds even require passage through a tortoise’s digestive system to germinate effectively, a process that researchers are gradually mapping in several rewilding projects across Rodrigues and Mauritius, where tortoises are reintroduced to regenerate lost forests.

These tortoises are instrumental in hydration for certain plant species; for example, the ebony tree germinates only 2-3% of its seeds without passing through a tortoise, yet that figure skyrockets to 80% afterwards. Observations reveal a fascinating pattern on Mauritius: seedlings sprouting in shapes that mirror tortoise dung. Similar treatments with Galápagos tortoises show increased germination success for the Galápagos tomato after digestive passage.

Additionally, tortoises are capable of transporting seeds over significant distances, aiding plants in expanding their territories. Unlike quick-seed passers like blue pigeons, which often eat and excrete seeds within brief intervals, tortoises’ digestive processes can last three weeks or more, allowing them to traverse a half-mile in a day and up to five miles per season. This long-distance movement is critical for separating seeds from their parent plants, as Falcón explains.

Furthermore, tortoise dung serves as a foraging ground for various birds and larger land crabs that follow the hermit crabs. “They’re not only dispersing these various seeds, but they’re also integrating them into the food chain,” notes Falcón. Berries consumed whole by a tortoise often become meals for other animals.

“Ecologically, Aldabra is the only location on Earth dominated by a reptilian megaherbivore in significant quantities,” Bunbury states. Centuries of giant tortoises have transformed the landscape into lush grassy areas, with much of this specialized habitat found along the coastline of Grande Terre, Aldabra’s principal island. “They’re basically glorified lawn mowers,” says Hansen, explaining how they foster their own productive grasslands.

On a joint expedition with Simon Watkins, the station’s science coordinator, and a couple of assistants, we navigated to southeastern Grande Terre, one of the regions with the highest tortoise populations. As we crossed the expansive blue lagoon—twice the size of Manhattan—Watkins guided our skiff into a mangrove area alive with the sounds of thousands of red-footed boobies and frigatebirds. After tying up, we trekked for about 15 minutes through rocky terrain and bushy thickets until we reached a small elevation that offered expansive views of the ocean, dunes, and our field hut near a pristine white beach. The tallest dune here is only 60 feet above sea level, marking Aldabra’s pinnacle point.

When tortoises gather en masse—more than 30 individuals per acre—their biomass surpasses that of large mammals in many African national parks, a spectacle in its own right. They create openings in the undergrowth, allowing sunlight to filter through, facilitating the growth of a multitude of other species. Golden orb-weaver spiders spin their intricate webs in the newfound light gaps, while sunbirds nest among exposed branches. Flightless rails often follow tortoises, scavenging for invertebrates disturbed by their slow movements.

Tiger beetles depend heavily on this turf, as noted by Hansen, who explains, “It’s easier for them.” Adult tiger beetles actively hunt for mites within this cropped habitat, while their larvae lie in wait in small holes, reliant upon tortoises to shape their environment. Research indicates that removing tortoises leads to a marked decrease in both adult and larval populations of these beetles within a year.

In the late afternoon, I made my way from the beach to the dune summit, noting sea turtle tracks and nests along my route. At the top, I spotted a pair of drongos, roughly robin-sized, perched amongst heliotrope. The expanse of the atoll stretches before me, displaying a 60-foot-wide strip of tortoise turf that runs alongside the rocky cliffs for miles, resembling a parched golf course dotted with stones and animal droppings. The tortoises meander across this grassland, maintaining their orientation towards the sun—each step followed by a quick bite at the ground. Co-evolved grasses and dwarf sedges have adapted to this continuous foraging, keeping their reproductive structures low to evade grazing.

Aldabra stands as a nearly pristine ecological laboratory, with its natural rhythms dictated by tides and climate, supporting an intricate web of life—from scuttling crabs engaging with dung heaps to vibrant ecosystems maintained by reptiles. Had Charles Darwin visited Aldabra during his time in Mauritius in 1836, it might have garnered the same recognition as the Galápagos Islands in both fame and scientific significance. However, to this day, its wonders remain largely undiscovered by the wider world.

“Giant tortoises symbolize an incredible connection to ancient ecosystems and their inner workings,” remarks Bunbury. “Gazing into the eyes of an adult giant tortoise reveals the past, invoking feelings of nostalgia for what has been lost and hope for their continuity and flourishing presence.”

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