Thousands of Giant Tortoises Anchor a Thriving Ecosystem on Aldabra, a Remote Atoll in the Indian Ocean
The animals graze the vegetation into a picturesque turf, fertilize the soil with their dung and disperse seeds over large distances
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Life on Aldabra atoll, on the western fringe of the Indian Ocean, moves at the pace of its oldest inhabitants. I’ve already dodged a couple of giant tortoises along the white sand path on my morning walk from the research station on Picard Island to the lagoon, but now I’m stopped in my tracks to admire one of the atoll’s peculiar charms—a large heap of tortoise dung lying on the ground like a pile of yams. The dung heap is being dismantled by a scrum of hermit crabs. When I crouch for a closer look, they freeze, waiting for me to make the next move. Yesterday, the dung was fresh—wet, dense and swarming with tiny flies. In another week, it will be reduced to a dry half-digested mass of grass and chopped leaves mixed with seeds, its nutrients absorbing back into a landscape built upon ancient coral. The breakdown of the dung—one of the richest nutrient streams here—has sustained Aldabra for thousands of years.
On an atoll dominated by giant tortoises, the real work of ecosystem engineering begins at the back end of this slow-moving grazer. Aldabra’s tortoises—much like elephants—alter the landscape and create rich habitats for other life. Their influence runs deep. They fertilize the thin soil and spread seeds with their dung. They mix up and aerate the soil through their lumbering movements. And they create a unique grassland ecosystem known as “tortoise turf” that supports a wealth of other species.
Giant tortoises once flourished in vast numbers across a large swath of islands dimpling the western Indian Ocean until seafarers during the Age of Exploration from the 1400s to 1600s plundered almost all of them to extinction. Tortoises were the perfect sustenance—and not just because they could be kept alive onboard a ship for a year or more without food or water. They were also delicious. Only Aldabra’s survived.
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Aldabra’s only inhabitants today are two dozen scientific and support staff living and working at its research station in a handful of low buildings clustered around a red-and-white lighthouse. In 1982, Aldabra became the first atoll to gain UNESCO World Heritage protection status. Today it is under the stewardship of the Seychelles Islands Foundation, which manages Aldabra as well as Praslin Island’s Vallée de Mai Nature Reserve, home to the rare and remarkable coco de mer—a double coconut.
Aldabra, the second-largest atoll in the world, belongs to the Seychelles, a nation of scattered islands off the coast of East Africa. “It feels like a true wilderness, which is a rare feeling for a comparatively small tropical island,” says Nancy Bunbury, the Seychelles Islands Foundation’s director of research and conservation. The atoll is home to some 150,000 giant tortoises (Aldabrachelys gigantea), which are as large as their famous Galápagos cousins but around five times more numerous. Enough of them live on the island to fill every seat in Yankee Stadium three times over.
The crabs wait a few seconds, but I don’t leave, so they abandon their feast and scramble in all directions. I dig around to see what they found so compelling. Besides decomposing fibers, there are some seeds—but fewer than I expected. This dung pile is just one small piece of a much larger network of interactions that wildlife biologists like Wilfredo Falcón have spent years unraveling. Falcón, now with the U.S. Forest Service, spent two field seasons studying Aldabra’s networks of seed dispersers. He mapped ten animals—including birds, bats and crabs—against the seeds they consumed. Tortoises ate 20 species, second only to the blue pigeon, which ate 26.
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Dennis Hansen, from the University of Zurich, spent parts of several years on Aldabra leading a long-term research project. “Tortoises are brilliant seed dispersers because they don’t have teeth,” he says. “They don’t masticate. They don’t chew.” If something can fit in their mouth, they’ll swallow it whole—unlike fruit bats, which eat figs on location and spit out the seeds.
Some seeds benefit from spending time in a tortoise’s gut. Several rewilding projects on the islands of Rodrigues and Mauritius, about 700 miles south of here, are using tortoises from Aldabra to do the work formerly provided by three extinct species. They’re helping restore the ancient forests lost to logging. They not only carry seeds of numerous species but also help them sprout, including endangered trees like the ebony. “For many, like the ebony, 2 or 3 percent germinate without going through a tortoise. And 80 percent germinate after,” says Hansen. On the forest floor on Mauritius, it’s not uncommon to see a patch of seedlings growing in the shape of a piece of dung. Experiments with Galápagos tortoises have shown that the endemic Galápagos tomato also needs a tortoise’s help to germinate. On their own, only a small number of seeds sprouted. But after spending several weeks passing through a tortoise, sprouting success skyrocketed from the single digits to 80 percent.
And tortoises can carry seeds a long distance—helping plants expand their range. Unlike the fruit-eating blue pigeons, which pass seeds in as few as 15 minutes, tortoises take three weeks or more to process food and are long-distance walkers. They can move a half mile in a day and up to roughly five miles in a season. “That’s also very important,” says Falcón, “how far away from the mother plant you move the seeds.”
Tortoise dung doubles as a seed bank for birds and larger land crabs, which arrive after the hermit crabs have moved on. “They’re not only dispersing these various seeds, but they’re also entering them into the food chain,” says Falcón. Berries can pass through a tortoise whole and become another creature’s meal.
“Ecologically, Aldabra is the only place on the planet dominated by a reptilian megaherbivore in huge numbers,” Bunbury says. A hundred thousand generations of giant tortoises have transformed the landscape into short-cropped grassy lawns. Much of this specialized habitat—less than 3 percent of the total land area—appears along the coastline of Grande Terre, Aldabra’s largest island. The tortoises are “basically glorified lawn mowers,” says Hansen. “They create their own highly productive grasslands.”
I joined Simon Watkins, the station’s science coordinator, and two assistants on an expedition to southeastern Grande Terre, where some of the greatest masses of tortoises on Aldabra can be found. Watkins steered the skiff across a clear blue lagoon twice the area of Manhattan to the eastern end. Watkins eased our skiff through a mangrove swamp of squawking and soaring red-footed boobies and frigatebirds numbering in the tens of thousands. From our mooring, the four of us hiked for 15 minutes across jagged limestone rocks, thickets of bushes and clumps of low trees that have somehow managed to get a roothold, and finally topped a small rise where we could see the ocean, a line of low dunes, and our field hut next to a small white beach. The tallest dune is just 60 feet above sea level—Aldabra’s highest point.
When tortoises hit the turf here en masse—more than 30 per acre—their collective biomass density surpasses the vertebrate biomass of large mammals in many African national parks and, to me, is every bit as spectacular. As they push through Aldabra’s undergrowth, they create open space, letting in sunlight for countless other species to flourish. Golden orb-weaver spiders spin their webs in the light gaps. Sunbirds build their nests from the branches that hang out in the open, Hansen says. And flightless rails often stalk along after tortoises, eating the invertebrates they kick up.
Tiger beetles “rely on the cropped turf as a hunting ground,” Hansen says. “It’s easier for them.” The adult beetles are visual running hunters that chase tiny mites. Their larvae live in small holes in the turf and wait for their prey to walk by. They depend “100 percent on the tortoises engineering this ecosystem,” he says. Tests with exclusion fences—keeping tortoises out—have shown that the turf makeup changes quickly. Within a year, both adult and larval tiger beetles virtually vanish.
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In the late afternoon I climbed from the beach to the top of the dune, past sea turtle tracks and nests along the way. At the top, a pair of drongos about the size of robins landed in a clump of heliotrope beside me, then flew off. I could see the edge of the vast atoll curving away in the east. Below, a 60-foot-wide band of tortoise turf ran along the edge of the rocky cliffs for miles in both directions, like a patchy water-starved golf course peppered with occasional rocks and dung. The tortoises were all drifting on the turf in more or less the same direction and keeping their backs to the sun—walking into their own shadows, a step, a bite, a step, a bite. A mixture of dwarf sedges and low grasses have co-evolved to survive nonstop foraging by holding their flowers and seeds low and out of reach.
Aldabra is one of the world’s best-preserved ecological laboratories—its rhythms set by tides and weather, supporting layers of life, from the crabs scuttling over a dung pile to the gardens tended by reptiles. Had Charles Darwin made it here when he was in neighboring Mauritius, in 1836, Aldabra might have come to rival the Galápagos Islands in name recognition as well as status as a scientific mecca. Instead, it and its tortoises remain virtually unknown to most of the world.
“Giant tortoises are so iconic because they give us an incredible insight into ancient ecosystems and how they might have worked,” says Bunbury. “When you look into the eyes of an adult giant tortoise you see the past, and it makes you ponder and feel sadness for all that has been lost and all they have seen, but also enormous hope and optimism that they are still here and thriving.”