When Charles Darwin made the famous voyage that took him to the Galapagos, he marveled at the giant tortoises that lumbered across the islands. He tried to ride them. He ate their flesh. He followed the paths they created in their ponderous travels. And he mused at their differing shapes on different islands, insights that helped steer him toward his theory of evolution by natural selection.
“It is the circumstance, that several of the islands possess their own species of the tortoise, mocking-thrush, finches, and numerous plants, these species having the same general habits, occupying analogous situations, and obviously filling the same place in the natural economy of this archipelago, that strikes me with wonder,” Darwin wrote in his account of the trip, The Voyage of the Beagle.
What he didn’t fully appreciate—at least judging by his writings—is the critical role the enormous reptiles played in shaping the plant communities of these islands.
But the return of thousands of giant tortoises to the Galapagos island of Española is giving scientists new insights into the transformative power these behemoths wield. It is the latest in a long list of scientific discoveries emerging from the tiny cluster of islands off the coast of Ecuador. And it holds out the potential for restoring island ecosystems in part by reviving reptile herbivores.
“While we want more tortoises due to their intrinsic interest and our fascination with them as unusual organisms, they are also key to restoring whole ecosystems here,” James Gibbs, a conservation scientists and acting president of the Galapagos Conservancy, wrote in an email.
The giant tortoises today number roughly 10% of the 200,000 thought to have once roamed the islands. Whalers and colonists took the biggest toll by killing them for food. Rats and pigs introduced by humans preyed on the slow-moving creatures as well. Grazing goats destroyed their habitat. On Española, the tiny southernmost island of the archipelago, their numbers dwindled to just 14 in the 1960s.
Thanks to a captive breeding program they have rebounded to 3,000 today. Gibbs and a colleague, Washington Tapia Aguilara, who previously headed the conservancy’s tortoise restoration program, wanted to understand how the return of the island’s main herbivore influenced the vegetation there.
Once covered in savannah-like grasslands, much of the island had been overtaken by small trees as the tortoises that once ate and trampled such trees vanished and feral goats devoured the grasses. The goats have since been removed from the island. To study the effect of the tortoise revival, the scientists fenced off 36-square-meter plots of land to keep the reptiles out. Then they monitored what happened to these tortoise no-go zones compared to nearby areas where the beasts could browse.
To get a broader view, the scientists also matched satellite imagery spanning 15 years to surveys showing where tortoises were concentrated across the center of the island.
After eight years, the fenced areas had higher concentrations of woody trees, while grass cover increased in plots the tortoises could reach. Satellite pictures revealed that in 85% of the places where tortoises were absent tree coverage increased, while it fell in places where tortoises were found. The higher the concentration of the animals, the bigger the drop in trees, the scientists reported in June in Conservation Letters.
“You need 2-3 tortoises per hectare and can completely change the ecosystem and all the other species within it by shifting it from forest to savannah,” Gibbs wrote.
A single tortoise eats more than a thousand kilograms of plants in a year. Then there is the crushing effect as the animals, which can weigh 400 kilograms, bulldoze their way across the island. While not reported in the current paper, the tortoises are also spurring a comeback of an endangered tree cactus (Opuntia megasperma) and helping to maintain nesting habitat for the waved albatross by opening habitat for nesting areas, according to Gibbs.
While the researchers expected the reptiles to have an effect, Gibbs said he was surprised at how dramatic it was at both local and landscape levels.
“The tortoise population is growing well, if slowly, and ultimately they will restore this island to what it once was, perhaps in 200 years when the tortoise population gets back to 10,000 or so,” he wrote.
The success here could hold promise not just for the Galapagos but for other tropical islands that have lost their major herbivores, which often were reptiles. While in many cases those animals have vanished completely, Gibbs suggested that looking for “analog species” could be a way to help recover islands, even if they were never originally home to history-making tortoises.
Tapia, et. al. “Rewilding giant tortoises engineers plant communities at local to landscape scales.” Conservation Letters. June 1, 2023.
Photo: Baby Giant Tortoise emerging from shell ©Pete Oxford