In the late 19th century, whalers, settlers and pirates changed the ecology of the Galapagos Islands by poaching some native species – such as the Galapagos giant tortoise – and introducing others, such as goats and rats. The latter species became pests and seriously destabilized island ecosystems. The goats overgrazed the fruit and plants the tortoises ate, while the rats feasted on their eggs. Over time, the turtle population declined sharply. On Española, an island in the southeastern part of the archipelago, the number of tortoises dropped from over 10,000 to just 14. Along the way, with goats eating all the plants they could, Española – once savanna-like – became barren.
A century later, conservationists set out to restore Hispaniola’s Galapagos giant tortoise—and the island’s ecosystem. They began eradicating the introduced species and capturing the remaining Española tortoises and breeding them in captivity. After the goats were destroyed and the turtles in the cages, the ecosystem was transformed again. This time the grassland was overgrown with densely clustered trees and shrubs. The full restoration of Española to its savanna-like state will have to await the return of the tortoises.
From the time these 14 tortoises were taken into captivity between 1963 and 1974 until they were finally released in 2020, conservationists from the non-governmental organization Galápagos Conservancy and the Galápagos National Park Directorate have reintroduced nearly 2,000 bred in captive giant Galapagos tortoises in Española. Since then, the tortoises have continued to reproduce in the wild, causing the population to blossom to approximately 3,000. They have also seen Española’s ecology transform once again as the tortoises reduce the range of woody plants, expand grasslands, and spread the seeds of a keystone species.
Not only that, but the return of the turtles has also helped the critically endangered woolly albatross, a species that breeds exclusively on Española. During the island’s arboreal era, according to Maud Quinzin, a conservation geneticist who previously worked with Galapagos tortoises, people had to repeatedly clear the areas the seabirds used as runways. Now, if the landing strips get overgrown, they will move turtles into the area to take care of them.
The secret to this success is that – like beavers, brown bears and elephants – giant tortoises are ecological architects. As they browse and dig and rummage around, they change the landscape. They trample young trees and shrubs before they can grow large enough to obstruct the albatrosses’ path. The giant tortoises also have a powerful impact on the giant prickly pear cactus species that call Española home—one of the tortoises’ favorite foods and an essential resource for the island’s other inhabitants.
When the turtles graze on the fallen leaves of the cactus, they prevent the paddle-shaped pads from taking root and competing with their parents. And after eating the cactus fruit, they drop the seeds around the island in dung balls that offer a protective coating of dung.
The extent of these and other ecological effects of the tortoise is documented in a new study by James Gibbs, conservation scientist and president of the Galápagos Conservancy, and Washington Tapia Aguilera, director of the Galápagos Conservancy’s giant tortoise recovery program.
To study these impacts up close, they fenced off some of the island’s cacti, giving them a way to gauge how landscapes develop when exposed to or freed from the turtles’ influences. They also examined satellite images of the island taken between 2006 and 2020 and found that while parts of the island still saw an increase in bush and tree density, the places where the turtles had rebounded were more open and similar to savannah.
Just one or two turtles per hectare (2.5 acres), the scientists write, is enough to cause a change in the landscape.
Dennis Hansen, a conservationist who has worked with the turtles living on Aldabra Atoll in the Indian Ocean, said that while the findings met conservationists’ expectations, it was good to have their suspicions confirmed. The results bode well for other wildlife restoration projects that include giant tortoise recovery as a cornerstone of their efforts, he says, such as those taking place on other islands in the Galapagos archipelago and in the Mascarene Islands in the Indian Ocean.
But on Hispaniola itself, although the turtles have been busy trampling down shoots and scattering seeds, they still have work to do. In 2020, 78 percent of Española was still dominated by woody vegetation. Gibbs says it may take several more centuries for Hispaniola’s giant tortoises to restore something like the ratio of grasses, trees and shrubs that existed before Europeans landed on the archipelago. But this long transformation is at least underway.
This article is from the Hakai Journal, an online publication for science and society in coastal ecosystems. Read more stories like this one at hakaimagazine.com.