Homo sapiens has had a pretty rocky relationship with the rest of the planet’s largest animals.
In the geologic blink that it’s taken mankind to spread across the globe, a pattern has emerged: where humans dispersed out of Africa, there often followed crashes of megafauna. Scientists quarrel over the precise role of human hunting, habitat change, and climate change in each extinction event, but the phenomenon is well-studied and especially apparent in the Americas, Australia, and on islands. In the past 50,000 years, for example, over 70 percent of America’s largest species went extinct. The mammoths, saber-toothed cats, dire wolves, van-sized ground sloths and armadillos that humans first encountered in North America—all of these disappeared shortly after the arrival of Paleoindians.
Megafauna have proportionately large impacts on their environments, often as keystone species that underpin whole webs of ecological relationships. Where these top predators and grazers disappear, huge ecological holes open up. Without the competition of apex carnivores, numbers of intermediary predators like foxes or coyotes can explode—a situation called mesopredator release—and destabilize, or even extirpate, local prey populations.
Growing evidence points to the importance of preserving these kinds of ecological relationships—the interactions between “plants, fruit eaters, grazers and browsers, predators, pollinators, and decomposers,” says writer Sharon Levy—rather than simply focusing on individual species for biodiversity’s sake.
Rewilding is conservation with an eye toward preserving intact, functional ecosystems. But true to its name, the idea has a wild side. Some of the original and most outspoken advocates of rewilding, accomplished scientists in their own right, have argued to take the theory to its radical conclusion: they want to see, for instance, the North American plains repopulated with African elephants, lions, and cheetahs that resemble the extinct megafauna of yore. A project called Pleistocene Park aims to similarly restore Siberia’s mammoth-steppe ecosystem by introducing large herbivores including musk oxen, horses, and bison.
This version of rewilding has engendered lively and outlandish debate in the pages the world’s top scientific journals. Behind the scenes, however, scientists have been making steady progress toward a humbler model of rewilding: all over the globe, teams are attempting to restore tortoises to their former ecosystems.
Tortoises are simply what we call turtles that stick to land.
Over recent centuries, overhunting and habitat destruction have led to precipitous declines for tortoise species. In some places, like the America Southwest, that meant drastic reductions in tortoise population size and distribution; in others, it meant utter extinction. By the mid-19th century, European sailors had obliterated all but one of eight species of island-dwelling giant tortoises from the Indian Ocean.
The loss of tortoises, often the dominant grazers in their landscapes, triggered a cascade of changes. Native plants that had for millennia depended on being eaten and spread by tortoises dwindled in number, disrupting nutrient cycles, losing associated insect populations, and paving the way for exotic invaders. In Mauritius, the sudden absence of tortoises led to takeover by woody and invasive species that outcompeted the native plants. We’ll never resurrect the Réunion or domed Mauritius giant tortoises, but scientists hope we can resurrect the ecological roles they played.
Nonthreatening, long-lived, and easy to control, tortoises are excellent candidates for rewilding. They became one of the first rewilding successes when, starting in the 1970s, researchers reintroduced the rare Bolson tortoise to its historic range in Arizona and New Mexico. Since then, scientists have envisioned bringing tortoises back to Madagascar, the Seychelles, the Galápagos, and Mauritius.
Now, more modest optimism comes from two papers published in the August edition of Conservation Biology. Both studies looked at the feasibility of replacing now-extinct giant tortoises with related species, but they did so on opposite sides of the world—one in the Galápagos and one on a tiny, remote island off Mauritius. In the Galápagos, researchers concluded that only one of the the “analog” species they studied would be an appropriate substitute for an island-wide reintroduction, demonstrating the importance of conducting extensive trials before unleashing tortoises on a landscape. In Mauritius, the team found that over the long term, tortoises could be a cost-effective way to keep nonnative plants down.
The havoc wrought by invasive species makes scientists understandably wary about rewilding with foreign tortoises, which may carry disease or negatively impact the ecosystems they’re intended to help. But if ecologists continue to move at the same cautious pace as their tortoises, the world may just see these mighty grazers restored to some of their former glory.
Rewilding: for now, at least, it’s tortoises all the way down.
For further reading about the megafauna extinctions, I highly recommend Sharon Levy’s book Once and Future Giants: What Ice Age Extinctions Tell Us About the Fate of Earth’s Largest Animals.
GRIFFITHS, C. J., ZUËL, N., JONES, C. G., AHAMUD, Z. and HARRIS, S. (2013), Assessing the Potential to Restore Historic Grazing Ecosystems with Tortoise Ecological Replacements. Conservation Biology, 27: 690–700. doi: 10.1111/cobi.12087
HUNTER, E. A., GIBBS, J. P., CAYOT, L. J. and TAPIA, W. (2013), Equivalency of Galápagos Giant Tortoises Used as Ecological Replacement Species to Restore Ecosystem Functions. Conservation Biology, 27: 701–709. doi: 10.1111/cobi.12038
ERIN WEEKS is a science and nature writer from Charleston, South Carolina. She currently works for Living on Earth.