About 61 percent of all modern turtle species are either threatened with extinction or already extinct, according to new research published in the journal BioScience.
Turtles are among the most threatened animal groups on Earth, the study's authors note, more so than birds, mammals, fish or even amphibians. Yet this crisis "is generally unrecognized or even ignored," they add, depriving turtles of public awareness that could help rally more resources for their struggle to survive.
"Our purpose is to inform the public of the many critical ecological roles turtles perform on a global scale, and bring awareness to the plight of these emblematic animals whose ancestors walked with the dinosaurs," says senior author Whit Gibbons, professor emeritus of ecology at the University of Georgia, in a statement.
Turtles have been around for more than 200 million years, but the traits that helped them outlast the dinosaurs are increasingly insufficient to save them from human-induced dangers like habitat loss, poaching, the pet trade and climate change.
"These modern descendants of an ancient lineage are touchstones for how human influences are causing the decline of so much of the world's wildlife," Gibbons adds. "Our hope is that everyone will be encouraged to engage in concerted efforts to conserve their well-earned legacy as part of our natural habitats."
The new study — led by researchers from the University of Georgia, the University of California-Davis, the U.S. Geological Survey and the Tennessee Aquarium Conservation Institute — synthesizes results from dozens of previous studies, both to shed light on the plight of turtles and to highlight what's at stake. It's the first major review of ecosystem services provided by turtles, which include perks like seed dispersal, maintenance of healthy food webs and habitat creation for other species.
One reason why turtles are so influential is that they can be carnivores, herbivores and omnivores, the researchers note, ranging from specialists that focus on a few food sources to generalists that eat almost anything. These diverse diets give many turtles broad power over the structure of other biological communities in their habitats, from sea turtles that protect seagrass meadows and coral reefs to freshwater turtles that alter environmental conditions like the pH, sediment accumulation and nutrient input of pond ecosystems.
Turtles help disperse plant seeds, too, and are even the main dispersers for certain species. North America's eastern box turtle, for example, is the only known seed disperser for a native plant called the mayapple, and several other plant seeds germinate more quickly after passing through its digestive tract. Galapagos tortoises also move large quantities of seeds over long distances, the study's authors point out, averaging 464 seeds of 2.8 plant species "per defecation event."
Turtles are also valuable food sources for other species, especially when they gather in large densities. This includes mass nesting "arribadas" of sea turtles like Kemp's ridleys, whose eggs and hatchlings provide an occasional bonanza for local predators. It also includes many less famous examples like pond sliders, which can boast as many as 2,200 individuals per hectare in some habitats.
And speaking of habitats, some turtles dig large burrows that provide housing for other species, too. Gopher tortoises in the U.S. Southeast, for example, can dig burrows more than 30 feet (9 meters) long, infrastructure that's used by hundreds of other species, from insects and spiders to snakes, amphibians, rabbits, foxes and bobcats. Even the mounds of soil leftover from digging the burrow can become habitat for certain plants, boosting floral diversity around burrow entrances.
"The ecological importance of turtles, especially freshwater turtles, is underappreciated, and they are generally understudied by ecologists," says Josh Ennen, research scientist at the Tennessee Aquarium Conservation Institute. "The alarming rate of turtle disappearance could profoundly affect how ecosystems function and the structure of biological communities around the globe."
Slow and steady
The radiated tortoise of Madagascar is critically endangered due to habitat loss and collection from the wild, both for food and for the international wildlife trade. (Photo: Andrzej Grzegorczyk/Shutterstock)
As with the majority of Earth's threatened wildlife, the most common problem facing turtles is destruction, degradation and fragmentation of their natural habitat. Many turtles are also hunted unsustainably for food or the international wildlife trade, which targets them both as live pets and for their shells.
Climate change is another threat for some species, both because of its effects on weather patterns and because of how temperature changes can affect turtle eggs. For species ranging from painted turtles to sea turtles, the ambient temperature determines the sex of baby turtles in their eggs, with cooler temps favoring males and warmer temps favoring females. At one major sea-turtle rookery in Australia's tropical north, for example, research has found that female turtles now outnumber males by at least 116 to 1. As more beaches warm up and produce fewer and fewer male hatchlings, researchers say this could lead to a crash in sea turtle populations.
And then there's plastic pollution. Sea turtles frequently clog their digestive tracts by eating plastic bags, which may resemble jellyfish, and have also been known to ingest things like plastic forks and straws, or to become entangled in abandoned plastic fishing line. In fact, according to one 2018 study, about half of all sea turtles on Earth have eaten plastic at some point, with younger turtles doing so more often than adults. Eating just one piece of plastic gives a turtle a roughly 22 percent chance of dying, the study found, while eating 14 pieces means a 50 percent chance of dying. Once a turtle eats more than 200 pieces of plastic, death is reportedly inevitable.
Because turtles have been around so long, it's easy to see them as invincible. Yet their habitats are now changing more quickly than many turtles can adapt, largely due to human activities, and six in 10 species are now either threatened with extinction or are already gone. If we don't act quickly to protect turtles, the study's authors warn, these ancient animals could fade away with surprising speed.
"We must take the time to understand turtles, their natural history, and their importance to the environment, or risk losing them to a new reality where they don't exist," says co-author Mickey Agha, a Ph.D. candidate in ecology at UC-Davis. "Referred to as a shifting baseline, people born into a world without large numbers of long-lived reptiles, such as turtles, may accept that as the new norm."