A seven-year study has revealed that removing invasive American bullfrogs from Yosemite National Park is helping the native northwestern pond turtle make a comeback – along with a host of other species.
When researchers from the University of California first began investigating Yosemite’s ponds in 2016, they were met with a cacophony of deep, booming bellows. These were the calls of American bullfrogs (Rana catesbeiana), a non-native species posing a significant threat to native local wildlife, says Sidney Woodruff, lead author of the new study published earlier this week in Biological Conservation.
“At night, you could look out over the pond and see a constellation of eyes blinking back at you,” recalls Woodruff. “Their honking noise is iconic, and it drowns out native species’ calls.”
But today, these waters sound very different. After years of conservation work to remove the bullfrog, native frogs, salamanders and northwestern pond turtles (Actinemys marmorata) are being heard and seen once again.
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Rise of the bullfrog
The northwestern pond turtle is one of just two subspecies of western pond turtle, California’s only native freshwater turtle.
Found from Baja California to Washington, these reptiles have disappeared from more than half their natural range, and invasive predators are a major reason why.
One of the biggest threats is the American bullfrog. Native to the eastern US, bullfrogs were introduced to the west in the 20th century and are now considered one of the world’s most harmful invasive species.
“One reason American bullfrogs are among the top worst globally introduced pests is because they eat everything – anything that fits into their mouth,” says senior author Brian Todd, a UC Davis professor in the Department of Wildlife, Fish and Conservation Biology.
During their, study, the team found baby turtles, newts, snakes, small birds and even rodents in the frogs' stomachs.
“They’ve been causing declines to native species everywhere they’re introduced, which is around the world,” adds Todd.
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Tracking turtles
To understand just how much bullfrogs were affecting turtle numbers, researchers tracked four sites in Yosemite between 2016 and 2022 – two with bullfrogs and two without.
Where bullfrogs were present, only older, larger turtles remained. At sites without bullfrogs, there were far more turtles – up to 100 times more in some cases – and crucially, young turtles were present.
It wasn’t until the invasive amphibians were all but eradicated from the sites in 2019 that juvenile turtles began reappearing in the previously bullfrog-infested ponds.
“As bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around,” says Woodruff, adding that the return of the turtles – and other native species – offers strong evidence that bullfrog removal could be vital to the species’ survival.
Why turtles matter
Western pond turtles do more than just bask on logs – they help recycle nutrients in aquatic ecosystems and are a key part of California’s natural history, the researchers explain.
“All across the state we have one native freshwater turtle, and it’s the western pond turtle,” Todd says. “If it disappears, we have no other freshwater turtles that are supposed to be here. It’s part of our natural heritage.”
While the study shows the benefits of bullfrog removal in key areas, the researchers warn it’s not a one-size-fits-all solution. Priority habitats with low re-invasion risk and high conservation value are the best candidates for future work.
Main image: Yosemite National Park. Credit: Getty
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