A Battle for Yosemite’s Wetlands
For decades, Yosemite National Park’s ponds and streams echoed with an alien soundscape — the deep, resonant croaks of American bullfrogs. Originally introduced to California in the mid-20th century, these amphibian invaders thrived in the park’s waterways, outcompeting and devouring native wildlife. Their unchecked spread pushed one resident species to the brink of disappearance: the northwestern pond turtle, California’s only native freshwater turtle.
This delicate reptile, already proposed as a threatened species under the U.S. Endangered Species Act, faced an existential threat from an adversary that had no business being there. “One reason American bullfrogs are among the top worst globally introduced pests is because they eat everything—anything that fits into their mouth,” explained Brian Todd, a professor in UC Davis’ Department of Wildlife, Fish and Conservation Biology and senior author of a new study on the issue.
From Japan to Brazil, the bullfrog’s appetite is notorious, and in Yosemite, its victims included not only young turtles but also native newts, salamanders, and other frog species.
Researching the Unwelcome Guests
Starting in 2016, UC Davis Ph.D. candidate Sidney Woodruff set out to uncover whether removing bullfrogs could revive Yosemite’s pond turtle populations. Over six years, Woodruff and her team carefully documented amphibian and turtle numbers at four sites within the park. The results confirmed what biologists had long suspected but hadn’t rigorously proven on this scale: bullfrogs and turtles cannot comfortably coexist.
“At the start, you’d shine a light into a pond and it looked like the stars were staring back,” Woodruff recalled. “You’d hear this deafening chorus of bullfrogs, and you could barely tell other wildlife was there at all.”
The research team’s goal was clear but challenging: remove the bullfrogs and see if nature could heal itself. Their method combined day and night operations. During daylight, scientists scoured pond edges to find and collect bullfrog egg masses using fine mesh nets. The egg clusters were left out in the sun to dry — ensuring they would not hatch. At night, when adult bullfrogs were most active, the team would wade into the dark, capturing the noisy amphibians by hand or net. Captured adults were then euthanized humanely.
Proof in the Ponds
As the bullfrogs vanished, the transformation was remarkable. Ponds once dominated by the invasive frogs began to resound with the calls of native amphibians once again. “As bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around,” said Woodruff. “It’s nice to go back to these sites and hear a chorus of native frogs calling again that previously would not have been heard.”
Population surveys painted an even clearer picture: in ponds without bullfrogs, the number of pond turtles was often two to 100 times higher than in ponds still infested with bullfrogs. Where bullfrogs lingered, the surviving turtles tended to be older and larger, indicating that juveniles were being consumed before they could reach maturity.
The discovery of a juvenile turtle in an area previously overrun by bullfrogs was a moment of hope for Woodruff and her colleagues. It was the first young turtle they had ever seen at that site, and it came only after successful frog eradication.
A Glimpse of a Balanced Future
The work conducted by Woodruff and her team is more than a localized conservation success. It offers a replicable model for how targeted invasive species removal can restore fragile ecosystems to a more natural state. The American bullfrog has long been an ecological problem in regions far beyond Yosemite — its spread to South America, Europe, and Asia has threatened countless native species worldwide.
Back in California, these findings may have broader implications for managing other wetlands and habitats where bullfrogs have displaced local species. Todd emphasized that restoring the pond turtle population is crucial for preserving the state’s biodiversity. “These turtles are part of our natural heritage,” he told UC Davis Press. “If we lose them, we lose a piece of what makes California’s wild places unique.”
An Ongoing Effort
Despite the victory in these four sites, conservationists caution that keeping bullfrogs out of Yosemite’s wetlands will require persistent vigilance. Their hardy nature and prolific breeding make them capable of rebounding quickly if left unchecked.
Still, for now, the ponds and streams that run through this iconic national park are a little quieter, more balanced — and more welcoming to the wildlife that belongs there. With the return of the northwestern pond turtle, Yosemite’s waters once again shelter an ancient Californian resident that predates the state itself.
The hope is that, with continued management and public support, these turtles will not merely survive but thrive, serving as a living testament to what is possible when humans decide to give nature a fighting chance.
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