Yosemite National Park, California l
The quiet ponds of Yosemite are once again alive with the gentle splashes and sunbathing shells of native northwestern pond turtles. This welcome return comes after years of hard work to remove a notorious invader: the American bullfrog, whose appetite and dominance had pushed the turtles to the brink in parts of the park.
The Bullfrog Problem
The American bullfrog is native to the eastern United States but was introduced in the West. Voracious eaters with large mouths, their diets can include birds, bats, rodents, newts, snakes, and turtles. (National Park Service). Credit: National Park Service, physics.org
The American bullfrog, native to the eastern United States, was introduced to California and Yosemite in the mid-20th century. Bullfrogs quickly became a major problem for local wildlife. They are voracious eaters, known to consume nearly anything that fits in their mouths—including insects, fish, birds, and, crucially, young turtles. Their booming calls once drowned out the native frogs and turtles, and their presence led to a sharp decline in the park’s only native freshwater turtle species, the northwestern pond turtle.
A Seven-Year Rescue Mission
Beginning in 2016, scientists from the University of California, Davis, Yosemite National Park, and the U.S. Geological Survey launched an ambitious project to study and reverse the impact of bullfrogs on native turtles. They focused on four sites: two where bullfrogs and turtles coexisted, and two where only turtles were present. Over seven years, the team removed more than 16,000 bullfrogs, their larvae, and egg masses from the affected ponds, working tirelessly through the spring and summer months.
Bullfrog removal was no easy task. Researchers camped in remote areas, used nets and flashlights to catch frogs by hand, and carefully collected egg masses from the water. Adult and juvenile bullfrogs were humanely euthanized, and their stomach contents were examined. The findings were stark: juvenile turtles, as well as newts, snakes, small birds, and rodents, were regularly found in the stomachs of captured bullfrogs.
Turtles on the Edge
Before the bullfrog removal, the turtle populations at bullfrog-infested sites looked very different from those at bullfrog-free ponds. Only large, older turtles—too big for bullfrogs to eat—remained where bullfrogs were present. Juvenile turtles were almost entirely missing, and the few that existed were found only inside bullfrog stomachs. At these sites, turtles were up to 36% larger and 97% heavier than those at bullfrog-free ponds, a clear sign that only the oldest survivors were left.
(Northwestern pond turtle. Image: AI Generated.)
In contrast, ponds without bullfrogs had turtles of all ages and sizes, and turtle numbers were 2 to 100 times higher. The lack of young turtles in bullfrog ponds meant that, without intervention, these turtle populations would eventually disappear.
Signs of Hope
The turning point came after 2019, when bullfrog numbers at the targeted sites were nearly eradicated. For the first time in decades, scientists began to find juvenile turtles in these ponds. The return of young turtles is a promising sign that the population is beginning to recover, freed from the constant threat of bullfrog predation.
The study’s results are clear: removing invasive bullfrogs allows native turtle populations to rebound by easing the pressure on hatchlings and juveniles. While such eradication efforts are difficult and may not be practical everywhere, they offer a powerful tool for protecting endangered native species in key conservation areas.
Why Native Turtles Matter
Western pond turtles play an important role in their ecosystems, helping to cycle nutrients and maintain healthy aquatic environments. As California’s only native freshwater turtle, their survival is tied to the state’s natural heritage. If they disappear, Yosemite and much of California would lose a unique and irreplaceable part of their wildlife.
Looking Ahead
The Yosemite project shows that, with dedication and the right strategies, it is possible to reverse the damage caused by invasive species. As the chorus of native frogs and the sight of young turtles return to the park’s ponds, scientists and visitors alike have reason to celebrate a hopeful chapter in the story of wildlife conservation.
“It’s nice to be able to go back to these sites and hear a chorus of native frogs calling again that previously would not have been heard,” said lead researcher Sidney Woodruff.
The fight for native species is far from over, but in Yosemite, the turtles are making a comeback—one pond at a time.
