Removal of Invasive Bullfrogs Leads to Increase of Native Pond Turtles in Yosemite, Study Finds

By reversing a takeover by non-native species, native species have a chance at recovery in their natural habitats, according to a new study led by scientists from the University of California, Davis (UC Davis).

The study, published in the journal Biological Conservation, explored how removing invasive bullfrogs could affect the native western pond turtles, which have experienced a major decline in their range. As UC Davis reported, the turtles have disappeared from about half of their original range.

Some threats to the western pond turtles come from habitat destruction, development, pollution and disease, but invasive species are also a major threat, according to the Center for Biological Diversity. Reed canary grass and other invasive plants make it more difficult for turtles to find their nesting sites. Non-native animals such as bullfrogs, red-eared slider turtle and largemouth bass also pose threats by increasing competition for food, while bullfrogs prey on the pond turtles, furthering their decline.

“One reason American bullfrogs are among the top worst globally introduced pests is because they eat everything — anything that fits into their mouth,” Brian Todd, senior author of the study and a professor in the Department of Wildlife, Fish and Conservation Biology at UC Davis, said in a statement. “They’ve been causing declines to native species everywhere they’re introduced, which is around the world.”

From 2016 to 2022, researchers studied four sites in Yosemite National Park, two that maintained the bullfrog population and two that had the bullfrogs removed. To remove the bullfrogs, scientists captured bullfrog eggs and left them out to dry. For adult bullfrogs, they captured the frogs and euthanized them based on American Veterinary Medical Association guidelines.

In the ponds without bullfrogs, the researchers found that pond turtles were between two and 100 times more abundant compared to the ponds where bullfrogs remained.

The team also found that only larger, older pond turtles survived in the ponds with bullfrogs, with the turtles at these sites being up to 36% bigger and up to 97% heavier. The only juvenile pond turtles the researchers found at bullfrog-present sites were in the bullfrogs’ stomachs.

The eradication of the bullfrogs made the presence of other native animals, in addition to the pond turtles, more noticeable to researchers.

“As bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around,” said Sidney Woodruff, lead author of the study and Ph.D. candidate at UC Davis. “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.”

Although the authors noted that bullfrog eradication may not work at all ponds due to cost and time constraints, they explained that the study showed promise for using this approach in the most vulnerable conservation areas.

“While the challenges associated with invasive species removal are significant, our study shows the potential benefit of reversing species declines and restoring freshwater ecosystems,” they concluded.

This article by Paige Bennett was first published by EcoWatch on 21 May 2025. Lead Image: A northwestern pond turtle hatchling at a study site in Yosemite. Sidney Woodruff / UC Davis.

Wildlife in catastrophic decline

The 73% decline in vertebrate wildlife populations during the last 50 years (mammals, birds, amphibians, reptiles and fish), is primarily driven by human activities that threaten biodiversity.

Habitat Loss and Degradation – human activities like deforestation, urbanization, and agriculture have led to the destruction and fragmentation of natural habitats, reducing the space and resources available for wildlife.

Pollution – air, water, and soil pollution from industrial activities, agriculture, and other sources can harm wildlife directly or indirectly by affecting their habitats and food sources.

Climate Change – changing weather patterns, rising temperatures, and sea levels are disrupting ecosystems and impacting species’ ability to adapt and survive.

Overexploitation of Resources – unsustainable hunting, fishing, and logging practices can lead to the depletion of populations and the loss of biodiversity.

Invasive Species – the introduction of non-native species can disrupt ecosystems, compete with native species, and threaten their survival.

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