Native turtles return to Yosemite after removal of over 16,000 invasive bullfrogs
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Yellow-legged frogs make comeback in Sierra-Nevada lakes
Non-native fish and the amphibian chytrid fungus nearly wiped out the yellow-legged frogs in the lakes of the High Sierra at Yosemite National Park.
A "constellation of eyes" welcomed Sidney Woodruff the first time she went nighttime frog spotting at Yosemite National Park. Her headlamp lit up thousands of eyes that belonged to the invasive American bullfrog, peering up at her from where they lurked in the water.
Woodruff, a PhD candidate at University of California, Davis, is the lead author of a recently-published study that analyzed how the removal of American bullfrogs affected the population of native northwestern pond turtles at Yosemite National Park. Over the course of six years, Woodruff and her team removed more than 16,000 American bullfrogs from the park − an invasive species that makes life for the northwestern pond turtle nearly impossible. But now, with less frogs in the midst, the native turtle is reappearing at the park.
"We focused on the impact to turtles, but anecdotally, we're out there and can see the change and response in the environment," Woodruff told USA TODAY. "It's really exciting to see that, see what these sites should look like."
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When, why did the American bullfrog invade Yosemite?
Native to the eastern United States, American bullfrogs were intentionally introduced to Yosemite in the 1950s, with the first bullfrog documented in The Ahwahnee Hotel's reflection pond in 1955, according to the U.S. Fish and Wildlife Service.
Yosemite isn't the only victim of American bullfrogs' invasive nature. In fact, the bullfrog is one of the top 100 world's worst invasive species, according to the Global Invasive Species Database.
How do American bullfrogs affect pond turtle populations?
American bullfrogs are a "highly successful invasive species," Woodruff said. This is due to their size and particularly, their stomachs. Bullfrogs, which can weigh 2-3 pounds, are adaptable and known for eating a variety of prey, including insects, crustaceans, other amphibians, reptiles, fish, small birds and mammals, Woodruff's study outlines. If it fits in their mouth, they'll eat it.
During her research, Woodruff reported numerous instances of small pond turtles found inside American bullfrog stomachs. And the pond turtles that were in the habitats were old and large, indicating that they only survived because they wouldn't fit in the frogs' mouths.
How were the American bullfrogs removed from Yosemite?
Woodruff's team studied and removed American bullfrogs in areas between Yosemite's Lake Eleanor and Hetch Hetchy Reservoir, which are about 40 miles north of Yosemite Valley.
Woodruff said there were three ways her team removed the bullfrogs from the park: Catching the frogs from shore, riding inflatable boats out to catch frogs further in the water and collecting egg masses from the water. In total, Woodruff and her team collected and removed 16,384 bullfrogs, larvae and whole egg masses from Yosemite, the study states. Once removed, the frogs were "humanely euthanized," Woodruff said.
"It is very unfortunate with invasive species work, particularly when you are dealing with the numbers we had," Woodruff said. "There's just no other solution, unfortunately."
What's happening now? What's next?
The sites where Woodruff and her team removed the bullfrogs are "completely different," she said. Northwestern pond turtle hatchlings and youth are swimming about, the native California red-legged frog has reappeared and several other native species are back − ones that would have previously been eaten up by bullfrogs.
Woodruff said the National Park Service is continuing the monitor the areas and their habitats and more bullfrog extraction may occur in the future.
Greta Cross is a national trending reporter at USA TODAY. Story idea? Email her at gcross@usatoday.com.
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