The call of American bullfrogs was deafening when scientists from the University of California, Davis, first began researching the impact of invasive bullfrogs on native northwestern pond turtles at Yosemite National Park.
"At night, you could look out over the pond and see a constellation of eyes blinking back at you," said UC Davis Ph.D. candidate Sidney Woodruff, lead author of a study chronicling the effects of removal. "Their honking noise is iconic, and it drowns out native species' calls."
But the ponds of Yosemite sound different today, with a chorus of native species making themselves heard. The researchers' study, published in the May issue of the journal Biological Conservation, found that as the park was depopulated of bullfrogs, northwestern pond turtles began to return. The study suggests that removing invasive bullfrogs may be necessary in priority conservation areas to help pond turtle populations recover.
A western icon
The northwestern pond turtle is one of two species of western pond turtle, the other being the southwestern pond turtle. Together, they are the only native freshwater turtles in California. Yet the western pond turtle has disappeared from over half of its range, which stretches from Baja, California to Washington state.
One of the biggest threats to the species is the introduction of American bullfrogs, which are native to the eastern United States but were introduced in the West, where they prey upon small, young turtles.
"One reason American bullfrogs are among the top worst globally introduced pests is because they eat everything - anything that fits into their mouth," said senior author Brian Todd, a UC Davis professor in the Department of Wildlife, Fish and Conservation Biology. "They've been causing declines to native species everywhere they're introduced, which is around the world."
Turtles return after bullfrogs croak
American bullfrogs were introduced throughout Yosemite in the 1950s and were well-established in multiple regions in the park by the mid-1970s. Field observations of bullfrogs eating native turtles suggested their arrival could be a factor in the northwestern pond turtle's drastic decline, but little research was available to confirm that theory. To research a potential link, the authors conducted a seven-year study to examine the impact of removing bullfrogs on native turtle populations. Between 2016 and 2022, they monitored four sites at Yosemite where native turtles persisted - two with bullfrogs and two without.
Where bullfrogs were present, only older, large turtles - too big to fit in bullfrogs' mouths - remained. Researchers even found juvenile turtles - along with newts, snakes, small birds and rodents - in the stomachs of captured bullfrogs. Turtles were up to 36% larger and up to 97% heavier at sites where bullfrogs were present, suggesting younger turtles are not surviving at sites shared with bullfrogs.
Turtles were 2 to 100 times more abundant where bullfrogs were absent. Not until bullfrogs were nearly eradicated from the sites in 2019 did scientists observe the first juvenile turtles at the formerly "bullfrog-present" ponds.
The value of native turtles
Western pond turtles, including northwestern pond turtles, play important ecological roles, cycling nutrients and energy through aquatic systems as they peacefully go about their lives. Proposed as a threatened species under the Endangered Species Act, their intrinsic value goes beyond the legal and ecological, Todd said.
"All across the state we have one native freshwater turtle, and it's the western pond turtle," Todd said. "If it disappears, we have no other freshwater turtles that are supposed to be here. It's part of our natural heritage."
The authors emphasize that eradicating non-native bullfrogs is not a likely solution for every location but could make sense for priority conservation areas where the risk of reinvasion is low and where native turtle recovery is most promising.
"As bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around," Woodruff said of their field work. "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."
Additional co-authors include Robert Grasso of Yosemite National Park and Brian Halstead of U.S. Geological Survey.
The study was funded by the Western Pond Turtle Range-wide Conservation Coalition, Yosemite Conservancy, U.S. Geological Survey and USDA National Institute of Food and Agriculture.
