Invasive bullfrogs ‘eat everything' — including turtles — at Yosemite, study says
A study has found that removing invasive bullfrogs from Yosemite National Park ponds has generated a resurgence in the population of native pond turtles, experts said.
When University of California, Davis, researchers first began studying four ponds at the park, they were overwhelmed by non-native American bullfrogs, a news release said.
'At night, you could look out over the pond and see a constellation of eyes blinking back at you,' said Sidney Woodruff, a UC Davis Ph.D. candidate and lead author of the study. 'Their honking noise is iconic, and it drowns out native species' calls.'
The invasive frogs had decimated the native population of northwestern pond turtles, according to the study, published in the May issue of the journal Biological Conservation.
Together with the southwestern pond turtle, northwestern pond turtles are the only native freshwater turtles in California, the university said.
Northwestern pond turtles have vanished from over half their range, which stretches from Baja California to Washington state.
At Yosemite, the only surviving turtles in the ponds surveyed were the ones that were too big for bullfrogs to eat, the study found.
American bullfrogs are native to the eastern United States but don't belong in the West.
'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.'
The bullfrogs were introduced to Yosemite National Park in the 1950s and quickly spread throughout the park, researchers said.
While their arrival was believed to be linked to the decline in pond turtles, it wasn't confirmed until the study took place, according to researchers.
Between 2016 and 2022, researchers monitored four ponds at Yosemite, two with bullfrogs and two without, the study said.
Turtles were 2 to 100 times more prevalent at the ponds where bullfrogs were absent, researchers said.
When bullfrogs were removed from the other two ponds in 2019, researchers found juvenile pond turtles in them for the first time, the study said.
'As bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around,' Woodruff said. '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.'
The Western Pond Turtle Range-wide Conservation Coalition, Yosemite Conservancy, U.S. Geological Survey and USDA National Institute of Food and Agriculture funded the study.
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