Welcoming Western Pond Turtles Back to Yosemite National Park

Here’s some uplifting news about the cherished Western Pond Turtles at California’s Yosemite National Park, an iconic protected wilderness area. As The Wildlife Society reports, these treasured reptiles are bouncing back at the iconic reserve’s two ponds in the nick of time,  after the removal of an invasive species, the American bullfrog.

Documented in a study published in the May 2025 edition of the Biological Conservation journal, the efforts to remove the predatory, grapefruit-sized  bullfrogs from the reserve’s ponds have been able to successfully reverse the worrying depletion of local freshwater turtles.

The Intrepid Research Pinpointing the Threat of the American Bullfrog
Sidney Woodruff, lead author of the above-mentioned study from the University of California, Davis (UC Davis), first began researching the impact of American bullfrogs on native northwestern pond turtles, one of two species of Western pond turtles at Yosemite National Park, and California’s only native freshwater pond turtle species, when she heard the frogs’ deafening honking noises. Her eyes followed the croaking sounds, and saw their constellation of eyes blinking back at her at night across the ponds.

Woodruff, accustomed to camping by ponds and lakes in remote corners of Yosemite, tells NPR that in the darkness of night, illuminated only by moonlight, the quiet would be broken by the telltale simultaneous croaking of thousands of these bullfrogs she saw when she flashed her headlamp over the water. Earth describes them as the sounds of imbalance, of an invasive species overpowering native life.

As UC Davis reports, these frogs were introduced into Yosemite in the 1950s, becoming entrenched there by the mid-1970s. USA Today details how these frogs, among the top 100 of the world’s worst invasive species according to the Global Invasive Species Database,  are native to the eastern United States. But field observations of bullfrogs eating native turtles suggested that their arrival could be the cause of the subsequent drastic decline of the turtles, though this had not yet been supported by research.

In order to examine the link, UC Davis researchers conducted a seven-year study between 2015-2022 to zoom in on the impact of removing bullfrogs on native turtle populations. They monitored four different Yosemite sites where native turtles were found, two with bullfrogs and two without them.

A Demonstration of the Successful Removal of an Invasive Species
Sure enough, the study confirmed the prevailing suspicions. Where bullfrogs were present, only older, larger turtles, too big to fit in the mouths of the bullfrogs, remained, as these frogs will eat whatever fits in their mouths! The researchers found juvenile turtles as well as turtle hatchlings, and other small creatures in the stomachs of captured bullfrogs. Turtles were up to 36 percent larger, and almost 100 percent heavier at sites inhabited by bullfrogs, indicating that younger turtles didn’t stand a chance in their presence.

Meanwhile, turtles were up to 100 times more abundant where there were no bullfrogs. It was only when bullfrogs were nearly eradicated from the sites in 2019, that scientists were able to spot the first juvenile turtles in the ponds formally populated by bullfrogs.

Woodruff shares that after several rounds of bullfrog removal that involved catching frogs, their larvae, and their egg masses, “we came across our first couple of small pond turtle hatchlings and juveniles swimming out in the environment.” Once they had removed the heavy bullfrog presence, these younger turtles were free to grow up unthreatened.

The Added Value of Native Turtles and Other Local Species
As the above-mentioned study details, native Western pond turtles have an important ecological role to play. They feed on a variety of plants, insects, and carrion, as Discover Wildlife explains, cycling nutrients and energy through aquatic systems, besides being part of the State’s natural heritage.

According to the researchers, the food web looks better when the native species that have evolved with each other are allowed to keep each other in check, without one particular species [the bullfrog] allowed to dominate.

Removing the invasive bullfrogs produced a pleasant chorus made up of other native species as well as Western pond turtles: “As the bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around,” says Woodruff. These include the native California red-legged frogs.

This conservationist enjoys returning to these sites today and hearing a chorus of native frogs calling again. She now refers to the sounds of native frogs as displaying a pleasant crooning rather than croaking timbre, a song she fondly calls  “this iconic Hollywood ribbit sound!”

The success of this work in Yosemite suggests that the targeted removal of invasive species, while not practical for every habitat, can be a vital strategy in restoring native wildlife populations. The above Instagram post from Wildheart_500 calls this achievement not just good news for turtles, but “a win for the entire wetland ecosystem, showing that restoring balance works when done right.”