Scientists save native turtles by removing thousands of bullfrogs

In the wild heart of Yosemite National Park, nights once echoed with the loud, unrelenting calls of American bullfrogs. Their voices were more than just noise. They were a sign of imbalance, an invasive species overpowering native life.

These frogs, introduced decades ago, spread widely and disrupted delicate ecosystems. Hidden beneath their croaks was a much quieter story: the disappearance of California’s only native freshwater turtle.

The northwestern pond turtle, a shy and ancient species, once thrived in these waters. But it began to vanish silently. Scientists at the University of California, Davis, led by Ph.D. candidate Sidney Woodruff, set out to understand why.

The researchers uncovered a powerful link between the invasive frogs and turtle decline. What followed was a years-long effort to remove the intruders and give the native species room to breathe again.

“At night, you could look out over the pond and see a constellation of eyes blinking back at you,” said Woodruff. “Their honking noise is iconic, and it drowns out native species’ calls.”

Native turtles declined as bullfrogs invaded

The northwestern pond turtle is one of two subspecies of the western pond turtle. The other, the southwestern pond turtle, shares similar challenges.

Together, they represent the only native freshwater turtles in California. Historically, these turtles inhabited areas from Baja California to Washington state. But over the last century, they have lost more than half their habitat.

The decline is not random. It correlates closely with the introduction of American bullfrogs, voracious predators native to the eastern United States.

Introduced across the West in the 20th century, often as food or fishing bait, bullfrogs adapted quickly. They preyed on insects, amphibians, small birds, rodents, and critically, 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 Brian Todd, senior author and UC Davis professor.

Bullfrogs wiped out young turtles

To investigate the turtles’ disappearance, the researchers focused on four pond sites in Yosemite from 2016 to 2022.

Two sites hosted bullfrogs. Two did not. What they found was striking: where bullfrogs thrived, turtle populations were older and significantly larger, while young turtles were almost entirely absent.

Scientists captured 12,317 bullfrogs from one site and 4,067 from another. Inside their stomachs, they discovered a disturbing menu. They found six young pond turtles, newts, garter snakes, birds, rodents, and dozens of frogs. Hatchlings were the most common turtle victims.

Bullfrogs, being gape-limited predators, cannot eat adult turtles. But hatchlings and juveniles, with shells as small as 25 mm, remained at risk for years. Their slow growth and high hatchling mortality made them especially vulnerable.

This prolonged pressure explained the eerie silence of turtle recruitment in bullfrog territories.

The cost of coexistence

Even in sites where adult turtles persisted, their populations skewed older and top-heavy. Researchers noted that young turtles simply did not survive long enough to join the population. Their absence indicated a collapsing system.

In bullfrog-infested areas, adult turtles were 26 to 36 percent longer and 75 to 97 percent heavier than their counterparts elsewhere. These metrics painted a picture of isolated, aging groups. The researchers referred to this as the “perception of persistence” where adults linger for decades while juveniles quietly disappear.

Without intervention, such populations face local extinction. Though they seem stable, their future hinges on the survival of the next generation. That generation, in bullfrog-occupied waters, never arrived.

Turtles returned after bullfrog removal

The team’s multi-year effort to remove bullfrogs reached its peak between 2017 and 2019. By 2019, the team had achieved near-complete eradication at the two bullfrog-present sites. And then, something changed. They began to see young turtles again.

“We captured the first juvenile turtles at bullfrog present sites only after reaching near complete bullfrog eradication in 2019,” noted the researchers. These were not the doomed hatchlings found in stomachs. They were alive, active, and growing. Their presence was a hopeful sign that turtle recruitment had resumed.

The absence of bullfrogs did not just benefit turtles. Other native species began to reappear. Salamanders, native frogs, and aquatic insects became visible and audible again.

“As bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around,” said Woodruff. “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.”

Challenges of removing bullfrogs

Removing invasive species is never easy. It demands time, manpower, and patience. In this case, the work included day and night field efforts, capturing frogs with nets, spears, and sometimes even pellet rifles. Egg masses were collected by hand. Adult frogs were humanely euthanized under approved protocols.

Despite logistical hurdles – including remote locations, changing weather, and the COVID-19 pandemic – the team remained committed.

Their removal of more than 16,000 bullfrogs represents one of the largest such efforts documented in a national park setting. The biomass removed, over 300 kilograms, was immense and redirected energy back into native food webs.

At one site, over 6,000 bullfrogs were removed in a single year. Compared to similar efforts across North America, Yosemite’s project stands out not only in scale but in success.

Rebuilding what was lost

Western pond turtles have long lifespans and low reproductive rates. They lay just a few eggs each year. Hatchlings face predation from fish, mammals, and other reptiles. Add bullfrogs to the mix, and very few survive to adulthood.

This makes each successful recruitment, each young turtle, deeply significant. It signals not only a reduction in predation but also the possibility of future generations.

The data show clear demographic differences. At bullfrog-free sites, juvenile turtles made up 19 to 35 percent of captures. At bullfrog sites, there were virtually none before 2020.

Their recovery will not be instant. But the return of young turtles proves that the system, once unbalanced, is stabilizing.

Lessons beyond Yosemite

The implications go far beyond this one park. Bullfrogs have been introduced across the globe. Their impact on freshwater systems, especially turtles, frogs, and salamanders, is widespread and severe.

The Yosemite project shows that where reinvasion risk is low, sustained removal can work.

“Our study shows the potential benefit of reversing species declines and restoring freshwater ecosystems,” concluded the researchers.

Other efforts, like reintroducing California red-legged frogs after bullfrog removal, further support this strategy. Yosemite’s experience can guide similar actions elsewhere and offer hope for beleaguered native species.

Quiet victories and new beginnings

Thanks to years of effort, the ponds of Yosemite now sing a different song. Bullfrogs no longer dominate the soundscape. In their place are the quiet rustlings and chirps of species that belong.

“All across the state we have one native freshwater turtle, and it’s the western pond turtle,” said Todd. “If it disappears, we have no other freshwater turtles that are supposed to be here. It’s part of our natural heritage.”

The battle is not over, but it is no longer silent. With vigilance and care, the ripples of this recovery can spread beyond Yosemite and bring back what was nearly lost.

The study is published in the journal Biological Conservation.

Image Credit: UC Davis

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