In Yosemite, scientists monitored 4 ponds for 7 years as invasive bullfrogs were removed; juvenile native turtles reappeared at affected ponds by 2019

For years, scientists who were working at Yosemite National Park observed fewer young turtles than adult ones. The absence of the younger turtles meant that the population had no chance of sustaining itself. That raised suspicions among scientists that an invasive predator introduced decades earlier was interrupting the turtles' life cycle. A study published at UC Davis states that scientists spent seven years testing the hypothesis. In 2019, juvenile turtles reappeared in the monitored ponds after invasive bullfrog populations had been reduced, suggesting that bullfrog removal was associated with the return of young turtles.

How invasive bullfrogs threatened Yosemite’s turtles

The American bullfrog is native to the eastern United States but was introduced to the West during the twentieth century. In Yosemite, bullfrogs became established in the 1950s and had spread to several parts of the park by the mid-1970s. Bullfrogs, unlike many frogs, have a tendency to eat anything they can find. They feed on nearly anything they can swallow, such as insects, birds, snakes, salamanders, rodents, and young turtles. Since newly hatched northwestern pond turtles are small enough to fit into a bullfrog's mouth, scientists believed many of the young turtles were eaten before they could grow up. The northwestern pond turtle was particularly significant to the study because it is one of California’s two native freshwater turtle species and has lost more than half of its historical range.

A seven-year experiment in four ponds

To understand whether bullfrogs were driving the decline, scientists monitored four turtle ponds in Yosemite between 2016 and 2022. Two ponds already had bullfrogs, while two did not, allowing scientists to compare turtle populations over time. This seven-year experiment in four ponds involved much more than counting turtles. Scientists removed 12,317 bullfrogs, larvae, and egg masses from one site and 4,067 from another, gradually reducing the invasive population until it was almost eradicated by 2019. They also measured turtles, tracked the age groups, and examined how the populations changed as bullfrog numbers fell. The results revealed a pattern. Only at ponds where bullfrogs remained, older turtles were found because they had grown too large to be eaten. Young turtles were almost absent.

The return of baby turtles

The best news of the study didn’t come right away. Scientists had to wait until bullfrogs were nearly gone before they observed the first juvenile turtles at the formerly invaded ponds. In 2019, hatchlings and young turtles reappeared in the areas where they had not been seen before. Turtles were seen in all age classes at ponds without bullfrogs, and could be 2 to 100 times more common at these ponds than at ponds with bullfrogs. However, the turtles that were found with bullfrogs were generally 26 to 36 percent smaller and 76 to 97 percent lighter, indicating that most of the juveniles and subadults had been lacking. This was a sign that the population was now once again reproducing successfully, and not just by the return of older turtles. These observations indicate that removing one invasive predator can benefit some native species within the same ecosystem. Because bullfrogs prey on a wide range of animals, reducing their numbers can lessen pressure across an entire aquatic ecosystem rather than helping only one species.

Why saving one turtle species matters

The comeback of California's native turtles is important. First, it shows the population is recovering; second, the species plays an important role in freshwater ecosystems. Western pond turtles are involved in nutrient cycling and energy processing in aquatic ecosystems through feeding, moving between land and water, and interacting with other living creatures. Furthermore, they are among California’s native freshwater turtle species, making their conservation important for the state’s natural heritage. Although the Yosemite research dealt with four ponds, the results may inform conservation efforts elsewhere.