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American bullfrogs, which were once confined to much of eastern North America, have exploded around the world with dire consequences for native species. Now researchers say they may have found a way to help that affected wildlife rebound. Here's NPR's Ari Daniel.
ARI DANIEL, BYLINE: Sidney Woodruff has spent multiple summers hiking to a remote corner of Yosemite National Park, where she's camped out next to ponds and lakes.
SIDNEY WOODRUFF: At night, you have, you know, the moon reflecting off the water.
DANIEL: Inevitably, the dark quiet is broken by an American bullfrog.
WOODRUFF: Like, (imitating American bullfrog) kind of noise that they do. Once one starts, another one starts, and then it becomes this, like, big, deafening chorus.
DANIEL: Woodruff, who's an ecology Ph.D. candidate at UC Davis, remembers flashing her headlamp over the water and seeing a constellation of eyes blinking back at her, reflecting the light.
WOODRUFF: The bullfrogs were there by the thousands upon thousands.
DANIEL: But these frogs were not supposed to be there. For decades, people introduced them outside their native range, including in Yosemite, where the amphibians mounted an invasion of swaths of the park. And these frogs, they're massive.
WOODRUFF: Maybe the size of a grapefruit in your hand, and they will literally just feed on anything that fits into their mouth.
DANIEL: Like salamanders, snakes, frogs, small birds and rodents and...
WOODRUFF: Pond turtle hatchlings - we sometimes refer to them as little cookies because they're so cute.
DANIEL: The northwestern pond turtle is California's only native freshwater turtle, a species that's nearly vanished from the West Coast due to several reasons, one of them being the American bullfrog. The national park previously conducted a massive bullfrog eradication effort in Yosemite Valley, in part for the pond turtles, but it was too late. The turtles couldn't stage a comeback. At the remote bodies of water that Woodruff was studying, though, the situation was different. There were still some older, larger turtles and...
WOODRUFF: The only time that we are coming across young, small turtles is when they are popping up in these bullfrog stomachs. So you have no younger individuals coming up through the ranks.
DANIEL: Woodruff wanted to know if the bullfrogs from these backcountry ponds were removed while there were still older turtles present, might the population show signs of recovery? So she and her colleagues did a combo of night surveys to remove the adults and day surveys...
WOODRUFF: To go after those bullfrog egg masses.
DANIEL: Across two sites, Woodruff estimates they removed some 16,000 bullfrogs - a near complete eradication. And after multiple years...
WOODRUFF: We came across our first couple of small pond turtle hatchlings and juveniles swimming out in the environment. So once we removed that heavy bullfrog presence, those younger turtles were free from that predation and able to grow up.
DANIEL: The researchers also spotted certain snakes and newts making a comeback.
WOODRUFF: You see what the food web looks like and how it should look like - right? - with no one species really dominating.
DANIEL: Woodruff says the study offers a potential tool for restoring freshwater ecosystems under certain circumstances. The results are published in the journal Biological Conservation. Kaili Gregory is a Ph.D. student in wildlife conservation at the University of Georgia who wasn't involved in the research.
KAILI GREGORY: It was encouraging to see that it's effective, but this northwestern pond turtle goes from central California up to Washington. That's a lot of area to remove bullfrogs.
DANIEL: And she says keeping bullfrogs out requires near constant vigilance, so wildlife managers will have to be strategic.
GREGORY: Maybe it's parts of the population that are really important for genetic diversity, or it's high-quality habitat. Maybe that's where we really focus our efforts.
DANIEL: Meanwhile, in Yosemite, Sidney Woodruff says there's been a partial return to an earlier soundscape.
WOODRUFF: As the bullfrog population went down, you started to hear some of our native chorus frogs again.
DANIEL: In other words, once the bullfrogs croak, the native frogs can finally croon.
Ari Daniel, NPR News.
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