On summer evenings in the Midwest, the muggy air comes alive with a chorus of crickets, cicadas, and frogs — especially bullfrogs. Their booming mating calls sound like something between a foghorn and a didgeridoo.
As far as we know, summer here has always sounded like this. Bullfrogs are native to most of the Eastern US, from the Mississippi River to the Atlantic Coast. They evolved here. They belong here. I, for one, adored them as a kid growing up in Iowa, and spent countless summer days trying to catch them to get a closer look.
What’s unusual is that a few states west — into Colorado and on to California — summer nights are similarly marked by the iconic call of the American bullfrog. But here, they don’t belong. They’re unwanted. And they threaten the very existence of some of the West’s other amphibious animals, such as the Oregon spotted frog, which is found only in the Pacific Northwest.
American bullfrogs are not native to the Western US. Humans brought them to the region more than a century ago, largely as a food source. And in the years since, the frogs — which are forest green and the size of a small house cat — have multiplied dramatically, spreading to countless ponds and gobbling up everything that fits in their mouths, including federally threatened and endangered species. Conservation scientists now consider them among the most dangerous invasive species in the Western US, and in the 40-plus other countries worldwide where they’ve been introduced.
That leaves bullfrogs in an unusual position. Invasive species are typically brought in from other countries — Burmese pythons in Florida and spotted lanternflies in New York City come from Asia, for example — but American bullfrogs are, as their name suggests, American. They’re both native and invasive in the same country. And the difference of just a few states determines whether we treat them like pests or as an important part of the ecosystem.
It’s easy to hate bullfrogs. They do cause a lot of damage and, like other non-native species, they’re leading to what some researchers call the Starbucksification of the natural world — you find the same thing everywhere you go, which can make ecosystems less resilient. Yet bullfrogs themselves aren’t the main problem, but rather a symptom of a much bigger one.
How bullfrogs took over the West
One reason is that people enjoy eating them. Or more specifically, their legs.
In the 1800s, as the human population in the West surged amid the Gold Rush, so did an appetite for frog legs, which were associated with fancy French cuisine. To meet that demand, people collected native amphibians from the wild, like the California red-legged frog. But as those species became rarer and rarer — in part, due to overharvesting, researchers suspect — entrepreneurs and farmers started importing American bullfrogs from the eastern US and tried to farm them.
For a time, it seemed like the bullfrog industry might take off.
“Bullfrog legs! Something to tickle the gustatory glands of the epicurean bon vivants,” a reporter wrote in the Riverside Daily Press, a California paper, in 1922. “Propagation of the bullfrog in this state already has become a successful reality. In the near future, bullfrog farming may be expected to take its rightful place as one of the prominent industries of California.”
That never really came to pass. Bullfrog farming proved challenging and financially risky: They take years to raise, they need loads of live food, and they’re prone to disease outbreaks, as Sarah Laskow wrote in Atlas Obscura. And for all that trouble, they don’t produce much meat.
But while the frog leg industry didn’t spread, the frogs, of course, did.
They escaped from farms and, with other accidental and intentional introductions, proliferated until they were common in ponds, lakes, and other water bodies throughout much of the West, including Arizona, California, and the Pacific Northwest. Now, in some portions of the region, “you see so many bullfrogs that it’s just sort of alarming,” said Michael Adams, an amphibian researcher at the US Geological Service, a government research agency that monitors wildlife. There are no reliable estimates of the total population of bullfrogs in the West, though a single pond can be home to thousands of individuals.
Part of what enabled their success is biology: A female bullfrog can lay as many as 25,000 eggs at one time, far more than most native species.
But as several researchers told me, humans have also modified the landscape in the West in ways that have helped bullfrogs take over. While western states have rivers and wetlands, permanent warm waterbodies weren’t common until the spread of agriculture and the need for irrigation, said Tiffany Garcia, a researcher and invasive species expert at Oregon State University. Now ponds, reservoirs, and canals — which bullfrogs love — are everywhere.
“It’s a story of human colonization,” Garcia said. “Bullfrogs were brought by people settling and industrializing the West, and they are maintained by people who are natural-resource users of the West. They wouldn’t be here and survive without us changing the landscape to create these systems where they do so well.”
Bullfrogs are often found alongside other non-native species, Garcia said, which typically tolerate landscapes modified by humans. And sometimes they even help each other succeed. Research has, for example, shown that bluegill sunfish — introduced in the West largely for sportfishing — can help bullfrogs survive. Sunfish will eat dragonfly larvae that might otherwise prey on bullfrog tadpoles.
“You can’t even consider them invasive species anymore,” Garcia said of bullfrogs. “You have to consider it an invasive community.”
Like unsupervised toddlers, bullfrogs will put pretty much anything in their mouths. Mice, birds, turtles, snakes, rocks, other bullfrogs — if it fits, they’ll try to eat it.
This is a big problem for species that are already rare, such as the Chiricahua leopard frog or the northwestern pond turtle. Bullfrogs are shrinking their paths to extinction.
“They’re implicated in the declines, along with habitat loss and drought, of many of our native reptile and amphibian species,” said Sidney Woodruff, a doctoral researcher at the University of California Davis who studies bullfrogs and other invasive amphibians.
In May, Woodruff published a study that found that waterbodies in Yosemite National Park that were full of bullfrogs had lower densities of northwestern pond turtles than those without invasive frogs. She also found that where bullfrogs were present, only large turtles could survive.
“Our study adds mounting evidence that hatchling and juvenile pond turtle losses to bullfrogs pose a serious threat to pond turtle population persistence,” Woodruff and her co-authors wrote.
And where bullfrogs live in communities with other invasive species, native animals often face even greater challenges, said JJ Apodaca, executive director of the Amphibian and Reptile Conservancy, a nonprofit environmental group. Non-native crayfish, for example, are voracious consumers of plants and other habitat features that native animals hide in. So, where you have invasive crayfish, the local fauna will be that much easier for bullfrogs to eat.
Invasive bullfrogs may also be spreading diseases. A study published in 2018 linked the arrival of bullfrogs in the West with the spread of a pathogen called chytrid fungus. While the pathogen typically doesn’t sicken bullfrogs, it has helped drive the decline and extinction of more than 200 amphibian species globally, including those in the West.
Okay, so let’s kill all the bullfrogs?
No, a bullfrog-killing spree won’t fix ecosystems in the West. They’re already everywhere, so even if scientists manage to eliminate them from a pond or 10 ponds — which often requires fully drying out the water body and hours and hours of effort — they’ll likely come back.
“It’s futile,” Garcia said. “We’re not getting rid of bullfrogs. Not really.”
Even if we could remove bullfrogs from large areas in the West, ecosystems wouldn’t suddenly revert back to some sort of natural state. Bullfrogs are both a problem themselves and a symptom of change — of the large-scale transformation of land in the West.
“There’s kind of an irony,” said Brendon Larson, a researcher and invasive species expert at the University of Waterloo. “We’re nurturing these agricultural systems — which are monocultures and non-native species — and then we’re turning around and saying we’re surprised when a non-native species does well in response to that.”
Doing nothing isn’t a great option either. Left alone, bullfrogs will continue to replace native species that comprise the ecosystems we depend on, including insects that pollinate our crops and salamanders that can help limit the amount of carbon dioxide entering the atmosphere and accelerating climate change.
The best approach, researchers told me, is to prioritize bullfrog control — to get rid of frogs in areas with endangered species or where conservation scientists are reintroducing native species that disappeared.
This works. For her study on pond turtles earlier this year, Woodruff and her colleagues caught more than 16,000 bullfrogs across two waterbodies in Yosemite — using nets, spears, air rifles, and other methods — which they then euthanized. It was only after her team shrank the invasive frog population that they detected small, baby pond turtles in those areas. That suggests that, absent bullfrogs, the turtles were finally able to breed and survive, “providing some hope for turtle population recovery once bullfrog predation pressures are alleviated,” the researchers wrote.
Woodruff says she noticed all kinds of other native animals return after clearing out the invasive bullfrogs, including native frogs, salamanders, and snakes. “The coolest thing to me was that the soundscape changed,” she told me. “Over time, you actually started to hear our native chorus frogs again.”
Managing bullfrogs is complicated, Woodruff says, and especially for her. She grew up in Alabama and Georgia, where the animals are native. She liked hearing them. But now she lives in California, where she’s studying how they harm the environment, and so hearing them makes her tense up.
“It sucks because I love these frogs,” Woodruff said. “It is not the animals’ fault. They are doing what they instinctively want to do — survive and procreate.” She tries to stay focused on the point, she said: “We’re doing this for the native species.”
