The North American bullfrog population is booming. That may sound like good news, but it isn't—not when the frog has leaped far beyond its native habitat.
"They are one of the most successful amphibians in the world, and they are causing trouble in several countries," said Cecil Schwalbe, a biologist with the U.S. Geological Survey at the University of Arizona in Tucson.
Native to North America east of the Rocky Mountains, bullfrogs are now found throughout the world. In many areas outside their native range, the frogs are outcompeting—and eating—just about everything in their path.
On wildlife refuges in Arizona where Schwalbe studies the amphibian, bullfrogs have nearly eliminated the Mexican garter snake and the Chiricahua leopard frog. Even during a recent trip to Japan, Schwalbe said he heard the frog's familiar croak everywhere he went.
Recent research suggests the amphibians may be carriers of—but mostly immune to—the chytrid fungus, Schwalbe said. The fungus has been implicated as a major culprit behind dwindling frog populations around the world, according to the biologist.
"That could explain the spread of the chytrid fungus in some areas such as the American Southwest. They carry it to the frog populations they are interacting with," Schwalbe said.
According to biologists, bullfrogs began their leap around the world in 1898, when they were imported to California to satiate a consumer appetite for frog legs. Similar importations spread the croakers to Asia, Europe, and South America.
In their native habitat, predators such as large water snakes, alligators, and snapping turtles keep adult bullfrogs in check, while fish slurp tadpoles. But in western North America and other regions of the world, effective bullfrog predators are absent.
In the absence of predators, the bullfrogs' prolific nature allows them to flourish. "A bullfrog may lay, in a single clutch, 20,000 eggs. Our native [Arizona] frogs are laying 2,000 to 3,000," Schwalbe said. "Bullfrogs have an order of magnitude advantage from the get-go."
Bullfrog tadpoles are also less palatable to [Arizona's] native and most non-native fish than the native tadpoles, according to Phil Rosen. A biologist at the University of Arizona, Rosen studies what insects and fish prey on bullfrog tadpoles.
"The tadpoles are so successful that our [Arizona] ecosystem is completely overrun with small and large bullfrogs," Rosen said. "Most native predatory fish will eat leopard frog tadpoles but not [the] bullfrogs'."
More tadpoles mean more bullfrogs with voracious appetites.
Studies of bullfrog intestines reveal the amphibians eat just about anything they can fit into their mouths: birds, rats, snakes, lizards, turtles, fish, other frogs, and especially each other. In southern Arizona the most common vertebrates found in bullfrogs are other bullfrogs, Schwalbe said.
Other frog species are also cannibalistic. But adult bullfrogs are acutely so, Schwalbe said. As long as tadpoles and young bullfrogs have enough algae and insects to eat, adult bullfrogs can subsist on the younger frogs. With such a reliable food source, the adult populations can grow well above what would normally be possible, putting additional pressure on the ecosystem.
Dennis Suhre is a graduate student who works with Schwalbe and Rosen at the University of Arizona. Surhe said this cannibalism, combined with competition for other food resources, gives younger bullfrogs incentive to leap far away from their hungry elders. And leap they do.
By marking and recapturing bullfrogs on and near the Buenos Aires National Wildlife Refuge in Arizona, Suhre has found that the young amphibians can move at least 6 miles (9.6 kilometers) in a few weeks.
To travel from one big pond to the next, the bullfrogs hop between small ponds interspersed throughout the arid landscape covered in grass and mesquite. "The wetter the year, the farther they will go," Suhre said.
Their lack of predators, prolific nature, and incentive to relocate make bullfrogs a difficult invasive species to eradicate. No single method has proved effective in eliminating them, according to Schwalbe.
Rotenone and other toxic chemicals can be applied to ponds to effectively kill fish and frog tadpoles. But bullfrogs have a simple defense to the tactic: They hop out of the water. Schwalbe also noted that such toxins kill indiscriminately and, thus, are a problematic approach for areas with endangered native species.
Researchers have had some success controlling bullfrogs at Buenos Aires National Wildlife Refuge, however. They have drained bullfrog-infested ponds during the dry season, killing bullfrog tadpoles and enabling researchers to capture and dispose of adult bullfrogs that attempt to escape.
The drained ponds fill back up when the monsoon rains arrive. Biologists can then reintroduce native leopard frogs.
The problem, Suhre said, is that the bullfrogs travel great distances. Unless eradication programs are done on a large enough scale to encompass whole landscapes, the bullfrogs return.
"All you need is two bullfrogs, a male and a female," Suhre said. "A female lays about 20,000 eggs. … Once that happens, it's very difficult to get the frogs out."
Predatory insects, such as dragonfly nymphs and diving beetles, may help mitigate bullfrog populations, according to Rosen, the biologist who studies bullfrog-prey relationships. However, to date, the fish found to be most effective at killing bullfrog tadpoles is the largemouth bass, a non-native species in Arizona.
"No conservationist wants to hear that, because bass are a significant problem in their own right," Rosen said.
Regardless, the biologist said that effective control of bullfrogs may require redesigned ecosystems that incorporate some level of non-native species to control invasive populations of bullfrogs and crayfish. (In the Southwest, according to Rosen, non-native crayfish are just as much a problem as bullfrogs.)
"At this stage we have no hope of completely eradicating bullfrogs or crayfish" in the complex stream systems in central Arizona and New Mexico, Rosen said. "And if we don't completely eradicate them, we don't eradicate them at all, unless we come up with a biocontrol mechanism," he said. (Biocontrol, or biological control, describes the process of controlling pest species by interfering with their ecology.)
Schwalbe added that control measures in the isolated waters of the arid, mountainous terrain of southeastern Arizona may prove more effective than in the more complex stream systems farther west.