Invading Bullfrogs Appear Nearly Unstoppable

John Roach
for National Geographic News
September 28, 2004

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.

Frog Leap

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.

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