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