Environment

This amphibian – loved for its legs – threatens its California cousins

A bullfrog suns himself on a bamboo stalk in a pond at the Effie Yeaw Nature Center in Ancil Hoffman County Park Saturday April 9 2004.
A bullfrog suns himself on a bamboo stalk in a pond at the Effie Yeaw Nature Center in Ancil Hoffman County Park Saturday April 9 2004. Sacramento Bee Staff Photo

American bullfrogs, native to the eastern United States, are hopping around Northern California ponds, gobbling up lizards, snakes, bats and birds – anything that fits in their mouths.

Among their prey are the adults and tadpoles of endangered native amphibians: Sierra Nevada and foothill yellow-legged frogs, said Colin Dillingham, a wildlife biologist with the Plumas National Forest.

He has applied for a state permit to eradicate bullfrogs in Snake Lake, Spanish Creek and other aquatic sites in the Meadow Valley area 16 miles west of Quincy. The plan is to use nets, hooks and air guns several nights a month over three or four years.

The problem is not just non-native bullfrogs eating Sierra Nevada yellow-legged frogs, a federal endangered species, and foothill yellow-legged frogs, under review for listing by the U.S. Fish and Wildlife Service. They are also consuming the tadpoles that dragon flies feed on and negatively affecting a wide range of species, said Dillingham.

"This is about destroying the whole food web for native species," Dillingham said.

Scientists consider the American bullfrog one of the world’s top 100 invasive species. It was introduced by the food industry for the culinary delicacy many find in frog legs. While amphibians are declining globally, the bullfrog continues to expand its range. A single female bullfrog can produce 20,000 to 40,000 eggs a year.

Along with its voracious appetite, this species is a known carrier of chytrid fungus, which causes a potentially fatal skin disease in frogs. Scientists believe the fungus is a leading cause of the decline of native amphibian populations all over the world and responsible for the extinction of over 100 species since the 1970s.

The Forest Service proposal for "total removal" in specific locations is the only bullfrog-removal project pending in the state, said Sarah Mussulman, a senior environmental scientist with the California Department of Fish and Wildlife. Before state officials will issue a permit, the federal agency must supply information about current population and range of bullfrogs as well as the details of the proposed eradication.

"You can't just go out and decide to remove a bunch of animals," Mussulman said. "You can't just shoot them."

The prospect of eradicating an invasive species is conjuring up grim memories of previous eradication projects in Plumas County. State biologists poisoned Lake Davis in 1997 to rid it of northern pike, a voracious predator they feared would decimate native fisheries downstream in the Sacramento San Joaquin Delta. When that chemical treatment failed, they pumped poisons again in 2007 into the reservoir that served as a back-up municipal supply for the city of Portola.

Larry Douglas, a Portola resident, said the bullfrog control project may need a full environmental review to reassure local residents. Lake Davis was a decades-long public relations disaster that is still being contested in court. More recently, Plumas anglers were furious when state biologists removed brook trout from Gold Lake to protect yellow-legged frogs.

The Plumas County Fish & Game Commission has not taken a formal position on the bullfrog project, said Gary Rotta, who represents the Meadow Valley area on the commission. Some members are concerned that the eradication might be incomplete, leading to a continuation of the bullfrog problem, he said.

Last year Yosemite National Park officials completed successful eradication of bullfrogs along the Merced River. That allowed reintroduction of native California red-legged frogs and western pond turtles. Both species disappeared 50 years ago, in part because of human reintroduction of the non-native bullfrog to the national park.

The Forest Service project is driven by the threat bullfrogs pose to populations of native species, said Dillingham. "We humans caused the problem. We should probably be responsible for fixing it," he said.

State officials consider the bullfrog a dangerous threat to a suite of native species, but they are currently taking no active steps toward removal, said Mussulman. The Forest Service permit application could take months to process, she said.

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