Amphibious invasion: Bullfrogs threaten native Northwest species

Several years ago, Michael Lucid pulled to the side of the highway to watch hundreds of frogs hopping across the road.

The Idaho Department of Fish and Game biologist didn’t realize it on that rainy, autumn night in Boundary County, but he was witnessing American bullfrogs’ aggressive, northward expansion.

Bullfrogs, known for their distinctive croak, are native to the southeastern United States, where they’re food for alligators. But there’s no native predator for them in the West, where they are multiplying rapidly and terrorizing the rest of the amphibian world. Bullfrogs take over ponds and marshes, gobbling up native frogs and other wildlife.

“They’re big – much bigger than native amphibians,” Lucid said. The golden-eyed, softball-size frogs weigh about a pound and will eat anything that fits into their gaping mouths. “Mice, ducklings. … They’re even cannibalistic of their own species,” he said.

Last week, Lucid and other biologists teamed up to count and catch bullfrogs at the Boundary Creek Wildlife Management area near the Canadian border. Representatives from three British Columbia entities worked alongside Idaho Fish and Game employees.

The bullfrogs’ advance concerns B.C. wildlife managers, who are trying to protect a rare population of Northern leopard frogs in the Creston Valley, about an hour’s drive north of Bonners Ferry. Populations of the once-common native frog have plummeted in Western Canada since the 1970s, and they haven’t been seen in North Idaho since the 1950s.

The Creston Valley has the last wild population of Northern leopard frogs in the Northern Rockies, according to Dennis Thoney, director of animal operations at the Vancouver Aquarium.

“Amphibians are key indicators of the health of the ecosystems in which they live, and the decline of one species can dramatically affect others,” Thoney said in a statement last year.

The aquarium has been raising Northern leopard frogs in captivity in an effort to keep the population from going extinct. Last year, biologists released 2,000 of the tadpoles from the breeding program into the marshes near Cranbrook, B.C.

The B.C. wildlife managers haven’t documented bullfrogs in their region, and they’re eager to keep the predatory frogs from spreading north. Officials from the Creston Valley Wildlife Refuge, the Central Kootenay Invasive Species Society and the B.C. Ministry of Forest, Lands and Natural Resource Operations took part in last week’s bullfrog operation.

Wildlife officials saw 116 of the predatory frogs but only caught and killed 20.

“They’re hard to catch,” Lucid said. “If you get close to them, they’ll go ‘EEEP!’ and all you’ll hear is a splash in the water. Sometimes, you can see their eyes in a pond, but they’ll just disappear into the muck.”

The frog-catchers had better luck after dark, when they were able to detect bullfrogs with the beam of high-powered flashlights. The frogs freeze in the spotlight and are easier to net, Lucid said.

The 20 frogs caught won’t put a dent in the local bullfrog population. A single female can lay 20,000 eggs, which is one reason bullfrogs have spread so rapidly in Western states. During the 1940s, Fish and Game employees released bullfrogs in Idaho in a misguided effort to provide frogs for people to catch and eat.

But last week’s frog-catching operation will help managers better understand bullfrogs’ distribution and spread, Lucid said. The invasive frogs occupy warm, moist valley bottoms.

“They’re a species that we expect to do quite well with global warming,” Lucid said.