Bullfrogs threaten native species in Idaho and Northwest

Several years ago, Michael Lucid pulled to the side of a North Idaho 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 including snakes, mice, ducklings — even other bullfrogs.

“They’ll eat just about anything they can shove in their face,” said Evin Oneale, regional conservation educator in Fish and Game’s Nampa office.

The golden-eyed frogs grow to softball size and weigh up to a pound. They likely arrived in Idaho, as in other parts of the West, in the late 1800s, as a potential food source for hungry settlers — miners, prospectors and the like.

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 Canada’s wildlife managers, who are trying to protect a rare population of northern leopard frogs in the Creston Valley, about eight miles north of the U.S.-Canada border. Populations of the once-common native frog have plummeted in western Canada since the 1970s. They haven’t been seen in North Idaho since the 1950s.

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. The aquarium has been raising northern leopard frogs in captivity to save the population from extinction. Last year, biologists released 2,000 of the tadpoles into marshes near Cranbrook, B.C.

Given the bullfrog’s predatory habits, B.C. wildlife managers are eager to keep them 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 caught and killed just 20.

In the Treasure Valley and other parts of southern Idaho, bullfrogs are “here to stay,” Oneale said. As invaders that threaten native species, they are regulated by the Department of Agriculture. There is no limit on catching them, but they must be killed on site and may not be transported alive. Their numbers in Southwest Idaho appear stable, but trying to eradicate them “would probably be a fool’s errand,” Oneale said. A single female can lay 20,000 eggs.

Charlie Justus, the Fish and Game regional conservation officer for Southwest Idaho, said the only frog species that could be threatened by bullfrog encroachment in southern Idaho would be the Columbia spotted frog in the Owyhees. But the elevation there is likely too high for bullfrogs to gain a foothold.

Elsewhere in southern Idaho, “They probably are crowding out other species, but not to the extent that they’re making population impacts, because the species they’re impacting are pretty cosmopolitan themselves,” he said.

John Cossel, a herpetologist who heads the biology department at Northwest Nazarene University in Nampa, agreed that the Owyhees were likely too high to be bullfrog friendly. He was surprised to hear of bullfrog sightings so far north in Idaho. The swarm that Lucid observed was likely an isolated mass migration event that is not uncommon, as juvenile bullfrogs look to “strike out on their own,” he said.

All else being equal, their northward migration is likely the result of climate change, Idaho’s experts said. Bullfrogs, like other amphibians, hibernate and can survive cold winters. But they couldn’t survive if ponds they depend on evaporate or freeze solid in winter.

“There’s no other reasonable explanation for bullfrogs suddenly expanding their range north,” Oneale said. “Winter is the great equalizer, and if winter temperatures are too cold, frogs cannot survive.”

Cossel said climate change “would be the first thing I think of” behind the frogs moving north. He said people in the Treasure Valley should not move bullfrogs around farther than they could travel on their own. Transporting live bullfrogs is, in fact, illegal.

People should also be wary of buying them in stores. Cossle said bullfrogs are more resistant to a fungus that is lethal to many frog species worldwide and, as potential disease carriers, could spread it to more susceptible species.

Statesman reporter Bill Dentzer contributed.

This story was originally published July 16, 2015 at 12:00 AM.