Bullfrogs' serenade signals trouble in Canada

American bullfrogs hopping over the border into western Canada are causing a stir on Vancouver Island

By Environmental News Network staff

Big, green, bug-eyed aliens with huge appetites are invading southern Vancouver Island.

You can see and hear the invasion happening this summer in several lakes and ponds around Victoria, Duncan, Nanaimo and Parksville.

The intruder? The American bullfrog.

The frog's distinctive bass serenade portends big trouble for Vancouver Island's native frog species and aquatic ecosystems in general.

"The biggest problem is that bullfrogs eat other frogs. Actually, they'll eat just about anything," says University of Victoria graduate student Purnima Govindarajulu, who is studying the biology of the bullfrog invaders for her doctorate degree.

Insects, fish, snakes, small mammals, birds and even other bullfrogs are fair game. "Whatever they can fit into their huge mouths," she says.

To find out where the bullfrogs are, how fast they grow and what they're eating, Govindarajulu spends her summers stalking, catching, measuring and tagging her slippery subjects in Victoria-area ponds and lakes.

In Canada, bullfrogs are not naturally found west of Ontario. It was people, probably looking to enhance their aquatic gardens or farm frogs, who brought the first bullfrogs to British Columbia's lower mainland and Vancouver Island several decades ago.

The frogs have been spreading in leaps and bounds ever since.

"Their range on the island is expanding by about 5 kilometres a year, mainly near urban areas," says Govindarajulu. So far, she's found them in several dozen local lakes and ponds, including Victoria's Elk and Beaver lakes.

Purnima Govindarajulu checks one of 50 artificial ponds she set up this summer to study the growth and survival of Pacific tree frog tadpoles in the presence of bullfrog tadpoles

Govindarajulu says evidence is mounting that bullfrogs are supplanting native frog species.

"Once bullfrogs get established they pretty much clean out the competition," she notes. Govindarajulu frequently gives public talks on ways to minimize the impact of this impressive but unwelcome amphibian.

"The easiest thing we can do is not move frogs around, which people still do, especially now that aquatic gardens and backyard ponds have become so popular," she says. "Wild frogs aren't going to stay in your backyard, they're going to hop away."

Although Govindarajulu releases the bullfrogs she captures in her study lakes, she will euthanize frogs discovered in new areas. Some people have difficulty understanding this, she says.

"They get very irate and say I'm playing God, but my answer is that we've already played God," she says. "Bullfrogs don't belong here and they're endangering our native frogs. It's important to make that distinction."

Copyright 2000, Environmental News Network, All Rights Reserved

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American bullfrog
University of Victoria
U.S. Geological Survey
Partners in Amphibian and Reptile Conservation
Savannah River Ecology Laboratory

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