July 14, 2000
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THE UNIVERSITY OF VICTORIA July 14, 2000 |
 An unwelcome easterer is invading vancouver Island - in leaps and bounds
by Valerie Shore They’re big, green, bug-eyed aliens with huge appetites and they’re invading southern Vancouver Island. If this sounds like a bad episode of The X-Files, it’s not. It’s reality, and you can see — and hear — the invasion happening in several lakes and ponds around Victoria, Duncan, Nanaimo and Parksville this summer. The intruder is the American bullfrog, and its distinctive bass bwum, bwum, bwum serenade is signalling big trouble for the 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 UVic graduate student Purnima Govindarajulu, who is studying the biology of the bullfrog invaders for her PhD. Insects, fish, snakes, small mammals and birds, even other bullfrogs, are all 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. It helps that she’s dealing with a giant of the froggy world. Bullfrogs are the largest frog in North America, measuring up to 20 centimetres in length (not including legs) and tipping the scales at up to three-quarters of a kilogram. Tadpoles can grow up to 15 cm long, with heads as big as golf balls. No one is sure how these goliaths found their way to Vancouver Island, but it most certainly wasn’t under their own hop-power. Bullfrogs are an eastern species, and in Canada are not naturally found west of Ontario. It was people — probably looking to enhance their aquatic gardens or farm frogs for their tasty legs — who brought the first bullfrogs to B.C.’s Lower Mainland and Vancouver Island several decades ago. The frogs have been spreading in leaps and bounds ever since.
Equally unnerving is the frog’s ability to reproduce. Females can lay up to 20,000 eggs at a time. “Their survival strategy is to eat as much as they can, grow as fast as they can, and make as many babies as they can,” says Govindarajulu. How the region’s native frogs are coping with this robust new neighbour is a major focus of Govindarajulu’s work. The red-legged frog — recently listed as a threatened species in B.C. — and the tiny tree frog are both easy meals for a large bullfrog. And while bullfrog tadpoles are vegetarian, they eat the same food as other frog tadpoles. “Once bullfrogs get established they pretty much clean out the competition,” says Govindarajulu, who has set up 60 artificial ponds on the UVic campus to see how the different species compete under controlled conditions. But perhaps the most intriguing part of her work happens at night when she and her assistant, Ron Patrick, go “frogging” in local ponds. Paddling quietly in a canoe, they use a bright flashlight to scan for the glare of beady frog eyes at the surface. Once they’re close enough, Govindarajulu lunges for the frog. “After two summers of this, I’m pretty quick,” she laughs. “I haven’t fallen in yet, but we’ve taken in water a few times.” The captives are plopped into a bucket and taken to the lab, where they’re anesthetized, sexed, measured, weighed and induced to throw up (to check stomach contents). Govindarajulu also injects tiny dabs of coloured plastic paint into the webbing of each hind foot to identify individuals if they’re recaptured. Within 24 hours of their “abduction,” the frogs are returned to their pond where they quickly resume their froggy business. Although she’s only halfway through her study, Govindarajulu says the evidence is mounting that bullfrogs are supplanting native frog species. For this reason, she frequently gives public talks — through the UVic Speakers Bureau and the CRD’s naturalist program — on ways we can all help 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.” As for kids, Govindarajulu encourages them to observe tadpoles, but to not take them home. Sometimes it’s a tough sell. “They ask why bullfrogs can’t be friends with other frogs,” she sighs. “They grew up with Walt Disney and here I am talking about predation. It’s hard.” Even some adults have difficulty understanding why Govindarajulu euthanizes bullfrogs captured in new areas. “They get very irate and say I’m playing God, but my answer is that we’ve already played God. Bullfrogs don’t belong here and they’re endangering our native frogs. It’s important to make that distinction.” Govindarajulu’s work is being funded by the Natural Sciences and Engineering Research Council and the Habitat Conservation Trust Fund. For more information on identifying frogs, go to her Web site at <web.uvic.ca/bullfrogs>. |
“Their range on the Island is expanding by about 5 kms a year, mainly near urban areas,” says Govindarajulu, who conducted her first bullfrog survey in the Victoria area in 1997. So far, she’s found them in several dozen local lakes and ponds, including Elk and Beaver Lakes. There is some transport by humans, she says, but the frogs are also colonizing on their own — not surprising given their long, muscular legs. “They can easily jump a metre high,” she marvels.