The American bullfrog has been labelled one of the world’s most invasive animals. From its Mississippi River-region homeland, it has spread — with help from human frog-leg-farming and pet-dumping friends — across the continent and elsewhere. It started emigrating to western Canada decades ago.
It wants the same things we do. It wants decent and adequate food and shelter, and a safe place to raise its kids — although that last requirement is a bit tricky, given the frog’s undiscriminating appetite.
When it finds its home getting too crowded or too noisy, the pickin’s getting too picked over or life getting too uncomfortable, the bullfrog looks for opportunities elsewhere. A bullfrog avoids Canada’s immigration requirements by sneaking across the border. It just packs up its enormous appetite and goes.
Without alligators, water snakes and other natural predators to keep its numbers down, bullfrogs thrive in their new homes. They spend their time competing against each other to be Top New Pond Predator Thought Least Likely to Succeed. In 2013, Victoria biologists Kevin Jancowski and Stan Orchard identified the stomach contents of more than 5,000 bullfrogs captured in the Greater Victoria area over five years. The researchers found remains of insects, fish, turtles, garter snakes, waterfowl, other frog species and bullfrog young and tadpoles. They also found remains of songbirds and small mammals.
The bullfrog considers most other wetland animals possible meals, and happily it eats its way through entire wetland ecosystems.
Humans are safe because of our size. Let’s hope they never experience a Godzilla-like mutation.
Since the mid-1980s, the frogs have invaded southern B.C., including much of Vancouver Island’s eastern edge and some of the Gulf Islands.
Last June, conservationists in southeastern B.C.’s Creston Valley sounded the alarm when an American bullfrog was found four kilometres south of the valley. The main concern there was for the leopard frog, native to B.C., smaller than the bully-frog, and endangered.
The Creston-area conservationists might find a new technique developed at the University of Victoria helpful for tracking the range of both frogs. Molecular biologist Caren Helbing and her team developed a way to detect a species’ presence through the DNA it leaves in an ecosystem. The DNA comes from the poop, mucus, scales, bits of skin and other tissues the animal scatters about as it goes about its business.
Helbing’s test focused on detecting environmental DNA, or eDNA, in lake water — bullfrogs in Victoria-area lakes and red-listed, native Rocky Mountain tailed frogs in several east Kootenay lakes. However, Helbing says it could be used to detect the presence of any species in samples taken from other environments — in soil or even air samples, for example.
“What we’ve done is introduce innovations and standards for the detection of eDNA,” she says. “It allows us to say with confidence whether an invasive species has entered an area, or determine the range of an endangered or threatened species.”
At Langford’s Florence Lake, the researchers detected bullfrog DNA.
Orchard, who has spearheaded a bullfrog-control program in the Victoria region since 2005, says that was to be expected. The CRD- and Langford-funded program aims to keep the frogs from invading Victoria’s watershed. Orchard and his three staff patrol a corridor from Saanich Inlet to Esquimalt Lagoon, capturing and euthanizing thousands of the frogs each year.
With Florence Lake on the inner side of the corridor, “it is subjected to continuous reinvasion from the north and east,” he says. “Since 2006, I have taken 7,646 adult and juvenile bullfrogs out of Florence Lake.”
Bullfrogs, he says, “are conspicuous by day because they sit out in the open, and they are easily detected at night with spotlights, so high-tech detection methods are really not required for managing bullfrog populations.”
But they might help researchers and conservationists identify when and where the first bullfrogs arrive in an area, and for tracking the presence of other species put at risk by a bullfrog invasion.
