Larry Licht’s recently published sentiments on invasive American bullfrogs in local habitats aren’t based on research. Bullfrogs are definitely a problem here.
The International Union for Conservation of Nature lists bullfrogs as one of the world’s 100 worst invasive alien species. They cite many examples of invasive bullfrogs threatening native species worldwide. My colleague, Kevin Jancowski, and I have examined the stomach contents of around 8,000 bullfrogs. Consequently, we speak with some authority on questions of bullfrog diet. We have also observed and documented bullfrog behaviour in local lakes and ponds throughout their active season (mid-April to early October) for the past nine years, and have so far collected and examined more than 30,000 adults and juveniles.
At the Beaver Lake Ponds, just south of Beaver Lake, during the last week of April through the first week of May, endangered hatchling western painted turtles will emerge from their nests and migrate directly into the mouths of hungry bullfrogs.
At Filberg Marsh bird sanctuary, north of Courtenay, residents try to control bullfrog numbers by annual cull. They have found as many as four ducklings in a single bullfrog stomach. This sanctuary hosts a nest box project for endangered wood ducks. So wood duck ducklings tumble from their nest boxes into ponds filled with large, predatory bullfrogs.
Many people have noted rapid declines in the numbers of calling Pacific tree frogs as bullfrogs invade and proliferate. We found bullfrogs were not only eating tree frogs, but that they were also eating egg-bearing adult females, thus taking out dozens of tree frogs in a single gulp.
Small mammals, lizards, snakes, songbirds, salamanders, frogs and even coho salmon are all on the bullfrog menu. There is scant evidence to support Licht’s assertion that adult (male) bullfrogs “readily eat smaller bullfrogs.” Our analysis found that instances of cannibalism of bullfrog tadpoles and juveniles by larger bullfrogs amounted to only 0.43 per cent (81 cases out of 18,814) of total prey remains. They swallowed twice as many native amphibians.
Licht asserts that “bullfrogs will rarely move away from the water’s edge.” This is not true. Bullfrogs did not get from Beaver Lake to Langford Lake and all points in between by rarely moving away from water. Our data demonstrate that you can routinely clear some ponds of all bullfrogs and then return at intervals to find more bullfrogs.
Time on land and distance travelled are unknown. Migration is a continuous process throughout the active season and involves both genders in all age classes, though the motivations and timing can vary for each group.
Red-legged frogs and tree frogs come into direct contact with bullfrogs throughout the active season. Western toads are probably extirpated from the Saanich Peninsula, but historically were widespread. Their disappearance from most of the western United States is attributed primarily to the introduction of a pathogenic chytrid fungus. Bullfrogs are an asymptomatic carrier of this fungus as well as other amphibian pathogens such as ranavirus.
Since the launch of my program, the B.C. Ministry of Environment has stated that bullfrog control and eradication is “impossible.” There is no scientific basis for this statement. It presumes that hypothesis testing is unnecessary and innovation only a pipe dream in the face of their brash presumption and pet notions.
Having stated that it’s impossible, the ministry proceeds, illogically, to cost it out to a million dollars per year or more. One million dollars per year would, in fact, sustain more than 20 two-person teams arrayed across the Saanich Peninsula.
Why is the province pumping money into conservation projects for listed species such as painted turtles and wood ducks while at the same time ignoring an obvious and expanding potent threat? For now, I have only one two-person team assigned primarily to keeping bullfrogs out of the Victoria watershed. I am grateful to Capital Regional District Water Services and CRD Parks, as well as the surrounding municipalities, for having funded this work. My program has resulted in an innovative, patented technique, unprecedented capture rates at modest cost ($40,000 to $50,000 a year), and the largest database of dead invasive bullfrogs in the world.
Last, why would anyone want to substitute the annual chorus of native Pacific tree frogs with the sonorous bellows of an interloping ecological menace?
Stan Orchard is a biologist and specialist in amphibians and reptiles.
