7 minute read
Bullfrogs are coming...no wait...they're here!
BY GABRIELLE LEE
If you take a wander down to a nearby lake or pond, you’re likely to see one animal that, by all accounts, should not be there at all.
In the past ninety years, a population of smooth, sizable green-brown frogs has emerged on Vancouver Island. The creatures are robust and distinct; they sport golden eyes, croak noisily, grow up to twenty centimetres in size and are easily identified by their large, circular tympanum, or exterior ear, located just behind the eye.
The American bullfrog is not native to Vancouver Island. Naturally belonging to the east coast of North America, they were introduced to BC in the 1930s and ’40s for farming frog legs, a culinary delicacy. However, workers soon realized that farming bullfrogs was uneconomical, and the animal was released. Through the bullfrog’s movement as well as human assistance, they became an invasive species, now found throughout Vancouver Island.
Dr. Tim Goater worked as a professor in the Biology Department at Vancouver Island University for 32 years. In addition to teaching various courses on zoology, evolution, parasitology and entomology, Goater has spent his career researching the ecological and evolutionary impacts of invertebrate animals, specifically parasites. This led him to the study of the American bullfrog.
Of course, the primary concern with any invasive species is its impact on the natural ecosystem. On Vancouver Island, bullfrogs are pervasive. “They have basically colonized virtually every aquatic habitat – from golf course ponds to fairly large lakes – [from Victoria] all the way up to Campbell River,” says Goater.
A constant threat to native fauna, bullfrogs are prolific predators with voracious and wide-ranging appetites, even outside their native land. Their primary prey are the insects found in aquatic environments, such as dragonflies, water beetles and yellowjacket wasps. But, Goater explains, “They’re not just feeding in the water… They come onto land, and that’s where they get the garter snakes, potentially mice,” and even sometimes ducklings.
“These frogs get so large that they will eat anything that they can shove in their mouth,” he states. “Especially in the very large bullfrogs – they’re cannibals. They’ll eat their own kind.”
Even more, the bullfrog population can increase rapidly. According to the Ministry of Environment and Parks, a single female bullfrog can lay up to 20,000 eggs annually. Goater notes that the abundant offspring can quickly deplete available habitat and resources. “Not only do the [bullfrog] tadpoles compete with our local frog species,” he says, “but they potentially eat them as well.”
The Invasive Species Council of Metro Vancouver notes that bullfrogs “are responsible for the decline of other amphibians, including some that are endangered… [They] also spread deadly viruses and fungi to other amphibians, and damage wetland habitats and water supply infrastructure.” Goater also notes that “our native fauna hasn’t evolved” to accommodate the sudden appearance of the bullfrog.
What’s more, bullfrogs disperse, and summertime is precisely when they do so. Mid-August brings about seasonal movement. “The juveniles… their first job, their first ecological feat is to get out of that pond and find a new one, because the population would be too big otherwise,” Goater clarifies. This, paired with human intervention and the bullfrog’s ability to travel “a couple kilometres, especially on a rainy night” means the population is easily spreading.
Along with the introduction of the bullfrog comes another unique concern: the introduction of a parasite. According to a 2013 study conducted by Goater and Colin W. Novak on introduced bullfrogs and their parasites, “introduced parasites have not had the time to coevolve with the local fauna,” which can subsequently lead to “new infectious diseases and parasites to native species.”
The lung fluke, or Haematoloechus longiplexus, is abundant in introduced bullfrogs. To understand how and if the bullfrog’s parasite is affecting local wildlife, Goater has spent the past several years monitoring the lung fluke population in various freshwater habitats. He and his students have utilized local sites, such as Quennell, Diver and Michael Lake, to conduct a long-term study that will hopefully yield further knowledge on the topic.
So far, Goater’s research has yielded promising results; the parasite does not appear to have spread to local fauna and has not notably affected local wildlife. Still, there is likely more to be discovered, and it remains a stretch to call these frogs harmless or a seamless addition to Vancouver Island’s ecosystems.
“These are pretty magnificent animals,” Goater expresses. “And yet, they cause ecological damage by their presence.”
Naturally, one might wonder what action can be taken to control the bullfrog’s presence in BC. Goater’s answer is simple: “[There’s] nothing we can really do about it.” Though researchers have managed to exterminate the frogs in a few manageable cases, populations in larger bodies of water, where “tadpoles keep increasing over the years,” make the complete removal of the amphibian virtually impossible. Goater says, “[We] will never be able to control them,” and that the bullfrogs’ rapid reproduction process means they will continue to reemerge, year after year.
Still, work can be done by the public. Though people may view the bullfrog as an unwelcome nuisance, Goater suggests simply letting “nature take course” if you find them on your property. Euthanizing introduced frog species is legal in BC only with landowner permission, authorization from provincial or federal authorities and the absence of local regulations that outlaw the practice. An environmental professional is also critical to the process; without one to identify the amphibian, the public runs a significant risk of harming or killing a similar-looking local frog – a crime under the province’s Wildlife Act. “The big problem is … [people] go out and kill what they think is a bullfrog, and they’ve killed a local frog... They don’t have the training to be able to recognize the other species that are here on the Island,” Goater says. “My most devastating one was, they said, ‘I’ve got bullfrogs in the freezer for you’ … and they weren’t bullfrogs. They were western toads, of all things”, a native toad designated as a species of Special Concern.
When Goater and his team do dispatch bullfrogs, they operate under strict guidelines issued by the Canadian Council of Animal Care and VIU to ensure safe, humane handling. Killing the frogs with pellet rifles, traps or chemicals, for example, is strongly discouraged and, indeed, illegal in many cases.
The British Columbia Ministry of Environment echoes Goater’s words, as the main objective at the moment is to contain bullfrogs and track their spread. To do this, the BC government instructs individuals to refrain from capturing, transporting, keeping or selling the frogs. Though tadpoles may tempt children who want to keep them as pets or gardeners who are looking to add life to their backyard, the government advises against both these practices, emphasizing that “transport seems to be one of the primary ways that bullfrogs are spreading in the province.” If one does encounter bullfrogs, the BC government’s Report Invasives app can be used to notify the government.
Although not a native of Vancouver Island, the American bullfrog has established itself as a long-term resident. Whether it is welcome or not, it seems like the bullfrog is a neighbour that will stay – at least for the foreseeable future.
