It鈥檚 a common fallacy that bullfrogs are a threat to native fauna, as expressed in a recent sa国际传媒 article.
Bullfrogs live along the margins of lakes and ponds and are restricted to this habitat. Except during rains, when they might disperse, bullfrogs will rarely move away from the water鈥檚 edge.
By contrast, three native species of anurans (frogs and toads) found on Vancouver Island, the red-legged frog, the western tree frog and the boreal toad, are not restricted to this zone. After metamorphosis, when aquatic tadpoles transform and move onto land, the juvenile frogs and toads of these three native species inhabit woodlands and fields away from the danger of bullfrogs near or in the water.
While bullfrogs may eat some young frogs, native predators of frogs include fish, herons, mink and many more kinds of birds and mammals. Natural mortality to tadpoles and juveniles is very high, likely more than 90 per cent, and bullfrogs are responsible for a very small part of this. Along with numerous other predators, bullfrogs may eat some baby ducks and turtles, but their diet is mainly the insects that live in or near the water edge.
Many other features of the bullfrog鈥檚 biology, such as longevity, reproductive capacity and territorial behaviour, should be understood and evaluated when programs of bullfrog control are being considered. For example, adult bullfrogs are highly territorial and will occupy a given region of a pond or lake. In that area, they readily eat smaller bullfrogs, which itself is a type of numbers control.
Also, a female can lay up to 20,000 eggs a year and the hatched tadpoles will spend almost two years in the water before moving onto land. Thus while an eradication program is based on removing adults from shorelines of lakes, ponds and reservoirs, many thousands of tadpoles remain in the water for later emergence. Even for a single pond or lake, without poisoning the entire body of water to kill all tadpoles (and other organisms in it), the probability of eradicating a species with this high fecundity and life history is very low.
Release by humans and global travel by boat are among means that have assisted redistribution of unwanted animals and plants. It is true that invasive species are a problem that should be taken seriously. Aggressive invasive plants may occupy and dominate habitat and thereby eliminate a native species from the same required land or water.
Invasive animals may affect native fauna by preying upon them or taking over nesting space that might be limited. Loss to humans from invasive species may be economic or esthetic.
Nevertheless, each case of an invading species must be considered with a broad understanding of the biology of that species. Each case should be judged by its own potential risks before knee-jerk reactions and costly, many futile, eradication projects are initiated.
Bullfrogs may be invaders, but with knowledge of the ecology of our local native amphibian fauna, they are not a serious threat. Hyperbole and unsubstantiated fear of any invasive species are responsible for the exaggerated concern. In the article, the statement that 鈥淏ullfrogs could also infest the environment with new bacteria or parasites鈥� is an example of such hype. I would be interested to know of scientific studies that document this declaration.
Funding for bullfrog eradication projects can be better spent on other conservation and environmental issues. It is highly unlikely a battle to eradicate Capital Region District bullfrogs and silence their deep resonant, wondrous mating calls can be won. Even if it were won, their removal would have little, if any, marked effect on water quality or our local native fauna.
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Larry Licht of Victoria is a herpetologist who has studied the amphibians of sa国际传媒.
