Frog Friend or Foe: The Problematic Legacy of the American Bullfrog
The American bullfrog is an amphibious powerhouse with an equally powerful impact on the ecosystems it inhabits. These frogs have top-notch swimming skills and tremendous leaping ability, not to mention an enormous appetite and mouth size, enabling them to triumph over almost anything smaller that crosses their paths.
It’s this athleticism and tenacity that has led to a problematic legacy, however—invasive success at the expense of native biodiversity.
Indigenous to the Eastern US, the bullfrog is vital for nutrient cycling and energy flows in ecosystems there. It’s also been long renowned by people in the region for its muscular physique, especially its large (and some would say tasty) legs.
Its powerful hind legs act as swim flippers, propelling it through the water at up to 4.5 miles per hour. With a single leap, this amphibian can cover distances of nearly six feet, or up to ten times its body length, showcasing unparalleled athletic ability.
Plus, during the breeding season, male bullfrogs become pro wrestlers, engaging in fierce territorial battles that involve grappling and jostling to gain access to potential mates. These confrontations, marked by aggressive shoving and underwater skirmishes, evidence the lengths to which bullfrogs will go to secure reproductive success.
All of this made them unstoppable when they were intentionally and accidentally introduced outside of their native range into the Western US.
Brought into California in the late 19th and early 20th centuries, bullfrogs were initially imported as a food source for the large numbers of miners flooding the state during the Gold Rush. The huge hind legs of bullfrogs are nearly twice as large as those of the native California red-legged frog initially targeted by the rapidly expanding human population there.
Then, during the Great Depression, the owner of the short-lived American Frog Canning Company sold bullfrog breeding pairs and instructions, along with big promises of prosperity, to people all over the country who were desperate to earn a living during a time when the unemployment rate was nearly 25%. Unfortunately for these folks and the local ecosystems where they lived, bullfrogs were more costly to raise in captivity than promised, and these efforts largely failed.
In addition to being purposely introduced to western states as a food source, as well as for biological control of insects, they’ve also been accidentally introduced. Bullfrogs have been unintentionally released during fish stocking events and have escaped from frog leg farms, pet owners, and research facilities to establish thriving populations and, unfortunately, wreak havoc on local ecosystems.
The issue is that bullfrogs are not only incredibly athletic but also voracious, “gape-limited” predators—meaning that what they eat is basically limited by how wide they can open their mouths. They gobble up everything from invertebrates to other frogs, salamanders, turtles, birds, bats, and small mammals.
Another secret to their success is their rapid reproduction rate. Female bullfrogs lay tens of thousands of eggs and can do so twice yearly. Those eggs hatch in just one week, and all those tadpoles are just as ravenous as the adults, eating anything they can, including each other and the tadpoles of other frog species.
As if all that weren’t enough, bullfrogs serve as carriers of the chytrid fungus Batrachochytrium dendrobatidis (Bd), responsible for the devastating amphibian disease known as chytridiomycosis. Bullfrogs can carry Bd without developing chytridiomycosis, enabling them to remain healthy and function normally while spreading it to more vulnerable amphibians. This deadly pathogen has decimated amphibian populations worldwide, driving numerous species to extinction.
As disease vectors with insatiable appetites and superior physical abilities, invasive bullfrogs are quite good at outcompeting native species for resources, including, unfortunately, other amphibians that were already struggling from a wide host of other threats. They imperil the Chiracahua leopard frog, northern red-legged frog, western toad, Pacific tree frog, many species of salamander, and more.
Although these challenges can seem insurmountable, there is hope. Land managers have been able to eradicate bullfrogs from relatively small focal areas with repeated and consistent efforts. With targeted management strategies like this and more, including large-scale removal programs and public education, we can expand on these successes.
Plus, most importantly, we must prioritize the protection and restoration of the habitats that provide crucial refuges for imperiled species. When habitats are healthy, populations of native species tend to be healthy, too. In fact, in a recent long-term study in Oregon, scientists found that when habitat conditions were favorable, several species of native frogs and salamanders were able to maintain stable populations even though they were sharing the landscape with bullfrogs (Rowe et al. 2019).
We’re carrying out all of these actions and more in western PARCAs, or Priority Amphibian and Reptile Conservation Areas, in order to bolster populations of native species against the effects of bullfrogs.
Together, we can work to curtail the impacts of this powerhouse of a frog in the West through removal, education, habitat restoration, and more while ensuring that it continues to be a star athlete in the East, where it belongs, by protecting and restoring its native habitat.
Reference
Rowe, J.C., Duarte, A., Pearl, C.A., McCreary, B., Galvan, S.K., Peterson, J.T., & Adams, M.J. (2019). Disentangling effects of invasive species and habitat while accounting for observer error in a long-term amphibian study. Ecosphere, 10: e02674. https://doi.org/10.1002/ecs2.
