by Whit Gibbons

November 7, 2010

Q: I have lived in Alabama, Michigan, and South Carolina and have heard bullfrogs bellowing on summer nights in all of these places. I assumed they were native to each region, but I have heard recently that bullfrogs are a big environmental concern in some parts of the country. How did bullfrogs get to places where they do not belong, and has something changed to make them become a problem?

A: Except for southern Florida, bullfrogs are native throughout the eastern United States into southern Canada. Within their native geographic range, where a diversity of their natural predators abounds, the population sizes of bullfrogs are constrained and kept under control. The problems you refer to are those that have occurred in the western states and other parts of the world--where bullfrogs are not a natural part of the ecosystem but have been introduced intentionally or accidentally.

The voracious, predatory nature of bullfrogs is well known. They eat small snakes, birds, other frogs, and even baby alligators. Apparently they will gobble up anything that moves and will fit in their big mouth. When introduced into areas where they have no natural predators to keep them in check, bullfrogs have acquired a reputation as a major predator on native vertebrates, including some endangered species. Among the endangered species reportedly eaten by introduced bullfrogs in California and Arizona have been California tiger salamanders, Chiricahua leopard frogs, and California red-legged frogs. Although data are often difficult to document with certainty, bullfrogs have been implicated in regional declines in native amphibians and fish in other countries where they have been introduced, including Germany, Italy, and France. Bullfrogs also have the potential for spreading parasites and diseases to native amphibians that have no natural defenses against them, as well as competing with native frogs for food resources.

One source of introduction of bullfrogs into parts of the country where they are not native is through the development of bullfrog farms. The big frogs, which can be up to eight inches in body length and a foot long with legs extended, are one source of frog legs, which some people consider a delicacy. Hence the reason for bullfrog farming. In the long run, bullfrog farming is generally an unprofitable venture.

As the Missouri Department of Conservation states in a document on pond management, "Successful frog farming is definitely more myth than reality." The Missouri message was for would-be bullfrog farmers. But commercial frog farming is not a self-sustaining occupation anywhere in the world. When sources of commercial frog legs sold to restaurants and grocery stores are carefully checked, the majority are found to be from captures of wild frogs. Nonetheless, bullfrog farming has been attempted in areas outside of the bullfrog's natural range, resulting in the dispersal of individuals into the surrounding habitats.

Bullfrogs are prolific breeders when any water is available, with females laying more than 20,000 eggs at a time. In contrast to many other frogs, the tadpoles of bullfrogs are typically resistant to predation by most fish, so that large numbers of juveniles are produced successfully from a single mating. One pair of introduced bullfrogs is capable of creating an enormous population of an invasive predator for which native species have no defenses.

The introduction of bullfrogs into states outside their native range and even into other countries can be an ecological disaster. In California the problem is perceived by some citizens and elected officials to be serious enough to warrant controls that will limit the introduction of bullfrog eggs, tadpoles, juveniles, and adults into the state. The intent is to prevent this invasive species from occupying natural ecosystems in a manner that will have a negative impact on native species. It is always unfortunate when legal roadblocks must be used that limit the free transport, sale, or even ownership of a wild species. But even more unfortunate are the consequences of the improper introduction of a species in places where it does not belong.

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