HAPPENS WHEN BULLFROGS GO WHERE THEY SHOULD NOT BE?
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?
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.
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.
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
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.
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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