Introduced Species Summary
Bullfrog (Rana castesbeiana)
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Common Name: (North American) Bullfrog
Scientific Name: Rana catesbeiana
Phylum or Division: Chordata
Class: Amphibia Linnaeus
Identification: The North American bullfrog is the largest frog in
North America. It can
reach over 6 inches in length, with males weighing up to one pound. One of its unique physical characteristics is the
absence of “dorsolateral ridges”. Many frogs have dorsolateral
folds which are lines of raised glandular skin in an area between the back
and the sides (see Figure 2). In
North American frogs, there are usually two ridges running parallel to the
midline of the body. The bullfrog is an exception
in that its dorsal folds start behind the eyes and go back and down around
the ears or the “tympanic membrane” (ear drum). Short
folds of skin extend from over the eye to behind this ear drum. In males, the ear drum is larger than the eye, while
in females it is about the same size or smaller. Thus,
the dorsolateral features and the tympanic membrane
help identify the bullfrog and distinguish it from other frogs. Other features include the following. The bullfrog’s hind feet are completely webbed except
for the last joint of the largest toe. It can
range in color from dark olive to pale green, but may be brownish. Tadpoles have a light ventral surface (belly, front
side) and the dorsal surface (back area) is greenish to yellow with numerous black spots. The bullfrog’s call is a deep “jug-o-rum” or “br-wum” bellow, made day and night, and can be heard
up to ¼ mile away. In breeding season,
the throat of the male is yellow, whereas the female’s is white. Unlike other frogs, it spends most of its time in
water from where it also does most of its hunting.
Figure 2: Green frog with dorsolateral
Original Distribution: The North American bullfrog is native to the central
and eastern United States
and the southern portions of Ontario
Current Distribution: North American bullfrogs are now found from Nova
Scotia to Central Florida,
from the East coast of the U.S.
to Wisconsin, and across the
Great Plains to the Rockies. The natural western limits of this species have reached
as far west as California and
Mexico. It is also found in Hawaii,
the Caribbean, South America,
Europe and Asia.
Site and Date of Introduction: Several theories exist, but it is believed to be
introduced from its original habitat in North America
in the early 1900s in Colorado
Introduction: There seem to be several
modes of introduction. Its original invasion
appears to have been through accidental introduction to trout streams and
lakes during the Colorado Divisions of Wildlife fish stocking operations
in the early 1900s. Bullfrogs occasionally invade
fish hatchery ponds and their larvae are caught along with fishes routinely
stocked in ponds and reservoirs. Other
modes of introduction include: (1) via the aquarium trade: as released pets
no longer suitable for home aquariums; (2) landscape/fauna “improvement”:
bullfrogs are deliberately introduced as a harvestable game animal (frogs
legs) or aesthetically pleasing wildlife; (3) pest control: bullfrogs have
been deliberately introduced to control agricultural pests; (4) self-propelled:
bullfrogs are capable of considerable overland travel and are known to disperse
throughout entire watersheds, provided they have interspersed patches of
suitable habitat (similar to the corridor concept); and (5) aquaculture:
bullfrogs are often imported into a country or area for commercial production
as food (frog legs), and as noted above can inadvertently be introduced along
with fishes raised in hatcheries where bullfrog larvae are abundant.
Reason(s) Why it has Become Established: Most fish appear to be averse to
eating bullfrog tadpoles because of their undesirable taste, so the bullfrog
has been able to thrive in freshwater lakes and rivers without these fish
predators. Bullfrogs are predators and
usually feed on snakes, worms, insects, crustaceans, frogs, and tadpoles,
so it has a varied menu and diet. They are also cannibalistic
and will not hesitate to eat their own kind. Bullfrogs
easily adapt to environments modified by human beings.
For example, increased water temperatures and
resulting aquatic vegetation, which are common factors in lakes polluted
by humans, favor bullfrogs by providing suitable habitats for growth, reproduction
and escape from predators. Bullfrogs also
have a longer breeding season and a higher rate of “pre-metamorphic survivorship”
(tadpoles), which also allows them to be more
successful than other frogs. One possible reason
the bullfrog has thrived in California
is that, in eastern North America (its native habitat)
bullfrogs evolved with a diverse predatory fish fauna, so that it has evolved
mechanisms that enable it to avoid predation
by fish in various environments.
As it consumes a wide variety of animals and insects, its main ecological
role is in controlling populations of insect pests and, where abundant, other
animals such as snakes and mice to control their populations.
American bullfrogs help to control insect pests and are important for medical
research because their skeletal, muscle, digestive, and nervous systems are
similar to those of higher animals. They are
often hunted or bred for meat (frog legs).
may be driving native frog populations and species to extinction in some
areas in the U.S.
negatively affecting biodiversity. Adults consume
any animal that can be swallowed, such as snakes, birds, fish, crawfish and
other frogs. To the extent that these animals
may be endangered in certain habitats shared by the bullfrog, extinction
could ensue. Another threat is that bullfrog
larvae can have a significant impact on algae, which can affect the aquatic
Control Level Diagnosis: Medium Priority: although the population may
be increasing in some regions, it may actually be declining or sustainable
in others, particularly where it is regularly harvested as game (e.g., Rhode
Island). The invasion
problem may be more acute in the western U.S.,
particularly in California
where Highest Priority may be appropriate in certain areas where other
frog populations are put in danger of extinction.
Therefore, I would recommend that the bullfrog population be more carefully
and selectively controlled in some regions of the U.S.,
while in others, natural or anthropogenic controls may already be in place
(harvesting, fish introduction, collection). Care
should be taken to avoid sweeping control measures where they are not appropriate.
Control Method: In California,
there have been attempts to control bullfrog populations by introducing new
fish species that are their predators. However,
as mentioned above, bullfrogs have evolved mechanisms to avoid predation
by fish, such as unpalatable eggs and tadpoles, as well as tadpoles that
have evolved to be inactive much of the time, which reduces their exposure
to predators. Some control measures are already
effectively in place. For example, in the U.S.,
both native and introduced populations are often managed as fisheries which
are held to low sustainable levels by harvesting pressure.
Adults can be taken by a variety of methods, including shooting, spears/gigs,
bow and arrow, clubs, nets, traps, angling, and by hand.
Collection at night is facilitated by the frogs’ immobilization in
response to bright light. Larvae may be killed
by water-borne poisons that are also used for fish. Collecting
egg material can also be an effective control method.
Global Invasive Species Database, Rana catesbeiana
Invasivespecies.gov, Species Profile,
Integrated Taxonomic Information System
(ITIS) Report, Rana catesbeiana, www.itis.usda.gov.
United States Department of Agriculture
(USDA), Forest Service, www.fs.fed.us/r4/amphibians/bullfrog/htm.
United States Department of Agriculture
(USDA) Southern Regional Aquaculture
Center (SRAC) Publication No.
436, “Bullfrog Culture”, March 1999, by C. Greg Lutz and Jimmy L. Avery.
United States Geological Survey, Northern
of Michigan’s Animal Diversity
of Victoria, “Natural History
of the American Bullfrog Rana catesbeiana in British Columbia”,
Photos and Map: From
U.S. Geological Survey, Northern
Author: Martin Murphy
Last Edited: February 17,