Rana catesbeiana
(American bullfrog)

The American bullfrog has been transported around the world primarily with the intention of cheaply cultivating its large and meaty hind legs for human consumption. It is also of commercial interest to biological supply houses, fish bait suppliers, the pet trade, and pond landscapers. In North America in the early 20^th Century it was released outside of its natural range by wildlife agencies keen on introducing a new game species (Lannoo, 1996; Tangley, 2003). Once liberated it is extraordinarily prolific, prone to migration, and highly adaptable to new environments. It grows into a relatively large, voracious, and indiscriminate predator that can come to dominate the margins of lakes and ponds. It competes with native anurans. Larvae can have a significant impact upon benthic algae, and thus perturb aquatic community structure. Adults may be responsible for significant levels of predation on native anurans and other aquatic herpetofauna, such as snakes and turtles. The species also contributes to the spread of pathogens such as the chytrid fungus, Batrachochytrium dendrobatidis (Garner et al., 2006). R. catesbeiana has been identified by the World Conservation Union (IUCN) as one of the world’s 100 worst invasive alien species (ISSG, 2005).

R. catesbeiana is native to eastern North America, ranging naturally from Nova Scotia, southern Quebec and Ontario in Canada, down through the eastern United States and Mississippi drainage, and southward along the east coast of Mexico. It has been introduced to Hawaii, parts of the western USA and Canada, Mexico and the Caribbean, South America, Europe and Asia (ISSG, 2005). In the central United States it is difficult to say for certain where natural populations end and alien populations begin, but there is no disputing the fact that all occurrences west of the Rocky Mountains are the result of translocation and release during the late 19^th century and throughout the 20^th. (It is known that bullfrogs were introduced to areas of California and Colorado in the early 1900s -- University of Michigan Museum of Zoology, 2005).

People continue to routinely transport bullfrogs from one place to another because they appear to be an easily cultivated and profitably marketable commodity for human consumption. Bullfrogs almost inevitably escape because they are, if not impossible, then certainly impractical to confine (see Liu and Lee (2009) for information on the effects of enclosure type and other factors on the risk of escape). However even in the face of wide recognition that it is not a good thing to do, some people persist in moving this ecologically damaging species from place to place for a complex of reasons that range from genuine ignorance to wilful mischief.      

Bullfrogs will colonize a wide variety of lakes, ponds, reservoir, irrigation ditches and marshes, but there are vital characteristics for supporting a population. Permanent water is necessary because bullfrog tadpoles generally take at least 12 months and as much as 48 months to reach metamorphosis; seasonal or intermittent pools, though useful as way stations for migrating bullfrogs, will probably not permit successful reproduction. Water temperature is also important because bullfrogs reproduce only in very warm water when summer temperatures exceed 25 degrees Celsius. For this reason, lakes and ponds that are not too deep < 6 m) and have good sun-exposure around the margins provide most favourable habitat for bullfrog reproduction. Riparian thickets and an abundance of aquatic and emergent vegetation are other factors that, while not vital, are often associated with thriving bullfrog populations because they provide cover and likely a more diverse prey base. However, golf course ponds with edges denuded of most riparian plants can also support healthy bullfrog populations. Acidification of waterways has been associated with bullfrog population declines in southern Ontario (Berrill et al., 1992), but the tolerance limits of bullfrogs to water chemistry extremes and various synthetic chemicals are still being explored.

Genetics

Recent studies have looked at genetic markers to try to identify the geographical origin, number of founders, and number of introductions of alien, invasive bullfrog populations (Ficetola et al, 2008a; Austin et al., 2004). While this line of research has demonstrated success at achieving its scientific goal, it has not yet been applied to bullfrog management and eradication. These studies do highlight the fact that American bullfrogs are physiologically adaptable and behaviourally flexible when moved between environments that are markedly dissimilar, and that they can invade huge areas starting from only a small number of founder individuals.

Bullfrog eggs are commonly attacked by fungi (Ruthig, 2009) and perhaps leeches (Licht 1969), while the tadpoles are preyed upon by aquatic invertebrates such as dragonfly larvae and predatory water beetles. Although fish sometimes predate bullfrog eggs, tadpoles and juveniles (Bury and Whelan, 1984; Casper and Hendricks, 2005), tadpoles avoid predation by being distasteful to many of the more widespread predatory fish, or use chemical-cued avoidance behaviour in their presence (Pearl et al., 2003). Chemical-cued avoidance behaviour is also used by bullfrog tadpoles in the presence of predatory dragonfly nymphs (Peacor, 2006). Larger aquatic-foraging garter snakes (Thamnophis spp.) might be effective predators of bullfrog tadpoles and juveniles, but garter snakes are commonly eaten by adult bullfrogs. Avian predators include herons and egrets (Ardeidae; Casper and Hendricks, 2005); various reptiles and mammals also eat bullfrogs.

Animal feed, fodder, forage

• Bait/attractant

General

• Botanical garden/zoo
• Capital accumulation
• Laboratory use
• Pet/aquarium trade
• Research model
• Sport (hunting, shooting, fishing, racing)

Human food and beverage

• Fresh meat
• Meat/fat/offal/blood/bone (whole, cut, fresh, frozen, canned, cured, processed or smoked)

The average person might find it difficult to distinguish between many species in the widely distributed and speciose family Ranidae (853 described species) because so many of them are green, brown and/or olive-coloured with black irregular patterns. In any case, colour and pattern are not the most reliable of bullfrog characteristics. Size narrows the possibilities where large adult specimens are concerned – a very big specimen is likely to be a bullfrog -- but juvenile bullfrogs can easily be mistaken for other species. The low, sonorous timbre of a male bullfrog’s advertisement call is distinctive compared to that of other frogs. It sounds more in the vocal range of a cow than a frog and amounts to multiple repetitions of “Rrrungh” that vary in volume and intensity depending upon the size of the frog and the temperature of the water in which it is sitting. Juvenile bullfrogs have a distinctive escape behaviour. They produce a characteristic “MEEP” alarm call as they propel themselves into the water when frightened. This action commonly involves skipping over the water surface by rapid multiple kicks of the hind legs before submerging and swimming away.  

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

There is a nearly universal agreement that American bullfrogs are one of the worst alien invasive species internationally. They are ecologically destructive and therefore eradication is acknowledged as the most desirable goal. However, there is also a collective sense of defeatism permeating all recent published discussions concerning bullfrog management (Adams and Pearl, 2007; Kraus, 2009). Adams and Pearl (2007) conclude that the “lack of obvious economic impacts” and a lack of reasonably feasible control methods inevitably translate into a lack of political will to supply the resources for large-scale management. Kraus (2009) lists some apparently successful eradication case studies that have been carried out in Europe, but then concludes that these may be exceptions to the general pattern of eradication failure. However, Kraus also observes that large-scale ecological problems are beyond the capabilities and outside the skill sets of the average volunteer organization or institutional research lab, though these are most often the people who are expected to take the initiative. In fact, eradication programs are more akin to military operations. The strategies, tactics and techniques for an effective eradication are to out-flank, out-pace, and ultimately to overwhelm your foe. Traditionally, biologists are trained to study a problem, collect data, and publish results but eradication is something quite different in the degree of dedication and improvisation required. It also needs sufficient long-term commitments from funding agencies as well as adequate support staff.      

World: Invasive Species Specialist Group (ISSG), Web based, http://www.issg.org/contact.htm

World: IUCN/SSC Global Amphibian Assessment, IUCN/SSC, International, http://www.issg.org

Italy: FAO (Food and Agriculture Organization of the United Nations), Viale delle Terme di Caracalla, 00100 Rome, http://www.fao.org/

Mexico: CONABIO - National Commission for Knowledge and Use of Biodiversity, Liga Periférico - Insurgentes Sur, Núm. 4903, Col. Parques del Pedregal, Delegación Tlalpan, 14010, http://www.conabio.gob.mx

USA: US Geological Survey - USGS, USGS National Center 12201 Sunrise Valley Drive, Reston, VA 20192, http://nas.er.usgs.gov

08/12/2009 Original text (Invasive Species Compendium) by:

Stan Orchard, BullfrogControl.com Inc., 69A Burnside Road West, Victoria, British Columbia, V9A 1B6, Canada

04/05/2005 Original text (Aquaculture Compendium) by:

Marta Stéfani, Departamento de Zootecnia da FCAV/Unesp Jaboticabal, Via de Ac. Prof. Paulo Donato Castellane, s/n Jaboticabal - SP - 14884-900, Brazil