The American bullfrog Rana catesbeiana is a very large frog weighing up to 0.5 kg and reaching a length of 20 cm. Native to the eastern parts of North America, it has been widely introduced to other parts of the world as a food crop, either for aquaculture or for harvesting from the wild. In some areas it was introduced as a biological control agent of agricultural pests, or as a pet for home aquaria and garden ponds.
Once introduced, the American bullfrog invariably succeeds in establishing populations in the wild. If it escapes from captivity, it is able to travel long distances overland to disperse into neighbouring waterbodies. It can flourish in areas disturbed by humans, as it tolerates the warmer temperatures and prolific aquatic vegetation typical of polluted waters. Furthermore, the eggs and tadpoles are unpalatable to fish, while the adults are generally inactive, making them inconspicuous to predators.
Once established, the bullfrog may have a detrimental impact on local biodiversity. It not only competes with indigenous frogs and toads, but is a voracious predator. It eats anything it can swallow – including other amphibians, snakes, turtles and even small birds and mammals – and is believed to be responsible for the decline of many species. The tadpoles are primarily grazers of benthic algae, and can significantly alter aquatic community structure. Also of concern is the potential for the bullfrog aquaculture trade to facilitate the spread of diseases that may threaten wild populations of amphibians. For example, in 1999 a bullfrog farm in Uruguay experienced a mass dieoff due to an outbreak of chytridiomycosis. This disease is caused by the fungus Batrachochytrium dendrobatidis, which has been implicated in the decimation of amphibian populations in a number of areas worldwide.
The bullfrog tends to be immobilised by bright light at night, so is usually hunted with a strong torch and then either caught or killed. Approved waterborne piscicides can be used to poison the tadpoles. The floating egg masses can also be collected for disposal, but since they sink to the bottom a day or two after laying, this method is usually impractical.
Bullfrogs in South America
- During the last few decades, a number of South American countries have begun rearing the American bullfrog to supply the lucrative trade in frogs’ legs for the restaurants of the United States and Europe. Brazil, Uruguay and Argentina are currently the largest South American producers.
- The bullfrog was introduced to Colombia in 1986 for research aimed at assessing its potential for aquaculture. It was bred in captivity at Calda and Buga, but in 1990 – when the risk to native species was fully realised – the government institute Inderena issued a resolution banning its cultivation in Colombia and ordering the destruction of the experimental collections. By then it was too late, however, as the bullfrog had already become established in the wild. Today the densest populations are found in the Sonso Lagoon in the Cauca Valley and areas around Buga. The region’s numerous irrigation dams and channels provide abundant suitable habitat, while the Cauca River has played a major role in the bullfrog’s dispersion, by carrying eggs laid on floating masses of aquatic vegetation downstream. The bullfrog not only competes with the native cane toad Bufo marinus for food, but also preys on its larvae, as well as those of other amphibians.
- In Venezuela, the bullfrog became established in 1998 following illegal releases into Andean waterbodies. Its area of distribution in Merida state is near the habitat of the critically endangered Venezuelan yellow frog Atelopus carbonerensis. In 2003, in an effort to control the bullfrog’s rampant spread, the Ministry of Environment introduced a bounty system to encourage licensed hunters to target the invader. It offered 1 000 bolivars – equivalent to about US 50 cents – per dead female frog, 500 bolivars for a male, and about 15 cents per kilogram of tadpoles. By the end of that year, more than US$1 640 had been paid for some 4 700 bullfrogs.
Reference: Matthews S. & Brandt K. South America Invaded: the growing danger of invasive alien species. Global Invasive Species Programme 2006