Life History, Abundance, Activity, and Special Behaviors
Introduced populations present great threats to native frogs, due to the bullfrog's voracious feeding habits and the size and competitive ability of the larvae. Although aquatic species and frogs constitute a major portion of its diet, other native species are also likely affected because bullfrogs have been reported to eat snake, birds, and small mammals as well. Furthermore, as bullfrogs are being introduced worldwide, they serve as carriers of the pathogenic fungus Batrachochytrium dendrobatidis (chytrid), which causes the lethal disease chytridiomycosis, believed to be a major factor in recent global amphibian declines (e.g., Garner et al. 2006 found that bullfrogs were consistently chytrid-infected in multiple countries). Infected bullfrogs appear to be rather resistant to chytridiomycosis, whereas the disease is lethal to many other amphibians, making the bullfrog an efficient carrier of the chytrid fungus (Daszak et al. 2004).Argentina: Introduced populations of R. catesbeiana have been recently reported in San Juan (Sanabria et al. 2005), Misiones (Pereyra et al. 2006), Buenos Aires (Barrasso et al. 2009), and Córdoba and Salta provinces (Akmentins and Cardozo 2009). Most introductions come from intentional or incidental releases from breeding facilities, except for Misiones where the bullfrogs there are believed to be the result of the range expansion of a Brazilian population (Pereyra et al. 2006). The bullfrog's adaptive ability has allowed it to invade a diversity of environments and disperse throughout Argentina. It has been observed to prey on native vertebrates. Although it has not yet been identified as a chytrid carrier in Argentina, its negative influence as a potential disease carrier remains to be further examined. One Argentinian species greatly affected by the presence of R. catesbeiana is Leptodactylus ocellatus, a generalist predator who shares a similar diet with R. catesbeiana and whose larvae are being preyed on by bullfrogs (Barrasso et al. 2009). The increase in number of captive-breeding facilities due to a large demand for human consumption and the lack of effective governmental control thereof are serious concerns in Argentina (Akmentins and Cardozo 2009). Belgium: Bullfrog larvae have been imported on a large scale for trade in pet shops. Many specimens were released into the wild and were able to survive to reach adulthood (Stumpel 1992). Free-ranging populations of R. catesbeiana have been observed in Belgium (Ficetola et al. 2006). Conservationists are concerned about the potential threat of R. catesbeiana to indigenous species, in particular Rana esculenta, which occupies the same niche (Stumpel 1992). Brazil: Introductions date as early as the 1930's in association with aquaculture. Except for the coldest months of the year, R. catesbeiana reproduces continuously during warm weather such as that of Brazil (Kaefer et al. 2007). This characteristic is shared with the cane toad (Bufo marinus, another invasive species in Brazil. The degree of native population loss brought on by the introduction of bullfrogs is still being speculated, but the reproductive ability of R. catesbeiana is worrisome. Giovanelli et al. (2008) propose that the Brazilian Atlantic Forest biodiversity hotspot in southern and southeastern Brazil may be the most susceptible to invasion, based on ecological niche modeling. Furthermore, a chytridiomycosis outbreak in an Uruguayan farm with stock that originated from Brazil suggests that Brazilian rearing facilities may contain specimens that are infected by B. dendrobatidis, and could thus be harmful to native anuran species if allowed to escape (Mazzoni et al. 2003). These authors urge the Brazilian government to regulate human introductions of R. catesbeiana more strictly. China: R. catesbeiana was first introduced to China in 1959. Since then, breeding populations have been established in most provinces of mainland China including Yunnan, Sichuan, Shanxi, and Zhejian Provinces (Wang et al. 2007). The bullfrog has been widely bred for both local consumption and for export since the 1980s (Wu et al. 2005). Escapes occur from rearing pens or in the process of transportation or trade (Wu et al. 2005; Li et al. 2006; Liu and Li 2009). R. catesbeiana poses a great threat to native anuran species due to its voracious feeding habits. Its body size is also much larger than any other native species, and it is known to consume at least 4 of the 10 native species in the Zhoushan archipelago of Zhejian Province. Wang et al. (2007) quantified the predatory impacts of R. catesbeiana and found that body size plays an important role in predator-prey interactions with native anurans of China. It has been suggested that the primary threat posed by juvenile bullfrogs is food competition, whereas the primary threat posed by adult bullfrogs is predation (Wu et al. 2005). Chytridiomycosis has also been reported from introduced bullfrogs (wild, farmed, and market-sold) in Yunnan province, as well as in native amphibians, suggesting that farmed and escaped bullfrogs may present a major threat to native species by carrying disease as well (Bai et al. 2010). Colombia: R. catesbeiana was introduced into Colombia in the 1990s and inhabits the inter-Andean valleys (Lynch 2006a). The bullfrog was originally introduced in hopes of exploiting it for food consumption, but it is now a biological plague in the valley of the Río Cauca and in certain localities on the western slopes of the Cordillera Oriental in Cundinamarca, as well as the lowlands of the Caribbean Region (Lynch 2006b). Its diet in Colombia has been reported to consist mostly of insects (56%), whereas vertebrates constitute only 2% of the diet (Daza and Castro 1999). Many endogenous Colombian frog species have been impacted by the lethal fungal disease chytridiomycosis; R. catesbeiana is a potential vector in Colombia of the fungal pathogen Batrachochytrium dendrobatidis (Ruiz and Rueda-Almonacid 2008). Cuba: In the 1920s, both adult and larval bullfrogs were observed in two small ponds near Rincon, a small village approximately 50 kilometers from Havana. It was not deemed a threat to the ecological system in Cuba at that time because it was thought that the bullfrog, which had a larval period of more than one year, would not be able to breed in the many temporary pools in Cuba (Hoffman and Nobel 1927). Now, bullfrogs are present throughout Cuba except the Sierra del Cristal National Park (Fa et al. 2002). France: Acclimatized R. catesbeiana populations have been recorded in France beginning in the 1960s (Ficetola et al. 2006). It was observed to be occurring near Bordeaux in 1997, in a limited area of gravel pits (Neveu et al. 1997). Currently, southwest France is the European area where the strongest expansion of R. catesbeiana is taking place; it also represents the second largest area in Europe where R. catesbeiana is present, constituting about 2,000 square km. Only three breeding populations have been reported, but observations of isolated individuals suggest that translocations are frequent. These secondary translocations facilitated by humans can substantially increase the rate of population expansion, which may further enhance capture and translocation. A large-scale eradication plan is being carried out in southwest France, including trapping of both adults and tadpoles, and education of local people (Ficetola et al. 2006). Unfortunately, samples of introduced populations of bullfrogs in Loir et Cher are found to be infected with B. dendrobatidis (Garner et al. 2006). Italy: R. catesbeiana was introduced to Italy in the 1930s, making it the first European country where this species was successfully introduced (Ficetola 2006). It is well established in northern Italy and breeds abundantly. Provinces affected at least since 1960 include Mantova, Pavia, and Verona (Lanza 1962). In northern Italy, the R. catesbeiana population does not appear to have expanded since the 1980's. Some populations are known to be infected by B. dendrobatidis (Ficetola 2006). Jamaica: Twenty-two R. catesbeiana specimens were first introduced to the Great Morass of the Black River of Jamaica in 1967 for commercial purposes. During the next 4 years it has spread in all directions from the point of introduction and eventually established themselves in the Upper Morass. Although no quantitative population estimates have been conducted, R. catesbeiana appears to have established extensive populations at suitable habitat areas of the island. Its expansion in Jamaica is further facilitated by temperature and lack of competition; the frogs breed continually throughout the year and displace local anurans such as Bufo marinus through habitat competition (Mahon and Aiken 1977). Japan: R. catesbeiana was first introduced to Japan by a professor at the Imperial University of Tokyo (now Tokyo University) around 1918 (Okada 1927). The frog was already well integrated into the Japanese herpetofauna by 1958-1959, approximately 40 years after its introduction (Telford 1960). R. catesbeiana is firmly established in at least the Kanto and Kansai Plains, the two largest lowland regions of Japan, and many local people recognize it as the "food frog" (Telford 1960). R. catesbeiana resides in freshwater habitats such as rice fields, ponds, and rivers. Studies reveal that it may negatively affect native anuran species such as the endangered Rana porosa brevipoda through predation and food competition. The removal of bullfrogs, along with other invasive exotic species, is highly recommended for conservation of local vertebrates (Hirai 2004). Studies reveal R. catesbeiana populations expanding in paddy fields prefer microhabitat with deep water; managing habitats to reduce immigration of bullfrogs may help prevent the spread of this invasive species (Minowa et al. 2008). A recent die-off of R. catesbeiana from ranavirus lasted from September through October 2008 in a 1000-m2 pond in western Japan. Infected feral populations of R. catesbeiana presents a serious threat to Japanese amphibians (Une et al. 2009). Fortunately, B. dendrobatidis does not seem to have infected introduced populations of R. catesbeiana, and no die-off from chytridiomycosis has been reported (Garner et al. 2006). Netherlands: bullfrogs were imported for trade in pet shops in the 1980s. Many specimens are released into the wild as larvae or freshly metamorphosed juveniles. In 1991, a reproducing population since 1989 was recorded in a garden pond in the city of Breda (Stumpel 1992). Eradication programs have been carried out in the Netherlands (Scalera 2007). Puerto Rico: in 1935, the Insular Department of Agriculture and Commerce of Puerto Rico introduced a total of 40 R. catesbeiana specimens from Florida to a specially constructed pond at Rio Piedras. The population expanded successfully, and by 1951 had invaded the neighboring towns of Mayaguez and Humacao. It did not appear to prey on other amphibians, but did consume a diversity of local insects (Perez 1951). Taiwan: the species was introduced into Taiwan from the United States via Japan in 1924 and 1951 (Hsu et al. 1970). The country actively participates in the production of bullfrog meat, and in fact is the world's top exporter of ranids. However, its native amphibians are at risk of infection by B. dendrobatidis. Taiwan's subtropical climate also creates a suitable environment for the growth of this pathogen (Schloegel et al. 2009). UR. catesbeiana was first introduced in 1987 for farming purposes, but at present most of the farms have closed down. A feral population was reported in 2008 at one of the closed farms at Rincón de Pando. Establishment of the population appears to be at an early stage but is potentially dangerous, as the invaded pond exhibits significant differences from non-invaded ones. For example, R. catesbeiana seems to have some type of positive interaction with fish, because the body size of common fish species are higher in the invaded pond where aquatic vertebrate richness is also highest. Furthermore, R. catesbeiana is the highest fraction of vertebrate biomass, displacing native amphibian species such as Hypsiboas pulchellus. Other negative effects on local amphibian fitness include reducing the premetamorphic period due to competition and predation pressures. The high level of anthropogenic disturbance and large amount of suitable habitat in Uruguay may facilitate R. catesbeiana expansion (Laufer et al. 2008). Recent mass deaths occurred at a large farming facility for bullfrogs 46 km from Montevideo, Uruguay, and the cause is suspected to be chytridiomycosis (Mazzoni et al. 2003). This is potentially dangerous to native anurans if R. catesbeiana were to serve as a carrier of B. dendrobatidis. However, no control program was implemented for these closed farms. Laufer et al. (2008) recommend taking measures against the population expansion as well as searching for new invasion points. Venezuela: the bullfrog was introduced as a food source around the 1990s, and has established dense populations in 14 natural and artificial ponds as far as 4.3 km away from the point of introduction. Examination of R. catesbeiana specimens in the Venezuelan Andes suggests that the bullfrog acts as a pathogen carrier that causes amphibian population declines in Venezuela (Hanselmann et al. 2004). 79.9% of the bullfrogs surveyed are infected with B. dendrobatidis and carrying an average of 2299 zoospores (Sánchez et al. 2008). Of the infected frogs, 96% appear otherwise healthy, making R. catesbeiana an efficient reservoir host. It is likely that year-round bullfrog breeding will heighten the impact of chytridiomycosis (Hanselmann et al. 2004). Chytridiomycosis is detected in native species occuring in pond, stream and terrestrial habitats from 80-2600 m. Dendropsophus meridensis, an endangered native species sympatric with R. catesbeiana, shows the highest infection prevalence and mean zoospore load (26.7% and 2749 zoospores). Although no clinical signs of disease were detected, environmental stress could potentially increase its vulnerability to the pathogen (Sánchez et al. 2008). B. dendrobatidis is also known to migrate through autoclaved late water to reach distant amphibian populations outside the range of R. catesbeiana dispersal (Hanselmann et al. 2004). R. catesbeiana has been introduced worldwide, and AmphibiaWeb is in the process of editing the Trends and Threats section of this page for each country.