Adult of Lithobates catesbeianus from province of Rome. © L. De Luca

By Vincenzo Ferri and Raoul Manenti, Commissione Conservazione Societas Herpetologica Italica

Exotic amphibians represent indeed a serious threat to biodiversity. In general they act as predators to the original fauna, or are competitors for resources, making other species disappear or, at least, severely decrease. More recently, it also became evident that several species may act as carriers of pathogens, like chytride fungus. Eradication projects are often invoked but rarely are put in practice. In Italy, there are at least three allochtonous species, which are the American bullfrog (Lithobates catesbeianus), the Balkan green frog (Pelophylax kurtmuelleri) and the African clawed frog (Xenopus laevis), although other species belong to Pelophylax are likely to occur. The American bullfrogs has a long history of presence in Italy, having been introduced in the first decades of last Century. Notwithstanding, the effects of its introduction are still largely unknown. For this, I guess it is interesting to host the text of some friends and colleagues about new findings of the bullfrogs. – Franco Andreone

The introduction of the bullfrog Lithobates catesbeianus is reported to occurred in Italy at least since 1932 form some drains near the village of Bigarello (Mantua). The species spread successively in several districts along the Po Plain mainly because of the fish trade that involved the accidental transfer of bull frog tadpoles from fish farms of the Mantua district to several basin used for game fishing. Successively a relevant role for bull frog spreading has been played by some tries to rear this alien species for commercial purposes that were carried out at the end of the eighties. Most of these attempts, that involved also other frog alien species, such as Pelophylax kurtmuelleri e P. ridibundus ended with a  failure or were implicated in commercial frauds. Consequently the survived bullfrogs were indiscriminately released in the wild.

Newly metamorphosed Lithobates catesbeianus from Lombardy. Photo:  V. Ferri

Newly metamorphosed Lithobates catesbeianus from Lombardy. © V. Ferri

Herpetologists deserved particular consideration to the bull frog since the beginning of its spreading in Europe, as it poses several problems from both the ecological and conservationist point of view in the damp biotopes that it colonizes. Different studies revealed that this from can significantly affect the structure of autochthonous amphibian communities (Veenvliet & Kus-Veenvliet, 2002). Moreover the bull frog may be a dangerous vector for the Batrachochytrium dendrobatidis, desease as it is relatively resistant to this patogenous (Lanza et al., 2007).

Tadpole of Lithobates catesbeianus from Lombardy. Photo: V. Ferri.

Tadpole of Lithobates catesbeianus from Lombardy. © V. Ferri

The European Commission (Veenvliet & Kus-Veenvliet, 2002) has repeatedly investigated the distribution of this alien species in all the historic localities of occurrence storiche (Albertini & Lanza, 1988). These investigations highlighted a new site were the bull frog occurs, but found that in all the other known localities the species was disappeared or the  populations density was quite decreased, as for example in Piedmont region (Lanza et al., 2007).

Habitats of Lithobates catesbeianus in Lombardy/ Photo: V. Ferri.

Habitats of Lithobates catesbeianus in Lombardy. © V. Ferri.

However the data collected during new researches performed very recently (by V. Ferri, A. Imperiale, L. De Luca, & M. Pandolfi,  pers. obs., 2014) underline that the bull frog is still well distributed in the already known localities of the Po river plain between the regions of Lombardy and Emilia Romagna and moreover, its spreading is still ongoing. The occurrence of the species has been recorded both around the historic localities, likely because of the individual spreading ability, and both in new region as in the case of Latium where we collected different new observations from natural and semi-natural water basins situated North and Northeast of Rome in the Tiber basin.

Habitats of Lithobates catesbeianus in Lombardy. Photo © V. Ferri.

Habitats of Lithobates catesbeianus in Lombardy. © V. Ferri

While the bull frog occurrence is linked to a strong decrease of autochthonous amphibian populations (including Pelophylax kl. esculentus, Hyla intermedia e Bufotes balearicus in Lombardy, P. kl. hispanicus e Hyla intermedia in Emilia Romagna) the results from the first samplings carried out in Latium seem quite interesting as the situation is different.

Pelophylax kurtmuelleri from Ligury. © V. Ferri

Pelophylax kurtmuelleri from Ligury. © V. Ferri

The autochthonous populations of amphibians, especially P. kl. hispanicus, H. intermedia, Bufo bufo, Lissotriton vulgaris, for the moment are apparently not affected by the massive occurrence and breeding success of the bull frog. However, further studies in these new discovered localities are needed both to survey autochthonous populations dynamics in the long period and both to date the introduction of the bullfrog and to understand how spread is currently this alien species. Moreover regional action plans should be carried out to evaluate eradication possibilities, taking into account the experiences of other European countries.

 References

ALBERTINI G. & LANZA B., 1988 – Rana catesbeiana (Shaw, 1802) in Italy. Alytes 6 (3/4): 117-129.

LANZA B., ANDREONE F., BOLOGNA M.A., CORTI C. & RAZZETTI E., 2007 – Fauna d’Italia, 42: Amphibia. Calderini Editore, Bologna.

Veenvliet, P. &  J. Kus – Veenvliet. 2002 – Review of the status of Rana catesbeiana in the European Union. In: Adrados L.C., L. Briggs (eds.) Study of application of EU wildlife trade regulations in relation to species which form an ecological threat to EU fauna and flora, with case studies of American bullfrog (Rana catesbeiana) and red-eared slider (Trachemys scripta elegans). Unpublished report to the European Commission, AmphiConsult, Denmark.