Disentangling the trophic interactions between American bullfrogs and native anurans: Complications resulting from post-metamorphic ontogenetic niche shifts

1. The American bullfrog, Lithobates catesbeianus (Shaw, 1802), is considered one of the world's worst invaders because of its potential to affect native fauna adversely, especially amphibians, through predation, competition, breeding interference, and disease transmission. 2. Here, the potential impact of introduced American bullfrogs (juveniles and adults) on native adult green frogs, Pelophylax kl. esculentus (Linnaeus, 1758), was investi- gated in a pond ecosystem by means of stomach‐content and stable‐isotope anal- yses. Specifically, this study was aimed at assessing the impact of the American bullfrog on native green frogs in terms of predation and competition (i.e. feeding ecology and habitat use), and uncovering changes in interspecific interactions linked to the ontogenetic dietary–habitat shifts of the invaders. 3. Stomach contents and isotope analyses suggest that L. catesbeianus changes tro- phic position in the food chain and undergoes ontogenetic dietary–habitat shifts during its life cycle. The species therefore has the potential to exert a double impact on native green frogs: as a predator at the adult stage and as a competitor (for food and habitat use) at the juvenile stage. 4. Stomach‐contentandisotopeanalysesgavesimilarresults,suggestingthatthepre- dation impact by adult American bullfrogs on green frogs is mediated by the pres- ence of the alien red swamp crayfish, Procambarus clarkii (Girard, 1852). Juvenile American bullfrogs and native amphibians overlap in their diet composition and in their habitat use, suggesting the existence of potential competition between the two species. 5. Ontogenetic dietary–habitat shifts in the American bullfrog, local conditions, and trophic interactions should be seriously considered in management approaches to promote the long‐term coexistence of L. catesbeianus and native amphibians. Such management options have been neglected in the literature, and are worthy of further investigation to understand their efficacy on native species.