Field data and worldwide literature review reveal that alien crayfish mitigate the predation impact of the American bullfrog on native amphibians

Biological invasion studies have focused mostly on the competition and predation impact of invaders on native species; however, introduced species frequently interact with each other and, contrary to the 'invasional meltdown theory', such interactions may have non-interactive effects on native fauna. Here, the effects of the interaction between American bullfrogs (Lithobates catesbeianus) and decapod crayfish on native amphibians have been investigated. Introduced American bullfrog populations were studied at two sites in central Italy (Rome, Latium), with their selection based on the presence or absence of the alien Procambarus clarkii. The pattern emerging from the fieldwork was then verified by a worldwide literature review on the trophic niche of the American bullfrog. Both field surveys and global diet analysis showed that the abundance of native amphibians in the American bullfrog diet was reduced when introduced crayfish occurred in both native and non-native ranges. Thus, an alternative invasion model can be proposed in which the occurrence of one invader (P. clarkii) may reduce the negative effect of an alien predator (L. catesbeianus) on native species. The present findings suggest that the removal of invaders may not always be the best solution for protecting native prey, especially when alien species are interspersed with other introduced species. In such cases, multispecies operations, sequenced appropriately or undertaken simultaneously, would be preferable. Moreover, the identification of mechanisms facilitating the coexistence of native species with invaders can offer important management options.