Invariant and vulnerable food web components after bullfrog invasion

Abstract

Alien species introductions produce strong impacts on invaded communities, altering their structure, diversity and functioning. These impacts are interrelated with changes in food web architecture. However, the reorganization or robustness of food webs in the face of invasion is a phenomenon poorly considered in ecology and conservation practices. In this article, we analyze the effects of the invasion of the American bullfrog, Lithobates catesbeianus, on the structure and function of invaded food webs. Specifically, we analyzed the integration of energetic channels by top predators, the relative use of alternative energetic paths by different functional groups and its dependence on body size among invaded and uninvaded communities in Uruguay, South America. The integration of energetic paths at high trophic positions by large body sized consumers was a pervasive feature of food webs among all studied ponds, in spite of turnover in top predator identity and large changes in community composition. Bullfrog post-metamorphs presented high trophic positions, integrating the primary producers and detritus paths, acting as apex predators in invaded food webs. The bullfrog tadpoles presented intermediate positions and were associated with the detritivorous pathway. On the other hand, the relative importance of the primary producers and detritus as carbon sources assimilated into the biomass of fish and invertebrates was altered in invaded systems. The robustness in the integration pattern of energy channels is congruent with its proposed central role in the stability of food webs. These results advance the understanding of the effect of invasions on key structural features of food webs, notably underrepresented in the invasion literature.