Changes in aquatic communities recently invaded by a top predator: evidence of American bullfrogs in Aceguá, Uruguay

Abstract

Effects caused by the invasive American bullfrog (Lithobates catesbeianus) are poorly understood at the community level. This study assessed the effects of a small feral bullfrog population (at lag invasion phase) on the different components of native aquatic communities (phytoplankton, zooplankton, invertebrates, fish and amphibians) of Aceguá, Uruguay. Our interest focused on exploring the early local effects of this invasion. We explored whether there existed any difference in taxa richness, abundances and size structure in association to bullfrog invasion, using four seasonal sampling of all the lentic system around the foci (two invaded and five non-invaded ponds). We analysed the occurrence of differences between the invaded and the non-invaded communities: for the taxa richness we used rarefaction, for the abundances and body sizes we used mean tests, and for the tadpole developmental stages we used G-test. We only found statistically significant bullfrog effects, in fish and anuran larvae. The fish assemblage was favoured, reaching greater abundance and body size in the bullfrog invaded ponds. In these ponds, the nektonic tadpoles diminished their abundances, and the benthic tadpoles also reached greater body sizes, but decreasing their recruitment. Our results suggest that bullfrog invasion could have complex effects, acting asymmetrically, affecting different trophic paths, and depending on native species’ habits and attributes. Understanding these effects, in early invasion foci, has a great relevance to awareness of local environmental authorities and the implementation of management plans.