Antagonistic effects of a native apple snail on other snails and macroinvertebrates in Southern Pampas waterbodies: a mesocosm approach

Abstract

Macroinvertebrates represent an important component of communities and trophic webs of freshwater ecosystems. Pomacea canaliculata (family Ampullariidae), an invasive apple snail native to the Pampas ecoregion, acts as a voracious grazer and plays a structuring role on submerged macrophytes that serve as food, habitat or foraging ground for many macroinvertebrates. Laboratory studies show that P. canaliculata can also act as a competitor or predator of other macroinvertebrates. Using a mesocosm approach, we sought to investigate if this species of apple snail affects other freshwater snails (Chilina parchappii, Heleobia parchappii, Biomphalaria peregrina and Melanoides tuberculata) as well as other macroinvertebrates and if consumption of submerged macrophytes and detritus mediates these effects. After twelve weeks we estimated in mesocosms with and without apple snails the abundance of snails and macroinvertebrates through samples taken separately from sediment and from macrophytes. We also estimated the coverage and biomass of macrophytes and the detritus biomass. Significantly lower abundance occurred in mesocosms with apple snails for all snail species and representative macroinvertebrates (i.e., Hirudinea, Ephemeroptera, Chironomidae and Odonata). Mesocosms with apple snails also resulted in lower total abundance of macroinvertebrates. We speculate that the negative effects of P. canaliculata on most other snails and macroinvertebrates living on macrophytes relates to their grazing on detritus and macrophytes. In contrast, the density of snails and macroinvertebrates in the samples from sediments were not affected by P. canaliculata. One snail species and Ephemeroptera were negatively affected by mechanisms not related to the reduction in macrophyte biomass and detritus. A decrease in the microhabitats provided by macrophytes probably mediated the negative effects on Odonata nymphs. The reduction in the abundances of snails and macroinvertebrates caused by P. canaliculata may have cascading effects on higher trophic levels in waterbodies from Southern Pampas but also in invaded regions.