The role of an invader in its native range: could differential grazing by apple snails structure the submersed macrophytes assemblages in Southern Pampas (Argentina)?

Apple snails are renowned for their strong impacts on aquatic vegetation and ecosystem function in invaded regions but the existing knowledge on their role in aquatic ecosystems and communities in their native range is scarce. The variation in palatability of five submersed macrophytes, the preference of Pomacea canaliculata for them and the potential effects of its differential grazing on macrophyte assemblages in Southern Pampas were investigated. In no-choice trials, macrophytes showed differences in palatability that were correlated to physical defences and not to nutritional values. In free-choice trials, preference was consistent with the palatability ranking. In a mesocosms experiment, Chara contraria and Zannichellia palustris were totally eradicated by apple snails, the coverage of Stuckenia striata increased only when they were absent and Ludwigia peploides and Myriophyllum quitense were not affected. The live biomass of Z. palustris, S. striata and M. quitense was reduced by apple snails. Total live and dead biomasses were lower, and chlorophyll-a concentrations higher, in the mesocosms with apple snails. Our results indicate that P. canaliculata has a keystone role in its native range and that its differential grazing could shape the structure and composition of submersed vegetation and promote shifts to turbid-water states or maintain them.