Effects of interactions between abiotic and biotic factors on growth of a non-native macrophyte

Invasive macrophytes are troublesome in a myriad of freshwater ecosystems worldwide. Before causing threats to recipient communities, invasive species face abiotic and biotic environmental factors during colonization and establishment that may hamper their success. We experimentally investigated the separate effects of one abiotic factor (sediment organic matter) and two biotic factors (competition and herbivory), along with their interactions, on the growth of the exotic species Hydrilla verticillata in its invasive range. We found that herbivory by the native mollusk Pomacea canaliculata, competition with the native macrophyte Egeria najas and high organic matter in sediment negatively impacted the performance of the exotic species in terms of plant height, biomass production and branching. However, the intensity of each limiting factor depends on the presence of a second factor, as indicated by significant interactions between sediment type x herbivory and competition x herbivory. Our most important finding was that these three factors act synergistically, with a disproportional significant negative impact on the exotic macrophyte. Because the native competitor and herbivore we used occur in lakes with high organic matter in the sediment, our findings help to explain why H. verticillata has still not colonized several aquatic habitats (i.e., floodplain lakes) in South America.