Effects of the density of the invasive macrophyte Hydrilla verticillata and root competition on growth of one native macrophyte in different sediment fertilities.
The occurrence of non-native species at high densities may generate competition for resources and possibly exclude native species in various environments. We evaluated the effects of increased densities of the non-native invasive macrophyte Hydrilla verticillata on the growth of the native species Egeria najas in different sediment types and with only root interactions or root+shoot interactions. We tested the hypothesis that the effect of the invasive on the native species is density dependent and that it is greater when competition for light and nutrients occurs (root+shoot interactions). The results of these experiments demonstrated that increased density of the invasive species H. verticillata significantly decreased the growth of the native species independent of sediment type (sand or mud sediments). When plants competed for water and sediment resources (root+shoot interactions), the native species was more impacted by the invasive than when they competed only for water resources (only shoots interacting). Our results show that E. najas is probably unable to colonize sites highly colonized by hydrilla, and this applies to both sand and mud sediments. This outcome suggests that H. verticillata is a threat for E. najas and likely other native submerged species in South America.