A native species does not prevent the colonization success of an introduced submerged macrophyte, even at low propagule pressure

Invasions are a threat to biodiversity because non-native species are generally more competitive than native species. Hydrilla verticillata is an invasive macrophyte that causes concern in many regions. We used field surveys and an experiment to test the hypothesis that H. verticillata colonization is negatively affected by a native competitor and positively affected by propagule pressure. We used data obtained in the field (a reservoir) to test whether H. verticillata was able to invade sites colonized by the native submerged macrophyte Helanthium tenellum. Then, we developed an outdoor experiment using H. tenellum as a competitor at different levels of H. verticillata propagule pressure. The field surveys indicated that H. verticillata invaded sites colonized by H. tenellum. Furthermore, our experiments showed that propagule pressure was effective in increasing H. verticillata colonization, while the presence of the native species was not enough to prevent early H. verticillata colonization, even at the lowest propagule pressure. We conclude that submerged macrophytes with creeping life forms, such as H. tenellum, do not provide biotic resistance to H. verticillata colonization independent of propagule pressure. These results are of concern because there are many important aquatic environments in Brazil that are colonized by creeping life forms macrophytes.