Interspecific associations between Hydrilla verticillata and three dominant native genera of submerged macrophytes are taxa dependent

Hydrilla verticillata is a submerged macrophyte that has invaded every continent except Antarctica. In this study, we tested the predictions that (i) H. verticillata invades sites with a higher prevalence of native species; (ii) co-occurrences between the invasive and natives depend on their degree of similarity in morphology and resource use and that (iii) native species morphologically similar to H. verticillata decreases in sites colonized by H. verticillata overtime, while occurrences of morphologically dissimilar species increase post-invasion. The incidences of H. verticillata and three taxa of dominant natives were inspected across 87 sites in a reservoir in South Brazil before and after invasion by H. verticillata. The predictions were tested through co-occurrence metrics and logistic regression analyses. The logistic regression indicated that H. verticillata invaded sites independently of the occurrence of dominant natives, but it co-occurred more than expected by chance with the morphologically dissimilar native Characeae species, Nitella sp. and Chara cf. guairensis. On the other hand, Egeria spp. (morphologically similar to H. verticillata) occurrences were not correlated with the presence of H. verticillata. Moreover, the probability of occurrence of Characeae increased significantly overtime in sites invaded by H. verticillata. These results indicate that H. verticillata invaded sites independently of environmental suitability and likely facilitated more dissimilar taxa, such as Characeae, but there was no evidence that it influenced Egeria spp. occurrences. The patches of H. verticillata probably provide favorable habitats for the establishment of Characeae.