Effects of an Invasive Alien Poaceae on Aquatic Macrophytes and Fish Communities in a Neotropical Reservoir

We investigated the potential effects of an invasive alien Poaceae (Urochloa arrecta) on the abundance, richness, and composition of native macrophytes and fish. Samples were collected in patches of macrophytes dominated by the invasive species and in others dominated by a native macrophyte (Eichhornia azurea). We assessed the potential effects of these two species on macrophyte and fish abundance and richness, as well as in fish assemblage composition. The effects of both macrophytes did not differ when they were in similar, low biomasses. Consistent with our expectations, however, the abundance of native macrophytes and fish decreased with increasing U. arrecta biomass, and both assemblages were eliminated at the extreme high levels of biomass attained by this species. In contrast, E. azurea did not affect the assemblages because this native species never reached levels of biomass as high as those recorded for the invasive species. Competition for space and light most likely determines the elimination of macrophytes in patches that are extensively colonized by U. arrecta, whereas fish assemblages do not occupy these patches due most likely to the limited space available for movement and foraging. Thus, U. arrecta is a source of concern for biodiversity conservation because it has negative influences on both assemblages when it is well developed, which is true in the later stages of the invasion process.