Effects of the exotic macrophyte, para grass (Urochloa mutica), on benthic and epiphytic macroinvertebrates of a tropical floodplain

1. We examined the effect of the exotic macrophyte, para grass (Urochloa mutica), on benthic and epiphytic macroinvertebrates of a tropical floodplain in northern Australia. Macroinvertebrates were sampled from four grass communities: (1) para grass, (2) hymenachne (Hymenachne acutigluma), a native perennial; (3) rice (Oryza meridionalis), a native annual, and (4) areas where para grass had been sprayed with herbicide. 2. Macroinvertebrate richness, abundance and community similarity showed very few differences among the grass communities, particularly in the epiphytic habitat. Benthic invertebrates showed some differences among grasses, with lower richness and abundance and different community structure associated with hymenachne. Herbicide control of para grass had no apparent effect on benthic invertebrates but reduced the abundance of epiphytic invertebrates in the short term. 3. The results of this study indicate that para grass has very little impact on macroinvertebrate communities, despite the changes to macrophyte communities. This is probably because para grass has similar physical structure to the native grasses and because none of these grasses contribute directly to aquatic food webs. Control of para grass using herbicide has little impact on aquatic invertebrates. This suggests that predicting the impact of weed invasion in wetlands requires an understanding of both the functional properties of macrophytes and the habitat preferences of the macroinvertebrates.