Impacts of invasive plants on plant communities are not universal, and effects may differ depending on the life form of the invading species. At three sites in the Brazilian Pampa region, we assessed the effects of invasion by Eragrostis plana Nees, considered to be the most problematic invasive plant in South Brazilian grasslands, on local plant communities. Specifically, we evaluated effects on species richness, community evenness and richness of principal functional groups along an invasion gradient. Based on plant community data, we also evaluated the effects of dominant native species on the same plant community features, thus being able to assess the relative impacts of an invasive species compared to abundant native species. Eragrostis plana invasion reduced total plant richness, with some distinct effects regarding plant functional groups, and differently from other coexisting native species that did not show the same effects even when of high abundance. Grasses were the most affected plant group when E. plana cover increased, consistent with the limiting similarity hypothesis. Overall, our results indicate clear negative effects of this invasive plant on the grassland communities. Comparing the effects of an invasive species to those of coexisting natives is crucial for disentangling the real impact of an invader. Our results demonstrate that effects are consequence of species identity, and not only of high abundances.

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