Invasive plant removal: assessing community impact and recovery from invasion.
Invasive species spread on natural ecosystems is one of the most important causes of biodiversity loss. To disentangle the invasive plant impact on communities it is essential to combine experimental and observation approaches, which enable the correct interpretations of results and lead to the right decisions for management. We examined the invasion of southern Brazil's grasslands by Eragrostis plana, which is currently the most problematic invasive species in the region. By means of an experiment on invaded communities complemented by observation of non-invaded communities, we assessed E. plana impact on vegetation, evaluated community response to its removal and discussed the effectiveness of removal methods. Fifty permanent 1×1 m plots were located on natural grassland that was partially invaded by E. plana. Removal was done annually from 2012 to 2015 and consisted of five treatments (n=10): (i) clipping above-ground biomass on one occasion; (ii) clipping above-ground biomass periodically; (iii) herbicide and (iv) hand-pulling, plus (v) control treatment with no-removal. In addition, 10 plots located in an adjacent non-invaded area were monitored. All removal treatments reduced E. plana cover across years, but were not enough to eradicate it. Our results revealed not only differences between invaded and non-invaded communities, but also an effect of E. plana removal on resident species richness and total cover. At the local scale, we demonstrated the impact of E. plana invasion on grassland vegetation, suggesting a reduction in resident species richness and total cover. Invasive species removal changed communities differently from invaded ones, but not resembling non-invaded references, suggesting that community recovery may need more time for reestablishment or that some restoration strategies are required. Synthesis and applications. This study demonstrated the impact on vegetation of the most important invasive species in southern Brazil's natural grasslands. We highlighted the advantages of combining observations of non-invaded communities and experimental studies on invaded communities, with and without invasive removal, to help infer causal relationships in ecological invasion research.