Biological invasions are considered as one of the main causes of the biodiversity erosion of at the global scale, even if on a local scale the impacts are sometimes difficult to quantify.

Locally the phenomena of invasion are often very fast and difficult to control when the populations are strongly implanted. The fight is then difficult and requires the deployment of important means. This implies a strong mobilization of the local stakeholders.

The Ria of Étel in Brittany is particularly concerned by the presence of Baccharis halimifolia which occupies more of 20ha of salt marshes. Considering this, a program of fight based on the organization of participative works was set up. This program of fight comes along with a program of follow-ups and with evaluation including several aspects: quantification and mapping of the population, evaluation of the impact of the invasion on several ecosystem components (flora, fauna, soils...) and evaluation of the management efficiency. The presentation will concern the two first points.

Population mapping made in 2017 allowed to underline the evolution of the invasion since 2009 date of the previous evaluation, with an increase of 4.1 ha. New populations were observed, which should be primarily eradicated.

On the base of this map, analyses are made to evaluate the impact of the presence of Baccharis halimifolia at local scale on vegetation and arthropods communities and soil characteristics. Links will be made between them and the population characteristics (density, age, ...).

The results will allow to precise the strategy of control of the invasion and also to evaluate the efficiency of this strategy. This last point will be implemented in 2019.