This study aims to analyse how the mosaic of habitats and the management measures of a marshy area in western France can influence the conservation value of a territory, based on the use of bird and arthropod assemblage indices. A multivariate analysis revealed that (1) marsh habitat heterogeneity that structures bird communities and (2) different management measures to control water levels are the most relevant environment indicators explaining the distribution of the meadow bird species. The occurrence of carabid (ground beetle) species and the distribution curve of the species abundance within the habitats proved the importance of flooding duration and grazing intensity on their species richness. Both the proportion of rare species and the intensity of rarity in spider species assemblages were used to assess the conservation value of the different habitats of the marsh. This revealed the importance of reed beds. Consequently, the conservation of a high-quality hedge network and the use of pastoral management techniques favouring reed-beds, are suitable both for aquatic passerines and arthropod communities, are finally advised to increase community richness.