Habitat-specific and pressure-driven differences in community composition and conservation status in Mediterranean coastal habitats

Mediterranean dunes and salt marshes are home to a wide range of organisms and unique and fragile plant species assemblages. To correctly assess dunes and salt marshes conservation status a thorough knowledge of community properties and how these respond to anthropic pressure is needed. Here we analyse data from 647 vegetation plots located along the coast of Tuscany. To first assess the variability among habitats and the most informative metrics and functional traits we calculated taxonomic, functional, and phylogenetic diversity metrics. We then assessed plant communities’ conservation status in terms of both taxonomic and functional Community Completeness Index (CCI) and the local contribution to phylogenetic beta diversity (phyLCBD), and obtained a total anthropogenic pressure index based on pressure type and intensity. We then compared habitats’ properties and assessed whether selected metrics varied in relation to pressures. We found multiple significant differences among habitats regarding all considered diversity and conservation metrics and shifts in multiple habitat metrics in relation to pressure. The most sensitive metrics were phylogenetic diversity and CCI, however all metrics, except few cases, increased in relation to increasing pressure. The functional traits community-weighted means (CWMs) that were influenced the most by pressure across habitats were specific leaf area, leaf density and leaf thickness, the latter consistently decreasing in relation to pressure. Observed changes are likely related to habitat homogenization and colonization by generalist species. The results of this study provide valuable information for coastal habitats mapping and conservation, pivotal in ensuring the provision of ecosystem services.