Enough Is Enough? Searching for the Optimal Sample Size to Monitor European Habitats: A Case Study from Coastal Sand Dunes

A robust survey method that samples the main characteristics of plant assemblages is needed to assess the conservation status of European habitat in the Natura 2000 network. A measure of variability, called pseudo-multivariate dissimilarity-based standard error (MultSE), was recently proposed for assessing sample-size adequacy in ecological communities. Here, we used it on coastal sand dune systems in three Special Areas of Conservation (SACs) in Tuscany. Our aim was to assess the minimum number of replicates necessary to adequately characterize sand dune environments in terms of differences between habitats and SACs, after a preliminary baseline assessment of plant diversity. Analysis of α and β diversity indicated that especially between habitats the three SACs protect different plant communities. The study of the MultSE profiles showed that the minimum number of replicates was related to habitat features and varied between 10 and 25 plots. Two-way PERMANOVA and SIMPER analysis on the full and reduced datasets confirmed that SACs and habitats host different plant communities, and that the contribution of the target species remained unchanged even with a reduced sample size. The proposed methodological approach can be used to develop cost-effective monitoring programs and it can be useful for plant ecologists and biodiversity managers for assessing ecosystem health and changes.