A complex interplay between natural and anthropogenic factors shapes plant diversity patterns in Mediterranean coastal dunes

A long history of human colonisation has profoundly altered Mediterranean coastal dunes, as well as their capacity of providing ecosystem services important for human well-being. The provisioning of these services depends on the integrity of the dune system, which is formed and maintained by coastal plant communities. Analysing the drivers of plant diversity loss is thus crucial for preserving Mediterranean coastal ecosystems. Using 20 cm resolution orthophotos, we mapped a wide Mediterranean coastal landscape and obtained a set of variables describing the distribution, abundance and size of natural (coastal dune habitats) and anthropogenic (urban areas and tourism facilities) patches. From the orthophotos, we also quantified the shoreline dynamism (coastal erosion and accretion) occurred in the area over a 10-year period. We then analysed how plant species richness, as well as the proportion of typical and ruderal plant species, related to the landscape variables and shoreline dynamism. Also, using piecewise structural equation modelling, we investigated the complex interplay between landscape variables and shoreline dynamism in shaping coastal plant diversity patterns. When focusing on plant species richness, we found no evidence of a negative effect of anthropogenic activities (urbanisation and tourism) on the diversity of coastal vegetation. However, analysing typical and ruderal plant species revealed that the latter are favoured under human-related disturbance, while typical species of the foredune decrease in areas subject to high anthropogenic pressure. This indicates that (i) looking only at plant species richness can lead to underestimating the impact of anthropogenic activities on coastal dune vegetation; and (ii) that human-related activities change the composition of dune vegetation, eventually promoting the establishment of ruderal species, which cannot support the functioning of coastal ecosystems and the provisioning of the related ecosystem services. Finally, results of the structural equation models highlighted that coastal erosion is an indirect driver of plant diversity loss, through its influence on the coastal landscape configuration.