Biological invasions and native plant diversity on Mediterranean coastal dunes.Diversity patterns of focal species can provide new information.

In Europe, coastal sandy habitats are considered highly endangered among those included in the EC Directive 92/43/EEC. Among the different threats which affect coastal communities, the spread of alien plants has been claimed to induce changes in community diversity and structure. We therefore set out to analyze diversity patterns of native and focal species (diagnostic and characteristic of coastal dune Habitats of European conservation interest) in sandy coastal habitats invaded by Carpobrotus aff. acinaciformis, a widespread alien plant. Focal species are a major conservation target for the EC Directive (92/43/EEC) and their decline should be considered a serious threat for the whole habitat. The study was performed in the central western coast of Italy. We randomly sampled the vegetation of the holocenic dune by 2x2m plots. First we split the collected data in two sets: invaded and non-invaded. We compared overall native and focal species richness patterns of the two sets by rarefaction curves. Then, in order to describe the singular aspects of species diversity (e.g. richness, Shannon index, Simpson index, Berger-Parker index), we also compared Rènyi’s diversity profiles and we tested the significance of the differences among them using a bootstrap procedure. Rarefaction curves of the non-invaded set rise quickly and reach higher accumulation values than the invaded set, but those differences were not significant. With respect to Rènyi’s profiles, the invaded dataset was always below the non-invaded one, but we registered significant differences among them only for some aspects of focal species diversity (Shannon, Simpson and Berger-Parker indices). In the analyzed case, the invasion is significantly associated with focal species diversity, instead those differences are not evident on overall native species pool. In the specific case, where the invasion is relatively recent, a consistent decline on focal species diversity could be used as an early alarm sign of diversity loss and may help to define specific conservation actions to prevent the decrease of overall biodiversity.