Focal species diversity patterns can provide diagnostic information on plant invasions

In Europe, coastal sandy habitats are considered highly endangered among those included in the EC Directive 92/43/EEC (Habitats Directive). 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 analyse 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 Habitats Directive 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 2 m × 2 m 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 between invaded and non invaded sets using a bootstrap procedure. Rarefaction curves of the non-invaded set rise quickly and reach higher accumulation values than the invaded set, but differences between the two curves were not significant. With respect to Rènyi's profiles, the profile for the invaded dataset was always below the non-invaded one, but differences in diversity were significant only when specifically considering the focal species (Shannon, Simpson and Berger–Parker indices). In the analysed case, the invasion is significantly associated with changes in focal species diversity, instead those differences are not evident on the all native species pool. In the case of recent invasions, a consistent decline on focal species diversity may represent an early alarm sign of diversity loss and may help define specific conservation actions to prevent the decrease of overall diversity.