Habitat type and community age as barriers to alien plant invasions in coastal species-habitat networks

Sand shore ecosystems are extremely vulnerable to alien plant invasions. While most of the abiotic drivers of alien success have been identified, less is known on the role of biological processes driving the invasion. Studying the interactions between alien and native plant communities across different habitats and along the ecological succession (i.e. community maturity) can elucidate the dynamics of alien invasions in dune systems. In this study, we sampled alien and native plant communities in 100 patches across 10 natural coastal landscapes in NE Italy. The patches represented three main habitat types (foredune, backdune and salt marsh, which differ in terms of sea storm-related disturbance and soil salinity) distributed along a gradient of community maturity (i.e. number of years since the plant community was completely eroded by a sea storm). We analysed the effects of alien/native status, habitat type and maturity on species richness and colonization potential of plant species pools. Colonization potential was estimated by applying for the first time on plant data a species-habitat network approach, which allowed us to assess in detail the effect of each plant community on the others. In backdune habitats, alien plant species richness was negatively related with community maturity, which in turn had a positive effect on native species richness. Colonization potential was positively influenced by age for native communities and negatively for alien communities in salt marshes. Among habitat types, backdune patches were also particularly prone to alien invasions and very efficient donors of alien plants to other patches. Salt marshes were in general very resistant to invasion but potentially acting as secondary reservoirs for some backdune alien species. This study identified backdune habitats as key nodes for alien plant introduction and spread in coastal ecosystems, underlining the importance of maintaining mature undisturbed patches as a barrier to alien invasions. This information could prove pivotal in optimizing monitoring and management efforts of alien plant species in these ecosystems, as well as in conservation prioritization.