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Evidence of the effects of alien plant colonisation on plant communities is often hindered by the fact that similar patterns in community composition can arise through a variety of processes. The objective of this study is to determine whether changes in species composition in coastal dune communities depend on the colonisation of a neophyte plant, Oenothera stucchii, or on concurrent processes that favour its colonisation. We hypothesised two scenarios: 1) a direct impact of O. stucchii on colonised communities, leading to displacement of native species; or 2) no direct impact of O. stucchii, i.e. the species colonises plant communities by exploiting disturbances that lead to the rearrangement of plant communities. We used the species-habitat network approach to identify potential drivers of changes in species composition, assuming that changes in the structure of the species-habitat network depend on the nature of the driving process. We demonstrated that changes in species composition in plant communities were due to species rearrangement, with colonised communities characterised by more homogeneous composition of species. We suggest that changes in plant communities may not depend on colonisation by O. stucchii per se, but on concomitant processes that affect coastal dune communities while promoting colonisation by O. stucchii. [ABSTRACT FROM AUTHOR]

We evaluated the ecological impacts of the invasive alien species (IAS) Acacia saligna on the vegetation composition and structure of two coastal dunes woody habitats of Union concern (2250*: coastal dunes with Juniperus spp. and 2260: dunes with sclerophyllous vegetation consisting of Cisto-Lavanduletalia). We sampled 20 paired plots per habitat type under invaded (A. saligna cover > 70%) and non-invaded conditions, following a stratified random protocol. We tested the differences between invaded and non-invaded plots in terms of species composition of the entire species pool and of different ecological guilds and growth forms. Our findings showed that the invaded maquis (habitat 2260) had a significant decline in median richness (5 vs 2) and median cover (24.50 vs 8.00) of focal species and a significant increase in median cover of ruderal species (2.00 vs 5.50). The invaded juniper shrubs (EU habitat 2250*) preserved the typical species composition, but with significant variations in vegetation structure. The approach adopted in this study, accompanied by the results obtained, contribute towards fulfilling EU Regulation 1143/2014 on IAS. Key policy insights Results reveal worrying levels of biodiversity loss on coastal dunes due to the invasion of the invasive alien species (IAS) Acacia saligna. We detected remarkable changes in the physiognomy of the Mediterranean maquis coastal dunes – a landscape that is losing its biological uniqueness. The spread of A. saligna is having negative ecological effects upon the conservation status of two habitats of Union concern (codes 2250* and 2260). Urgent preventive measures, effective control strategies and eradication of A. saligna must be implemented across the Mediterranean Nature 2000 Coastal Protected Areas Network. The approach adopted and results obtained contribute to fulfilling EU Regulation 1143/2014 on IAS. [ABSTRACT FROM AUTHOR]