ANALYSIS OF DIVERSITY PATTERNS OF NATIVE AND ALIEN SPECIES IN COASTAL PLANT COMMUNITIES: FROM LOCAL TO GLOBAL SCALE

Coastal ecosystems are deemed one of the most productive ecosystem in the world hosting habitat and species of great ecological and conservation values. As a direct consequence of urbanization and globalization, these ecosystems are undergoing several threats among which biological invasions represent one of major concern. A clear and comprehensive understanding of the invasion process in these habitats is still lacking; in this thesis, I explored the diversity patterns of native and alien species along with the main ecological determinants shaping coastal plant communities from local to global scale. In more detail, I investigated alien and native diversity patterns both in a disturbed site (Trieste port area) and in natural sites (northern Adriatic sand dunes habitats) at a local scale by using a probabilistic sampling design. Furthermore, I collated a global database of sand dune ecosystems consisting in 14,841 quadrats and phytosociological relevès across habitats and floristic kingdoms aiming at disentangling patterns of native and alien plant diversity and understanding which are their main determinants at the global scale. In urban areas, I found that habitat diversity enhances biodiversity and it could provide an effective filtering effect able to reduce the spread of alien species. For what concern natural ecosystems, my results highlighted the strong differentiation in community composition between native and alien species with the latter showing, on average, lower complementarity among habitats. Furthermore, in these habitats, the occurrence of alien species showed to be locally related to geomorphological variables and coastal dynamism more than climatic features. Additionally, functional traits reveal that alien species pool have higher values of performance-related traits such as Specific Leaf Area displaying also lower functional diversity. A potential signal of small-scale functional homogenization driven by the occurrence of alien species has been also detected, with deleterious effects for ecosystem functioning (loss of species endowed of unique traits composition and reduction in the functional space). The global scale study confirm that native species follow the well-known sea-inland gradient worldwide; on the contrary alien species show different patterns according to the floristic kingdom considered. The global model developed (Bayesian GLMM) confirmed that anthropogenic factors are the main drivers of alien species richness whilst native richness is explained by a combination of climate and habitat features. As a general conclusion, these findings suggest that in coastal environments plant diversity patterns remain constant across spatial scales, especially for native species: this is probably due to the same strong environmental gradients characterizing these natural ecosystems worldwide that make plant communities strongly spatially structured. In addition, a more diversified native community seem to cope better with alien species spread. Conversely, alien community does not present such a strong spatial structure both considering the species pool than the functional diversity patterns; anthropogenic factors do play a key role in their spread both at local and global scale. At last, controlling and eradication programs may be planned where higher levels of alien β diversity are observed optimizing time and resources.