Drivers of Species Richness and Community Composition in Marine Artificial Habitats Isolated from Natural Rocky Shores

The Theory of Island Biogeography (TIB) explains species diversity on islands as a balance between immigration and extinction rates, influenced by island size and distance from the mainland. Larger and closer islands support higher biodiversity due to increased colonization and lower extinction rates. Artificial hard structures, such as seawalls, piers, and offshore wind farms, are constructed for various human purposes in marine areas that typically consist of soft sediment habitats, altering local ecosystems by providing new habitats for marine organisms, often including Non-Indigenous Species (NIS). Due to their isolated positioning, they also function as artificial islands; however, TIB has never been applied to explain the dynamics of colonization in these systems. This thesis does a TIB extension to marine urban structures (MUS), specifically artificial hard substrates along the Italian coastline. This research treats this kind of infrastructure as island-like and investigates the relationship between species richness, structure size, and isolation from natural rocky shores. The present study integrated sampling across seven biogeographical zones and showed a strong negative correlation of species richness with distance from natural rocky shores, thus indicating that isolation is a primary driver of biodiversity variation on MUS. While higher species diversity was hosted by larger structures, the size effect was modulated by proximity, as structures close to natural rocky shores are subjected to increased connectivity with natural habitats. Isolated structures had unique community compositions when compared to those in proximity to natural rocky shores. This study also determined a higher proportion of NIS on more isolated and larger structures, suggesting that these kinds of environments facilitate the establishment and spread of NIS due to reduced biotic resistance. Therefore, geographical and biological elements must be considered in the design and management of marine urban structures for the future. This thesis puts TIB into current ecological knowledge to increase the understanding of marine urban environments and to propose practical suggestions for increasing the ecological value of such structures. The implications of this study provide critical insights into the biodiversity patterns and ecological functioning of marine urban structures within marine environments.