The global biodiversity loss is causing abrupt shifts in the structure and functioning of ecosystems with severe ecological and socio-economic consequences. Therefore, improving our understanding of ecosystem dynamics and regime shifts, as well as the stabilizing role of biodiversity across multiple scales is needed. Here we investigate the temporal dynamics and stability of marine ecosystems using high-resolution monitoring data on fish species composition, abundances and traits throughout European Seas. More specifically, we quantify and compare the direction and magnitude of community change at multiple spatial scales and levels of biological organization. Our results show less variability in community trajectories at larger spatial scales and higher levels of biological organization. The main underlying processes providing stability are statistical averaging arising from a larger pool of species, while at smaller spatial scales stability also emerge from functional complementarity channeled through the distribution of species traits within functional groups.

Our understanding of the community assembly processes acting on non-indigenous species (NIS), as well as the relationship with native species is limited, especially in marine ecosystems. To overcome this knowledge gap we here develop a trait-based approach based on the functional distinctiveness metric to assess niche overlap between NIS and native species, using high-resolution data on benthic invertebrate communities in the Baltic Sea. Our results show that NIS retain a certain degree of similarity with native species, but display one or a few singular traits (e.g., size, life span or bioturbation ability). Furthermore, we demonstrate that community assembly processes, including both environmental filtering and limiting similarity affect NIS establishment, but that their effects may be highly context dependent, as illustrated by pronounced spatial patterns in distinctiveness. Finally, our trait-based approach provides a generic framework applicable to other areas and organisms, to better understand and address biological invasions.