A trait-based approach to assess niche overlap and functional distinctiveness between non-indigenous and native species

Non-indigenous species (NIS) pose a major threat to biodiversity and the structure and functioning of ecosystems worldwide. However, our understanding of the underlying assembly processes acting on NIS, as well as the relationship with native species is limited, especially in marine ecosystems. To overcome this knowledge gap we here developed a trait-based approach to assess niche overlap and functional distinctiveness between NIS and native species, using high-resolution data on benthic invertebrate communities in the Baltic Sea as a case study. Our results show a moderate degree of functional distinctiveness of NIS if considering the entire trait space, but significant differences in individual traits, notably adult life span, size and bioturbation. This indicates that NIS may successfully establish by displaying only one or a few singular traits, while retaining a certain degree of similarity with the natives. Furthermore, we demonstrate pronounced spatial patterns in distinctiveness, largely explained by environmental conditions and biotic effects channeled through species richness and evenness. Hence, several community assembly processes, including environmental filtering and limiting similarity may affect the establishment and success of NIS, while their effect and relative importance may be highly context dependent. Finally, our trait-based approach provides a generic framework applicable to other areas and organisms, to better understand and address biological invasions and their impacts on biodiversity and marine ecosystem functioning at large.