Expanding on the portuarization syndrome from an ecological perspective: eDNA reveals rich diversity, non-indigenous hotspots, and biotic homogenization in ports

Ports are well-known entry points for marine non-indigenous species (NIS), which arrive as hitchhikers on ships. Ports are also expected to be gateways for the spread of NIS in the wild and resemble each other more than communities outside due to their singular characteristics. However, the uniqueness of species assemblages in ports and how they differ from natural habitats have only been marginally investigated, notably at regional scale. Using eDNA metabarcoding, we obtained a comprehensive and standardized overview of metazoan community diversity in 12 paired ports and adjacent natural areas along the northwestern Mediterranean Sea. As expected, we found that NIS are more abundant in ports than in natural habitats, and that the species assemblages in ports differ from those in natural habitats. In addition, we observed that communities in ports are far more homogeneous than their natural counterparts. This finding supports the hypothesis of biotic homogenization in highly anthropized habitats. We also observed a pattern that had previously been documented mainly in fish, but that we identified here in every phylum studied except Arthropoda: species richness detected in ports is comparable to, and in some case even greater than, that observed in natural habitats. Overall, our findings broaden, through an ecological perspective, the "portuarization syndrome" concept, which originally defined ports as unique replicated environments that promote specific evolutionary processes.