How do non-indigenous species affect native species and habitats?

Biological invasions are a key driver of global change, affecting biodiversity and natural ecosystem functioning. Non-indigenous species (NIS) with significant established and expanding populations have the potential to become invasive, with serious environmental, socio-economic, and/or human health consequences. Mediterranean Islands (including Marine Protected Areas “MPAs”), important hotspots of biodiversity, are particularly vulnerable to NIS invasions. Understanding the effects of NIS species on biodiversity, as well as their current distribution and expansion trends, is therefore critical for creating effective conservation strategies. Here, we present three different case studies from Sicily (North-Western Mediterranean Sea) that highlight how NIS species interact with native species and the habitat where they settle. The first case study, carried out along the coast of Favignana Island (Egadi Islands MPA), highlights how the ability of Caulerpa cylindracea to form multi-layered mats trapping the sediment promotes the establishment of other NIS, including the biofouler worm Branchiomma bairdi. The second case study, performed along the North-Western coast of Sicily, shows how Halophila stipulacea can inhibit the growth of the co-occurring native seagrass Cymodocea nodosa. Finally, the third study case, undertaken along the coast of Favignana Island, highlights how the shift in habitat from the native Ericaria brachycarpa to the invasive Asparagopsis taxiformis may drastically erode primary producer biomass and associated biodiversity. Overall, our findings not only emphasize the negative effects of NIS on native habitats, but also confirm that MPA protection does not limit the introduction and spreading of NIS.