Non-Indigenous Species (NIS) within Marine Protected Areas (MPAs). The case study of the Aegadian Islands MPA (Sicily, Tyrrhenian Sea) and the Dwejra MPA (Maltese Islands, Ionian Sea)

The introduction of non-indigenous species (NIS) has been pointed out as a major threat to biodiversity. NIS may become invasive alien species (IAS) and may cause biodiversity loss and ecosystem service changes. In the Mediterranean Sea, Sicily and surrounding islands, also including a high number of Marine Protected Area (MPAs), as a consequence of their geographic position and the intense maritime traffic, is a region particularly vulnerable and prone to NIS invasions. Since frontiers do not exist in the sea, biological invasions may severely affect MPAs, whose major aim is biodiversity conservation. Among the Sicilian MPAs, the Aegadian Islands MPA report the highest number of NIS (19). Maritime traffic has certainly spearheaded the introduction and spread of these NIS. The presence of Caulerpa cylindracea in all the Sicilian MPAs, confirms the invasiveness of this species. Within the Aegadian Islands MPA anchoring is the main vector for the spread of this NIS, which has affected algal diversity but also favoured the establishment of other NIS namely the sabellid Branchiomma bairdi. Within the Dwejra MPA (Maltese Islands), anchoring has favoured the colonization by a number of vegetal NIS, namely the phanerogam Halophila stipulacea and the alga Caulerpa taxifolia var. distichophylla, mainly by creating clearances within the previously continuous Posidonia oceanica meadow. The high presence of NIS in MPAs confirms that they are not immune to NIS invasions. The expansion of NIS by recreational vessels represents a risk for MPAs, which are popular touristic destinations. The dissemination of good practices may stop new introductions, therefore enhancing biodiversity conservation. The creation of permanent observatories and alarm systems involving Sicilian and Maltese MPAs, able to early detect new introductions and to follow the spread of species already present might be an effective tool in the management of present and future introductions of NIS in MPAs.