Impacts of invasive alien marine species on ecosystem services and biodiversity:a pan-European review

A good understanding of the mechanisms and magnitude of the impact of invasive alien species on ecosystem services and biodiversity is a prerequisite for the efficient prioritisation of actions to prevent new invasions or for developing mitigation measures. In this review, we identified alien marine species that have a high impact on ecosystem services and biodiversity in European seas, classified the mechanisms of impact, commented on the methods applied for assessing the impact and the related inferential strength, and reported on gaps in available information. Furthermore, we have proposed an updated inventory of 87 marine species in Europe, representing 13 phyla, which have a documented high impact on ecosystem services or biodiversity. Food provision was the ecosystem service that was impacted by the greatest number of alien species (in terms of both positive and negative impacts). Following food provision, the ecosystem services that were negatively affected by the highest number of alien species were ocean nourishment, recreation and tourism, and lifecycle maintenance, while the ecosystem services that were most often positively impacted were cognitive benefits, water purification, and climate regulation. In many cases, marine aliens were found to impact keystone/protected species and habitats. Thirty percent of the assessed species had an impact on entire ecosystem processes or wider ecosystem functioning, more often in a negative fashion. Forty-nine of the assessed species were reported as being ecosystem engineers, which fundamentally modify, create, or define habitats by altering their physical or chemical properties. The positive impacts of alien species are probably underestimated, as there is often a perception bias against alien species. Among the species herein assessed as high-impact species, 17 had only negative and 7 only positive impacts; for the majority (63 species), both negative and positive impacts were reported; the overall balance was often unknown. Although there is no doubt that invasive species have modified marine ecosystems, evidence for most of the reported impacts is weak, as it is based on expert judgement or dubious correlations, while only 13% of the reported impacts were inferred via manipulative or natural experiments. A need for stronger inference is evident, to improve our knowledge base of marine biological invasions and better inform environmental managers.