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

Our review identified 86 alien marine species, within 13 phyla, having high negative or positive 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 information gaps. Food provision was the ecosystem service impacted by the greatest number of alien species, positively or negatively. Following food provision, the ecosystem services being 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 negatively affect 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 way. Forty-nine of the assessed species were reported as being ecosystem engineers, fundamentally modifying, creating or defining habitats by altering their physical or chemical properties. The positive impacts of alien species are probably underestimated, mainly due to 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 (62 species), both negative and positive impacts were reported, the overall balance being often unknown. Although without doubts, invasive species have modified marine ecosystems, evidence for most of the reported impacts is weak, being based on expert judgement or dubious correlations, while only 13% of the reported impacts were inferred via manipulative or natural experiments. Evidently, stronger inferences are needed, to improve our knowledge of the consequences of marine biological invasions and to better inform environmental managers.JRC.H.1-Water Resource