Almost 1,300 marine and coastal non-indigenous species (NIS) and cryptogenic species have been recorded in Europe with more than 3700 introduction events. All European seas host NIS and some species are known with ephemeral populations of short duration but most have established self-sustaining populations. Of the latter, those spreading to form additional populations elsewhere and adversely affecting biological diversity, ecosystem functioning, socio-economic values or human health in invaded regions, are termed invasive alien species (IAS). IAS are considered as one of the key causes of biodiversity changes worldwide. The rate of new aquatic invasions has increased in recent decades, thereby increasing concerns. The increasing numbers of new arrivals document the need for species introduction vector management.
The impacts of IAS are immense, occasionally disastrous and almost always irreversible. IAS may alter the structures, dynamics or functions of aquatic communities, or impose significant economic costs. In monetary terms, it was recently calculated that the lost output due to all aquatic IAS, health impacts and expenditure to repair IAS damage costs EU stakeholders more than 100 million EUR annually. These concerns were noted by the United Nations which named IAS as one of the top five anthropogenic threats of the world's oceans.
Analyses of the long-term time series have demonstrated that despite dramatic changes in the environment, no uniform consequences to NIS have been observed. All shifts in population abundance are species-specific and exhibit no generic patterns. All planktonic non-indigenous invertebrates studied showed an abrupt increase, whilst the biomass of benthic non-indigenous invertebrates were either stable or displayed an abrupt increase over time. Despite similar environmental settings, the dynamics of NIS vary among sub-basins, suggesting the existence of basin/habitat specific attributes modulating the invasion outcome. However, it has been found that changes in temperature seem to be a common significant forcing factor for the population dynamics of most NIS.
The 2020 goal of the EU Biodiversity Strategy concerning marine IAS may not be fully attainable without prompt concerted actions. The setting of a new target date may have to be considered and should be accompanied by scientifically robust, sensible and pragmatic plans to minimise introductions of marine NIS and to study those present.