This Descriptor focuses on the assessment of the scale of the pressure and impacts of marine non-indigenous species (NIS) introduced as a result of human activities, in relation to the main vectors and pathways. New introductions of NIS and increases in the abundance and spatial distribution of established NIS should be prevented. It is recognized that there is only limited knowledge about the effects of the NIS on the marine ecosystem, which implies additional scientific and technical development focused on new potentially useful indicators.
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D2 Commission Decision 2010/477/EU
Descriptor 2 is a pressure descriptor that focuses on the prevention and reduction of impacts of non- indigenous species. New introductions of NIS and increases in the abundance and spatial distribution of established NIS should be prevented. Descriptor 2 interacts with several other GES Descriptors which have impact on native biodiversity, ecosystem functioning and seabed habitats as well as commercial marine resources (seafood), namely D, 3, 5, 7, 8, 9, 10. In particular, impacts that result from NIS should be managed, where feasible, so that the achievement of GES for the biodiversity Descriptors 1, 3, 4 and 6 is not compromised.

Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystem.

The identification and assessment of pathways and vectors of spreading of non-indigenous species as a result of human activities is a prerequisite to prevent that such species introduced as a result of human activities reach levels that adversely affect the ecosystems and to mitigate any impacts. The initial assessment has to take into account that some introductions due to human activities are already regulated at Union level ( 10 ) to assess and minimise their possible impact on aquatic ecosystems and that some non-indigenous species have commonly been used in aquaculture for a long time and are already subject to specific permit treatment within the existing Regulations ( 11 ). There is still only limited knowledge about the effects of the non-indigenous species on the environment. Additional scientific and technical development is required for developing potentially useful indicators ( 12 ), especially of impacts of invasive non-indigenous species (such as bio- pollution indexes), which remain the main concern for achieving good environmental status. The priority in relation to assessment and monitoring ( 13 ) relates to state characterisation, which is a prerequisite for assessment of the magnitude of impacts but does not determine in itself the achievement of good environmental status for this descriptor. 

D2 Documents
The following definition of non-indigenous species (NIS) was proposed by TG2: 

"Non-indigenous species (NIS; synonyms: alien, exotic, non-native, allochthonous) are species, subspecies or lower taxa introduced outside of their natural range (past or present) and outside of their natural dispersal potential. This includes any part, gamete or propagule of such species that might survive and subsequently reproduce. Their presence in the given region is due to intentional or unintentional introduction resulting from human activities. Natural shifts in distribution ranges (e.g. due to climate change or dispersal by ocean currents) do not qualify a species as a NIS. However, secondary introductions of NIS from the area(s) of their first arrival could occur without human involvement due to spread by natural means."

Invasive Alien Species (IAS), are defined by TG2 as "a subset of established NIS which have spread, are spreading or have demonstrated their potential to spread elsewhere, and have an adverse effect on biological diversity, ecosystem functioning, socio-economic values and/or human health in invaded regions". 

In addition, TG2 described the key terms ""levels that do not adversely alter the ecosystems" as the absence or minimal level of "biological pollution". Biological pollution is defined by TG2 as the impact of IAS at a level that disturbs environmental quality by effects on: an individual (internal biological pollution by parasites or pathogens), a population, a community, a habitat or an ecosystem.