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.
© Fotolia, Author: Shakzu
D2 Commission Decision (EU) 2017/848 of 17 May 2017
Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystems
D2C1 – Primary: The number of non-indigenous species which are newly introduced via human activity into the wild, per assessment period (6 years), measured from the reference year as reported for the initial asessment under Article 8(1) of Directive 2008/56/EC, is minimised and where possible reduced to zero. Member States shall establish the threshold value for the number of new introductions of non-indigenous species, through regional or subregional cooperation.

D2C2 – Secondary: Abundance and spatial distribution of established non-indigenous species, particularly of invasive species, contributing significantly to adverse effects on particular species groups or broad habitat types.

D2C3 – Secondary: Proportion of the species group or spatial extent of the broad habitat type which is adversely altered due to non-indigenous species, particularly invasive non-indigenous species. Member States shall establish the threshold values for the adverse alteration to species groups and broad habitat types due to non-indigenous species, through regional or subregional cooperation.
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.