Non-indigenous species pose a significant threat to biodiversity and are a key focus of European Union policy efforts. Around 779 have been reported in Europe's seas, with the rate of new introductions steadily rising since 1970. The average annual rate of new introductions quadrupled compared to the 1970's in the last six-year assessment period of 2012-2017. Half of these introductions occurred via the transport-stowaway/shipping pathway, with 14% through corridors such as manmade waterways that connect adjacent non-EU waters. Stronger action is needed to reduce the risk of new introductions, including harmonised monitoring for NIS across Europe's seas.
Figure 1. Number and cumulative number of marine non-indigenous species in Europe's seas, 1970-2021
Non-indigenous species (NIS) are widely recognised as a major global threat to biodiversity. They may also pose a risk to human and animal health, while impacting local economies and livelihoods, often causing irreversible harm. In favourable environmental conditions—such as the absence of natural predators— NIS can reproduce, spread rapidly and become invasive, potentially outcompeting native species for space and other resources, and introducing lethal parasites and diseases. At least 87 NIS are considered invasive in Europe's seas .
To address this growing threat, the EU has introduced targeted legislation and policies. The Invasive Alien Species (IAS) regulation sets out rules to prevent, minimise and mitigate the negative impacts of IAS across the EU. Additionally, the Marine Strategy Framework Directive (MSFD) aims to achieve good environmental status (GES) in Europe's marine waters. The directive mandates minimising or eliminating new NIS introductions through human activity and requires Member States to establish thresholds for GES regarding newly introduced NIS.
At least 952 NIS, excluding microalgae and parasite/pathogen species, had been recorded in Europe's marine waters by 2021, with 779 introduced since 1970. Most of these new introductions since 1970 are invertebrates (67.6%), followed by vertebrates (17.7%) and primary producers (14.7%). The types and rates of NIS introductions vary across Europe's regional and subregional seas.
The recent increase in NIS numbers has been largely driven by introductions to the Mediterranean Sea and North-East Atlantic. Average annual introductions were 17.7 and 15.5 new NIS, respectively, during the 2012–2017 assessment cycle (Figure 1). The Aegean-Levantine Sea is the most affected in the Mediterranean, with an annual average of 16 new NIS. Macaronesia in the North-East Atlantic recorded an annual average of 8.5 new NIS over the same period (see results by sub-region here).
The surge in the rate of new introductions during the last six-year assessment period (2012-2017) suggests a deteriorating state. It could also reflect enhanced scientific monitoring and the growing contribution of citizen science. This surge may also be linked to climate change, as evidenced by the influx of tropical species, such as Red Sea fish into the Mediterranean. Additionally, it may be a result of time lags in detecting new NIS. In conclusion, current data alone are insufficient to assess GES based solely on NIS introduction numbers.
Figure 2. Pathways of introduction of non-indigenous species to Europe’s seas, 1970-2021
The main pathway for marine NIS in Europe's waters has been through seafaring vessels, via hull fouling, ballast water and other transport-stowaway methods, accounting for 51.2% of new introductions. This is followed by the corridor pathway (14.3% of new introductions) primarily through manmade waterways connecting adjacent non-EU waters such as the introduction of Red Sea species into the Mediterranean. Other significant pathways include the unintentional transport of live organisms (transport-contaminant) and natural spread from neighbouring non-EU waters (unaided) (Figure 2).
During the last six-year assessment period, most pathways saw a steady rise in NIS introductions. This may reflect increased scientific efforts to identify NIS in hotspots such as ports and marinas. The findings underscore an urgency for stronger measures to reduce the risk of new introductions and spread of IAS. Key actions include deepening our understanding of NIS impacts on biodiversity and ecosystems, particularly under changing climatic conditions, and maintaining updated, validated NIS inventories to ensure effective monitoring of policy implementation.
Supporting information
The indicator presents trends in the number of new introductions of non-indigenous species for European marine waters, as well as by marine (sub)region and assessment period (six years), as per the MSFD Descriptor 2 criterion D2C1.
Additionally, the indicator presents the cumulative increase in the number of NIS, by marine (sub)region and by three main species groups: (1) primary producers (plants and algae), (2) invertebrates and (3) vertebrates. The pathways or processes that result in the introduction of NIS are also described. These are not part of the MSFD D2C1 but are useful for management purposes to provide more insight into the taxonomic groups of the newly introduced NIS and the modes of introduction.
National inventories of NIS were analysed, revised, and updated with published data from biodiversity and hot-spot campaigns, academic surveys, and citizen science observed until December 2021. The revised list was cross-checked against existing NIS databases and validated by national experts.
Species filtering and taxonomic aggregation
For each regional sea, marine and estuarine species are grouped by selected taxonomic group: primary producers (plants and algae), invertebrates and vertebrates. Species sorting follows recommendations from Tsiamis et al.
Oligohaline species are included if the relevant species have been found in estuarine or coastal systems of the assessed region. NIS that have spread from one region/subregion to another through natural dispersal are included in the assessment and the pathway is classified as ‘unaided’. This applies mainly to many Red Sea species that have invaded the eastern Mediterranean through the Suez Canal (Lessepsian immigrants) and are progressively moving towards the central and western Mediterranean.
Birds, cryptogenic and range-expanding species are excluded. Species currently reported as 'extinct' or ‘absent’ in the literature are excluded. Among the questionable records (species classified differently in the European Alien Species Information Network (EASIN) and the Information System on Aquatic Non-indigenous and Cryptogenic Species (AquaNIS), introductions were included or excluded based on the validation by national experts. Pathogenic and parasitic NIS are omitted since from a legislative perspective they are managed under the Animal Health Law. According to Tsiamis et al. parasitic NIS should be reported under D2 but not considered when assessing against a GES threshold until further data comes to light. Accordingly, pathogens and parasites are listed but not considered in the six-year analyses related to D2.
Similarly for microscopic algae (phytoplankton), there is a divergence of opinions among national NIS experts as to their native, cryptogenic or NIS status, as reflected in the literature and in EASIN or AquaNIS and as such, are listed but not considered in the six-year analyses related to D2.
Data analysis
Individual species records are analysed to determine the year of first reporting from the environment by sub-regional sea. Data are then used to calculate the number of new NIS, the cumulative number of NIS and the rate of new recordings as a proxy of new introductions, presented per 6-year intervals.
NIS that have been registered in more than one regional sea are recorded in each (sub)region, meaning that regional assessments do not add up to the pan-European assessment since the latter only considers species once. NIS that have been recorded with an unknown year of introduction are considered when providing the total number of NIS, both at the pan-European and regional sea levels.
Care has been taken to ensure that the nomenclature problems encountered have not resulted in multiple separate recordings (e.g. the same species recorded in different regions or species lists, databases with different synonyms for the same species).
Detection year
The year of introduction is based on the first collection dates reported. First detection/collection dates are not always documented, especially for records before 1940. In such cases, the date of the publication is used as the first record date. In cases where a range of dates (e.g. 1986-1994) or a decade (e.g. 1970s) is reported, the introduction date is taken as the average year (e.g. 1975).
Pathway description and assignment
Each species is assigned to the most likely primary pathway of introduction or pathway of secondary spread, as appropriate. Pathway descriptions follow Convention on Biological Diversity CBD definitions and include: release in nature (REL), including intentional release of aquarium species; transport-contaminant (TC), including contaminants on animals (except parasites, species transported by host/vector) and parasites on animals (including species transported by host and vector); escape from confinement (EC), referring to the accidental escape or unintentional release of aquarium species (see IUCN 2018); transport-stowaway (TS), including ship/boat ballast water (TS-ballasts), ship/boat hull fouling (TS-hull) and other means of transport (TS-other); corridor (COR), referring to interconnected waterways/basins/seas (in the current context, this refers to the Suez Canal and human-made canals); unaided (UNA), secondary spread, as appropriate; and, unknown (UNK).
The intentional release of aquarium species, previously classified as escape from confinement (EC) is now included under release in nature (REL).
The pathway assigned to each species follows the EASIN method (also reported in Pergl et al.). Where more than one pathway of introduction is suspected or documented, the analysis considers these modes of introduction, giving the same weight to each so that the total number of pathways per species equals to one. The analysis at pan-European level includes the primary pathway in Europe's seas and may differ from that at regional scale, which may be the result of secondary introduction (unaided or another pathway/vector involved).
Methodology for gap filling
Not applicable
According to the Convention on Biological Diversity (CBD), non‑indigenous species (NIS) are those species introduced outside their natural past or present distribution. Marine NIS may spontaneously arrive in new areas from areas already invaded through natural migration, but they are often introduced by human activities, such as maritime transport, aquaculture and canals. Climate change may provide new opportunities for NIS to proliferate and spread. Once introduced in a new area they can become 'invasive' and have severe ecological and socio-economic impacts on local ecosystems. Invasive NIS are considered a main threat to biodiversity after habitat loss/destruction. In recognition of this, NIS are one of the pressures on the marine environment being addressed by EU policy, including the Marine Strategy Framework Directive (MSFD).
While the Water Framework Directive (WFD) does not specifically refer to IAS, this was rectified for the marine environment with the MSFD, which makes explicit reference to ‘non-indigenous species introduced by human activities’ as one of the 11 Descriptors used to assess Good Environmental Status (GES). NIS are treated as a distinct Descriptor (D2) of GES: 'Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystem'.
Under the MSFD Descriptor 2 (D2), Member States need to consider NIS in their marine management strategies. The MSFD requires that ‘The number of non-indigenous species which are newly introduced via human activity into the wild, per assessment period (6 years), is minimised and where possible reduced to zero’ and also states that ‘Member States shall establish the threshold value for the number of new introductions of non-indigenous species, through regional or subregional cooperation’. The initial reporting of Member States in 2012 did not provide regionally harmonised information. While refined baseline inventories of NIS per EU Member State have recently been established, the number of introductions compatible with GES (or threshold values) are still currently being discussed.
The most efficient (and cost-effective) way to deal with the threat of NIS to biodiversity and society is through a combination of preventive measures, early detection, and rapid response to new incursions, with permanent management only as the last option. Efforts under the MSFD are focused on limiting the environmental pressure of NIS by reducing the rate of their introduction and spread by managing pathways through which NIS move. A similar approach is also the main driver behind the IAS regulation, or Regulation (EU) No 1143/2014 on the prevention and management of the introduction and spread of invasive alien species. It requires Member States to prevent the introduction and spread of regulated IAS (IAS of Union concern), enforce effective early detection and rapid eradication mechanisms for new introductions, and adopt management measures for species already widely spread. Thus, the IAS regulation supports Natura 2000 through the Habitats Directive and the Birds Directive Directives by providing Member States with a dedicated legal tool aimed at preventing the introduction and spread of IAS of Union concern, and at mitigating their impact. Currently, only one marine fish species, Plotosus lineatus, is on the list of IAS of Union Concern. The other marine species on the list is the Chinese mitten crab, Eriocheir sinensis, which can be considered as marine, brackish and freshwater species.
The European Green Deal and the Biodiversity Strategy for 2030 both stress the importance for the EU to put nature on a path to recovery by 2030 by better protecting and restoring healthy ecosystems. Specifically, the Biodiversity Strategy commits to reduce by 50% the number of red list species threatened by invasive alien species.
Additionally, Regulation (EC) No 708/2007 concerning use of alien and locally absent species in aquaculture, is applicable to all aquatic species used in aquaculture and for stocking to enhance wild stocks, with the exception of species used in the ornamental trade only.
Also related is the International Convention for the control and management of Ships Ballast water and sediments (BWM), which entered into force globally on 8 September 2017 and constitutes a crucial step towards the reduction of the spread of non-indigenous species regionally and worldwide. This global response aims to prevent the potentially devastating effects of the spread of invasive species carried by ships' ballast water from one region to another, through the strict control and management of ships' ballast water and sediments.
Due to a lack of information and awareness, and in the absence of comprehensive and harmonised legislation and monitoring framework at the European level, the issue of NIS and their impacts has largely been underestimated.
Uncertainty in alien status
It is common to miscategorise NIS as native. Many pseudo-indigenous species occur in the Mediterranean, precisely because many old taxonomic works originated in the region.
There are many records of cryptogenic species in Europe's seas. Phylogenetic studies have revealed that some of these are truly non-indigenous. For more details see Zenetos et al.
Knowledge on the diversity of marine microalgae in Europe's seas is limited, making it difficult to determine whether a suspected microalgal invader was already present as part of rare, hidden and unsampled phytoplankton.
Uncertainty in introduction/detection year
It is not always feasible to know the exact year of first collection/observation of a NIS. This is common in older publications that provide checklists with no further details. The year of introduction is based on the first collection dates reported but these do not necessarily indicate the true year of introduction, which may have been years earlier.
Due to the time lapse between observation and publication, the rate of introduction for the most recent years (2020-2021), is an underestimate as information on species collected during this period is expected to be published over the next two years.
Moreover, later inspection of archived samples has often led to the backdating of recent findings. This is the case for (1) unreported records, particularly by amateur conchologists who publish their earlier findings only after a species’ identity has been published; (2) overlooked species in museums and/or private collections; and (3) phylogenetic methods revealing the backdating of species, since these methods are now used to identify alien and cryptogenic species.
Uncertainty in pathway description
In many cases, it is impossible to identify the introduction pathway/vector. For example, for species that are most frequently associated with hull fouling, this form of transport is assumed to be the responsible vector. For planktonic taxa and microscopic resting stages, ballast water is deemed to be the most likely vector as species that are associated with hull fouling can be expected to be flushed away during journeys at sea. Human activities near the site of first recording are generally assumed to be responsible for the introduction event. However, such deductions are not always well founded, especially in cases where the most likely vector is unclear (e.g. for ship-transferred NIS, ballast and hull fouling are assigned as potential vectors).
Data sets uncertainty
Data sets on NIS in official sources, such as AquaNiS, EASIN, HELCOM and NOBANIS are often contradictory. This can be because these sources include freshwater species, which are encountered in the upper estuarine reaches, in their inventories. HELCOM also defines the regional borders of the Baltic Sea differently in comparison to EU MSFD.
Geographical comparability
Partly native species have been included in the regional- and subregional-level analyses but not considered at EU level. Such NIS mostly include Mediterranean molluscan transported through shellfish movements to the North-East Atlantic and vice versa but also sessile macrobiota, such as tunicates, and Ponto-Caspian species that have been introduced to the Baltic Sea and other EU MSFD regions outside of the Black Sea. Species native to a subregion (e.g. the North Sea) that have been anthropogenically transferred to another country within the subregion are not included in the analyses of the subregions, although they do count as NIS in the countries that they have invaded. Of the validated NIS in EU waters, 46 are species native to at least one EU regional sea and have not been included in the analyses at pan-European level.
Descriptive indicator (Type A - What is happening to the environment and to humans?)SDG14: Life below water
FIG1: Number of new NIS and cumulative number of new NIS FIG2: Percentage and number of new NIS
Once a year
References and footnotes
For example, the striped eel catfish (Plotosus lineatus), now occurs in the Mediterranean Sea, having invaded as a Lessepsian migrant through the Suez Canal. It is reported to compete for resources with similar predators leading to changes in native community structure; impacts fisheries as by-catch in trawl catch, causing loss of working hours due to injuries in fishers and can sting fishers and beachgoers.
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