Non-indigenous species threaten biodiversity in Europe’s seas and are targeted by EU policy. Since 1970, 640 species (excluding microalgae, pathogens and parasites) have invaded European waters, with an increasing average annual rate of new introductions. Between 2012 and 2017, on average 21.5 species per year were detected, up from seven in the 1970s. Introduction rates vary by species group, pathway and marine region, with the numbers of new species in the Mediterranean Sea being particularly notable. Transport – Stowaway/Shipping remains the main pathway of introduction, accounting for almost half of all new invasions, followed by unaided introductions from neighbouring non-EU waters (23%).
Figure 1. Number and cumulative number of marine non-indigenous species in Europe's seas, 1970-2020
Non-indigenous species (NIS) can become invasive and spread rapidly, and this is widely recognised as a major threat to biodiversity worldwide. NIS can also threaten human and animal health and affect economic activities and livelihoods, and their impacts can be irreversible. Under ideal environmental conditions and ecological circumstances (e.g., in the absence of natural predators), NIS may spread and reproduce excessively, feeding on native species or out-competing them for space and other resources; sometimes also carrying parasites and diseases that can be lethal to native species or dangerous to human and animal health. Around 87 NIS are currently considered invasive in Europe's seas.
To address this, invasive alien species (IAS) are targeted by EU legislation and policy. Regulation (EU) No 1143/2014establishes rules to prevent, minimise and mitigate the adverse impact on biodiversity of the introduction and spread of IAS within the EU. Moreover, the Marine Strategy Framework Directive (MSFD), aims to achieve good environmental status (GES) for all EU marine waters, such as by ensuring that NIS do not adversely alter ecosystems. It requires that the number of NIS newly introduced into the wild through human activity is minimised or reduced to zero and that Member States establish thresholds for NIS introductions.
Until 2020, at least 804 NIS (excluding microalgae and parasite/pathogen species) have been recorded in European marine waters, of which 640 have invaded since 1970. The vast majority are invertebrates (64.2%), followed by primary producers (18.4%) and vertebrates (17.3%). The proportion of each type of NIS and their annual rates of introduction vary among regional and subregional seas. The increase in the number of NIS at European level in recent years has been largely driven by introductions in the Mediterranean Sea and the North-East Atlantic, with an average of 14.0 and 11.7 new NIS per year between 2012-2017, respectively (Figure 1). Within the Mediterranean region, the Central Mediterranean is most prominent with an average of 10.5 new NIS introductions per year between 2012-2017. In the North-East Atlantic, the Bay of Biscay and Iberian Coast leads with seven new NIS per year during the same period (Figure 1 – extra).
The large increase, particularly of vertebrates and invertebrates, observed between 2006-2011 and 2012-2017 could indicate a general worsening of GES, but could also be the result of increased scientific effort and contribution of citizen science (Lehtiniemi et al., 2020). This increase could also be related to climate change, the role of which is further supported by the influx of tropical biota into European waters (e.g. Red Sea fish species in the Mediterranean). On the other hand, the decrease in the rate of introductions more recently, most notably of primary producers, does not necessarily indicate an improvement in GES but could be the result of the time elapsed between NIS being introduced and detected for the first time (Zenetos et al., 2019). In conclusion, the information available is currently insufficient to confidently assess GES based on the numbers of NIS introduced.
Figure 2. Pathways of introduction of non-indigenous species to Europe’s seas, 1970-2020
The transfer of NIS by seafaring vessels (transport-stowaway by hull fouling, ballast water and other) remains the main pathway of NIS dispersal, accounting for 46.6% of new introductions in European waters. This is followed by unaided introductions from neighbouring non-EU waters (22.8%), transport-contaminant, e.g. unintentional movement of live organisms (11.9%) and release in nature, i.e., intentional release (4.3%). Overall, the proportional increase in new introductions has risen over the last two six-year assessment periods for most of the pathway modes, most noticeably by hull fouling and escape from confinement, e.g., from aquaria, aquaculture and mariculture. These findings could be explained by increased scientific interest and effort to identify new NIS in hot spot areas such as ports and marinas (Azevedo, 2019; Tempesti et al., 2020). Nonetheless, they highlight that further action is needed to minimise future risks of new introductions and spread of invasive NIS and more knowledge on their impacts on biodiversity and ecosystems, in particular in a changing climate. For this it is crucial to have updated and validated NIS inventories, including baselines and threshold values so that policy implementation progress can be effectively monitored.
Supporting information
The indicator presents trends in the number of new introductions of non-indigenous species by marine (sub)region and assessment period (6 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 group of the newly introduced NIS and the channel 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 2020 (reported until June 2022). The revised list was cross-checked against official NIS datasets 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' 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), very few have been included.
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, 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 but for which the year of introduction is unknown 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
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; 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. 2020). 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 European waters 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 Non-Indigenous Species (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 European seas. Phylogenetic studies have revealed that some of them are truly non-indigenous. For more details see Zenetos et al. (2017).
Knowledge on the diversity of marine microalgae in European 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 (2019-2020), is an underestimate as information on species collected during this period are expected to be published in the next 2 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 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 NOBANIS, HELCOM, EASIN and AquaNiS, are often contradictory. This can be because these sources include freshwater species, which are encountered in the upper estuarine reaches (oligohaline waters with salinities < 5 psu), 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 include sessile macrobiota, such as tunicates. 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, 48 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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