3.5 Invasive alien species


Alien species are plants and animals that have deliberately or accidentally been introduced outside their natural range. When finding good living conditions such species may spread quickly and thus become “invasive”. Once established, they are difficult or impossible to control.

In the aquatic environment, alien species are non-native plants or animals that compete with and could even eradicate natural aquatic species. Invasive alien species (IAS) are thus a significant pressure to the good ecological status of surface waters, aquatic habitats and species in general. Within the 2nd RBMPs, 15 European countries reported IAS as a significant pressure for ca. 2 700 water bodies (2 % of total) with the highest proportion being reported in Spain, the Netherlands, Norway, and Slovakia.

It is estimated that there are ca. 750 freshwater species which are established aliens or suspected to be alien in European inland waters (Nunes, et al., 2015). Species such as the Chinese mitten crab or the zebra mussel are a major threat to Europe’s aquatic biodiversity. The number of IAS in European freshwaters has been rising, having increased sevenfold over the last 100 years (European Network on Invasive Alien Species - EASIN, (Cid, and Cardoso, 2013)). According to data from EASIN, the highest numbers of freshwater alien species have been registered in river basin districts in France, Germany, Netherlands, Belgium, Sweden and Ireland (Figure 15).

Figure 15          Number of European freshwater alien species in river basin districts



Source: European Commission - Joint Research Centre - European Alien Species Information Network (EASIN) https://easin.jrc.ec.europa.eu/; Map created on 6 November, 2020.

Alien species are mainly introduced to freshwaters via aquaculture followed by releases and escapes of pet/aquarium species. Furthermore, introductions through inland canals or shipping (e.g. with ballast water) and fisheries/angling are also quite widespread but make up a lower share of alien species introductions (Nunes, et al., 2015). Climate change is obviously an additional reason, e.g. if temperature increases, currently natural thermal barriers which normally limit the establishment of IAS will become more suitable for alien species. This will potentially lead to a geographical redistribution of species and create alien invasive aquatic communities (IUCN, 2017).   

In European seas, more than 1 360 marine alien species have been observed, of which almost 1 100 have been introduced since 1950. These consist primarily of crustaceans and molluscs, followed by plants, micro-organisms, and fish. The rate of introductions in the marine environment is continually increasing, with almost 300 new species reported since the year 2000 (EEA 2012).

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Invasive alien species (IAS) threaten native wildlife, alter communities, affect the food webs, and introduce new constraints to the recovery of the native biodiversity. Some also cause economic damage.

Examples of invasive plants are curly waterweed (Lagarosiphon major), floating pennywort (Hydrocotyle ranunculoides) and large flowered waterweed (Egeria densa). Such plants may cover large areas of water and wetlands making natural vegetation and ecosystems impossible. Invasive plants have disrupted navigation and damaged waterworks by blocking pipes and pumps. This also included pumping intakes for cooling water of nuclear power plants resulting in safety problems (Sarat, et al., 2015). For example to control damage of floating pennywort in the Netherlands the total annual control costs have been  around 1 million Euro (BirdLife International, n.d.).

Some invasive aquatic invertebrates have had major effects on the ecosystems that they invade, e.g. the red swamp crayfish and the distribution of the crayfish plague. The plague is estimated to have economic cost in Europe of over €53 million/year (EC 2019c). The zebra mussel Dreissena polymorpha forms dense encrustations, which provoke serious damage to infrastructure, clogging up the water-intake of industrial and drinking water plants. The killer shrimp Dicerogammarus villosus can feed on a variety of freshwater invertebrates, including other native shrimp species, fish eggs and young fish, and can significantly alter ecosystems (BirdLife International, n.d.). Alien species may also act as carrier of fungus organisms or spread diseases (Strayer, 2010).

Invasive freshwater fish e.g. from stocking disrupted the food web, when predating the native smaller fish and their food, and simplified the original communities (BirdLife International, n.d.). Escapes from aquaculture (e.g. salmon) changed genetic behaviour of natural populations.

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Measures and management challenges

According to the Invasive Alien Species (IAS) EU Regulation (1143/2014/EU), all Member States should implement strategic plans and measures to combat the adverse effects of IAS. These should include prevention measures, early detection and rapid eradication, as well as management measures. 

Prevention measures are pathway oriented and aim at preventing the intentional or unintentional introduction. One example is ballast water management (under the Ballast Water Management Convention), where the ballast water of ships has to be treated or filtered, or exchanged in the open sea before entering the freshwater ecosystems to avoid introduction of IAS e.g. Chinese mitten crab. To avoid further spread of invasive plants between unconnected water bodies by e.g. water sport equipment (such as boats and trailers), public awareness raising, also for angling, hunting or zoos is carried out. Other measures are reducing nutrients for plant reduction or physical barriers.

Basis for early detection and rapid eradication measures are surveillance monitoring to detect the presence of IAS by e.g. establishing an early detection network, citizen science initiatives, eDNA monitoring or remote sensing techniques to detect invasive floating plants. Cutting or mowing or hand weeding of submerged plants, and trapping, hunting and fishing for fish and crustacean are also measures to eradicate IAS.

Management measures aim at minimizing the harm IAS cause. Examples are the commercial use of the Chinese mitten crap for food consumption, biological control (manipulation) of the food web of an ecosystem or the use of herbicides to control massive invasive plant growth.    

Besides the IAS Regulation, other policies tackle aquatic alien species as well. Under the WFD, alien species were identified and monitored as a pressure in European water bodies. But, only a low number of measures to reduce the pressure of alien species were implemented within the 2nd RBMP (EC 2019d). Other relevant policies include the Marine Strategy Framework Directive which sets specific objectives for managing alien species (non-indigenous species) in European seas to reach good environmental status, and the Regulation concerning use of alien and locally absent species in aquaculture (7087/2007/EU).

Currently, there is no direct cross-linkage between management strategies under the IAS Regulation, the WFD or the MSFD. However, there is immense potential for more efficient protection of naturally occurring aquatic communities, since measures to protect aquatic species under the IAS Regulation are also suitable for fulfilling the goals of the WFD and the MSFD. Nearly all Member States have national strategies for preventing and mitigating the impact of IAS and these should be more closely coordinated with the programme of measures of the WFD RBMPs as well as the programmes of measures under the MSFD. Under consideration of the significant increase of alien species in freshwaters and the marine environment in recent decades, there is risk that the number of alien species continues to rise with high impact on biodiversity, if no harmonization and efficient management strategies are implemented.

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