A good understanding of the mechanisms and magnitude of the impact of invasive alien species on ecosystem services and biodiversity is a prerequisite for the efficient prioritisation of actions to prevent new invasions or for developing mitigation measures. In this review, we identified alien marine species that have a high impact on ecosystem services and biodiversity in European seas, classified the mechanisms of impact, commented on the methods applied for assessing the impact and the related inferential strength, and reported on gaps in available information. Furthermore, we have proposed an updated inventory of 87 marine species in Europe, representing 13 phyla, which have a documented high impact on ecosystem services or biodiversity.
    Food provision was the ecosystem service that was impacted by the greatest number of alien species (in terms of both positive and negative impacts). Following food provision, the ecosystem services that were negatively affected by the highest number of alien species were ocean nourishment, recreation and tourism, and lifecycle maintenance, while the ecosystem services that were most often positively impacted were cognitive benefits, water purification, and climate regulation. In many cases, marine aliens were found to impact keystone/protected species and habitats. Thirty percent of the assessed species had an impact on entire ecosystem processes or wider ecosystem functioning, more often in a negative fashion. Forty-nine of the assessed species were reported as being ecosystem engineers, which fundamentally modify, create, or define habitats by altering their physical or chemical properties.
    The positive impacts of alien species are probably underestimated, as there is often a perception bias against alien species. Among the species herein assessed as high-impact species, 17 had only negative and 7 only positive impacts; for the majority (63 species), both negative and positive impacts were reported; the overall balance was often unknown. Although there is no doubt that invasive species have modified marine ecosystems, evidence for most of the reported impacts is weak, as it is based on expert judgement or dubious correlations, while only 13% of the reported impacts were inferred via manipulative or natural experiments. A need for stronger inference is evident, to improve our knowledge base of marine biological invasions and better inform environmental managers.

Fourteen species of hydroids, including two anthoathecates and 12 leptothecates, are reported from the west coast of North America on debris from the tsunami that struck Japan on 11 March 2011. Six species were found on a dock that stranded at Agate Beach, Newport, Oregon, five from a boat at Gleneden Beach, Oregon, four from a dock in Olympic National Park, Washington, and two from a boat in Grays Harbor, Washington. Obelia griffini Calkins, 1899, the most frequently encountered species, was collected on three of the four derelict substrates. Eight of the species are known to be amphi-Pacific in distribution. Of the rest, at least five (Stylactaria sp.; Eutima japonica Uchida, 1925; Orthopyxis platycarpa Bale, 1914; Sertularella sp.; Plumularia sp.) are not previously known from the west coast of North America. Hydroids of E. japonica occurred as commensals in the mantle cavity of the mussel Mytilus galloprovincialis Lamarck, 1819. Obelia griffini, O. gracilis Calkins, 1899 (not its secondary homonym Laomedea gracilis Dana, 1846) and O. surcularis Calkins, 1899 are taken to be conspecific. Of the three simultaneous synonyms, precedence is assigned to the name O. griffini under the Principle of the First Reviser in zoological nomenclature. The species is sometimes regarded as identical with O. dichotoma (Linnaeus, 1758).

Community development of ascidians was monitored weekly in Mission Bay (San Diego, California, USA) from September 2011 through November 2012 to assess how ascidian communities might be structured by spatial differences in environmental conditions. We repeatedly photographed PVC plates that were deployed on floating docks at four sampling locations: Seaforth Landing, Mission Bay Yacht Club, South Shores, and Hilton. Environmental variables (temperature, salinity, visibility, flow and dissolved oxygen) were measured during each sampling period. Eleven of the fifteen ascidian species observed on plates were invasive, three were native, and one was cryptogenic. Ascidian community composition varied significantly by time and sampling location within Mission Bay. Spatial differences in community dissimilarity were correlated to visibility, salinity, and temperature. Ascidian communities established in the warm, hypersaline waters furthest from the mouth of Mission Bay were typically most different from those near the mouth. Comparisons of % dissimilarity at the beginning and end of sampling indicated that ascidian communities were more dissimilar at the end of sampling. Ascidians persisted year round, and invasive and cryptogenic species were generally more abundant in winter months than native species. Below average rainfall, and the consequent relatively warm sea surface temperature and high salinity in a relatively mild climate region like San Diego, probably prevented the winter ascidian die-offs that typically occur in this area. Such environmental changes may be advantageous to invasive ascidians, not only with regard to mediating their persistence, but also in terms of facilitating the establishment, spread, and dominance of future invaders.

Scientists recognize the importance of ecological data prior to invasion by non-native species in order to evaluate changes in the recipient community. Here we assess the potential impact of the invasion of the bivalve Isognomon bicolor (C.B. Adams, 1845) on Brazilian rocky shores through the use of surveys both before and after the arrival of this non–native species. The invader was mostly distributed across the mid and low shore levels of the intertidal zone with relative abundance ranging from 9.0 to 36.7 percent cover. The mid shore, previously dominated by the native barnacle Tetraclita stalactifera (Lamarck, 1818), was co-dominated by this barnacle species and I. bicolor after invasion. The relative abundance of these species, and presumably the interaction strength between them, differed between sites. At the site where I. bicolor reached the highest abundance (around 30% on average), the abundance of T. stalactifera decreased on average 70% compared to baseline values obtained before the I. bicolor invasion. Finally, conspicuous and extensive I. bicolor beds such as those observed in this study have not been reported in its original distribution range. Beds of I. bicolor may create a much more intricate biogenic matrix than the extents of bare rock and barnacle clumps it replaced. This bivalve may act as an ecosystem engineer and, thus a functionally different component of the intertidal community in its invaded range compared to its native distribution.

Coastal Patagonia, often regarded as a pristine area of the world, has been invaded by non-indigenous species that are rapidly modifying local ecosystems. One of the most conspicuous invaders is the kelp Undaria pinnatifida. First recorded near the city of Puerto Madryn (Argentina, 42.75°S) in 1992, Undaria’s range has expanded more than five degrees of latitude to the south during the last 15 years. By 2007 it was first detected north of Valdés Peninsula (a natural barrier to dispersal), beginning its northward expansion, and between 2007 and 2011 spread along the coasts of San José and San Matías gulfs at an average rate of ~50 km yr-1. A small population was detected in Mar del Plata (Argentina, 38.04°S) in 2011, more than three degrees of latitude to the north, suggesting a new human-mediated inoculation. A thermal tolerance window is hypothesized based on experimental information and remotely sensed sea-surface temperature (SST) at the latitudinal range limits of U. pinnatifida populations worldwide. The window is defined by average SSTs ranging between -0.6 °C and 16.8 °C in the coldest month, and between 13°C and 28°C in the warmest month. Using climatologic satellite SST from several locations, a potential latitudinal thermal range extending between Puerto Deseado (Argentina, 47.75°S) and Cape Torres (Brazil; 29.35°S) is predicted. Salinity and substrate quality however constrain suitable habitat in the Southwest Atlantic to discrete stretches of coastline, suggesting that the northern potential boundary would be around La Coronilla (33.90°S), on the Uruguayan coast.

The recruitment and population structure of the non-indigenous brackish water mytilid Xenostrobus securis (Lamark, 1819), native to Australasia, were studied in the lower reaches of the Kino River, central Japan. Post-larvae > 2 mm in shell length were found exclusively among barnacle stands of Fistulobalanus kondakovi (Tarasov and Zevina, 1957) and algal fronds of Gloiopeltis tenax (Turner) Decaisone, 1842 from January to May and from July to October. Density of the post-larvae was much greater in the latter period than in the former. Small mussels with shell lengths of 2–4 mm settled to the conspecific mussel beds in April and September. Primary and secondary settlements onto different substrates were observed in the early stages of the life cycle. Two or three cohorts were identified in conspecific mussel beds, and they grew at a rate of ca. 1 mm per month. Density in the beds fluctuated monthly and was greater in the lower than in the middle intertidal zone, suggesting greater spatial heterogeneity in density due to predation in the former.

The establishment of non-indigenous aquatic invertebrates may strongly affect community and food web structures in invaded systems. The invasion of the Ponto-Caspian amphipod Dikerogammarus villosus in European waterways, has led to the displacement of other macroinvertebrates, especially other amphipod species. The key factor for the decline of both native and invasive amphipods is assumed to be strong intraguild predation (IGP) by D. villosus, as observed in laboratory studies. However, the importance of IGP under field conditions is nearly unknown. Here, we investigated the trophic annidation of the invasive D. villosus in a benthic food web in one season in 2011, focusing especially on the relevance of IGP at an invasion front in Switzerland. We combined stable isotope analyses with molecular analyses of D. villosus´ gut content using a group-specific primer for gammarid amphipods. Analyses of the standard ellipse area showed a strong overlap of the isotopic niches of the amphipod taxa. Niche width analysed by Stable Isotope Bayesian Ellipses in R (SIBER) revealed no significant differences between D. villosus and the native gammarid species. Furthermore, we did not detect DNA of native gammarid species in any of the 62 analysed gut contents of D. villosus as direct evidence for predation. However, diet of amphipods may vary over the year and we cannot exclude completely the hypothesis that D. villosus does predate upon native gammarids. Nevertheless our results indicated that IGP is not the key factor in the displacement of native amphipods and does not seem to play an important role in invasion of the investigated Swiss water body. Hence, factors other than IGP seem to be the driving forces for the observed displacement of native amphipods.

Invasive species are often in focus in the non-indigenous species (NIS) research while low-abundance species receive relatively little attention. However, tracking NIS dynamics since the early stages of an invasion provides valuable information on the ecology of invasions. In the current paper, we investigated the invasion history and population dynamics of the small-bodied cladoceran Evadne anonyx G. O. Sars, 1897 in the Gulf of Riga (Baltic Sea) almost since its first detection in 2000. The species already was widespread the Gulf of Riga in 2001 and has been found in nearly every subsequent sample collected during summer months. However, the abundance of the species remained low, seldom 100 individuals m-3. Both, salinity and water temperature affected the spatial distribution and population abundance of E. anonyx. The species was found to occur only sporadically at salinities below ca. 6. To obtain reliable presence/absence and density estimates on this small-bodied cladoceran, the entire zooplankton sample needed to be analysed.

Aquatic invasive species are a major threat to native freshwater ecosystems and cause enormous ecological and economic damage worldwide. Didymosphenia geminata (Lyngbye) M. Schmidt is an emerging invasive aquatic species that is spreading aggressively in Southern South America. Using niche analysis and species distribution models (SDMs), we examined niche requirements of D. geminata using climatic, topographic, and biological variables. We compared the realized niche of the species in the United States (US) with the niche in Chile. Within Chile, we also examined the environmental conditions (environmental envelopes) of rivers with and without this alga, to assess whether this species has the potential to colonize more rivers. Finally, we compared the SDMs from the US and Chile projected to Chile. Results showed that the potential distribution of D. geminata varied significantly between US and Chile. The US-based model predicted a geographic distribution in Chile which ranged from 32º to 55º S latitude while the Chile-based model predicted suitable habitats only from 36° to 48º S, and not so in the coastal ranges nor in Southern Patagonia (52–55°S). In Chile, we found no differences in the environmental envelopes of the invaded and uninvaded rivers, thus suggesting that this species has even more suitable habitats to invade. These results can be used for prioritizing survey sites in Southern Chile for an early detection and management of D. geminata and for the conservation of native flora and fauna in freshwater ecosystems in Chile.

In this study, we performed a scientometric analysis of published scientific papers on the biological invasion of macrophytes in freshwater ecosystems to assess the main trends and gaps in research concerning this group of organisms. The analysis showed that publications on invasive macrophytes increased exponentially in the last decade. However, the activity index (a comparison of the quantitative trends of invasive macrophytes in relation to all of the papers on macrophytes) indicated that invasive macrophytes have not been consistently studied more than other topics in the field of limnology over the last decade. The most studied macrophyte species were Myriophyllum spicatum, Hydrilla verticillata, Phragmites australis and Eichhornia crassipes. Certain gaps were related to the limited number of studies on the important species threatening tropical ecosystems, under-representation of investigations on the impacts of invasive macrophytes on fish and lack of studies associating macrophytes with microorganisms (bacteria and fungi). Studies that encompassed several levels of biological complexity were also scarce, indicating that the studies were fragmented at specific levels. Finally, there was a clear geographical bias, with fewer studies occurring in Neotropical and Afrotropical regions. Identification of these gaps may be useful for addressing future studies that might help evaluate the causes of invasion by macrophytes and the impacts of such invasions on freshwater ecosystems.

Great Britain has been subject to an increasing rate of invasion from freshwater species of Ponto-Caspian origin. A recent horizon-scan of potential invaders into Great Britain named the Ponto Caspian quagga mussel, Dreissena rostriformis bugensis (Andrusov, 1897), as the non-native species least wanted. On 29th September 2014 quagga mussels were discovered in the Wraysbury River, Surrey, during a routine kick sample collected by the Environment Agency. Identity was confirmed using genetic markers (Cytochrome Oxidase I - COI) on five individuals encompassing a broad morphological variation. The absence of very large individuals (max. length 16 mm), absence of shells and absence of quagga mussels in samples collected during March 2014 point toward a recent invasion. The quagga mussels were found attached to submerged rocks, vegetation, bridge walls and shells of zebra mussels, Dreissena polymorpha (Pallas, 1771). The collection site is a small (<5m wide), shallow (<50cm deep) stream that is not navigable or regularly fished. This suggests that the species is more widely distributed than the current location, because such a system is unlikely to be the point of introduction. The shallow depth (30cm) in open water at which quagga mussels were abundant is surprising given that zebra mussels have failed to establish in such habitats, despite being present in the catchment for over 100 years. Previously published models predict quagga mussels will establish widely across England, western and southern Wales and central Scotland. The high abundance and inter-connectivity of waterways adjacent to the Wraysbury River suggest further spread is likely. Containment through national biosecurity measures (e.g. ‘Check, Clean, Dry’ of boats and equipment, as promoted by the UK Environment Agency) is recommended, although ultimately it can be assumed that quagga mussels will cause similar widespread ecological and economic harm in Britain as has been experienced in invaded regions of Western Europe and North America.