The Chinese mystery snail, Bellamya (Cipangopaludina) chinensis, was recorded for the first time in 2007 in the Netherlands. By 2016, twelve water bodies (mostly riverine ecosystems) had been colonized by this freshwater snail. These records were the first known introductions of this alien species in the European Union (EU). Insight into the invasiveness and (potential) risks of ecological, socio-economic and public health effects of B. chinensis in Europe is urgently needed due to multiple introductions, permanent establishment and continuing secondary spread. A field survey was carried out to determine dispersal rate, habitat conditions and population characteristics of B. chinensis in the floodplain Eijsder Beemden along the Meuse River. The natural dispersal rate in this area was 0.1 km/yr and the average population density was 0.33 individuals/m². This species has colonized several floodplain lakes that are hydrologically connected to the Meuse River. New introductions and colonization of the main channels of large rivers are expected to accelerate the dispersal of this species through water flow and shipping vectors. A risk assessment of B. chinensis was performed using the Harmonia+ protocol. Evidence of deliberate and unintentional introductions led to a high score for introduction risk. Risk of establishment was also assessed as high. The risk assessment resulted in a medium score for spread risk due to dispersal by human action. The assessed impact on plant targets or animal targets was very low. A medium risk was assigned to impacts on environmental targets. Risk of impacts on human targets received a low score. The overall invasion risk was classified as high and environmental impact was medium, resulting in a medium overall risk score. Regulation of B. chinensis trade and an increase in public awareness about its impact are required to prevent new introductions and further spread of this species in Europe. Moreover, there is an urgent need for research concerning the effects of B. chinensis on native biodiversity and ecosystem functioning, and cost-effective management of this species (e.g., eradication, population control and containment measures).

The spiny water flea (Bythotrephes longimanus), an aquatic invasive zooplankton species native to Eurasia, was first recorded from Manitoba waters at the Pointe du Bois Generating Station on the Winnipeg River using a larval fish drift net with 950 µm mesh, on 18 July, 2009. Bythotrephes drift density upstream and downstream of the station was highly variable with maximum densities of 23.3 individuals/100 m³ (9 samples) in June 2010 and 9.4 individuals/100 m³ in June 2012 (60 samples). In August and October of 2011, Bythotrephes were identified from the stomachs of eight cisco (Coregonus artedi) collected from the South basin of Lake Winnipeg near the mouth of the Winnipeg River, indicating that the invader had become part of the local food web. Subsequent targeted sampling for Bythotrephes with ship-based vertical plankton net (76 µm mesh) tows at 65 offshore/pelagic sites in Lake Winnipeg during summer and fall of 2012 indicated that the invader had colonized the South basin by early August and had expanded its distribution into most of the North basin by early October. No individuals were captured at a site just past the lake outlet in the Nelson River. Densities of Bythotrephes in Lake Winnipeg ranged from 0 to 93.1 and 0 to 39.2 individuals/m³ among sites for the summer and fall sampling, respectively. The highest mean density (9.2 individuals/m³) was observed for the South basin in the summer of 2012. Non-targeted kick-net sampling at Playgreen Lake, located on the Nelson River approximately 27 km north of the Lake Winnipeg outlet in August of 2012, confirmed the rapid northward expansion of Bythotrephes through Lake Winnipeg. This represents the first evidence for further downstream dispersal. However, as of August 2015, multiple kick-net and Ekman/Ponar grab samples from Cross and Sipiwesk lakes further downstream on the Nelson River have not captured any Bythotrephes.

The relative role of propagule pressure, abiotic and biotic variables as determinants of alien species occurrence differs among studies. This limits the synthesis of emergent patterns in invasion ecology and generalisations for conservation actions. In order to produce a broad and general assessment of the invasion pattern in aquatic habitats we here propose a macroecological approach to assess the drivers of occurrence of alien species within the whole biota (microorganisms, plants and animals) across several natural freshwater ecosystems of Italy. We simultaneously tested three groups of variables (propagule pressure, abiotic and biotic characteristics), selected as putative predictors of invasibility of a site. Propagule pressure, expressed as proximity to larger inhabited areas, and differences in the native species richness of the receiving community, had a significant role in determining the number of alien species occurrences. Furthermore, body size influenced the occurrence and colonization processes of alien species. Finally, climatic characteristics were relevant in determining the chances that a site was invaded, confirming the role of these abiotic filters in the invasion process.

The ability of non-native species to establish populations, expand their range, and ultimately affect native species is influenced by the behaviour and dispersal potential of individuals. Spatial sorting theory predicts that individuals with greater dispersal tendencies will accumulate at the invasion front and promote further territorial expansion. The theory predicts that the reproduction of such individuals at the front as a result of their proximity leads to selection for better dispersers. To test for evidence of this phenomenon in successful invasive populations, we compared boldness and dispersal tendency in one native North American and two invasive European populations of pumpkinseed (Lepomis gibbosus) reared in a common environment, with a second wild native population from a local water body added to the comparison. Tests were conducted in a 4-metre long artificial flume with a video capture system recording movement. Boldness was measured using the time it took an individual to emerge from a sheltered area, and dispersal tendency was assessed using a combination of movement metrics as the fish explored the flume. Contrary to expectations, native North American populations were bolder than invasive European populations, and there was no significant difference in dispersal tendency between native and invasive pumpkinseed. Females were bolder and showed greater dispersal tendency than males. Post-hoc analysis indicates that populations originating from lotic water bodies had significantly lower dispersal tendency than those from lentic water bodies regardless of native/invasive status, suggesting that habitat of origin may affect dispersal tendency.

We conducted two indoor experiments to test the competitive strength of the invasive plant Crassula helmsii in comparison to that of two native moorland species from northwest Europe, Littorella uniflora and Hypericum elodes in terrestrial conditions. In both experiments nutrient-poor moorland soil was used. The total cover of C. helmsii increased gradually in monocultures, until after 7–20 weeks a maximum of 12% in experiment 1 and 15–20% in experiment 2 was reached in the control conditions. Nitrogen content of C. helmsii plants was very low at the end of the latter experiment. Adding additional N had little effect in experiment 1, but in experiment 2 C. helmsii cover doubled. In the mixed cultures, C. helmsii fared worse than the native species. In experiment 1 the increase in cover was higher for both L. uniflora and H. elodes than in their monocultures, while in experiment 2 fresh weight at the end of the experiment was 3 times higher for L. uniflora and did not differ significantly between H. elodes and C. helmsii. The results indicate that the native species are better competitors for nutrients than C. helmsii due to their larger root system. No allelopathic effect of L. uniflora on C. helmsii growth was observed. These observations are discussed in the light of C. helmsii management in the field.

The potentially toxic, invasive cyanobacterium Cylindrospermopsis raciborskii, originating from sub-tropical regions, has spread into temperate climate zones in almost all continents. Potential factors in its success are temperature, light and nutrient levels. Grazing losses through zooplankton have been measured in the laboratory but are typically not regarded as a factor in (failed) invasion success. In some potentially suitable lakes, C. raciborskii has never been found, although it is present in water bodies close by. Therefore, we tested the invasive potential of three different isolates introduced into natural plankton communities using laboratory mesocosm experiments under three grazing levels: ambient zooplankton densities, removal of large species using 100 µm mesh and a ca. doubling of large species. Three C. raciborskii isolates originating from the same geographic region (North-East Germany) were added separately to the four replicates of each treatment and kept in semi-continuous cultures for 21 days. Two isolates disappeared from the mesocosms and were also not viable in filtered lake water indicating that the lake water itself or the switch from culture medium to lake water led to the decay of the inoculated C. raciborskii. Only one out of the three isolates persisted in the plankton communities at a rather low level and only in the treatment without larger zooplankton. This result demonstrates that under potentially suitable environmental conditions, top-down control from zooplankton might hamper the establishment of C. raciborskii. Non-metric multidimensional scaling showed distinct variation in resident phytoplankton communities between the different grazing levels, thus differential grazing impact shaped the resident community in different ways allowing C. raciborskii only to invade under competitive (= low grazing pressure) conditions. Furthermore, even after invasion failure, the temporary presence of C. raciborskii influenced the phytoplankton community.

Invasions of tropical and subtropical aquatic plants threaten biodiversity and cause ecological and economic impacts worldwide. An urgent question is whether native herbivores are able to inhibit the spread of these alien species thus providing biotic resistance. The potential for biotic resistance to these plants depends on plant traits that affect palatability to herbivores, i.e., plant nutritional quality and the presence of secondary metabolites related to anti-herbivory defenses. Studies across latitudinal gradients suggest that aquatic plants from lower latitudes may be less nutritious and better defended than high latitude plants. Therefore, we hypothesized that native herbivores prefer temperate plants over plants from tropical and subtropical regions which would limit the strength of biotic resistance that native temperate herbivores can provide against alien tropical and subtropical plants. Drawing upon the published literature we (1) investigated whether native temperate herbivores reduce the establishment or performance of tropical and subtropical alien plants in the field, and (2) analyzed herbivore consumption of tropical and subtropical versus temperate plants in laboratory feeding trials. In our literature survey, we found only three field studies which, in contrast to our hypothesis, all demonstrated that the native herbivores (beavers, coots or generalist insect herbivores) significantly reduced the success of invading tropical and subtropical plant species. The analysis of the feeding trials yielded mixed results. Ten out of twelve feeding trials showed that (sub)tropical and temperate plants were consumed in equal amounts by both temperate and tropical generalist ectothermic herbivores. The remaining trials showed higher consumption rate of both temperate and tropical plants by tropical snails. Although a body of evidence suggests that tropical plants are nutritionally poor and better defended (i.e., less palatable) compared to temperate plants, we conclude that in the majority of cases, herbivores would eat tropical plants as much as temperate plants. Thus, in agreement with the available field studies, evidence suggests that there is potential for biotic resistance from native generalist herbivores to tropical invasive plants in non-tropical areas.

The host range expansion of the specialist milfoil weevil, Euhrychiopsis lecontei, from the native Myriophyllum sibiricum (northern watermilfoil) to invasive M. spicatum (Eurasian watermilfoil) is one of the few examples of a native insect herbivore preferring, growing and surviving better on a nonindigenous host plant than it does on its native host plant. The milfoil weevil’s preference for the nonindigenous plant can be induced during juvenile development or through exposure to Eurasian watermilfoil as an adult. We evaluated how the fecundity of the milfoil weevil was affected over time by juvenile and adult exposure to the native, invasive and invasive × native hybrid milfoils and whether fecundity was correlated with host plant quality. Weevils reared on Eurasian watermilfoil laid more eggs than those reared on northern or hybrid watermilfoils. When weevils were collected from and exposed to milfoils collected directly from a lake, Eurasian-reared weevils had higher fecundity and greater preference for Eurasian over northern watermilfoil. When weevils were reared on and allowed to oviposit on milfoils grown in a common environment, the differences in fecundity and preference for Eurasian over northern or hybrid watermilfoils diminished. In Eurasian-northern experiments, milfoil weevils laid more than 80% of their eggs on Eurasian watermilfoil, but that value decreased when plants from common environments were used. Despite this preference, most weevils continued to use both hosts indicating that a complete host switch is unlikely. Weevils showed no oviposition preference between Eurasian and hybrid milfoils. The milfoil weevil had higher fecundity on Eurasian watermilfoil, which had a higher concentration of carbon, polyphenols and lignin than did northern watermilfoil, which had a higher concentration of ash. The milfoil weevil’s preference for Eurasian watermilfoil was affected by changes in plant chemistry, and ash appeared to act as a deterrent to oviposition on northern watermilfoil. The milfoil weevil can modify its response based on host-plant chemistry.

Freshwater plants affect the ecosystem functioning of shallow aquatic ecosystems. However, because native plants are threatened by environmental change such as eutrophication, global warming and biological invasions, continued ecosystem functioning may be at risk. In this study, we explored how the growth of native and non-native plant species in eutrophic, warm conditions impacts two plant ecosystem functions: regulation of phytoplankton growth and methane emission. We expected that plants would inhibit phytoplankton growth, while for methane emission both inhibition and stimulation are possible. We conducted an outdoor experiment using monocultures of four native and four non-native freshwater plant species planted at three different densities, as well as a no-plant control. Monocultures of each species were planted in 65 L mesocosms and after three weeks of acclimatisation each mesocosm was inoculated with phytoplankton. Subsequently, we added nutrients twice a week for eight weeks, before harvesting the plant biomass. During these eight weeks, we measured chlorophyll-a concentration thirteen times and the diffusive methane emissions once after four weeks. The mesocosms amplified the temperature of a warm summer so that plants were exposed to higher-than-average temperatures. We found that five plant species lost biomass, two species increased their biomass only at the highest initial plant density (native Myriophyllum spicatum and non-native Lagarosiphon major) and a single species increased its biomass at all densities (on average 14 times its initial mass; amphibious non-native Myriophyllum aquaticum). Overall, the mean biomass change of non-natives was positive, whereas that of natives was negative. This difference in biomass change between native and non-native plants did not relate to overall differences in phytoplankton mass or diffusive methane emissions. In mesocosms where submerged plant species gained biomass, chlorophyll-a concentration was lower than in the no-plant control and mesocosms with biomass loss. Diffusive methane emissions were highest in mesocosms where plants lost considerable biomass, likely because it increased substrate availability for methanogenesis. However, mesocosms where plant biomass increased had emissions similar to the no-plant control, hence we found no inhibitory effects of plant presence on diffusive methane emission. We conclude that plant growth in eutrophic, warm conditions varies strongly with plant identity. Our results furthermore suggest that plant identity determines whether the replacement of native by non-native freshwater plants will alter ecosystem functions such as regulation of phytoplankton growth and methane emission.

More than one-third of non-native freshwater organisms are known to cause ecological impacts, but there is still a knowledge gap on how impacts are related to non-native plant abundance. We investigated the relationship between coverage of three aquatic non-native invasive species Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum and phytoplankton and zooplankton abundances. We hypothesized that uninvaded ponds, with a diverse submerged macrophyte community, would have higher plankton biodiversity compared to invaded ponds. In addition, we predicted that invasive plant cover would have a negative impact on plankton biodiversity. We sampled the phytoplankton and zooplankton communities of 18 ponds in Belgium, concurrently with 11 local environmental variables including cover of the invasive species. Our results showed that uninvaded ponds, with a high abundance of submerged vegetation, were associated with indicators of a clear water state and several zooplankton taxa. Variation partitioning showed that both environmental variables and invasive species cover explained zooplankton community variation. We observed a negative relationship between invasive species cover and zooplankton density as expected. In particular, large cladocerans and littoral taxa were absent when the cover of invasive species was high. This is possibly due to low dissolved oxygen concentrations, physical obstruction and a putative allelopathic property of the invasive plants. Invasive species cover was not significantly correlated with phytoplankton biovolume, although some evidence indicated that the biovolume of functional groups (FG) reflected the differences in habitat between invaded and uninvaded ponds. Reynolds functional Y group (Cryptomonas spp.) shows a tolerance to low light availability and was the dominant FG in the invaded ponds. Our study provides evidence that it is necessary to include cover of an invasive species to disentangle patterns of impact.

Several species of invasive crayfish have become established in the Netherlands, the most recent addition being Orconectes virilis. Since crayfish are known to impact water quality and aquatic macrophytes in areas they invade, this study investigated whether this was also the case for this species under Dutch conditions and if so, whether a crayfish density producing “no effects” could be established. We focussed on the potential impact of O. virilis on water quality variables (pH, electrical conductivity, dissolved oxygen and turbidity), as well as their impact on submerged and emergent macrophytes. In a compartment experiment with different densities of O. virilis, statistically significant effects were observed at crayfish densities of 1.25 crayfish/m² on electrical conductivity, turbidity, submersed macrophyte biomass, and the emergent plant Sagittaria sagittifolia, due to crayfish actively severing plants and physically disturbing the sediment. No statistically significant differences with controls were observed at a density of 0.63 crayfish/m². Since densities of 0.03 to 5 crayfish/m² have been found in different water types in the Netherlands, this indicates that the water quality and macrophyte biomass in Dutch waters are being negatively impacted by invasive crayfish. As a consequence, attempts to reach a good ecological status as required in the Water Framework Directive will be frustrated by the presence of this invader.

Regulated rivers in Western Europe have rapidly been colonized by invasive alien Ponto-Caspian gobies. In particular, the round goby (Neogobius melanostomus) can reach high densities in habitats with hard substratum, such as groynes and dams made of basalt stones. High densities of Ponto-Caspian gobies negatively impact native benthic fishes. It is hypothesized that natural complex three-dimensional structures in Western European rivers, such as (pieces of) large wood (e.g., trees that fell into the river), are a less attractive habitat for Ponto-Caspian gobies. These bottom-dwelling fishes are strongly associated with sheltered places on the river bottom and may avoid the three-dimensional structure of large wood in the water column. The colonization of littoral zones provided with large wood (i.e., entire trees) by round goby and native fishes was studied in the River Lek (a distributary of the River Rhine) in the Netherlands during the period 2014–2016. The fish assemblage of four reference sites dominated by basalt stones was compared with that of four large wood sites. Counts of round goby in large wood habitats were significantly lower than in habitats dominated by basalt stones, while native fishes were more abundant in large wood habitats. In large wood habitats counts of native fishes were significantly higher than those of round goby, whereas the reverse was true in the reference habitat. Counts of the entire fish assemblage did not significantly differ between habitat types. These results suggest that large wood in regulated Western European rivers predominantly functions as a suitable habitat for native fishes whereas the invasive bottom-dwelling round goby only uses large wood habitats to a limited extent. Large wood may be applicable as a management tool to stimulate native fish fauna with minimal facilitation of the round goby.