In the past several decades, Ponto-Caspian species have established in freshwater and brackish habitats of the North and Baltic Sea and Great Lakes-St. Lawrence River region in much higher numbers than expected based on introduction effort (i.e. shipping frequency) and environmental conditions between these regions. Several studies have suggested that Ponto-Caspian taxa are euryhaline and might be able to adapt rapidly to changing salinity conditions, and therefore may be more successful colonizers than species from other regions. To determine the realized niche in the native and introduced habitats of Ponto-Caspian invaders and to assess whether they have expanded their salinity tolerance during the invasion process, we conducted a literature search to assess the salinity ranges of 55 Ponto-Caspian species in their native and introduced ranges. Our results confirmed that the majority of those species occupied a wide range of salinity in their native and introduced habitats. Approximately 50% of species were reported from both freshwater and brackish habitats (0–18 PSU) in their native region, occupying a salinity range of at least 10 PSU difference. In general, the number of species tended to decline as salinity increased in both native and introduced habitats. More than 90% of Ponto-Caspian invaders occupied freshwater (< 0–0.5 PSU) habitats, while 18% did not occur above 18 PSU. Overall, 46% of species were reported from a narrower salinity range in their introduced compared to their native region. Consequently, our study revealed significantly broader salinity ranges in native compared to introduced habitats. As our results suggest that the majority of Ponto-Caspian invaders occupy freshwater and brackish habitats in their native region, the recent numerous invasions of freshwater and brackish areas by these species should not be a surprise. Therefore, managers and policy makers should take into account that the majority of Ponto-Caspian invaders originate from fresh water or low salinities and develop new regulations to prevent future invasions from the Ponto-Caspian region, as well as from areas highly invaded by Ponto-Caspian taxa such as Northern Europe.

Two of the most invasive freshwater bivalve species in temperate regions worldwide are the Ponto-Caspian dreissenids Dreissena polymorpha and Dreissena rostriformis bugensis. Throughout their range, observations have been made of a dominance shift favouring D. r. bugensis where the two species co-occur. Although both dreissenids have been widely studied, the mechanisms driving this dominance shift are not completely understood. Our long-term and short-term field experiments assessed a selection of species traits related to growth and settlement, which may be linked to a competitive benefit for either of both species. We assessed relative population densities in time and space, mortality, and inter- and intraspecific interactions in relation to environmental factors like temperature, salinity, and light intensity, using 14 × 14 cm PVC settlement plates. Dreissenids were identified, counted and measured over 11 years in the waterbody where D. r. bugensis was first discovered in Western Europe. Dreissena polymorpha appeared to have a competitive benefit over D. r. bugensis by being able to settle earlier, i.e. in spring, while other studies indicate the opposite. As salinities in the study region were relatively high in spring, this discrepancy may be explained by a higher salinity tolerance of D. polymorpha, as is known from the literature. In addition, Common Coots predated D. r. bugensis clusters. Dreissena polymorpha were not usually found in such clusters. Regardless of these competitive benefits for D. polymorpha, D. r. bugensis is the dominant species in the region. One possible explanation could be the fact that D. r. bugensis was found in similar densities on both light exposed and shaded fouling plates. Dreissena polymorpha on the other hand, was practically absent on light-exposed plates after 12 months. Therefore, a wider range of habitats seems to be suitable for the settlement of D. r. bugensis. Another driver of the dominance shift can be linked to it having faster growth than D. polymorpha, especially during spring. More generally, D. r. bugensis occurred in higher population densities on plates with D. polymorpha than on plates with only D. r. bugensis. Dreissena polymorpha individuals may thus induce the settlement of additional individuals of D. r. bugensis. A final reason for the dominance of D. r. bugensis concerns its low winter mortality. While the number of D. polymorpha individuals more than halved over the winter 2016–2017, no significant decrease in numbers of D. r. bugensis was recorded.

We conducted a mesocosm experiment to determine how multiple stressors including invasive zebra mussels (ZM), a nutrient pulse (NP), and the addition of herbivorous large-bodied zooplankton (Daphna pulex and Simocephalus vetulus; LBZ) affected resident plankton (phytoplankton and zooplankton) in both single- and multiple-factor treatments. We also assessed whether the establishment of LBZ was influenced by ZM and/or NP. Algal and rotifer biomass was significantly greater in treatments with nutrients than in treatments with both nutrients and zebra mussels indicating antagonistic effects of the two stressors. Zebra mussels also had several effects on zooplankton that were independent of the other stressors: zebra mussels reduced the biomass of the invertebrate predator Chaoborus sp., while the biomass of resident cladocerans and large-bodied zooplankton tended to be higher in at least some of the treatments with zebra mussels. Large-bodied zooplankton did not affect algal biomass or resident zooplankton in any of the treatments. Our results contribute to a growing body of research showing that invasive zebra mussels can be the primary drivers of plankton dynamics when multiple stressors are present in aquatic ecosystems.

The zebra mussel, Dreissena polymorpha (Pallas, 1771), is considered to be one of the world’s worst invasive species with a large impact on local biodiversity and ecosystem services in Europe and North America. Recently, a large-scale displacement of the invasive zebra mussel by the similarly invasive quagga mussel, Dreissena rostriformis (Deshayes, 1838), is occurring in large parts of Western and Central Europe. While the exact reasons for the competitive advantage of the quagga mussel remain unknown, its potentially higher fitness might play a role. This replacement of one invasive species by a closely related invasive species offers a unique opportunity for unravelling patterns and processes of competition. To test whether the quagga mussel derives its competitive advantage from higher growth rates, a fully closed and controlled microcosm system was used to subject specimens of both species to different intensities of intraspecific and interspecific competition. The study revealed that both species reacted qualitatively similar to the different treatments. However, under all competition scenarios the quagga mussel showed substantially higher growth rates and larger growth ranges. Therefore, these characteristics might provide the quagga mussel with a higher flexibility in fluctuating environments and allow it to reach adult size earlier. This, in turn, can make the quagga mussel less prone to parasite pressure and other biological constraints during growth, and provides an advantage in the competition for space (hard substrates) and food.

The introduction and spread of alien mollusc species is strongly related to human activities such as connecting river basins through canal construction and shipping. Economic growth has caused an increase in commercial and recreational navigation on rivers and led to the development of extensive networks of waterways. Ships alter flow velocity in littoral zones via water displacement and propeller jet streams, thereby affecting structure and functioning of riverine mollusc communities and their ecosystem services, such as water purification and nutrient cycling. A literature review was performed to derive data for determining field based upper flow velocity occurrences for 37 native and 8 alien mollusc species present in the rivers Rhine and Meuse. Next, these upper flow velocities were used to construct species sensitivity distributions (SSDs) representing the species assemblages of native and alien molluscs in the littoral zone of these rivers. The SSDs were used to derive the potentially occurring fractions (POFs) of both species assemblages in groyne fields or in channels behind longitudinal training dams (LTDs), due to shipping induced changes in flow velocity conditions. POFs were calculated for various types of ships, in three river Rhine distributaries (Nederrijn: impounded; Waal and IJssel: free flowing) and the river Meuse (impounded). The SSDs of native and alien species assemblages did not differ significantly. Alien species with the lowest and highest tolerances were Musculium transversum (Say, 1829) and Dreissena polymorpha (Pallas, 1771), respectively. Valvata cristata Müller, 1774 and Radix balthica (Linnaeus, 1758) were the native species with the lowest and highest flow velocity tolerance, respectively. Freight ships were associated with the lowest POF in impounded rivers (0.76) as well as in free-flowing rivers (0.61). Shipping was associated with lower POFs in groyne fields of free-flowing rivers than those of impounded rivers. The highest POFs were found in channels behind an LTD in a free-flowing river. Shipping is associated with a shift of the mollusc species assemblage towards flow resistant species and could thereby affect ecosystem functioning and services.

The ability to track changing distributions and long-term interactions between native and introduced species provides insights into forecasting impacts of invaders. We used data from deliberate field introduction experiments of a freshwater Gammarus amphipod (Crustacea) to the rivers of a small British island, The Isle of Man. The deliberate introductions commenced in 1949 and re-sampling occurred in the ‘60s, ‘70s ‘80s, ‘90s, 2005 and finally in 2015. The findings generally confirmed the failure of introductions of Gammarus pulex into sites previously dominated by the native Gammarus duebeni celticus even with high propagule pressure in terms of numbers of individual invaders released and repeated introductions. Despite this, the island-wide 2015 survey revealed that G. d. celticus occurred in fewer sites and G. pulex in more sites than a decade earlier, indicating at this island-wide scale, the displacement of G. pulex by G. d. celticus seems to be continuing. In addition, one of the introduction sites, Arbory Stream which contained G. d. celticus from 1951–2005 contained only G. pulex in 2015, showing that even after six decades, assumptions cannot be made as to the limits of invader range expansion. Multivariate analysis of physico-chemical data from the 2015 island-wide survey revealed water quality as a crucial environmental gradient that influences the distributions of the native and invader species, with the former in higher organic water quality sites than sites with the invader or where invader and native co-occurred. The North American amphipod Crangonyx pseudogracilis had expanded its range since 2005, including invading low biological water quality sites previously devoid of both Gammarus spp.. Pollution from historic metal mining is implicated as a potential major factor excluding amphipods. This deliberate ecological experiment, through long-term monitoring, continues to provide insights into the factors determining the distributions of natives and invaders.

The round goby (Neogobius melanostomus), a small Ponto-Caspian bottom-dwelling fish species, was first observed in the St. Lawrence River in 1997. After twenty years, it is now considered as one of the most successful invaders of the St. Lawrence River. Despite the elevated abundances observed throughout the river, little is known about its impacts on the littoral/mid-shore fish community. In this study, we used a large dataset obtained from an annual standardized fish survey held in the St. Lawrence River to analyze the impacts of round goby on littoral native fish community and native predator assemblages within five distinct segments of the river characterized by different round goby abundances. Throughout the system, the round goby negatively impacted the abundances of tessellated darter (Etheostoma olmstedi) likely as a consequence of competitive exclusion. In contrast, the small pelagic/demersal brook silverside (Labidesthes sicculus) and emerald shiner (Notropis atherinoides) increased in abundance. Indirect food web effects, such as predation pressure relaxation, are suspected. Native predator abundances exhibited contrasting trends in the different sectors analyzed, with the exception of smallmouth bass (Micropterus dolomieu) abundances that increased in all fluvial lakes. We hypothesize that trophic relationships between littoral fish and native predators are influencing the outcomes of the round goby invasion of the St. Lawrence River.

In the last two decades, rapid range expansions of the invasive Ponto-Caspian round goby (Neogobius melanostomus) have been reported from many large waterbodies in Europe and North America. A high invasion potential of this species has been reported through its opportunistic life-history traits and high phenotypic plasticity. Nevertheless, it appears that unlike many other invasive fish species, N. melanostomus has not been able to colonize small and mid-size tributaries of large rivers (streams of order higher than two or three). Given that the invasion history of N. melanostomus in Europe is still rather short, and its invasion potential so high, its future possible expansion represents a big question with important ecological, conservation and management implications. One of the ways to answer this question is to identify the key environmental parameters important for further expansion of N. melanostomus, and to compare these with other successful invaders with longer histories, e.g. topmouth gudgeon (Pseudorasbora parva) and pumpkinseed (Lepomis gibbosus). In this study, basic environmental, physicochemical and chemical parameters, biological elements and human disturbances in all types of streams in Slovakia were analysed in order to identify the key factors that discriminate habitats where the three invasive species were present relative to those where they were not. Random Forest Analysis was used to predict and assess the relationship among the large number of potential predictor variables and a dependent variable. This method is useful especially when large numbers of correlated predictors are evaluated and both quantitative and qualitative predictors are involved. The main aim of this study was to estimate potential for further spread of the three invasive species, with an emphasis on the possibility of further expansion of N. melanostomus into small and mid-size tributaries of large rivers. The habitat parameters that characterize current distribution limits of N. melanostomus were found to be the wetted width, slope, pH, temperature and conductivity. The species was found to prefer wider wetted width and shallower slope, higher oxygen saturation and slightly alkaline pH. Nevertheless, the altitude (often considered a surrogate of several habitat parameters) limits current distribution of N. melanostomus both in Slovakia and other regions of Europe. Thus, in contrast to other successful invaders (L. gibbosus and P. parva) the analysis of key factors regulating current distributions of N. melanostomus suggests that its future spread into small and mid-size tributaries of large rivers is unlikely.

Invasive alien species are a major threat to aquatic biodiversity. Europe’s second largest river, the Danube, is affected by several invasions, including those by four closely related Ponto-Caspian goby species. To investigate population dynamics of invasive alien gobies, we surveyed population trends of all goby species, from absence until full establishment of the round goby (Neogobius melanostomus), along a 248 Km river section of the upper Danube River. Distribution and abundance of non-native gobies was highly species-specific and varied with both time (year) and space (location). Gobies strongly dominated the fish community along the entire sampled study area, comprising 76% of all caught individuals, whereas abundances of typical native species such as chub (5%), barbel (4%) and ide (3%) were low. Ten years after its first introduction, round goby is most successful in terms of abundance, tubenose goby is most successful in terms of range expansion and persistence, whereas bighead goby seemingly lost the race. While being decoupled from industrial shipping, goby invasion success strongly appears to be triggered by man-made ecosystem alterations and community changes.

Few investigations have examined whether population genetic changes occur over the course of nonindigenous species invasions, which would facilitate understanding their trajectories and ecological successes. The Eurasian ruffe fish Gymnocephalus cernua was unintentionally introduced from ballast water released into western Lake Superior of the North American Laurentian Great Lakes in ~ 1986, likely originating from the Elbe River region where ruffe is indigenous. The ruffe spread quickly to several other areas in the upper Great Lakes, but despite early predictions, has not invaded the lower Great Lakes. In ~ 1991, the ruffe was introduced to Bassenthwaite Lake in northern England through bait bucket releases from southern England where it is indigenous (and genetically distinct from the Great Lakes invasion). The genetic trajectories of these two independent invasions offer intriguing evolutionary and ecological comparisons. This study tested two alternative hypotheses for the genetic compositions of the two invasions, with reference to two native northern European populations (Elbe River and Vistula Lagoon): whether they have (1) experienced temporal consistency or (2) changed over their respective 30-year time courses. At each invasion stage (early, middle, or later), population genetic diversity may (A) remain similar, (B) decrease due to drift, or (C) increase over time due to addition and establishment of new propagules. Analyses of variation at 10 nuclear DNA microsatellite loci and entire mitochondrial DNA control region sequencing revealed that both invasions overall have lower effective population sizes, allelic richness, and observed heterozygosity compared to the native populations, indicating founder effects. The genetic compositions of both invasions significantly changed over 30 years, supporting hypothesis 2. Diversity has remained relatively consistent overall (A), with decreased allelic richness at the middle invasion stage (B), followed by recovery (C), suggesting arrival of new propagules. Moreover, population differentiation occurred with spread in the upper Great Lakes, with no overall differences in genetic diversity levels (scenario 2A). Ruffe numbers in the Great Lakes have been declining over time, and its continued absence from the lower Great Lakes may reflect limited genetic diversity stemming from a single source population area and few new supplements. The genetic diversity reservoirs of both ruffe invasions, along with continued measures to prevent new inoculants, may constrain their populations and limit long-term ecological success.

Predator presence may result in non-lethal costs to prey; these costs may be even larger if both predators and prey do not naturally co-occur. We experimentally evaluated the survival of two aquatic invertebrates from the upper Paraná River basin (Chironomus sancticaroli – benthic and Daphnia magna – pelagic) exposed to a native mesopredator fish (Astyanax lacustris) in three non-lethal predation treatments: (i) the absence of piscivorous fish, (ii) the presence of a native piscivorous fish (Hoplias aff. malabaricus), or (iii) the presence of a non-native piscivorous fish (Astronotus crassipinnis). The treatments were crossed with the absence and presence of vegetated habitats. We predicted that the mesopredator might not have adapted foraging strategies to eat in the presence of an unfamiliar predator while invertebrate prey use vegetated habitats for protection. Therefore, we expected that the combination of piscivorous fish presence and vegetated habitat would lead to a classical additive response on invertebrate survival, which will be higher in the presence of non-native piscivorous fish than in the presence of native piscivorous fish. The presence of vegetated habitat and piscivorous fish increased invertebrate survival, but together they did not promote an additive response. Instead, the non-lethal cascade effects of the native piscivorous fish was similar between vegetation treatments, whereas the non-native piscivorous fish led to higher survival of the invertebrates in non-vegetated versus vegetated habitats. Although refuges are understood as elements that enhance predator-prey stability, the effectiveness of submerged macrophyte stands as safe habitats is dependent on some predator traits, such as origin and hunting mode. Our results also indicate that the invasion of A. crassipinnis has the potential to change mesopredator prey selectivity from a diet based on pelagic prey with the native H. aff. malabaricus to the random consumption of pelagic and benthic prey, which may have important consequences for the stability of natural aquatic food webs.

The Paranapanema River is most frequently invaded by non-native fish from the Upper Paraná River Freshwater Ecoregion. To understand how the diet of non-native fishes varies based on habitat type, we studied 12 populations of 6 non-native fish species with the aim of verifying whether diet, niche breadth, or trophic guild vary between lentic and lotic habitats. Fish were sampled in both habitats in the Paranapanema River basin between August 2014 and March 2016. A permutational multivariate analysis of variance – PERMANOVA was applied so that the composition of fish diet could be compared between habitats. Levin’s standardized index was used to evaluate the trophic niche breadth of the species, revealing differences in the diets of Ossancora eigenmanni, Auchenipterus osteomystax and Trachelyopterus galeatus based on habitats. Seven trophic guilds were identified, and those for Serrasalmus marginatus, Loricariichthys platymetopon, and T. galeatus were the same (piscivores, detritivores, and omnivores, respectively) in both habitats. In contrast, the trophic guilds of O. eigenmanni, A. osteomystax, and Plagioscion squamosissimus varied between the habitats. Habitat-specific variability in the diet composition of the species and, for the most, the niche breadth in the lotic habitats increased; therefore, we conclude that this type of environment can provide access to a greater variety of food resources for non-native fishes.

The North American channel catfish Ictalurus punctatus has been widely introduced to Europe, but no in-depth studies on its ecology and potential impacts in the introduced European range have been carried out. In 2016, 248 specimens of I. punctatus were collected from the Arno river, Florence (Central Italy), and analysed for their length, weight, size, sex, and stomach contents to assess their diet. Specimens < 30 cm total length (TL) represented the majority of the sampled population. Detritus and phytoplankton dominated the diet, while in larger fish (≥ 30 cm TL) two invasive alien species, the topmouth gudgeon Pseudorasbora parva and the red swamp crayfish Procambarus clarkii, were dominant prey items. Diet composition of I. punctatus significantly varied among size classes, but not between sexes. The results indicate an opportunistic but gape size limited feeding behaviour, suggesting an intra-specific competition avoidance mechanism.