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Current velocity shapes co-existence patterns among invasive Dikerogammarus species
Abstract
Facilitative interactions among co‐evolved representatives of the endemic Ponto‐Caspian fauna are regarded as a major factor of their invasion success. Nevertheless, the most renowned examples represent interactions between different trophic levels or functional groups, while ecologically similar species can be expected to show competition‐based niche partitioning.
Here, we test for differences in the realized niche of three invasive Dikerogammarus species (Crustacea: Gammaridae) in their co‐occurring range. We sampled multiple habitats within sites distributed along the River Danube to test whether some environmental variables could reveal spatial niche differentiation among the three species of Dikerogammarus , and if so, to test a predictive model outside the zone of co‐occurrence.
Spatial niche differentiation was present among the species, primarily determined by current velocity (and associated substrate preference), likely reflecting a stress tolerance–competitive ability trade‐off. Suspended matter concentration was also relevant, suggesting food resources (through filter feeding) might represent another important niche axis, somewhat loosening the terms of co‐existence between D. haemobaphes and the other two species.
Environmental variables could effectively explain the absence of D. bispinosus in the Lower Danube, implying that the co‐existence of the three species is possible only along a sufficiently wide current velocity gradient, and the observed turnovers are the result of niche expansion in the absence of the stronger competitor.
Hence, differences in invasion success may be attributed to a stress tolerance–competitive ability trade‐off. Our results suggest the advantage of D. villosus is attributable to its competitive dominance, allowing it to monopolize lentic and/or structured habitats, which represents a fortunate pre‐adaptation to anthropogenic alterations of aquatic ecosystems. The presence of D. villosus does not greatly affect the expansion of D. haemobaphes ; however, the exclusion of D. bispinosus from lentic habitats by D. villosus probably strongly limits its potential to spread by active dispersal.
... In the early 1990s it was abundant in the German part of the Danube, but soon after the appearance of the killer shrimp, the population of demon shrimp in that area declined (Weinzierl et al. 1996;Kley and Maier 2006;Borza et al. 2018). Similar observations were made in other rivers: Rhine, Lahn, Drava, Oder, and Vistula (Weinzierl et al. 1996;Bernauer and Jansen 2006;Kley and Maier 2006;Grabowski et al. 2007b;Chen et al. 2012;Ćuk et al. 2019; own unpublished data), suggesting effective replacement of this species by D. villosus, even if in some conditions co-existence of D. haemobaphes and D. villosus was observed (Borza et al. 2017;Hellmann et al. 2017). Taking into account all the above, we cannot exclude a possibility that the haplotype diversity in the Southern Corridor had once been richer but pauperized after the populations of the demon shrimp declined following lengthy interaction with the killer shrimp. ...
... The killer shrimp is known to be a very active and robust competitor, which may force less competitive species (such as D. haemobaphes) to leave their optimal ecological niches and extend their ranges into habitats yet unoccupied by competitors (Kobak et al. 2016 and references therein). Additionally, the presence of stronger competitors may lead to niche partitioning as it was observed for Dikerogammarus spp. in the Danube (Borza et al. 2017). Such factors have possibly promoted range extension of the less competitive species and may help to explain why D. haemobaphes conquered rivers along the Central Corridor (Kobak et al. 2016). ...
... These differences may be explained by interspecific competition with the killer shrimp (Kobak et al. 2016) that could force D. haemobaphes to leave the native range to reduce the competitive pressure. Another reason may be competitive niche partitioning, as observed by Borza et al. (2017) and perhaps different habitat characteristics of the rivers making up the two invasion corridors. Finally, we conclude the demon shrimp extended its range rapidly and that the expanding population was large as evidenced by the lack of loss of genetic diversity in the populations studied along the Central Corridor. ...
The region of the Black, Caspian, and Azov seas are known both as (i) the place of extensive crustacean radiation dated to the times of Paratethys and Sarmatian basins, and (ii) present donor of alien and invasive taxa to many areas worldwide. One amphipod morphospecies, Dikerogammarus haemobaphes, is known both as native to rivers draining to the Black and Caspian seas as well as a successful invader (nicknamed demon shrimp) in Central and Western European rivers. Based on mitochondrial (COI and 16S) and nuclear (28S) datasets and 41 sampling sites, representing both the native (19) and the invaded (22) range, we assessed cryptic diversity, phylogeography and population genetics of this taxon. First, we revealed the presence of two divergent lineages supported by all markers and all species delimitation methods. The divergence between the lineages was high (18.3% Kimura 2-parameter distance for COI) and old (ca. 5.1 Ma), suggesting the presence of two cryptic species within D. haemobaphes. Lineage A was found only in a few localities in the native range, while lineage B was widespread both in the native and in the invaded range. Although genetic divergence within lineage B was shallow, geographic distribution of 16S and COI haplotypes was highly heterogeneous, leading us to the definition of four Geo-Demographic Units (GDUs). Two GDUs were restricted to the native range: GDU-B1 was endemic for the Durugöl (aka Duruşu) Liman in Turkey, whereas GDU-B2 occurred only in the Dniester River. GDU-B3 was both present in several localities in the native range in the Black Sea drainage area and widespread in Central and Western Europe. The GDU-B4 was found exclusively in the Moskva River in Russia. Extended Bayesian Skyline Plot indicated steady growth of GDU-B3 population size since 30 ka, pointing to the rather old history of its expansion, first in the late Pleistocene in the native range and nowadays in Central and Western Europe. The analysis of haplotype distribution across the present distribution range clearly showed two invasion routes to Central and Western Europe. The first one originating from the lower Dnieper allowed the demon shrimp to colonize Polish rivers and the Mittellandkanal in Germany. The second one, originating from the Danube delta, allowed to colonize the water bodies in the upper Danube basin. The UK population has originated from the Central Corridor, as only a haplotype found exclusively along this route was recorded in the UK. Population genetics analysis showed that the invasion of the demon shrimp along the Central Corridor was not associated with the loss of genetic diversity, which might contribute to the success of this invader in the newly colonized areas.
... On the contrary, Ponto-Caspian peracarids can usually coexist with D. villosus within the same water body despite the population declines in some cases, which can be ascribed to their extraordinarily high densities before the appearance of the stronger competitor/predator (i.e. 'niche extension' or 'enemy release'; Borza, Huber, Leitner, Remund, & Graf, 2017a;Van Riel et al., 2006). As D. villosus can displace all studied species from their preferred habitat (i.e. ...
... Differences in habitat use are obvious in some cases, e.g. several Ponto-Caspian amphipods are psammo-pelophilous (Borza, Huber, Leitner, Remund, & Graf, 2017b) and mysids are epibenthic or semipelagic; however, the factors of niche differentiation among lithophilous Ponto-Caspian amphipods are only partially known (Borza et al., 2017a). ...
... All target species were present in almost the entire section of the Danube studied, except for D. bispinosus (Table 3; Borza et al., 2017a). Dikerogammarus villosus proved to be the most widespread andon averagemost abundant species during the survey, followed by C. ischnus and O. obesus, which in turn reached a maximum density even higher than D. villosus (Table 3). ...
• Studying interactions among co‐evolved invaders might help us in understanding, predicting, and perhaps mitigating the impact of the invading species on the native biota. The factors of spatial niche differentiation were investigated among invasive Ponto‐Caspian peracarids with the aim of revealing how co‐evolved species can coexist with the ‘killer shrimp’ Dikerogammarus villosus, an invasive gammarid replacing non‐Ponto‐Caspian species throughout Europe.
• Multi‐habitat samples from the third Joint Danube Survey were analysed by partitioning the variation in species density data between environmental and spatial explanatory variable sets. Relevant predictors were identified by forward selection and their role was interpreted based on the redundancy analysis (RDA) triplot. The effect of substrate types was further analysed in certain species using generalized linear models.
• The analysis revealed characteristic differences in habitat preference (i.e. spatial niche differentiation) among the species, allowing coexistence with D. villosus at different spatial scales. The relatively small and lean body of Chaetogammarus ischnus and Jaera sarsi might allow the avoidance of interference with large Dikerogammarus specimens by using narrow interstices among pebbles and stones (microhabitat‐scale differentiation). The remaining Ponto‐Caspian species included in the analysis showed an affinity for substrate types (Obesogammarus obesus) or current velocity intervals (Dikerogammarus bispinosus) that differed from those preferred by D. villosus (mesohabitat‐scale differentiation), presumably in connection with feeding preferences in some cases (Dikerogammarus haemobaphes and Trichogammarus trichiatus).
• These results provide a framework for a preliminary risk assessment concerning the continuing high potential for range expansion of D. villosus: i.e. the identification of the most vulnerable species in those regions of the world not invaded at present but potentially colonizable, based on their habitat preference and morphology. The lessons learned from Ponto‐Caspian peracarids can be applied to the whole macroinvertebrate fauna, as the same principles (i.e. the avoidance of interference) can be expected to determine their coexistence with D. villosus.
... We expected that gammarids originating from brackish water habitats (Rozengurt, 1971) would exhibit preferences for increased ionic content in water (Piscart et al., 2005), preventing them from entering small, more freshwater affluents of large rivers (Grabowski et al., 2009). We also assumed that both species would select lentic sites (Devin & Beisel, 2007;Borza et al., 2017), but that P. robustoides, as a more common inhabitant of stagnant waters would be relatively less adapted to flow. Finally, we expected that P. robustoides would be better adapted to counteract negative aspects of life in very shallow locations (Wawrzyniak-Wydrowska & Gruszka, 2005;_ Zytkowicz et al., 2008), such as extreme temperatures. ...
... The gammarids showed no affinity for any of the tested flow velocities (Fig. 5A), which may limit their distribution to lentic areas or structured habitats _ Zytkowicz et al., 2008;Grabowski et al., 2009;Borza et al., 2017). Avoidance of fast flowing waters distinguishes invasive amphipods from those staying within their native ranges (Devin & Beisel, 2007). ...
... Perhaps the adaptations of P. robustoides to commonly occupied sandy areas (Wawrzyniak-Wydrowska & Gruszka, 2005;_ Zytkowicz et al., 2008), which are poorly sheltered and exposed to wave actions, allowed it to counteract the water flow in our experiment. In a field study, D. villosus was found to be the least rheotolerant compared to D. haemobaphes and D. bispinosus and the most associated with structured habitats offering protection against flow (Borza et al., 2017). Our experiment confirmed its ability to locate and utilize lentic microhabitats which can help it thrive in flowing waters. ...
Identification of habitat preferences of invasive organisms is crucial for predicting their distribution and impact. We conducted laboratory experiments to determine preferences of Dikerogammarus villosus and Pontogammarus robustoides, invasive in Europe, with regard to ionic content (450 and 830 µS/cm in a Y-maze), flow (0–30 cm/s in a flow-through tank with stagnant water refuges) and temperature (a gradient 5–35°C). D. villosus selected the lower salinity, whereas P. robustoides did not discriminate between salinities, suggesting that, the raised ionic content in large European rivers cannot explain their absence in small tributaries, as previously postulated. Gammarids never preferred flowing over stagnant water, but could stay in a flow B10 cm/s without behavioural changes. They avoided flow C15 cm/s. Gammarids selected warmer water than their acclimation temperatures. Temperature selection by D. villosus was related to its acclimation temperature. The choice of P. robustoides depended on the season: it preferred a lower temperature in summer than in autumn. Therefore, its thermal preferences were more stable throughout the year, which might be advantageous in shallow, thermally variable areas. Gammarids used environmental variables to select warm, lentic, freshwater locations, which may help them locate an optimum microhabitat and thrive in temporally and spatially variable environments.
... Others were intentionally introduced in reservoirs to facilitate fish production (Arbačiauskas et al., 2010). Most peracarid crustacean species from the Ponto-Caspian region were introduced in the Middle and Upper Danube River during the 20 th century (Borza et al., 2017). Ponto-Caspian amphipods are considered the most important and most successful invasive peracarid crustaceans because of their significant role in the homogenization of the benthic macroinvertebrate community (Rewicz et al., 2014;Van den Brink et al., 1993;Van der Velde et al., 1998). ...
... However, in the present study, D. haemobaphes was not found at sites in the Drava River, suggesting that D. villosus has outcompeted D. haemobaphes from the Drava River. On the other hand, C. sowinskyi was likely the first invader, with the first record in Hungary in 1917, expanding its range from the Ponto-Caspian region in the Danube River (Borza, 2011;Borza et al., 2017). Therefore, C. sowinskyi was likely the first invader of the Sava and Drava, while C. curvispinum arrived second and colonized the Drava more successfully than the Lower Sava. ...
Contribution of alien peracarid crustaceans to the biocontamination of benthic macroinvertebrate assemblages in Croatian large rivers Peracarid crustaceans (orders Amphipoda, Isopoda and Mysida) are one of the most important groups of invaders in fresh and brackish waters. Although large rivers in Croatia have been heavily invaded by peracarid crustaceans, little is known about their impact on local benthic macroinvertebrate assemblages. The main aim of this study was to investigate the contribution of alien Peracarida to the biocontamination of macroinvertebrate assemblages in Croatian large rivers as a proxy measure of their impact. Quantitative sampling was conducted twice (2015 and 2016/2017), at 48 sites on four large rivers (Danube-4, Sava-21, Drava-20, Mura-3). Physicochemical parameters were measured eight times during 2015 and 2016. In total, 16 species were recorded, five native and 11 alien species. The highest number (10 alien species, most from genus Dikerogammarus and Chelicorophium) were found in the Danube River, while subsets of alien species were found in the Sava (5 spp.) and Drava Rivers (6 spp.), and no alien species were found in the Mura River. The most upstream reaches of the Mura, Drava, and Sava Rivers in Croatia have not yet been colonized by invasive peracarids and the native peracarids are still abundant there. In the Drava River, alien Peracarida had the highest densities. Significant negative correlations were established between the number of native taxa and alien Peracarida species. Proportions of alien peracarids in the total density of benthic macroinvertebrates were spatially variable, the highest average values in the Drava River (24.4 %), and lower in the Danube (20 %) and the Sava (17.7 %) Rivers. Alien Peracarida had the most important contribution to the richness and abundance contamination of macro-invertebrate assemblages. The upstream spread of invasive peracarids will increase biocontamination and could cause a decline of native species richness and abundance in the most upstream reaches of the Sava and Drava Rivers. To prevent further upstream spread of alien Peracarida, "check, clean and dry" protocols should be included in the national regulations.
... In the Danube Basin, Cărăuşu (1943) reported D. v. bispinosus from the lower Danube (near Olteniţa). There are also records of D. bispinosus from the middle Danube (Germany, Austria, Hungary, Slovakia, Croatia, Serbia, Romania) (Dudich 1947;Borza et al. 2015Borza et al. , 2017Borza et al. , 2021Brtek 2001;Copilaș-Ciocianu & Arbačiauskas 2018, Figure 3) where it prefers riverine habitats in the middle stretches of large rivers (Borza et al. 2017). To sum up, almost all the historical records of D. bispinosus from the Black Sea basin are of riverine freshwater origin. ...
... In the Danube Basin, Cărăuşu (1943) reported D. v. bispinosus from the lower Danube (near Olteniţa). There are also records of D. bispinosus from the middle Danube (Germany, Austria, Hungary, Slovakia, Croatia, Serbia, Romania) (Dudich 1947;Borza et al. 2015Borza et al. , 2017Borza et al. , 2021Brtek 2001;Copilaș-Ciocianu & Arbačiauskas 2018, Figure 3) where it prefers riverine habitats in the middle stretches of large rivers (Borza et al. 2017). To sum up, almost all the historical records of D. bispinosus from the Black Sea basin are of riverine freshwater origin. ...
The Ponto-Caspian amphipod Dikerogammarus bispinosus Martynov, 1925 was originally described from the Black Sea basin. Its recent discovery in the Caspian Sea basin was puzzling because it was unknown whether it was an invasive or an overlooked native species in this area. Here, we examined specimens collected from both the Black and Caspian Sea basins by means of molecular species delimitation based on nuclear (28S) and mitochondrial (COI) DNA sequences, as well as scanning electron microscopy (SEM). Our analyses reveal that D. bispinosus comprises three evolutionary independent lineages that are molecularly and morphologically distinct. One lineage occurs throughout rivers in the Black Sea basin, while the other two inhabit the Caspian Sea and were found in sympatry, further reinforcing that they are distinct species. Our time-calibrated phylogeny indicates that these lineages split during the Late Miocene-Pliocene, a period corresponding with the separation of the Black and Caspian basins via the Caucasus mountain uplift. SEM imaging revealed morphological differences with respect to setal patterns on the gnathopod propodi among all three lineages. Therefore, our results clearly indicate not only that D. bispinosus is native in the Caspian region, but that it has been overlooked for a long time. Additional populations covering the entire range of this species complex need to be further studied in order to gain a more complete picture of its evolutionary history and resolve its taxonomy.
... Peracarid crustaceans represent the most diverse group of Ponto-Caspian invaders with considerable ecological impacts in the North-Atlantic region (Gallardo and Aldridge 2015;Pagnucco et al. 2015;Borza et al. 2017a). The previous surveys have already made a significant contribution to our knowledge about their distributional patterns (Borza et al. 2010(Borza et al. , 2015(Borza et al. , 2017a as well as their ecological interactions (Borza et al. 2017b(Borza et al. , 2018a. In this paper, we present the faunistic results of the fourth survey pertaining to this group. ...
... The survey also yielded a notable record for Dikerogammarus bispinosus Martynov, 1925; it was found at rkm 1073 although previously it was missing in the downstream section (< ~ 1200 rkm; Borza et al. 2015). This peculiar distributional pattern was attributed to the competitive exclusion of the species by D. villosus under low current velocity (Borza et al. 2017b). The record in the Iron Gates section might indicate that the flow conditions required by the species are still present in the narrower parts of the gorge despite the impoundment. ...
The River Danube has played a pivotal role in the range expansion of Ponto-Caspian faunal elements in recent decades; therefore, the monitoring of its biota is of high scientific and conservation importance. In this publication, the records on peracarid crustaceans yielded by the macrozoobenthos samples for the 4 th Joint Danube Survey (2019) are presented. Altogether 21 species (16 Amphipoda, 2 Isopoda, 3 Mysida) were recorded at 44 sites in the Danube (between river km 2581 and 18) and its major tributaries. Invasive Ponto-Caspian species showed the most common occurrence, some of which (Chelicorophium robustum, Chelicorophium sowinskyi, and Paramysis lacustris) have been observed beyond their previously known distribution. Nevertheless, the records of Gammarus spp. in the German Danube section might potentially indicate an improvement in the status of these native species in the region. The survey also confirmed the intensifying colonization of the River Tisza by Ponto-Caspian species, showing the first record of C. robustum in the river as well as additional occurrences of the previous invader, Pontogammarus robustoides. Besides these notable records, the dataset will also serve as a useful reference for potential further range expansions.
... Interspecific interactions among amphipods can be quite complicated: they can live together in diverse habitats (Piscart et al., 2010;Borza et al., 2017b), allowing spatial segregation of species (Kley and Maier, 2005;. Alternatively, a stronger competitor can displace weaker ones to other habitats (Kley and Maier, 2003;Grabowski et al., 2007;Kobak et al., 2016). ...
... Similarly, Hellmann et al. (2017) and Koester et al. (2018) found positive correlations between D. villosus and E. ischnus. This phenomenon could be attributed to habitat complexity (MacNeil et al., 2008;Piscart et al., 2010) or variability of flow conditions (Borza et al., 2017b) allowing spatial segregation. Perhaps, small E. ischnus utilizes smaller interstices among substratum particles, inaccessible for larger species (Borza et al., 2018). ...
Knowledge of habitat requirements and interspecific interactions of invasive species helps predict their impact and spread. We determined the relationships within the invasive freshwater Ponto-Caspian amphipod assemblage, and their associations with macroinvertebrates in the near-shore zone of a central European lowland dam reservoir. We sampled five habitat types: bare sand at the water line, bare sand (0.2 m depth), bare sand (0.5 m depth), macrophyte-overgrown sand (1 m depth), stones (0.3 m depth) on four dates (October 2015–October 2016). Pontogammarus robustoides occurred in all habitats, Dikerogammarus villosus and Echinogammarus ischnus were limited to the stony bottom. Amphipod densities were positively associated with one another except Dikerogammarus juveniles, negatively correlated with adults. The occurrence of D. villosus, juvenile Dikerogammarus and E. ischnus was positively related to the presence of the shelter-forming bivalve Dreissena polymorpha . Pontogammarus robustoides was positively associated with sphaeriid clams and gastropods (shelters), as well as oligochaetes and chironomids (potential prey items). Dikerogammarus villosus and E. ischnus were positively related to chironomids and oligochaetes, respectively. Coexistence of various alien amphipods in the studied area, indicated by prevailing positive relationships in their assemblage, may be enabled by the abundance of shelters and rich food sources allowing habitat partitioning.
... following the landmarks in Fišer et al. (2009). Occurrence data for D. bispinosus was collected from the literature in order to provide an up to date overview of its distribution (Martynov 1925;Yaroshenko 1957;Dudich 1947;Dedyu 1967;Jażdżewski and Konopacka 1988;Eggers and Martens 2001;Müller and Schramm 2001;Brtek 2001;Müller et al. 2002;Sayapin 2003;Voronin and Yermokhin 2004;Tischikov and Tischikov 2005;Kley and Maier 2005;Žganec et al. 2009;Son et al. 2010;Labat et al. 2011;Filinova and Sonina 2012;Lipták 2013;Borza et al. 2015Borza et al. , 2017Gallardo and Aldridge 2015). ...
... So far, D. bispinosus, D. villosus and Shablogammarus shablensis appear to be the only Black Sea native amphipod species that have spread into the Caspian basin (Grigorovich et al. 2002). Further upstream dispersal of D. bispinosus along the Volga and Ural rivers may be expected given its rheophilous affinity (Borza et al. 2017). ...
The Ponto-Caspian amphipod Dikerogammarus bispinosus is regarded as a native species throughout the lower stretches of rivers that drain into the Black Sea. Its occurrence in the Caspian Sea basin was uncertain due to conflicting reports. Here, we provide the first conclusive evidence for its presence in this basin. Individuals of both sexes, including ovigerous females, were collected in May 2000 from the Ural River in Kazakhstan, suggesting full establishment. If it was a recent invasion, the most probable dispersal pathway into the Caspian basin would have been via the Volga-Don canal as D. bispinosus was reported in the early 2000s from the lower Don River and the Saratov reservoir on the Volga River. However, given that until relatively recently D. bispinosus was considered a subspecies of D. villosus, we cannot rule out that it has been overlooked in earlier reports from the Caspian Sea basin by being mentioned as D. villosus or even D. haemobaphes. We also provide new data on the distribution of Gammarus lacustris, Obesogammarus platycheir, Pontogammarus abbreviatus, P. robustoides, Turcogammarus aralensis and Wolgagammarus dzjubani in western Kazakhstan and southwestern Russia.
... Nonetheless, its tolerance is not always broader than that of other amphipods Gergs et al. 2013;Poznanska et al. 2013). However, D. villosus is often able to compensate for this by exhibiting different types of behaviour, such as hiding, low locomotor activity or dominance in competition for refuges (Gabel et al. 2011;Borza et al. 2017). In addition, the species is omnivorous and has highly flexible and opportunistic feeding behaviour which also contributes to the aforementioned compensation (e.g. ...
... Dick and Platvoet 2000;MacNeil et al. 2011) or displacement from microhabitats (e.g. Casellato et al. 2008;Borza et al. 2017), indirect effects by exploitation competition may also contribute to the negative consequences for invaded communities. ...
Invasive species can affect native communities by replacing competitors, overexploiting prey species or altering ecosystem structure. One example is the Ponto-Caspian amphipod Dikerogammarus villosus which has established large populations in European rivers and is widely considered as the main cause for the decline of native benthic invertebrates. This effect has been mainly associated with direct predation, whereas the indirect effects via competition for primary resources are poorly understood and possibly underestimated. To assess the probability of these indirect effects, we performed five outdoor flow-through mesocosm experiments in three European rivers, manipulating the density of D. villosus. We quantified its in-situ food consumption during three 24-h gut content surveys in the mesocosms. Gut evacuation rates for correction were measured in the laboratory for different food sources and under continuous feeding. We analysed the invader’s effects on primary resources by quantifying periphyton biomass and community leaf litter decomposition in the mesocosms at different D. villosus densities. The remarkably high food consumption rates (0.38–1.27 mg mg⁻¹ d⁻¹, in dry mass/dry body mass) of D. villosus can be attributed to its high gut evacuation rates. The leaf litter decomposition rates indicate that D. villosus is an efficient shredder; however, there was no effect on periphyton biomass. Our results indicate that D. villosus may be a strong competitor with primary consumers in benthic food webs of invaded rivers, with both direct and indirect negative effects on benthic communities. High consumption rates together with opportunistic feeding behaviour probably promote the invasion success of this amphipod.
... Nonetheless, its tolerance is not always broader than that of other amphipods ( Gergs et al. 2013;Poznanska et al. 2013). However, D. villosus is often able to compensate for this by exhibiting different types of behaviour, such as hiding, low locomotor activity or dominance in competition for refuges ( Gabel et al. 2011;Borza et al. 2017). In addition, the species is omnivorous and has highly flexible and oppor- tunistic feeding behaviour which also contributes to the aforementioned compensation (e.g. ...
... Dick and Platvoet 2000;MacNeil et al. 2011) or displacement from microhabitats (e.g. Casellato et al. 2008;Borza et al. 2017), indirect effects by exploitation competition may also contribute to the negative consequences for invaded communities. ...
Freshwater ecology in the Elbe and the Rhine: Dikerogammarus villosus, the “killer shrimp”, appears to have a number of different strategies for survival. It is able to change its hunting and feeding behaviour to suit its surroundings and the environmental conditions.
... villosus, especially large individuals, to oxygen limitation may explain its occurrence in the nearshore zone of rivers and dam reservoirs, close to the water line, where water dynamics provides good oxygen saturation (Borza et al., 2017;Verberk et al., 2018). Various factors, such as water dynamics and stratification, decomposition processes, salinity, eutrophication and temperature influence the concentration of oxygen in water. ...
Climate change has a profound impact on ectotherms, which suffer suboptimum thermal conditions in their native areas or spread to previously unavailable locations. However, it is often neglected that responses to environmental changes are likely at a population level, rather than a species‐specific phenomenon.
Two groups (Western and Eastern) of the invasive Ponto‐Caspian amphipod Dikerogammarus villosus independently spread in Europe from two geographically isolated and genetically distinct source populations living in the Danube and Dnieper deltas, respectively. They are expected to meet in the near future in Polish inland waters. This makes this species a perfect model to investigate the intraspecific variability in responses to environmental changes as well as the effects of hybridisation between different genetic units on invasiveness.
We scaled maximal metabolic rate (MMR), standard metabolic rate (SMR) and aerobic scope (AS, a measure of physiological performance) of D. villosus at different temperatures (15 and 25°C) and oxygen levels (9.5 or 7.5 mg/L at a temperature of 15 and 25°C, respectively, and 5.5 mg/L at both temperatures) to test for differences in responses to environmental changes among the distinct invasive groups and their hybrid.
Large Western Group individuals showed a lower AS at a high temperature, compared to low temperature and to the Eastern group individuals, whereas small animals exhibited similar AS irrespective of temperature and group identity. Moreover, large individuals from both groups experienced a reduction in MMR and AS under low oxygen conditions, whereas small animals did not change these parameters in response to oxygen reduction. Although parental groups reduced their AS in response to oxygen reduction, AS of the hybrid remained unchanged.
In conclusion, organism responses to climate change depend on intraspecific variation, which should be considered in studies related to this topic. Furthermore, D. villosus appears sensitive to oxygen deficits and temperature changes (especially the Western group and large individuals), but intraspecific hybridisation can enhance the environmental tolerance of invaders.
... In this instance, it is likely that competitive interactions dictate that coexistence may only be possible with spatial segregation with the less competitive non-native taxa occupying suboptimal habitat at reduced abundances (Platvoet et al., 2009). This has been observed with Dreissena bugensis (quagga mussel) displacing D. polymorpha (Stoeckmann, 2003) and Dikerogammarus villosus (killer shrimp) outcompeting Dikerogammarus haemobaphes (demon shrimp; Borza et al., 2017). However, the number of studies that observe such interactions under field settings remains rare as most studies exhibit snapshot observations rather than undertaking repeated surveys or experimentally examine such interactions. ...
Abstract Biological invasions remain one of the most pressing threats to biodiversity globally, with unprecedented rates of establishment that are unlikely to abate in the future. As such the occurrence of multiple interacting invasive non‐native species (INNS) is becoming more common. Despite this, much of the research on the interaction of multiple INNS is conducted within laboratory settings, with field studies remaining rare or being conducted as one‐off sampling. There is therefore a deficit of knowledge pertaining to the natural population trajectories of multiple INNS and the wider ecological implications for the structure and function of the native communities. Here we present multiple years of data collected from a reservoir in the United Kingdom, which has undergone invasion by three congener non‐native amphipods. We initially observed the coexistence of Dikerogammarus haemobaphes with Crangonyx pseudogracilis under habitat segregation in the reservoir. However, the following year saw complete displacement of C. pseudogracilis and reduced abundances of D. haemobaphes once the more competitive Dikerogammarus villosus established abundant populations in the reservoir. D. villosus exhibited strong effects for the structure and function of the wider macroinvertebrate community composition in addition to driving reductions in taxa and functional richness. Gastropoda taxa appeared to be unaffected by the invasion dynamics. Significant increases in functional divergence (and less so functional dispersion) values were also observed following D. villosus establishment, with this response metric possibly representing an important tool in detecting stress from biological invasions that we urge scientists to test more extensively. In contrast to D. villosus, D. haemobaphes implications for the wider ecological community appeared to be limited. Importantly, we observed evidence to suggest the presence of boom–bust cycles with the explosion of D. villosus leading to a dramatic drop in their abundances the following year. We believe that the wider ecological implications of their abundant populations were so great that resources were not sufficient within the reservoir to support their abundant populations, leading to the observed population collapses. We call for further studies that investigate the population trajectories and wider ecological implications of multiple non‐native species in field settings to further our limited knowledge base.
... At the same time, in the geographical areas indicated as potentially highly suitable for both Dv and Dh (overlapping range), local factors and behaviour mechanisms may affect their co-existence but also the exclusion of one or the other, meaning that additive or synergic effects cannot be automatically assumed. The species share some environmental preferences: both have been described as predominately lithophilous (Borza et al. 2017a), with an affinity for hard substrates and cobbles (Clinton et al. 2018;Macneil and Platvoet 2013). However, Borza et al. (2017b), studying sites along the River Danube (East Europe), also reported differences in their current velocity preferences, with Dv being primarily associated with lentic/slow-flowing systems, but able to use refugia provided by obstacles in faster flowing systems. ...
Biological invasions represent one of the main contemporary pressures facing freshwater ecosystems, and a better understanding of invasive species potential distributions is essential to prepare for future stressors. Crustacean invaders contribute significantly to global invasions with the Ponto-Caspian region being one of the primary donor areas for the Palearctic. The amphipods Dikerogammarus villosus and Dikerogammarus haemobaphes, popularly known as "killer" and "demon" shrimps, are emblematic of successful Ponto-Caspian invaders of European freshwaters. However, the geographical areas in which the abiotic environment is potentially suitable for them have not been investigated. To address this gap, current and future potential distributions were studied for the European Western Palearctic considering two scenarios and time periods (2050 and 2070) as well as the association between anthropogenic activities and individual species habitat suitability. Results show large areas of central-western Europe are currently suitable for both species and indicate some potential for range expansion within colder European areas. In particular, D. haemobaphes has the potential to expand its range further west and within southern parts of Europe. Scenarios of future climate change don't provide evidence for further range expansion compared to the current conditions and suggest a reduction of range overlap within the most suitable areas. Results reveal lowland areas are at greatest risk of colonisation as well as a significant association with anthropogenic activities for both amphipods. The outcomes of the research could be used by resource managers for preparing and managing future changes of both species distributions and facilitate decision-making for monitoring and control.
... Among them, Dikerogammarus villosus and Dikerogammarus haemobaphes have colonized most of the European main inland water bodies in less than 20 years, having a deteriorating effect upon local benthic communities [6][7][8]. Dikerogammarus bispinosus (a third invasive representative of the genus Dikerogammarus) reached the Rhine estuary via the southern invasion corridor [1]; however, populations of this species have been declining in the lower section of the Danube in recent decades [9,10]. Dikerogammarus villosus and D. haemobaphes earned their nicknames "killer" and "demon" shrimp, respectively, due to their impacts on the most recent studies show that, while D. villosus seems to be a well-defined species over its geographic range [27], both D. haemobaphes and D. bispinosus contain divergent phylogenetic lineages, which may represent yet undescribed, cryptic, or pseudocryptic species [6,41]. ...
The Ponto-Caspian region is the main donor of invasive amphipods to freshwater ecosystems, with at least 13 species successfully established in European inland waters. Dikerogammarus spp. and Pontogammarus robustoides are among the most successful, due to their strong invasive impact on local biota. However, genomic knowledge about these invaders is scarce, while phylogeography and population genetics have been based on short fragments of mitochondrial markers or nuclear microsatellites. In this study, we provide: (i) a reconstruction of six mitogenomes for four invasive gammarids (D. villosus, D. haemobaphes, D. bispinosus, and P. robustoides); (ii) a comparison between the structure of the newly obtained mitogenomes and those from the literature; (iii) SNP calling rates for individual D. villosus and D. haemobaphes from different invasion sites across Europe; and (iv) the first time-calibrated full mitogenome phylogeny reconstruction of several Ponto-Caspian taxa. We found that, in comparison to other gammarids, the mitogenomes of Ponto-Caspian species show a translocation between the tRNA-E and tRNA-R positions. Phylogenetic reconstruction using the mitogenomes identified that Ponto-Caspian gammarids form a well-supported group that originated in the Miocene. Our study supports paraphyly in the family Gammaridae. These provided mitogenomes will serve as vital genetic resources for the development of new markers for PCR-based identification methods and demographic studies.
... Such a scenario was observed in the Oder and Vistula rivers where the populations of D. haemobaphes were abundant. However, soon after, the appearance of the D. villosus began to decline (summarized by Jazdzewska et al., 2020), even if, in some conditions, those two species may coexist (Borza et al., 2017). On the contrary, C. curvispinum may regain this area and coexist with A. lacustre. ...
Apocorophium lacustre (Vanhoffen, 1911), which is a native brackish amphipod species of the North Atlantic and Baltic coasts, was recorded in the upper Oder River for the first time in 2017. Prior to that, only alien amphipod species had been found in this area. The aims of the study were to describe the distribution pattern of A. lacustre in the upper Oder River catchment, to investigate the composition and structure of the amphipod assemblages against the background of the habitat conditions and to provide a genetic identification of the species using DNA barcodes.
In total, 16 sites were studied. Apocorophium lacustre was recorded in 14 of them. It was not found at one site in the Oder River and at one location in the lower Klodnica River. Apocorophium lacustre outnumbered the other amphipods in the Oder, in the mouth section of the Klodnica and at one site in the Gliwice Canal. The alien species Gammarus tigrinus Sexton, 1939 was dominant in the amphipod communities at the southernmost site in the Oder River, in the Klodnica Canal and at most sites along the Gliwice Canal. In the Klodnica River, another alien species, Dikerogammarus villosus (Sovinsky, 1894), was dominant in the amphipod fauna at two sites. While the density of A. lacustre was high in the Oder River, it was much lower in both the canals and the Klodnica River. In our study, the depth and river velocity both contributed to the distribution of this species.
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... The WHPT scoring system and other very similar approaches have been widely applied globally to characterise the ecological quality of flowing waterbodies (Buss et al., 2015;Guareschi et al., 2017). The PSI and LIFE indices work with a specific sensitivity lists and associated scoring systems and were specifically investigated because flow velocity, substrate composition and suspended sediment concentrations have been identified as potential abiotic parameters that may affect the distribution of Dikerogammarus species (Borza et al., 2017). Functional diversity responses were investigated through calculating two metrics: (a) functional richness (FRich) and (b) functional redundancy (FRed) for each sample. ...
Biological invasions could have major implications for the management and conservation of freshwater systems if they lead to a misclassification of waterbodies. However, there is limited understanding of the sensitivity of existing biomonitoring tools to invasive species in rivers; and even less known regarding how they influence community taxonomic and functional measures.
This research explores the response of freshwater macroinvertebrate communities to biological invasion using taxonomic and functional indices. Utilising a long‐term dataset (spanning 2000–2019, 5,988 samples) from rivers in England, the performance of four biomonitoring tools (WHPT, WHPT‐ASPT, LIFE and PSI) and two community functional indices (functional richness and redundancy) was examined before and after the colonisation of the invasive species, Dikerogammarus haemobaphes (Eichwald, 1841; Crustacea: Gammaridae). This species represents a recent (first record 2012) and highly successful invader, allowing its range expansion within waterbodies to be examined in detail. Spatial (national and basin level) and seasonal (spring and autumn) effects were investigated using a before–after control–impact (BACI) experimental framework and linear mixed effects models.
Results indicated that invasion by D. haemobaphes resulted in significant reductions to the WHPT index and functional diversity metrics (richness and redundancy) while more subtle patterns were observed for other metrics. Analysis of seasonal and individual river basins (River Trent and R. Thames) identified largely consistent responses. The establishment of D. haemobaphes also resulted in some modifications to the functional composition of aquatic communities primarily associated with voltinism and resistance features.
Synthesis and applications. Our findings indicate that Dikerogammarus haemobaphes should be considered a significant pressure to riverine communities. These results have implications for biomonitoring, which informs managerial actions as effects may not be detected using a single taxonomic index. Community functional measures are useful in characterising the effects of invasive species and may form a valuable part of the ‘toolbox’ used for studying biological invasions in rivers. The research illustrates the need to consider the wider threats posed by invasive species on the long‐term integrity of freshwaters and the efficacy of freshwater biomonitoring tools.
... N. hrabei and N. valachicus are bound to the muddy, dimly lit and densely vegetated bottom of stagnant or slow flowing lowland waters (Copilaș-Ciocianu et al., 2017a and references therein). These habitats are characterized by eutrophic conditions, high temperature and frequent hypoxia (Junk et al., 1989;Parr and Mason 2004;Graeber et al., 2013) and are avoided by the presumably ecologically closest competitors, the oxyphilic and mostly rheophilous gammarid amphipods (Meijering, 1991;Meijering et al., 1995;Henry and Danielopol, 1999;Copilaș-Ciocianu et al., 2014;Copilaș-Ciocianu and Boroș, 2016;Borza et al., 2017;Mauchart et al., 2017). Experimental evidence has shown that niphargids can tolerate hypoxia and other adverse environmental conditions much better than surface-water gammarids Hervant et al., 1995;Malard and Hervant, 1999;Coppellotti Krupa and Guidolin, 2003;Simčič et al., 2005;Simčič and Brancelj, 2006;Flot et al., 2014). ...
... However, that study was spatially restricted, by only considering Lake Constance, and other studies on larger spatial scales indicate that abiotic environmental factors, such as current velocity and type of substrate, could be relevant for the influence exerted by D. villosus (e.g. Borza et al., 2017;MacNeil et al., 2013). Thus, it remains unknown whether D. villosus has such a high impact on the benthic community of river systems on a larger geographic scale. ...
Dikerogammarus villosus, one of the most successful invaders in European river systems, is commonly regarded as a threat to native biodiversity and a main factor structuring the benthic community of invaded systems. The impact of D. villosus has been intensively studied in small-scale experiments and field observations, but its impact on natural communities on a larger scale remains unclear. Here, we investigated the benthic community structure at ten sites covering a broad range of habitats along the River Rhine (Central Europe) and its tributaries, to determine whether D. villosus is one of the main factors structuring the benthic community. Community composition was analysed using non-metric multidimensional scaling, distance-based redundancy analysis, and correlation analyses. D. villosus was one of nine relevant taxa present that altogether reflected a large part of the variation in the benthic samples, but further analyses indicated that the species might be less important for the community structure than other relevant taxa. Moreover, all nine relevant taxa together can explain only a similar amount of variation in our samples than the five relevant non-faunal environmental factors (water temperature, pH, conductivity, percentage of medium-sized gravel and macrophytes). Overall, our results suggested that rather a combination of non-faunal environmental factors than D. villosus mainly structure the benthic community composition on this larger spatial scale.
... We modeled the spatial structure of the study using the asymmetric eigenvector map (AEM) method (Blanchet et al. 2008a(Blanchet et al. , 2011 allowing the consideration of directional spatial processes, induced by the currents in our case. Since the locations of the samples within sites were not recorded, we considered only sites, using river km distances (d ij ) weighted as 1=d ij (conforming to Borza et al. 2017a), and replicated the values of the generated spatial variables (AEM eigenfunctions) for all samples within each site. We excluded two sites (8 samples) in the two minor arms of the Danube delta (Sulina and Sf. ...
After Chelicorophium curvispinum, two other Ponto-Caspian tube-dwelling, filter feeding amphipod species (C. robustum and C. sowinskyi) have colonized several catchments in Central and Western Europe in recent decades. To reveal the mechanism of niche differentiation among them, we measured the mesh sizes of their filtering apparatus and analyzed multi-habitat sampling data from the River Danube using RDA-based variance partitioning between environmental and spatial explanatory variables. Morphometric data showed clear differentiation among the species by filter mesh size (C. curvispinum > C. robustum > C. sowinskyi). Field data also indicated the relevance of suspended matter; however, the mere quantity of suspended solids included in the analysis could not explain the abundance patterns effectively. Current velocity, substrate types, and total nitrogen content also had a non-negligible effect; however, their role in the niche differentiation of the species is not evident. In summary, differences in their filter mesh sizes indicate a niche differentiation by food particle size among the invasive Chelicorophium species, allowing their stable coexistence given sufficient size variability in their food source. Consequently, the two recent invaders increase the effectiveness of resource utilization, resulting in a more intensive benthic-pelagic coupling in the colonized ecosystems.
... The resulting irregular, high shear stress at the river banks is particularly negative for the sensitive land-water interface (Liedermann et al., 2014), which is essential for hemilimnic organisms during metamorphosis. Native taxa prefer organic and rather fine substrates, whereas alien FIGURE 4 Temporal development of the number of alien species (dark grey dots) including molluscs, peracarid and eucarid crustaceans, fishes, and reptiles in the middle Danube in Hungary (according to Bódis et al., 2012), the amount of transported goods by inland navigation in Austria (framed triangles), and the number of hydropower plants (HPPs) along the whole Danube River (grey rectangles) species favour larger, stabile substrates, especially rip-rap-dominated habitats (Borza, Huber, Leitner, Remund, & Graf, 2017;Graf, Csányi, et al., 2015). Riverbank restoration can create more lentic areas, such as shallow riverbanks with fine sediments and large woody debris accumulations, which support the distribution of the indigenous fauna (Graf, Hartmann, & Leitner, 2011). ...
For centuries, rivers have experienced massive changes of their hydromorphic structures due to human activities. The Danube River, the second largest river in Europe, is a case in point for long-term societal imprint. Resulting human-induced pressures are a key issue for river management, aiming to improve the ecological conditions and guarantee the provision of ecosystem services. As the most international river basin in the world, the management of the Danube is particularly challenging and needs a well-organized cooperation of 19 nations. The recent river basin management plan has identified pollution and hydromorphological alterations as most pressing problems, but it has also acknowledged newly emerging issues. In this article, we present 3 specific examples of highly relevant issues for the future river basin management of the Danube: (a) long-term impacts in the catchment such as changes in flood patterns and potential ecological consequences; (b) complex feedback loops linking the spread of neozoa with intertwined stressor responses due to river engineering for different purposes; and (c) linkages between different assessment approaches based on European legal frameworks to analyse the specific pressures at different spatial scales. These examples highlight the need for a more integrated approach in future Danube River Basin management schemes. Furthermore, large-scale effects such as climate change and interactions of multiple pressures need to be addressed in future management to increase resilience of the river system and to allow a sustainable ecosystem-based management of rivers.
... N. hrabei and N. valachicus are bound to the muddy, dimly lit and densely vegetated bottom of stagnant or slow flowing lowland waters (Copilaș-Ciocianu et al., 2017a and references therein). These habitats are characterized by eutrophic conditions, high temperature and frequent hypoxia (Junk et al., 1989;Parr and Mason 2004;Graeber et al., 2013) and are avoided by the presumably ecologically closest competitors, the oxyphilic and mostly rheophilous gammarid amphipods (Meijering, 1991;Meijering et al., 1995;Henry and Danielopol, 1999;Copilaș-Ciocianu et al., 2014;Copilaș-Ciocianu and Boroș, 2016;Borza et al., 2017;Mauchart et al., 2017). Experimental evidence has shown that niphargids can tolerate hypoxia and other adverse environmental conditions much better than surface-water gammarids Hervant et al., 1995;Malard and Hervant, 1999;Coppellotti Krupa and Guidolin, 2003;Simčič et al., 2005;Simčič and Brancelj, 2006;Flot et al., 2014). ...
Groundwater is an extreme environment due to its absence of light, resource scarcity and highly fragmentary nature. Successful groundwater colonizers underwent major evolutionary changes and exhibit eye and pigment loss (troglomorphies). Consequently, their chances of dispersal and survival in the well-connected surface waters are greatly decreased, resulting in significant endemism. The West Palaearctic subterranean amphipod genus Niphargus comprises hundreds of narrowly endemic and troglomorphic species. Nevertheless, a few are known to occur in surface waters, two of which, N. hrabei and N. valachicus, have extremely large ranges that even exceed those of many surface-water amphipods. We tested if this pattern results from a secondary colonization of the relatively well-connected epigean environment, and whether this ecological shift promoted the large-scale dispersal of these species. Results showed that despite their ecological and zoogeographic similarities, N. hrabei and N. valachicus are not closely related and independently colonized surface waters. Their phylogeographic patterns indicate Middle to Late Pleistocene dispersal episodes throughout the Danube lowlands, and relatively modest yet significant genetic differentiation among populations. Clustering based on morphology revealed that the two species are phenotypically closer to each other than they are to most other epigean congeners. We presume that the ecological shift to surface environments was facilitated by their ability to thrive in hypoxic waters where rheophilic competitors from the family Gammaridae cannot survive. In conclusion, our results indicate that adaptation to groundwater is not a one-way evolutionary path and that troglomorphic species can occasionally recolonize and widely disperse in surface waters.
The global spread of non-native species is leading to an increasing frequency of multiple co-occurring non-native species. We examined the co-occurrence of the bivalve mollusc Dreissena polymorpha (zebra mussel) with three Ponto-Caspian amphipods (Dikerogammarus villosus, Dikerogammarus haemobaphes, and Chelicorophium curvispinum) across England and Wales in association with in-situ substrate size. For all three amphipod species, substrate grain size where amphipods co-occurred with D. polymorpha was significantly finer than when recorded in isolation. Subsequently, we confirmed this via aquarium experiments. We examined the occurrence of D. villosus with D. polymorpha when present with cobbles, gravel, or sand from three population sources (co-location with abundant D. polymorpha populations, co-location with low populations, and naïve). Experiments demonstrated that D. villosus actively sought shelter on or near D. polymorpha, with their co-location being significantly more prevalent in finer grained substrates (sand > gravel > cobble). The strength of this co-location differed by population source, with those co-located with high D. polymorpha densities demonstrating a greater association. Our analyses and experiments indicate that D. polymorpha may facilitate Ponto-Caspian amphipod establishment in otherwise suboptimal locations, whereby mussel shells provide favourable structural habitat for the amphipods, analogous to the presence of coarse-grained benthic sediment.
Ponto-Caspian gammarids have invaded European waters, affecting local communities by predation and competition. Their ranges and dispersal rates vary across Europe, which may result from their interspecific interactions, accelerating or reducing migrations. We checked this hypothesis by testing interference competition among co-occurring invaders: Dikerogammarus villosus, D. haemobaphes and Pontogammarus robustoides. We used 140-cm long tanks (gravel substratum), divided into seven compartments. We introduced 25 “residents” into the outermost compartment, separated with a barrier. After 1h, we introduced 25 “intruders”. After the next 1h, we removed the barrier and the gammarids dispersed in the tank. After 4 or 20h, we counted the gammarids in the compartments. We tested all pairwise species combinations and single-species controls. Dikerogammarus villosus displaced other species (P. robustoides only after 4h) and reduced its own motility after 20h in their presence. Pontogammarus robustoides stimulated the short-time migrations of D. villosus intruders and of D. haemobaphes. As P. robustoides migrated spontaneously much more than Dikerogammarus spp., its impact decreased after longer time. Dikerogammarus haemobaphes stimulated the short-time movement of P. robustoides intruders but reduced the long-time relocation of this species. In general, gammarid dispersal increased in the presence of stronger competitors (D. villosus and P. robustoides, especially residents) and decreased in response to weaker competitors (D. haemobaphes). Thus, competitive interactions may affect dispersal of invasive gammarids and contribute to the fastest spread of the weakest competitor, D. haemobaphes observed in the field, whereas the strongest species, D. villosus was the latest newcomer in many novel areas.
Examining congeners can help identify critical differences between species that affect invasion and spread. We examined Dreissena polymorpha (zebra mussel) and Dreissena rostriformis bugensis (quagga mussel), which are important invaders in freshwater and share general ecological characteristics, to determine whether they had similar rates of invasion at different spatial scales (global, regional, local, and individual water bodies) from the time of first introduction to the present. We also contrasted differences in ecological and population characteristics that could influence speed of spread. Although D. polymorpha and D. r. bugensis are relatives and share a Common native habitat, morphology, lifestyle, life history, and dispersal potential, D. polymorpha was found to be a better invader than D. r. bugensis at most spatial scales throughout their invasion history. Spread at the regional scale in North America was the same for both species, but the initial rate of invasion by zebra mussels far outpaced the spread of quagga mussels both in the United States and in Europe. The estimated lag time between initial introduction and maximal population size is 5 times shorter for D. polymorpha than for D. r. bugensis, which may be an important factor affecting the speed with which this species can spread. Differences in population dynamics may facilitate the increased rates of spread of D. polymorpha relative to D. r. bugensis, especially at smaller spatial scales.
The amphipod Dikerogammarus villosus has colonized most of the European main inland water bodies in less than 20 years, having deteriorating effect on the local benthic communities. Our aim was to reveal the species phylogeography in the native Black Sea area, to define the source populations for the colonization routes in continental Europe and for the newly established UK populations. We tested for the loss of genetic diversity between source and invasive populations as well as along invasion route. We tested also for isolation by distance. Thirty three native and invasive populations were genotyped for mtDNA (COI, 16S) and seven polymorphic nuclear microsatellites to assess cryptic diversity (presence of deeply divergent lineages), historical demography, level of diversity within lineage (e.g., number of alleles), and population structure. A wide range of methods was used, including minimum spanning network, molecular clock, Bayesian clustering and Mantel test. Our results identified that sea level and salinity changes during Pleistocene impacted the species phylogeography in the Black Sea native region with four differentiated populations inhabiting, respectively, the Dnieper, Dniester, Danube deltas and Durungol liman. The invasion of continental Europe is associated with two sources, i.e., the Danube and Dnieper deltas, which gave origin to two independent invasion routes (Western and Eastern) for which no loss of diversity and no isolation by distance were observed. The UK population has originated in the Western Route and, despite very recent colonization, no drastic loss of diversity was observed. The results show that the invasion of the killer shrimp is not associated with the costs of loosing genetic diversity, which may contribute to the success of this invader in the newly colonized areas. Additionally, while it has not yet occurred, it might be expected that future interbreeding between the genetically diversified populations from two independent invasion routes will potentially even enhance this success.
Dreissena polymorpha (zebra mussel) and D. rostriformis bugensis (quagga mussel) continue to spread in Europe and in North America, and have large ecological and economic impacts where they invade. Today many more waterbodies are invaded by zebra mussels, and therefore the extent of their impact is greater than that of quagga mussels. Both species provide additional space and food for invertebrates in the littoral zone, increasing their diversity and density. In contrast, in the profundal zone, quagga mussels may compete for space and food resources with benthic invertebrates, decreasing their diversity and density. The system-wide effect of dreissenids depends on water mixing rates, lake morphology, and turnover rates. Because quagga mussels are found in all regions of a lake, and form larger populations, they may filter larger volumes of water and may have greater system-wide effects, especially in deep lakes, than zebra mussels, which are restricted to shallower portions of lakes. Shortly after initial invasion, as populations increase, both dreissenids will have their largest effects on communities, and most of them will be direct effects. After the initial stage of invasion, impacts are less predictable, and more likely to be caused by indirect effects through changes in the ecosystem.
http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12348/abstract?campaign=wolearlyview
1. The outcome of multiple invasions from a common origin may lead to facilitative interactions
because the invaders have co-evolved under similar environmental conditions. This outcome
is often referred to as invasional meltdown, with a resultant increase in invasive species
and a decline in native species richness and abundance.
2. This study seeks to assess the full scope of the threat posed by a high-risk group of 23
freshwater invaders originating from the Ponto–Caspian region (south-east Europe) across
Great Britain. Ponto–Caspian invaders constitute a group of special concern because they
have recently caused a large-scale invasion into western Europe.
3. According to a literature review, 76% of reported interactions between Ponto–Caspian
invaders are positive (mostly provision of food and commensalism) or neutral (habitat partitioning).
Negative interactions (mostly predation) were rare, thus highlighting the ability of
Ponto–Caspian invaders to coexist.
4. At least 14 out of the 23 Ponto–Caspian organisms investigated are well-established in the
Rhine estuary and Dutch ports. Four of them (Hemimysis anomala, Dikerogammarus villosus,
D. haemobaphes and Hypania invalida) have recently established in Great Britain. Regression
models suggest the rest are under a critical risk of being transported, with four species predicted
to have arrived already to Great Britain: Echinogammarus ischnus, Jaera istri, Limnomysis
benedeni and D. bispinosus.
5. According to species distribution models, the cumulative risk of invasion of multiple Ponto–
Caspian species, thus invasional meltdown, is highest in the south-east of England and
decreases to the north and west.
6. Great Britain might be on the brink of invasional meltdown, and as a consequence, confronting
the problem of Ponto–Caspian invasive species is a vital element for national biosecurity.
7. Synthesis and applications. The predictive models and maps developed in this study provide
a means for an evidence-based prioritization of species and habitats for the management
of existing and future invasions of Ponto–Caspian species. This integrated approach can be
easily applied to risk assess other groups of species and habitats.
The 'killer shrimp', Dikerogammarus villosus, has been recognised as one of the 100 worst alien species in Europe, in terms of negative impacts on the biodiversity and functioning of invaded ecosystems. During the last twenty years, this Ponto-Caspian amphipod crustacean has rapidly spread throughout Europe's freshwaters and its invasion and continued range expansion represents a major conservation management problem. Although a great deal of research has focussed on this almost 'perfect' invader as its damaging impacts, realised and potential, have become evident, we now present the first comprehensive review of D. villosus taxonomy, morphology, distribution, community impacts, parasites, life history, physiological tolerance and finally, possible eradication methods. We show the direct and indirect ecosystem impacts of this invader can be profound, as it is a top predator, capable of engaging in a diverse array of other feeding modes. It can quickly dominate resident macroinvertebrate communities in terms of numbers and biomass, with subsequent large-scale reductions in local biodiversity and potentially altering energy cycling, such as leaf litter processing. This damaging European invader has the potential to become a key invader on a global scale as it may be capable of reaching North American freshwaters, such as the Great Lakes. One positive aspect of this invader's spread and impact is increased interest in alien species research generally, from decision-makers, stakeholders and the general public. This has resulted in greater financial support to study invasion mechanisms, preventative measures to stop invasion spread and ways to minimise damaging impacts. Our review provides a specific example, that studies identifying management strategies that mitigate against a potential invader's spread should be undertaken at the earliest possible opportunity in order to minimise potentially irreversible ecosystem damage and biodiversity loss.
Biological invasions continue to exert pressure on ecosystems worldwide and we thus require methods that can help understand and predict the impacts of invasive species, on both native species and previously established invaders. Comparing labora- tory derived functional responses among invasive and native predators has emerged as one such method, providing a robust proxy for field impacts. We used this method to examine the likely impacts of the Ponto–Caspian amphipod Dikerogammarus hae- mobaphes, known as the ‘‘demon shrimp’’, a little investigated invader in European freshwaters that has recently established in the British Isles. We compared the functional responses on two prey species of D. haemobaphes with two other amphipod species: Dikerogammarus villosus, a congeneric invasive with well-documented impacts on macro-invertebrate com- munities and a native amphipod, Gammarus pulex. Prey species were native Chironomus sp. and the invasive Chelicorophium curvispinum, a tube-building amphipod also originating from the Ponto– Caspian region. D. villosus showed higher Type II functional responses towards both prey species than did D. haemobaphes and G. pulex, with the latter two predators exhibiting similar impacts on the native prey. However, D. haemobaphes had higher functional responses towards the invasive C. curvispinum than did G. pulex, both when prey individuals were tubeless and resident in their protective mud tubes. Thus, we demonstrate that functionally equivalent invasive congeners can show significantly different impacts on prey, regardless of shared evolutionary history. We also show that some predatory invaders can have impacts on native prey equivalent to native predator impacts, but that they can also exert significant impacts on previously introduced prey. We discuss the importance of invasion history and prey identity when attempting to understand and predict the impacts of new invaders.
Drifting can be an effective way for aquatic organisms to disperse and colonise new areas. Increasing connectivity between European large rivers facilitates invasion by drifting aquatic macroinvertebrates. The present study shows that high abundances of invasive species drift in the headstream of the river Rhine. Dikerogammarus villosus and Chelicorophium curvispinum represented up to 90% of the total of drifting macroinvertebrates. Drift activity shows seasonal and diel patterns. Most species started drifting in spring and were most abundant in the water column during the summer period. Drift activity was very low during the winter period. Diel patterns were apparent; most species, including D. villosus, drifted during the night. Drifting macroinvertebrates colonised stony substrate directly from the water column. D. villosus generally colonised the substrate at night, while higher numbers of C. curvispinum colonised the substrate during the day. It is very likely that drifting functions as a dispersal mechanism for crustacean invaders. Once waterways are connected, these species are no longer necessarily dependent on dispersal vectors other than drift for extending their distribution range
A widely cited hypothesis in ecology is that species-rich communities are less vulnerable to invasion than species-poor ones, owing to competition for limiting resources (the "biotic resistance" model). However, evidence for biotic resistance in aquatic ecosystems is equivocal. Contrary to the view that communities become more resistant to invasion as they accumulate species, the rate of invasion has increased over the past century in areas that have received frequent shipping traffic. Furthermore, introduced species may facilitate, rather than compete with, one another. A review of invasions in the Great Lakes indicates that direct positive (mutualistic and commensal) interactions among introduced species are more common than purely negative (competitive and amensal) interactions. In addition, many exploitative (e.g., predator-prey) interactions appear to be strongly asymmetric in benefiting one invading species at a negligible cost to another. These observations, combined with an increasing invasion rate in the Great Lakes, tentatively support the Simberloff - Von Holle "invasional meltdown" model. The model posits that ecosystems become more easily invaded as the cumulative number of species introductions increases, and that facilitative interactions can exacerbate the impact of invaders. It provides a theoretical argument for substantially reducing the rate of species introductions to the Great Lakes.
Among ca. 750 species of Crustacea recorded from Poland, 18 representatives of 5 orders of macrocrustaceans have been identified as alien species that have invaded or have been introduced to Polish waters. Out of 44 freshwater, brackishwater and semiterrestrial species of Amphipoda occurring in Poland (not counting several stygobiotic species), 8 species from three families may be included in this group. They are Corophiidae: Chelicorophium curvispinum (=Corophium curvispinum), Gammaridae: Gammarus roeselii, G. tigrinus, Chaetogammarus ischnus (=Echinogammarus ischnus), Pontogammaridae: Dikerogammarus haemobaphes, D. villosus, Pontogammarus robustoides and Obesogammarus crassus. It is noticeable that most of them (C. curvispinum, C. ischnus, D. haemobaphes, D. villosus, P. robustoides, O. crassus) are of Ponto-Caspian origin, one species was introduced from North America (G. tigrinus), and one from south-eastern Europe (G. roeselii). All the species listed above have spread widely in Poland, in large rivers and artifical reservoirs (Chelicorophium curvispinum, Dikerogammarus villosus, D. haemobaphes, Pontogammarus robustoides, Gammarus tigrinus) or in medium sized rivers (Gammarus roeselii), in brackish coastal waters (Obesogammarus crassus) or both in fresh and brackish waters (Gammarus tigrinus, Pontogammarus robustoides). In most places they successfully outnumber or even completely replace native amphipod species. This paper presents data on biogeography, history, biology and ecology of alien species, as well as the consequences of their invasion in Poland with an extended bibliography and references to other European countries.
The invasive gammarid Dikerogammarus villosus is known to be a strong predator preying voraciously on a wide array of prey organisms including other gammarids. Predation by D. villosus on other gammarids is regarded as a main reason which led to the elimination of native and invasive gammarids in many reaches of European streams. At several sites in the River Danube and in a Rhine tributary, D. villosus was found co-existing with other gammarids. We studied whether predatory D. villosus is spatially seg- regated from other gammarids which would reduce predatory interactions. Two sites were investigated, one in the Danube (site 1) where D. villosus co-existed with the invasives Echinogammarus ischnus and Dikerogammarus bispinosus and another site in a Rhine tributary (site 2) where D. villosus co-existed with the invasive gammarid Echinogammarus berilloni and two native gam- marids, Gammarus pulex and Gammarus roeseli. At site 1, D. villosus was spatially segregated from E. ischnus but not from D. bispinosus; E. ischnus lived at the uppermost shoreline in boulder substrate while Dikerogammarus spp. lived in greater depth in stony to gravelly substrate. At site 2, D. villosus prevailed in greater depth and gravelly to stony substrate where Corbicula molluscs were present while the other species lived in near-shore areas in macrophytes. Our results suggest that co-existence of predatory D. villosus with gammarids is possible by niche partitioning and that high habitat complexity as at site 2 facilitates co-existence. The fact that at site 1 D. bispinosus occupied approximately the same spatial niche than D. villosus cannot be explained at present time.
Invasive species drive important ecological and economic losses across wide geographies, with some regions supporting especially large numbers of nonnative species and consequently suffering relatively high impacts. For this reason, integrated risk assessments able to screen a suite of multiple invaders over large geographic areas are needed for prioritizing the allocation of limited resources. A total of 16 Ponto-Caspian aquatic species (10 gammarids, one isopod, two mysids, and three fishes) have been short-listed as recent or potential future invaders of British waters, whose introduction and spread is of high concern. In this study, we use multiple modeling techniques to assess their risk of establishment and spread into Great Britain. Climate suitability maps for these 16 species differed depending on the eastern and western distribution of species in continental Europe, which was related to their respective migration corridor: southern (Danube-Rhine rivers), and northern (Don and Volga rivers and Baltic lakes). Species whose suitability was high across large parts of Great Britain included four gammarids (Cheliorophium robustum, Dikerogammarus bispinosus, D. villosus, and Echinogammarus trichiatus) and a mysid (Hemimysis anomala). A climatic "heat map" combining the results of all 16 species together pointed to the southeast of England as the area most vulnerable to multiple invasions, particularly the Thames, Anglian, Severn, and Humber river basin districts. Regression models further suggested that alkalinity concentration > 120 mg/L in southeast England may favor the establishment of Ponto-Caspian invaders. The production of integrated risk maps for future invaders provides a means for the scientifically informed prioritization of resources toward particular species and geographic regions. Such tools have great utility in helping environmental managers focus efforts on the most effective prevention, management, and monitoring programs.
Niche partitioning among close relatives may reflect trade-offs underlying species divergence and coexistence (e.g., between stress tolerance and competitive ability). We quantified the effects of habitat and congeneric species interactions on fitness for two closely related herbaceous plant species, Mimulus guttatus and Mimulus laciniatus, in three common habitat types within their sympatric range. Drought stress strongly reduced survival of M. guttatus in fast-drying seeps occupied by M. laciniatus, suggesting that divergent habitat adaptation maintains this niche boundary. However, neither seedling performance nor congeneric competition explained the absence of M. laciniatus from shady streams where M. guttatus thrives. M. laciniatus may be excluded from this habitat by competition with other species in the community or mature M. guttatus. Species performance and competitive ability were similar in sympatric meadows where plant community stature and the growing season length are intermediate between seeps and streams. Stochastic effects (e.g., dispersal among habitats or temporal variation) may contribute to coexistence in this habitat. Habitat adaptation, species interactions, and stochastic mechanisms influence sympatric distributions for these recently diverged species.
Theoretical models predict that the net outcome of biotic interactions among plants is the sum of co-occurring negative and positive interactions, with facilitation gen- erally increasing in importance with increasing abiotic stress. However, species differ in fundamental niche optima; thus the intensity of stress is relative among species and im- portant only in the context of these relative differences. We tested the hypothesis that the facilitative response of a species is relative to how much abiotic conditions deviate from the optimum conditions for that particular species (stress), and the competitive ''response'' ability of the species (i.e., its ability to tolerate the inhibitory effects of neighbors). In a field experiment, we examined the responses of three co-occurring species with different ecological optima, Bromus erectus, Brachypodium rupestre, and Arrhenatherum elatius ,t o the alleviation of a primary limiting resource (water), and to biotic interactions in a me- soxeric grassland in eastern France. We found that A. elatius had a strong positive response to watering, the response of B. rupestre was moderately positive, and B. erectus did not respond significantly, suggesting that water stress was only important for the first two species. Most importantly, the net outcome of the interaction between each species and its neighbors depended on the degree of water stress it experienced in its natural habitat. For survival, in the control plots we found no significant interactions for B. erectus (not stressed) whereas B. rupestre and A. elatius (stressed species) were facilitated. Enhancing water availability suppressed facilitation of B. rupestre and A. elatius and led to competitive exclusion of B. erectus. In contrast to survival, there was no facilitation for growth in the control plots, and competition intensity increased for all three species with watering. In our experiment the competitive response of a species was inverse to its ability to tolerate stress, indicating a trade-off between these components of the species response. A facili- tative outcome appears to be a function of a species having both a low tolerance to a particular abiotic stress and a strong competitive-response ability.
The Ponto-Caspian gammarid Dikerogammarus villosus currently expands its range and replaces native gammarid populations in many large rivers of Europe. Results of small-scale, still-water experimentation fostered the idea that D. villosus reduces native gammarid populations in large rivers through predation. However, in large rivers, the prevailing flow conditions (potentially harsh or benign for predator and/or prey) and related flow-refuge use (potentially concentrating predator and prey in small spaces) could interfere in this predator-prey interaction. We combined a field survey with a mesocosm-flume experiment to assess flow-refuge use, mortality, and associated effects on leaf-litter decay through potential shredding by the gammarids of coexisting D. villosus and native Gammarus pulex across a shear stress (τ) gradient. In real running waters, both species had no well defined preference for a narrow τ range, suggesting that D. villosus would effectively prey on G. pulex across a wide τ range. In our experiment, however, the concentration of D. villosus and G. pulex in flow refuges increased with τ and decreased with the duration of the experiment (decrease of D. villosus < G. pulex). Overall mortality over the entire experiment was higher in G. pulex than in D. villosus and tended to decrease with τ in D. villosus, whereas it tended to increase with τ in G. pulex. Decay of alder leaf litter decreased with τ and time; however, the difference (low vs. high τ) of the temporal decay-rate drop was low. Our experimental results support the idea that at higher τ, both D. villosus and G. pulex are concentrated in flow refuges, which facilitates predation of the former on the latter. These flow effects on the interaction between D. villosus and a native gammarid (and potentially other rheophilic invertebrates) have particularly important implications for restoration projects that increase the discharge in by-passed sections of large regulated rivers, as the concurrent or future invasion of D. villosus could imperil the restoration of rheophilic invertebrate populations.
Range extensions of aquatic Ponto-Caspian macroinvertebrate species in Europe have mainly been facilitated by the interconnection of river basins through man-made canals and intentional introductions. Three inland migration corridors can be distinguished: (i) a northern corridor: Volga → Lake Beloye → Lake Onega → Lake Ladoga → Neva → Baltic Sea, (ii) a central corridor connecting the rivers Dnieper → Vistula → Oder → Elbe → Rhine, and (iii) a southern corridor connecting the Danube and Rhine rivers. Important trade harbours in Europe were connected via these corridors allowing further range extensions of macroinvertebrate species attached to a vessel's hull or in ballast water. The central corridor was the main migration route before 1992, after which the southern corridor became the most important migration route for the range expansions to the west because of the reopening of the Main-Danube Canal, connecting the Rhine and Danube basins. Especially the water level maintenance in the upper part of the canal, with water supply from the Danube basin, facilitated migration of mobile animals (e.g., crustaceans) from the Danube basin towards the Rhine basin; however, contribution of other transport mechanisms (e.g., shipping) is expected in the near future.
There are about 100 nonindigenous species recorded in the Baltic Sea. Invasive species have resulted in major changes in nearshore ecosystems, especially in coastal lagoons and inlets that can be identified as "centres of xenodiversity". Fewer than 70 of these species have established reproducing populations. Dominant invasion vectors include unintentional introductions via ballast water, tank sediments, and hull fouling, aquaculture, and the construction of canals that have facilitated active or passive natural dispersal. Of the approximately 60 unintentionally introduced species with a known invasion history, 38 are transoceanic (including 19 Atlantic species of American origin) and 18 of Ponto-Caspian origin. Species that have caused economic damage to fisheries, shipping, and industry include the hydrozoan Cordylophora caspia, the barnacle Balanus improvisus, the cladoceran Cercopagis pengoi, and the bivalve Dreissena polymorpha. The Baltic Sea has served as a secondary source of nonindigenous species to the North American Great Lakes. Further study is warranted to quantify large-scale ecosystem changes in the Baltic associated with establishment and population growth of nonindigenous species and to prevent future invasions.
Twenty sites along the St. Lawrence River were sampled to determine if the distribution and abundance of invasive mussels (zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena bugensis)) are explained by physicochemical variables. Calcium concentration, substrate size, and depth independently explained significant proportions of variation in biomass for both species. Zebra mussel populations occurred at calcium levels as low as 8 mg Ca center dot L-1, but quagga mussels were absent below 12 mg Ca center dot L-1, suggesting that they have higher calcium requirements. Both species increased in biomass with increasing substrate size but displayed contrasting patterns with depth. Using combinations of these environmental variables, we developed stepwise multiple regression models to predict zebra mussel biomass and quagga mussel biomass. The zebra mussel model included calcium concentration, substrate size, and depth (r(2) = 0.36, P < 0.0001), while the quagga mussel model included only substrate size and depth (r(2) = 0.32, P < 0.0001). These results suggest that dreissenid mussel abundance (and correlated impacts) will vary predictably across environmental gradients, but the same predictive model will not be accurate for both species.
: I give an overview of the observational and experimental evidence for ecological character displacement in adaptive radiation. Sixty-one published cases of character displacement involving closely related species (congeners) make up the observational data set. All cases involve divergence, even though parallel and convergent dis-placement are theoretically possible. Character ratios in sympatry were greatest when displacement was symmetric (mean 1.54) and least when asymmetric (mean 1.29), perhaps because the most sym-metric resource distributions are also the broadest. Carnivores are vastly overrepresented in the data compared with other trophic groups, with herbivores the next most common category. I consider five hypotheses to explain this pattern, including the possibility that the likelihood of divergence via competition depends on position in food webs. Overall, the quality and completeness of observational data has improved in recent years, as judged by the extent to which individual cases satisfy six standard criteria. All but one of the criteria are met in over half the cases. Most often lacking is independent evidence that the species involved compete for resources. For this reason, we cannot be sure that divergence in sympatry is usually the result of resource competition rather than some other interaction. Field experiments on character displacement, which explore how interaction strength changes per unit change in phenotypic traits, are only just beginning. I summarize research on threespine sticklebacks that used experiments in ponds to test three predictions: that present-day differences between sympatric species are a "ghost" of compe-tition past; that adding a competitor alters natural selection pressures on a species already present, favoring divergence; and that divergent natural selection stemming from resource competition is frequency dependent. In total, the evidence suggests that character displacement occurs frequently in nature, and it probably plays an important role in the evolution of diversity in many adaptive radiations.. All rights reserved. radiation. The idea is simple: depletion of shared resources by closely related, morphologically similar species would favor phenotypes exploiting new resources as yet un-tapped, leading to divergence. Darwin (1859) referred to the process many times, but Lack's (1947) study of the Galápagos finches really solidified the view. Lack presented several examples in which beak differences between species were greater where they occurred together (sympatry) than where they occurred separately (allopatry). His most mem-orable case was of the two ground finches Geospiza fuli-ginosa and Geospiza fortis. Both have similar beak sizes on islands where each occurs alone, but G. fuliginosa has a smaller beak and G. fortis a larger beak on islands where the two coexist. Brown and Wilson (1956) later compiled a list of similar examples from other birds and dubbed the phenomenon "character displacement." These studies had a profound influence on evolutionary theory, and the idea that competition greatly influenced the evolution of diversity soon gained widespread acceptance. Later views of competition's role in divergence were more skeptical. Doubts about its importance spread fol-lowing a series of critiques that questioned the quality and completeness of the evidence and the conclusions that could be drawn from it. For example, Grant (1975) pointed out that two species with partly overlapping geographic ranges might exhibit greater differentiation in sympatry than allopatry if each was responding independently to the same environmental gradient. Arthur (1982) observed that, in most of the putative cases of character displace-ment available at the time, evidence was lacking that dif-ferences between sympatric and allopatric character states had a genetic basis. The patterns might instead have been the outcome of phenotypic plasticity, which, however in-teresting, is not character displacement. Developing sta-tistical approaches suggested that morphological differ-ences between species in sympatry were often not greater than would be expected purely by chance (Strong et al. 1979; Simberloff and Boecklen 1981). Randomly generated "null" assemblages of species often exhibited patterns sim-ilar to those seen in real communities. In other words, alleged instances of character displacement usually also had simple alternative explanations, and at that time, few data sets were complete enough to rule them out.
In plant communities, invasion resistance may increase with diversity because empty niche space decreases simultaneously. However, it is not clear if this only applies to exotic species or also to native species arriving at a site with few other native species during community assembly. We tested the latter by transplanting four native species into experimental grassland communities varying in species richness form 1–16 (−60) species. In addition, we tested the hypothesis that invasion is less successful if the invading species belongs to a functional group that is already present in the community. The test invaders included a grass species (Festuca pratensis, FP), a short (Plantago lanceolata, PL) and a tall herb species (Knautia arvensis, KA), and a legume species (Trifolium pratense, TP). The same four functional groups also occurred alone or in all possible combinations in the different experimental communities. The overall performance of the transplants was negatively related to the logarithm of the species richness of host communities. Plant biomass declined by 58%, 90%, 84% and 62% in FP, PL, KA and TP, respectively, from monocultures to 16-species mixtures, indicating lower invasiveness of the two herbs than of the grass and the legume. Resident grasses showed a strong negative effect on the performance of all test invaders, whereas resident small and tall herbs had neutral, and resident legumes had positive effects. The case of the legumes indicates that contributions to invasion resistance need not parallel invasiveness. Communities containing resident species of only one functional group were most inhibitive to transplants of the same functional group. These results indicate that invasion resistance of experimental plant communities is related to the degree of niche overlap between resident species and invaders. This niche overlap can be high due to generally low amounts of empty niche space in species-rich resident communities or due to the occurrence of the same functional group as the one of the invader in the resident community. Stronger within- than between-functional-group invasion resistance may be the key mechanism underlying diversity effects on invasion resistance in grassland and other ecosystems at large.
Obtaining a better knowledge of how flow variability affects lotic biota is of considerable importance to stream and river management. We contend that processes at different hierarchical levels of organization in lotic ecosystems are sensitive to variation in flow at related hierarchical temporal scales. Ecosystem disturbance caused by large-scale events (i.e. infrequent, but high magnitude flow events with a recurrence interval of years to many days) tend to determine high-level characteristics of ecosystem structure (e.g. determining species pools, periphyton versus macrophyte dominance) and function (e.g. balance between auto- and heterotrophy). The high-level ecosystem characteristics then set the stage for processes that are influenced by flow variation that occurs at smaller temporal scale (i.e. minutes to milliseconds) such as colonization, biotic interactions and mass transfer enhancement of production. We contend that large-scale temporal events predominantly affect lotic ecosystems through physical drag processes (‘drag-disturbance’), whereas small-scale flow variations affect ecosystems through mass-transfer processes (including invertebrate and fish food-uptake). Drag-disturbance and mass-transfer related processes mark the opposite ends of a continuum of flow variability controlled processes, with moderate temporal scale flow variability events affecting ecosystems through both drag-disturbance and mass-transfer processes in similar proportions. Flow variability, and associated effects on ecosystems, across these scales is discussed with reference to New Zealand rivers. We suggest that these concepts can be integrated across the full range of temporal scales based on a spectrum of velocity variations. This may provide a unifying conceptual model for how the structure and functioning of lotic ecosystems are linked to flow variability. Copyright © 2005 John Wiley & Sons, Ltd.
The main objective of the European Union (EU) funded project AQEM1was to develop a framework of an assessment system for streams in Europe based on benthic macroinvertebrates that fulfils the requirements of the EU Water Framework Directive. Initial assessment methods for 28 European stream types and more generally applicable tools for stream biomonitoring in Europe were generated.
The development of the system was based on a newly collected data set covering stream types in Austria, the Czech Republic, Germany, Greece, Italy, The Netherlands, Portugal and Sweden. Altogether, 901 benthic invertebrate samples were taken using a standardised multi-habitat sampling procedure and a large number of parameters describing the streams and their catchments was recorded for all sampling sites. From the stream and catchment characteristics measures of stress were derived. A large number of metrics was tested independently for each of the stream types, to identify the response of each metric to degradation of a site. This process resulted in up to 18 core metrics for the individual stream types, which were combined into a different multimetric index in each country. The multimetric AQEM assessment system is used to classify a stream stretch into an Ecological Quality Class ranging from 5 (high quality) to 1 (bad quality) and often provides information on the possible causes of degradation.
AQEM provides a taxa list of 9557 European macroinvertebrate taxa with associated autecological information, a software package for performing all the calculations necessary for applying the multimetric AQEM assessment system and a manual describing all aspects of the application of the system from site selection to data interpretation.
Invasions of alien gammarid species have led to reduced abundance of many native gammarid species and earlier gammarid invaders. Intra-guild predation (IGP) has been suggested as the main mechanism causing such species displacements. This study elucidates a mechanism for species displacement that is based on competition for shelter, viz. species excluding each other from a shelter place. Preferences of Dikerogammarus villosus and Gammarus roeselii for shelter space were studied in laboratory experiments. In contrast with night conditions both species showed a strong preference for shelter sites under daytime conditions so that all experiments were carried out under permanent light conditions. Single individuals of D. villosus sheltered more than those of G. roeselii. Intraspecific competition experiments with different size classes and sexes showed that in contrast with D. villosus, the size class had a significant effect on the mean sheltering proportion of individuals of G. roeselii. When both species were brought together in a basin with a shelter site to test interspecific competition, individuals of G. roeselii were actively pushed out of their shelters within 24 h, while D. villosus showed no change in shelter. This led to a significantly increased predation risk for G. roeselii, which was confirmed in an experiment in which a benthic fish was added. IGP only manifested itself after 48 h, indicating that competition for shelter preceded IGP. When shelter opportunities are in short supply, shelter exclusion may be one of the initial mechanisms for gammarid species displacements.
The River Danube plays a central role in the spread of Ponto-Caspian species as a part of the so-called
southern invasion corridor (Danube-Main-Rhine system); therefore, changes in its peracarid fauna (comprising
the bulk of invasives) merit special attention. The latest international research expedition (Joint Danube Survey
3, 2013) offered an opportunity for updating and synthesizing our knowledge about this group along the Danube,
previously based on studies covering only certain river sections and/or dealing with a subset of species. Altogether
17 amphipod, 7 mysid, 3 isopod, and one cumacean species were recorded at 55 sites investigated between Ulm
(river km 2581) and the Delta. Recent large-distance expansion of additional Ponto-Caspian species was not observed, but three species (Chelicorophium robustum, C. sowinskyi, and Echinogammarus trichiatus) have been able
to shorten their distributional gap in the Middle Danube, E. trichiatus being recorded for the first time in Serbia.
Ponto-Caspian peracarids are still gradually advancing in the German section, as well, implying retreat of native
Gammarus spp., and impeding the spread of non-Ponto-Caspian invaders. On the contrary, some Ponto-Caspian
species seem to have declined in certain river sections; Dikerogammarus bispinosus was entirely missing in the
Lower Danube, and several species characteristic of the lower reaches had been recorded previously much farther
upstream (most notably Chelicorophium maeoticum and Obesogammarus crassus). The analysis of current and
historical distributional patterns revealed that the crucial step in the large-scale spread of Ponto-Caspian species is
crossing the section between Baja and the Sava estuary (rkm ~1480 –1170) – characterized by an unfavorable combination of relatively strong currents and fine bed material – by passive transport. Presence immediately downstream
of this section does not appear to promote further expansion in most of the cases; the source region of large-distance
dispersal is most likely the Delta, implying that potential future invaders cannot be identified based on their previous expansion in the lower reaches of the river.
With accelerating rates of invasion being documented in many ecosystems, communities of interacting invasive species are becoming increasingly common. Opposing theories predict that invaders can either hinder or promote one another's success. Additionally, evidence suggests that co-occurring invaders can interact to amplify or mitigate one another's impacts on ecosystems. However, there has not been a quantitative review on interactions among multiple invasive animals. Here I use a meta-analysis approach to show that, across a global scale, the mean interaction among invaders was to reduce one another's performance. This pattern was consistent when considering interactions between marine animals but interactions were neutral overall in terrestrial and freshwater ecosystems. Crucially, individual studies showed that neutral interactions were the most common interaction type. Further, I demonstrate that the combined ecological impacts of multiple invaders were frequently the sum of their independent effects (additive) but the mean effect was non-additive and less than predicted (antagonistic). In both meta-analyses, the disparity between the most frequent and mean interaction type indicates that case studies of multiple invasions commonly have different outcomes to global trends. These results will help predict how co-occurring invasive animals interact and assist in developing management strategies for problematic invaders in our changing world.
Metacommunity ecology addresses the situation where sets of local communities are connected by the dispersal of a number of potentially interacting species. Aquatic systems (e.g. lentic versus lotic versus marine) differ from each other in connectivity and environmental heterogeneity, suggesting that metacommunity organisation also differs between major aquatic systems. Here, we review findings from observational field studies on metacommunity organisation in aquatic systems.
Species sorting (i.e. species are ‘filtered’ by environmental factors and occur only at environmentally suitable sites) prevails in aquatic systems, particularly in streams and lakes, but the degree to which dispersal limitation interacts with such environmental control varies among different systems and spatial scales. For example, mainstem rivers and marine coastal systems may be strongly affected by ‘mass effects’ (i.e. where high dispersal rates homogenise communities to some degree at neighbouring localities, irrespective of their abiotic and biotic environmental conditions), whereas isolated lakes and ponds may be structured by dispersal limitation (i.e. some species do not occur at otherwise‐suitable localities simply because sites with potential colonists are too far away). Flow directionality in running waters also differs from water movements in other systems, and this difference may also have effects on the role of dispersal in different aquatic systems.
Dispersal limitation typically increases with increasing spatial distance between sites, mass effects potentially increase in importance with decreasing distance between sites, and the dispersal ability of organisms may determine the spatial extents at which species sorting and dispersal processes are most important.
A better understanding of the relative roles of species sorting, mass effects and dispersal limitation in affecting aquatic metacommunities requires the following: (i) characterising dispersal rates more directly or adopting better proxies than have been used previously; (ii) considering the nature of aquatic networks; (iii) combining correlative and experimental approaches; (iv) exploring temporal aspects of metacommunity organisation and (v) applying past approaches and statistical methods innovatively for increasing our understanding of metacommunity organisation.
Since the first publication on Amphipoda in Lake Balaton (1897) altogether 8 species were found in different habitats of the lake. The species composition changed from time to time. In the 1980s 3 pontocaspian amphipods (Corophium curvispinum, Dikerogam marus haemobaphes and D. villosus) were found on the submerged macrophytes and on the stones of the littoral zone. We studied the life history and measured the biomass of C. cur vispinum and D. haemobaphes on the submerged macrophyte Myriophyllum spicatum near Tihany peninsula. Both amphipods reproduce from May (C. cun'ispinum till October, D. haemobaphes till the middle of August). They produce two generations in a year. Biomass of C. curvispinum and D. haemobaphes was roughly 11 and 0.1 g animal dry wt per m. res pectively, in an area covered by M spicatum in August 1986. Amphipods play an important role in water ecosystems as utilizers of organic matter and as food for fish. At the same time, they react sensitively to changes in the environment. The first data on the occurrence of amphipods in Lake Balaton are from the last century (Table 1). Information was published on one of the spe cies (Gammants pulex) of the littoral zone. In the 1930s a pontocaspian species, Corophium curvispinum, appeared in the lake and rapidly invaded almost all of its habitats. This species might come from the Danube through the Sió canal, as they are building their tubes on the surface of water vessels (Sebestyén 1934, Entz 1943). In addition to this species, Gammarus roeseli appeared in the lake between the 1930s and 1950s. Species composition of amphipods of Lake Balaton changed considerably at the beginning of the 1950s, when Gammarus roeseli was replaced by pontocaspian Dikerogammarus species (D. haemobaphes and D. villosus). It is possible that the Dikerogammarus species came into the lake with Limnomysis benedeni, which was introduced into the lake as fish food (Woynárovich 1954). The Dikerogammarus species have been permanent elements of amphipod fauna in the lake. The spreading of pontocaspian amphipods in Lake Balaton was probably possible be cause of their ability to utilize the high detritus content of the lake water as filte ring organisms (Entz 1943, 1949, Ponyi 1955, 1956a). Another reason for the presence of these species in Lake Balaton might be that D. villosus has a higher tolerance to high water temperature than does G. roeseli, which is basically a river inhabitant (Lukacsovics 1958). In addition to the above mentioned amphipods Synurella ambuláns and Ni-phargus mediodanubialis has been found in Lake Balaton (in the marshy zone of the reed belt, close to the shore) (Ponyi 1962). I should mention here that my description of the amphipod species follows the "Limnofauna Europaea" (Pinks ter 1978), disregarding the description of subspecies.
The movement of species is one of the most pervasive forms of global change, and few ecosystems remain uninvaded by nonnative species. Studying species interactions is crucial for understanding their distribution and abundance, particularly for nonnative species because interactions may influence the probability of invasion and consequent ecological impact. Interactions among nonnatives are relatively understudied, though the likelihood of nonnative species co-occurrence is high. We quantify and describe the types of interactions among nonnative plants and determine what factors affect interaction outcomes for ecosystems globally. We reviewed 65 studies comprising 201 observations and recorded the interaction type, traits of the interacting species, and study characteristics. We conducted a census of interaction types and a meta-analysis of experiments that tested nonnative competition intensity. Both methods showed that negative and neutral interactions prevailed, and a number of studies reported that the removal of a dominant nonnative led to competitive release of other nonnatives. Positive interactions were less frequently reported and positive mean effect sizes were rare, but the plant characteristics nitrogen fixation, life cycle (annual or perennial), and functional group significantly influenced positive interactions. Positive interactions were 3 times more frequent when a neighboring nonnative was a nitrogen fixer and 3.5 times lower when a neighboring nonnative was an annual. Woody plants were 2 or 4 times more likely to have positive interactions relative to grasses or herbs, respectively. The prevalence of negative interactions suggests that managers should prepare for reinvasion of sites when treating dominant nonnatives. Though positive interactions were infrequent, managers may be able to anticipate positive interactions among nonnatives based upon traits of the co-occurring invaders. Predicting positive nonnative interactions is an important tool for determining habitat susceptibility to a particular invasion and for prioritizing management of nonnatives with a higher likelihood of positive interactions.This article is protected by copyright. All rights reserved.
Linkages between habitat structure, invasibility and direct species interactions were examined in a Central European river system, the Schipbeek in the Netherlands, invaded by the ‘killer shrimp’ amphipod Dikerogammarus villosus . The invader's arrival in other fresh waters is linked to declines in macroinvertebrate biodiversity, with previous studies indicating that the invader's impacts may be mediated by habitat heterogeneity and substrate type.
Using an extensive 50 km field survey, potential habitat factors influencing distribution were identified. Of 12 sites surveyed, two contained fish ‘passes’ (instream sections creating faster flows via concrete structures covered in cemented rocks). It was only in these sites that D. villosus was found and then only in the fish pass areas, as opposed to the surrounding river, which, like the remaining 10 sites of the survey, was characterized by sluggish flows and a muddy/silty substrate. Gammarus roeselii was the numerically dominant amphipod in these latter areas, with no D. villosus being detected. A repeated survey more than 2 years later, revealed the same distribution pattern.
Within both fish pass areas, D. villosus relative abundance was highest in faster flowing, less macrophyte dense patches, with the opposite true for G. roeselii . D. villosus numerical abundance decreased with increasing macrophyte cover in both passes.
A transplant experiment indicated high survivorship of D. villosus outside fish pass areas and showed that slow‐flowing watercourses with mud/silt substrates, previously deemed ‘unsuitable’, are not effective barriers to D. villosus spreading to more ‘suitable’, faster, rocky substrate watercourses.
We recommend that when the location and cost‐benefit analysis of instream rehabilitation structures, such as fish passes, is being considered, attention must be paid to the risk of hastening the spread of this invader. We also recommend that when monitoring programmes are designed for D. villosus in newly invaded regions, instream artificial structures should be included as part of the survey.
Copyright © 2013 John Wiley & Sons, Ltd.
The 50-ha long-term forest plot on Barro Colorado Island in Panama was 'ground zero' for the development of ecology's 'neutral theory' and comparisons with its 'niche theory' counterpart. In this issue, Garzon-Lopez and colleagues used tree distributions at this site to recast the unresolved (and unresolvable) debate to show that observational scale drives the perception of which processes predominate. The 50 ha long-term forest plot on Barro Colorado Island in Panama was 'ground zero' for the development of ecology's 'neutral theory' and comparisons with its 'niche-theory' counterpart. In this issue, Garzon-Lopez and colleagues used tree distributions at this site to recast the unresolved (and unresolvable) debate to show that observational scale drives the perception of which processes predominate.
1. Biological invasions are regarded as one of the greatest threats to biological diversity. One of the macroinvertebrate groups with the largest number of invasive species in fresh water are gammarid amphipods. Their omnivorous (including predatory) feeding behaviour may facilitate their spread and establishment in new areas.
2. Dikerogammarus villosus , the ‘killer shrimp’, is a well‐known example of a Ponto‐Caspian gammarid that is a very effective predator and successful coloniser in Europe. There are, however, other invasive Ponto‐Caspian amphipods, which have spread successfully in Northern, Central and Western Europe. Our aim here was to test whether two of such invaders ( Pontogammarus robustoides and Dikerogammarus haemobaphes ) are also more predacious than a native species ( Gammarus fossarum ).
3. Stable isotope analysis (δ ¹⁵ N and δ ¹³ C) of Ponto‐Caspian amphipods coexisting in a reservoir demonstrated that the trophic positions of P. robustoides and D. haemobaphe s were similar to that of D. villosus . Echinogammarus ischnus and Chelicorophium curvispinum occupied the lowest position in the food web, while the native Gammarus fossarum (collected from another waterbody) had an intermediate trophic position.
4. Stomach content analysis of P. robustoides , D. haemobaphes and G. fossarum collected in the field, as well as laboratory feeding experiments, was used to compare diet and feeding preferences among the two invasive and one native species. All three species were omnivorous and predacious. However, the two invasive species ( P. robustoides and D. haemobaphes ) were more effective predators than G. fossarum and showed a clear preference for animal prey and tissue.
5. Pontogammarus robustoides and D. haemobaphes may, like D. villosus , also be called ‘killer shrimps’ and could have a similar impact as invaders of European freshwater and brackish waterbodies.
The North American Great Lakes have been invaded and dramatically altered by more than 145 alien species. Many invasions have occurred during the past few decades because of the release of Eurasian ballast water from transoceanic ships. Current regulations require ships to exchange foreign ballast with highly saline water before entering the Great Lakes; this procedure should prevent colonization by strictly freshwater species, but species with broad salinity tolerance might survive transport in exchanged water. A recent series of invasions by euryhaline organisms from the Black and Caspian Seas region signals a new phase in the transformation of the Great Lakes - one that supports the concept of an 'invasional meltdown'.
Distributions of species, animals or plants, terrestrial or aquatic, are influenced by numerous factors such as physical and biogeographical gradients. Dominant wind and current directions cause the appearance of gradients in physical conditions whereas biogeographical gradients can be the result of historical events (e.g. glaciations). No spatial modelling technique has been developed to this day that considers the direction of an asymmetric process controlling species distributions along a gradient or network. This paper presents a new method that can model species spatial distributions generated by a hypothesized asymmetric, directional physical process. This method is an eigenfunction-based spatial filtering technique that offers as much flexibility as the Moran's eigenvector maps (MEM) framework; it is called asymmetric eigenvector maps (AEM) modelling. Information needed to construct eigenfunctions through the AEM framework are the spatial coordinates of the sampling or experimental sites, a connexion diagram linking the sites to one another, prior information about the direction of the hypothesized asymmetric process influencing the response variable(s), and optionally, weights attached to the edges (links). To illustrate how this new method works, AEM is compared to MEM analysis through simulations and in the analysis of an ecological example where a known asymmetric forcing is present. The ecological example reanalyses the dietary habits of brook trout (Salvelinus fontinalis) sampled in 42 lakes of the Mastigouche Reserve, Québec.
Much as Rachel Carson's "Silent Spring" was a call to action against the pesticides that were devastating bird populations, Charles S. Elton's classic "The Ecology of Invasions by Animals and Plants" sounded an early warning about an environmental catastrophe that has become all too familiar today-the invasion of nonnative species. From kudzu to zebra mussels to Asian long-horned beetles, nonnative species are colonizing new habitats around the world at an alarming rate thanks to accidental and intentional human intervention. One of the leading causes of extinctions of native animals and plants, invasive species also wreak severe economic havoc, causing $79 billion worth of damage in the United States alone. Elton explains the devastating effects that invasive species can have on local ecosystems in clear, concise language and with numerous examples. The first book on invasion biology, and still the most cited, Elton's masterpiece provides an accessible, engaging introduction to one of the most important environmental crises of our time. Charles S. Elton was one of the founders of ecology, who also established and led Oxford University's Bureau of Animal Population. His work has influenced generations of ecologists and zoologists, and his publications remain central to the literature in modern biology. "History has caught up with Charles Elton's foresight, and "The Ecology of Invasions" can now be seen as one of the central scientific books of our century."-David Quammen, from the Foreword to "Killer Algae: The True Tale of a Biological Invasion"
It is suggested that evolution in plants may be associated with the emergence of three primary strategies, each of which may be identified by reference to a number of characteristics including morphological features, resource allocation, phenology, and response to stress. The competitive strategy prevails in productive, relatively undisturbed vegetation, the stress-tolerant strategy is associated with continuously unproductive conditions, and the ruderal strategy is characteristic of severely disturbed but potentially productive habitats. A triangular model based upon the three strategies may be reconciled with the theory of r- and K-selection, provides an insight into the processes of vegetation succession and dominance, and appears to be capable of extension to fungi and to animals.
In laboratory experiments, quagga mussels (Dreissena bugensis) survived as well as zebra mussels (Dreissena polymorpha) and equaled or exceeded their growth rate (-3 to 242% change in wet mass) when reared at 6 or 23°C and fed natural seston or Chlamydomonas at food levels ranging from 0.05-7.4 µg·L-1 chlorophyll a (chl a). Superior growth of quagga mussels was most pronounced at low food levels. We found no significant differences in per capita clearance rates (CR), functional responses, or feeding behavior between zebra and quagga mussels fed Chlamydomonas, Nannochloris , or mixed suspensions of Nannochloris and clay. Per capita CR ranged from 0.018 to 0.402 L·mussel-1·h-1 for zebra mussels and from 0.010 to 0.407 L·mussel-1·h-1 for quagga mussels. Because quagga mussels had more biomass per unit shell length, we found lower biomass-specific CR for quagga mussels. When fed natural seston, zebra and quagga mussels could selectively reject inorganic material and at the lowest seston level the assimilation efficiency of quagga mussels (81%) was significantly higher than that of zebra mussels (63%). Our experi- ments suggest that quagga mussels can survive, grow, and feed as well or better than zebra mussels in epilimnetic wa- ters with either low or high productivity.
I measured respiration, shell growth, body mass, and reproduction in sympatric populations of zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels in western Lake Erie to determine if the species differ in physiological parameters and if any differences provide an explanation for the recent displacement of zebra mussels by quagga mussels. Between May and August, I measured respiration (1998), shell growth of marked mussels suspended in cages in the lake (19992000), soft body mass and mass of gametes released (19982001), and the number of gametes released by individual mussels (1999). Quagga mussels had lower respiration rates and greater shell growth and body mass. There was no difference in the percentage of spawning mussels or the number of sperm released by individuals, but zebra mussels generally released more eggs and a greater mass of gametes than did quagga mussels. Similar reproduction at a lower body mass indicates that zebra mussels devote a greater proportion of body tissue to reproduction. Lower respiration rates and larger size give quagga mussels a competitive advantage and may explain their displacement of zebra mussels.
The Mantel correlogram is an elegant way to compute a correlogram for multivariate data. However, recent papers raised concerns about the power of the Mantel test itself. Hence the question: Is the Mantel correlogram powerful enough to be useful? To explore this issue, we compared the performances of the Mantel correlogram to those of other methods, using numerical simulations based on random, normally distributed data. For a single response variable, we compared it to the Moran and Geary correlograms. Type I error rates of the three methods were correct. Power of the Mantel correlogram was nearly as high as that of the univariate methods. For the multivariate case, the test of the multivariate variogram developed in the context of multiscale ordination is in fact a Mantel test, so that the power of the two methods is the same by definition. We devised an alternative permutation test based on the variance, which yielded similar results. Overall, the power of the Mantel test was high, the method successfully detecting spatial correlation at rates similar to the permutation test of the variance statistic in multivariate variograms. We conclude that the Mantel correlogram deserves its place in the ecologist's toolbox.
At present, the invasive round and bighead gobies are the most abundant benthic fish species in the near shore zone of the Middle Danube. We compared their diet seasonally in natural and artificial habitats and contrasted it with the food supply. The composition of the macroinvertebrate community was determined mainly by seasonal changes, whereas habitat type had smaller effect. Round gobies followed these changes flexibly. They consumed mainly chironomids in the spring, whilst amphipods and molluscs in the summer and autumn. Bighead gobies relied on amphipods in each season and in both habitats, and consumed fish, too, including round goby (intraguild predation). Diet overlap was determined by the morphological differences of the species allowing a varying degree of differentiation according to the seasonally changing food supply. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
1. Mean motion and near‐bed flows in streams and rivers can be described using a classification derived from fairly simple field measurements. Our proposed classification is ecologically useful because it incorporates the combined effects of velocity, depth and substrate roughness to provide a means of quantifying the flow regimes occurring within the microhabitats of stream benthos.
2. Mean motion is characterized by the Reynolds number and the Froude number. Both are easily calculated, and because they are dimensionless they provide a means of comparing flows at different sites.
3. Five categories of near‐bed flows (i.e. the flow microenvironments of stream benthos) are recognized. Flow may be hydraulically smooth or hydraulically rough and the latter category is subdivided further into: chaotic flow, wake interference flow, isolated roughness flow and skimming flow. Hydraulically smooth flows occur in sections of a river bed with fine sediments (e.g. sands, muds and clays). over flat sheets of bedrock, or in association with the flat blades of submerged macrophytes. Hydraulically rough flows occur where the substrate elements are larger (e. g. pebbles, cobbles and boulders) and are a function of substrate roughness and the depth of flow relative to the height of the roughness elements. Chaotic flows and wake interference flows predominate in riffles whilst isolated roughness flows and skimming flows are more likely to be a feature of runs.
4. Conventional stream sampling methods (e.g. the Surber and box or cylinder samplers) may collect across several different flow microhabitats. Our classification should enable different flow microenvironments to be recognized and so sampled more appropriately which, in turn, may reduce apparent clumping and the wide confidence intervals of benthic population estimates. Because our classification identifies ‘patches’ within the flow regime associated with the stream bed it enables stream ecologists to generate testable hypotheses regarding the distribution and abundance of benthic species in response to flow.
5. Our classification identifies spatial patterns in the flow regimes associated with the stream bed. Temporal patterns have not been identified: however, predictable changes in spatial patterns will resuh from temporal changes in stream discharge.
SUMMARY 1. Biological invasions often involve close taxonomic relatives either as native/invader pairs or as invader/invader pairs. Precise identification and differentiation of species is therefore of paramount importance to reconstruct the invasion history. Genetic studies are indispensable in the case of morphologically conservative taxonomic groups.
2. We analysed the Pontocaspian freshwater amphipods Dikerogammarus that have successfully invaded the benthos of large Central European rivers. Taxonomic uncertainties were clarified by phylogenetic analyses of mitochondrial 16S and COI genes. The three-way partitioning of allozyme genotypes in a syntopic population further corroborated the taxonomic status of the three species Dikerogammarus haemobaphes, D. villosus and D. bispinosus. Dikerogammarus bispinosus had been prior misidentified as a subspecies of D. villosus. The conspicuous colour types of D. villosus, however, appeared to be conspecific.
3. The genetic identification of the previously more abundant D. haemobaphes individuals in old samples supported the ‘successive invasion wave’ hypothesis with D. haemobaphes as the first invader displaced by the second invader D. villosus. Dikerogammarus bispinosus could be a potential future invader.
4. Haplotype differentiation was apparent between two invasion lines of D. haemobaphes, but the occurrence of a single widespread haplotype indicates genetic impoverishment during rapid colonisation.
The gammarid composition at 25 sites in the rivers Danube, Main and the southern reaches of the Rhine were studied during the years 2002–2004. Dikerogammarus villosus and Echinogammarus ischnus were the most frequent species prevailing at 17 sites. Sympatric occurrence of D. villosus and E. ischnus was observed at 12 sites. Dikerogammarus haemobaphes was recorded at 7 sites; this species prevailed in the Danube, west of the Weltenburger Enge and in the Isar mouth where it co-existed with native species (Gammarus pulex and/or G. roeseli) at 6 sites. Dikerogammarus bispinosus and E. berilloni were found at only 1 site, where they co-existed with D. villosus and E. ischnus, and with D. villosus and native species, respectively. Investigation of reproductive characteristics at 3 sites showed that females of D. villosus and D. haemobaphes produced the biggest clutches with more than 100 eggs. Females of E. ischnus produced much smaller clutches (10–35 eggs on an average), but very big eggs. Clutch sizes and egg volumes of D. bispinosus and E. berilloni resembled those of native species. Our results suggest that the most successful invaders (D. villosus, D. haemobaphes and E. ischnus) display reproductive traits that facilitate their success. Both Dikerogammarus sp. allocate energy into production of many but small eggs, thus maximizing offspring number, while E. ischnus allocates its energy into production of fewer but large eggs which could be beneficial at sites where food is scarce.
Dikerogammarus villosus, a Ponto-Caspian species, is known to be a particularly successful invader, and is currently the prevailing invasive gammarid
in Southern German large waters. Field observations suggest that D.villosus has replaced the native G.pulex and the invasive D.haemobaphes, also a Ponto-Caspian species, in some reaches of the German Danube. Dikerogammarus villosus is also believed to be the reason why Echinogammarus berilloni, a Mediterranean species, and Dikerogammarus bispinosus, a further Ponto-Caspian gammaridean species, could not build significant populations in Southern German rivers. Because
intraguild predation (IGP) is regarded as a major force leading to species displacements, we hypothesized that superiority
in predation by D.villosus is responsible for the disappearance or limited occurrence of several native and invasive gammarideans in many reaches of
Southern German waters. To test this hypothesis, we conducted a series of laboratory IGP experiments with 1:1 combinations
in which D.villosus was allowed to prey on other gammarids and vice versa. We also studied the extent of cannibalism within species. Dikerogammarus villosus was a stronger predator than G.pulex and E.berilloni. However, D.villosus was not stronger in predation than its relatives D.haemobaphes and D.bispinosus, although results with the combinations of D.villosus and D.bispinosus were less conclusive, especially in the female–female combination. Cannibalism rate was high in D.haemobaphes compared with other species. We conclude that superiority in IGP experiments could explain the disappearance of G.pulex and the missing or limited occurrence of E.berilloni since the arrival of D.villosus in Southern German rivers. However, the results of IGP experiments do not explain why D.haemobaphes was displaced by D.villosus in many places and why D.bispinosus could not build significant populations in Southern German waters. Possibly specific combinations between mutual predation
and cannibalism or factors other than predation could have affected displacement and occurrence patterns of Dikerogammarus species in Southern German waters.