The river Rhine: a global highway for dispersal of aquatic invasive species

The river Rhine is heavily influenced by human activities and suffers from a series of environmental constraints which hamper a complete recovery of biodiversity. These constraints comprise intensive navigation and habitat modification by hydraulic engineering. Improving water quality while these constraints remain in place has led to increased colonization by aquatic invasive species. This tendency has been accelerated by the construction of canals connecting river basins. Over the last two centuries, the total surface area of river catchments connected to the river Rhine via inland waterways has been increased by a factor 21.6. Six principal invasion corridors for aquatic species to the river Rhine are discerned. The extensive network of inland waterways has allowed macroinvertebrate species from different bio-geographical regions to mix, changing communities, affecting the food webs and forming new constraints on the recovery of the native biodiversity. From the eighteenth century onward, in the freshwater sections of the river Rhine, a total of 45 non-indigenous macroinvertebrate species have been recorded. The average number of invasions per decade shows a sharp increase from <1 to 13 species. Currently, the contribution of non-indigenous species to the total species richness of macroinvertebrates in the river Rhine is 11.3%. The Delta Rhine and Upper Rhine exhibit higher numbers of non-indigenous species than other river sections, because the sea ports in the Delta Rhine and the Main-Danube canal function as invasion gateways. Important donor areas are the Ponto-Caspian area and North America (44.4 and 26.7% of the non-indigenous macroinvertebrate species, respectively). Transport via shipping and dispersal via man made waterways are the most important dispersal vectors. Intentional and unintentional introductions are highest for the period 1950–1992. The cumulative number of non-indigenous species in time is significantly correlated with the increase in total surface area of other river catchments connected to the river Rhine by means of networks of canals. The species richness of non-indigenous macroinvertebrates is strongly dominated by crustaceans and molluscs. Invasive species often tolerate higher salt content, temperature, organic pollution and current flow than native species. Spatiotemporal analyses of distribution patterns reveal that average and maximum dispersal rates of six invasive species vary between 44–112 and 137–461 km year⁻¹, respectively. Species arriving in upstream sections first show a shorter time lag between colonisation of the Delta and Upper Rhine than species initially arriving in downstream areas. Temporal analyses of macroinvertebrate assemblages in the littoral zones indicate that native species are displaced by non-indigenous species. However, established non-indigenous species are also displaced by more recent mass invaders.     

Potential impact of invasive amphipods on leaf litter recycling in aquatic ecosystems

The impact of biological invasions on local biodiversity is well established, but their impact on ecosystem functioning has only been sketchily documented. However, biological invasions may impede services provided by aquatic ecosystems, such as, for example, the decomposition of organic matter, a key process in most small streams. To address this question, we experimentally quantified the leaf litter breakdown activity of native and invasive amphipod species, which are keystone species in aquatic ecosystems. The breakdown rate of each species was used to estimate the potential leaf litter recycling in the Rhône and Meurthe Rivers in sites occupied solely by native species and sites dominated by invasive species. We found that invaders were not able to compensate for the activity of native species and that the replacement of native species led to a decrease of at least 66% in the rate of leaf litter recycling. Our approach provides empirical evidence of the functional impact of non-indigenous species on leaf litter recycling, using standard protocols and literature data.     

Impact of river management history on the community structure,species composition and nutrient status in the Rhine alluvial hardwood forest

The present-day Rhine alluvial hardwood forest (Querco-Ulmetum minoris, Issler 24) in the upper Rhine valley (France/Germany) is comprised of three vegetation units, one still flooded by calm waters (F) and the two others unflooded, one for 30 years (UF30) (after the river canalisation) and the other for about 130 years (UF130) (after river straightening and embankment work in the middle nineteenth century). In the three stands, species composition, structure and diversity of vegetation and nutrient content of mature leaf, leaf litter and soil have been studied. Fungi (Macromycetae) were only studied in two stands (F and UF130). The intensity of nutrient recycling was exemplified by comparing the chemical composition of rainwater, flood, throughfall, mature leaf, leaf litter, soil and groundwater in two of these stands (F and UF30).The elimination of floods has caused a change in floristic composition, tree density and plant diversity. Tree density was higher in the two unflooded stands and was related to a large increase in sapling (< 6 cm dbh) density more than to a change of stem (> 6 cm dbh) density. Sapling density increased 2 times and three times in the UF30 and the UF130 respectively, whereas the stem density increased only 12% in the first stand and decreased 29% in the second one. The saprophytic macromycete communities have been supplemented with mycorrhizal species. Leaf litter production was slightly greater in the flooded (4.44 T ha^-1 yr^-1) than in the two unflooded stands ( 3.72 T ha^-1 yr^-1). Foliar N level is twice as high in the flooded stands in spite of an opposite soil status. P level decreased in soil and leaves with the duration of isolation and was at the same level in the groundwater in two stands (F and UF30). K, Mg and Ca contents of green leaf and leaf litter were high due to the geochemistry of the Rhine substrate (calcareous gravels and pebbles) and similar in all the stands studied, even though there are large inputs of these three elements by floods. Moreover we showed that the groundwater chemistry reflected the variations of nutrient inputs and thus could be a good indicator of the functioning of an alluvial ecosystem and of its change as a result of human activities. The restoration of floods in hardwood forest contributes to the preservation of alluvial vegetational structure and composition, the stimulation of biological processes and a better plant mineral nutrition and water supply.     

Assessing the functional turnover of species assemblages with tailored dissimilarity matrices

It is often suggested that community functional diversity is an appropriate predictive measure of ecosystem functioning, particularly if relevant species traits for the ecological property of interest are carefully selected. However, methods for selecting traits are often based on expert knowledge or on theoretical models of ecosystem functioning, but usually do not include explicitly developed quantitative procedures. Here we propose to construct a so‐called ‘tailored dissimilarity matrix’ between species assemblages to emphasize their functional turnover in response to some user‐defined ecological property. First, a subset of community weighted mean trait values (CWM) is selected by stepwise regression on the ecological process of interest. The selected CWM values are then replaced by the residuals of the least‐squares regressions of each single CWM on the ecological process of interest and pairwise Euclidean distances between the residual values at each sampling site are calculated. We illustrate the advantages of the tailored approach using two distinct plant and bee communities under contrasting fire regimes in temperate forests of southern Switzerland. Our results demonstrated that, unlike for the original CWM values, the tailored approach optimized the degree of functional differentiation among bee and plant species assemblages, i.e. the species functional turnover, with respect to different fire regimes.     

Successional status of trees in gallery forest along the river Rhine

Abstract. 20 alluvial forest stands of different ages along the river Rhine in central Alsace, France, are described. A natural complex landscape occurs which is formed by erosion activity of the river. Recent human impact (canal construction, cutting) has affected river hydrology: flooding is eliminated over large areas and the underground water levels are stabilized. The forest vegetation varies in species diversity and structure, from young pioneer to older, mid-successional forests. The forests were classified into four associations: Salici-Populetum nigrae, Ligustro-Populetum nigrae, Fraxino-Populetum albae and Querco-Ulmetum minoris. The first three communities are ‘softwood’ because of the dominance of Salix and Populus, the fourth, dominated by Quercus robur, Fraxinus excelsior and Ulmus minor, is ‘hardwood’. Differences in structure, species composition and diversity in 10 widely varying stands in 30-yr and 150-yr old forests are quantified and interpreted in relation to the processes and gradients (moisture and texture) involved. A model of forest succession is developed as follows:

• 1 Whatever the topographic level, Salix and Populus species are the most competitive in colonizing bare sediments.
• 2 Under natural conditions, pioneer softwoods are generally replaced by hardwoods in less than 100 yr.
• 3 Old Querco-Ulmetum is basically the terminal stage of the alluvial succession.
• 4 Old softwood forests result from an interruption of the natural course of succession. Softwoods may be an intermediate or late-successional phase depending on the interruption.
• 5 Successional processes change according to hydrological and edaphic gradients.
• 6 Allogenic processes of flooding are fundamental in the space-time species pattern.
• 7 Allogenic processes are responsible for the high species and community diversity.

     

Comparison of nitrification rates in three branches of the lower river Rhine

The nitrogen cycle in the lower river Rhine was analysed, using data on concentrations of ammonium, nitrite and nitrate, measured in the period from 1972 to 1986. The massive discharge of ammonium in densely populated areas in the Federal Republic of Germany led to microbial nitrification, detectable as decreases in ammonium and nitrite concentrations in the lower river Rhine over reaches 85–133 km long. The distribution of the nitrogen-rich Rhine waters over three different branches in the Netherlands permits some of the factors governing microbial nitrification in the river bed to be discriminated. In the fast-flowing main channel, intensively used by ships, nitrification is more important than in the smaller branches, despite the short residence time of the water in the main channel. Differences in the flow rate of water, in grain size distribution of sediments, and in intensity of shipping (aeration, turbulence) seemed to be responsible for the different rates of nitrification.     

Seed bank survival of an invasive species,but not of two native species,declines with invasion

Soil-borne seed pathogens may play an important role in either hindering or facilitating the spread of invasive exotic plants. We examined whether the invasive shrub Lonicera maackii (Caprifoliaceae) affected fungi-mediated mortality of conspecific and native shrub seeds in a deciduous forest in eastern Missouri. Using a combination of L. maackii removal and fungicide treatments, we found no effect of L. maackii invasion on seed viability of the native Symphoricarpos orbiculatus (Caprifoliaceae) or Cornus drummondii (Cornaceae). In contrast, fungi were significant agents of L. maackii seed mortality in invaded habitats. Losses of L. maackii to soil fungi were also significant in invaded habitats where L. maackii had been removed, although the magnitude of the effect of fungi was lower, suggesting that changes in soil chemistry or microhabitat caused by L. maackii were responsible for affecting fungal seed pathogens. Our work suggests that apparent competition via soil pathogens is not an important factor contributing to impacts of L. maackii on native shrubs. Rather, we found that fungal seed pathogens have density-dependent effects on L. maackii seed survival. Therefore, while fungal pathogens may provide little biotic resistance to early invasion by L. maackii, our study illustrates that more work is needed to understand how changes in fungal pathogens during the course of an invasion contribute to the potential for restoration of invaded systems. More generally, our study suggests that increased rates of fungal pathogen attack may be realized by invasive plants, such as L. maackii, that change the chemical or physical environment of the habitats they invade.     

An experimental assessment of biodiversity and species turnover in terrestrial vs canopy leaf litter

Forest canopies support diverse assemblages of free-living mites. Recent studies suggest mite species complementarity between canopy and terrestrial soils is as high as 80–90%. However, confounding variation in habitat quality and resource patchiness between ground and canopy has not been controlled in previous comparative studies. We used experimental litter bags with standardized microhabitat structure and resource quality to contrast the colonization dynamics of 129 mite species utilizing needle accumulations on the ground vs in the canopy of Abies amabilis trees in a temperate montane forest in Canada. Mite abundance and species richness per litter bag were five to eight times greater on the ground than in the canopy, and composition differed markedly at family-, genus-, and species-level. Seventy-seven species (57%) were restricted to either ground or canopy litter bags, but many of these species were rare (n<5 individuals). Of 49 ‘common’ species, 30.6% were entirely restricted to one habitat, which is considerably lower than most published estimates. In total, 87.5% of canopy specialists had rare vagrants on the ground, whereas only 51.9% of ground specialists had rare vagrants in the canopy. Canonical correspondence analysis of mite community structure showed high species turnover through time and a high degree of specialization for early-, mid-, and late-successional stages of litter decomposition, in both ground and canopy mites. In addition, distinct assemblages of ground-specialist mites dominated each elevation (800, 1000, and 1200 m), whereas few canopy-specialist mites had defined elevational preferences. This suggests that canopy mites may have greater tolerance for wide variation in environmental conditions than soil mites. The degree of species turnover between adjacent mountains also differed markedly, with 46.5% turnover of ground species, but 63.4% turnover of canopy species between the two montane areas. While ground and canopy assemblages are similar in total biodiversity, it appears that local mite richness (alpha diversity) is higher on the ground, whereas species turnover between sites (beta diversity) is higher in the canopy. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Variation in vernal species composition in alluvial forests of the Rhine valley,eastern France

Patterns of vernal synusiae in alluvial forests of the upper Rhine valley were analysed phytosociologically. Five synusial types were described. Environmental factors included in the analysis were spring water level, texture, trophic status and soil reaction. A separate environmental study was undertaken in pure stands of six competitive geophytes in the most fertile habitats. In a laboratory experiment effects of water stress and interspecific competition were studied for three competitive geophytes. The vernal species can be grouped into eight groups with respect to species position along the environmental gradients occurring in the floodplain. These groups can be used for bioindication.     

Seasonal patterns in water use and leaf turnover of different plant functional types in a species-rich woodland,south-western Australia

Woodlands in south-western Australia are evergreen and transpire throughout the year despite the long, hot and dry summers of the Mediterranean climate. Results from a case study in a species-rich Banksia woodland are used to discuss the ecological and physiological properties that appear to be essential features of this and similar communities. Tree, shrub and perennial herbaceous species with long-lived leaves dominate the community, whereas winter-green herbaceous species with short-lived leaves constitute a minor group. The total leaf area index is therefore reasonably constant in all seasons. Leaf area index is low and canopies are open, causing good coupling between the vegetation and the atmosphere, and making stomatal control an effective regulator of transpiration. Mean maximum (winter) stomatal conductances were high at approximately 300 mmol m^–2 s^–1. Deep-rootedness allows the dominant species to access soil moisture throughout the unsaturated zone, and down to the capillary fringe of the saturated zone. Shrubs and herbs with shallow roots experience greater drought stress during summer. Rates of community evapotranspiration are limited by leaf area index in the wet season, and further reduced by stomatal closure in the dry season. Deep-rooted plants appear to decrease their stomatal conductance before the development of severe drought stress. Such conservative behaviour, possibly related to plant hydraulic constraints, is a contributing factor to the limited seasonality in community water use.     

Nitrogen addition,but not pulse frequency,shifts competitive interactions in favor of exotic invasive plant species

Biological Invasions - Temporally dynamic resource supplies may alter or lead to fluctuations in competitive outcomes. Resource pulses have been theorized to promote incursion by exotic species...     

Intraguild predation and species exclusions in amphipods: the interaction of behaviour, physiology and environment

1. Data from field surveys, laboratory experiments and computer simulations of community dynamics revealed that a novel interaction among intraguild predation, physiological adaptation and environment may explain the complex distributions of two putatively competing aquatic amphipods. 2. Gammarus pulex and G. tigrinus both thrive in fresh and oligohaline waters in western Europe. However, the native European G. pulex excludes the invading North American G. tigrinus from freshwaters of relatively low conductivity, whereas the reverse occurs at higher conductivities. Additionally, there is much spatio-temporal fluctuation in the patterns of coexistence of these species. 3. Laboratory experiments in The Netherlands and Ireland revealed that mutual predation of moulting individuals occurred frequently between these species. However, predation frequencies were differentially in favour of G. pulex under the ionic conditions to which this species is physiologically adapted (freshwater). On the other hand, predation was not differential under the ionic conditions to which G. tigrinus is physiologically adapted (oligohaline water). 4. A mathematical model, which extends the logistic equation to include mutual intraguild predation, simulated interactions over a range of values of relevant population parameters. This indicated that G. pulex would be excluded when balanced instantaneous rates of mutual predation were combined with the known greater reproductive output of G. tigrinus. However, this reproductive advantage is overcome by any relatively small bias in the instantaneous rate of predation favouring G. pulex, leading to the exclusion of G. tigrinus. This occurs even when the reproductive advantage to G. tigrinus is relatively large. Moreover, the model generated ‘switches’ in species dominance that are determined by the relative values of reproductive rate and mutual predation. The time taken to ‘switch’ may explain the transient periods of apparent coexistence of these species observed in the field. 5. The complex community dynamics of such species may thus be understood in terms of variation in the intensity of species interactions mediated by behavioural, physiological and environmental factors.     

Relationships among leaf functional traits,litter traits,and mass loss during early phases of leaf litter decomposition in 12 woody plant species

Slow bee paralysis virus (SBPV)—previously considered an obligate honeybee disease—is now known to be prevalent in bumblebee species. SBPV is highly virulent in honeybees in association with Varroa mites, but has been considered relatively benign otherwise. However, condition-dependent pathogens can appear asymptomatic under good, resource abundant conditions, and negative impacts on host fitness may only become apparent when under stressful or resource-limited conditions. We tested whether SBPV expresses condition-dependent virulence in its bumblebee host, Bombus terrestris, by orally inoculating bees with SBPV and recording longevity under satiated and starvation conditions. SBPV infection resulted in significant virulence under starvation conditions, with infected bees 1.6 times more likely to die at any given time point (a median of 2.3 h earlier than uninfected bees), whereas there was no effect under satiated conditions. This demonstrates clear condition-dependent virulence for SBPV in B. terrestris. Infections that appear asymptomatic in non-stressful laboratory assays may nevertheless have significant impacts under natural conditions in the wild. For multi-host pathogens such as SBPV, the use of sentinel host species in laboratory assays may further lead to the underestimation of pathogen impacts on other species in nature. In this case the impact of ‘honeybee viruses’ on wild pollinators may be underestimated, with detrimental effects on conservation and food security. Our results highlight the importance of multiple assays and multiple host species when testing for virulence, in order for laboratory studies to accurately inform conservation policy and mitigate disease impacts in wild pollinators.     

Net-building behaviour,tolerance and development of two caddisfly species from the river Rhine (Hydropsyche contubernalis and H. pellucidula) in relation to the oxygen content

Summary Tolerance, growth and development of H. contubernalis and H. pellucidula were analyzed at different O₂-concentrations to determine the species specific potency amplitude in relation to O₂-concentration. In addition netbouilding behaviour was compared at different O₂-concentrations and current velocities. Under anaerobic conditions slightly greater tolerance was observed in H. contubernalis than in H. pellucidula. But at a higher O₂-concentration H. contubernalis showed a distinctly higher O₂-deficiency tolerance than H. pellucidula in relation to net-building activity, growth and development. In addition, more than 50% of H. contubernalis instars built capture nets with regular meshes at an O₂-saturation of only 30% (15°C). High growth rates were also found at 50% O₂-saturation and more than half the larvae developed into imagos. In addition to diminished net-spinning activity the mean growth rate started to decrease at an O₂-saturation of 70% to 38% and the development into an imago was not possible in H. pellucidula. Because H. contubernalis corresponds to a regulative type, being able to maintain a high level of netbuilding activity, growth rate and development over a wide range (50% to 100% O₂-saturation). On the other hand H. pellucidula corresponds to a conform type, because physiological efficiency decreases rapidly with falling O₂-concentration, so that development into an imago is no longer possible below 85% O₂-saturation. Positive correlation between net-spinning activity and tested current velocity between 10 cm/s and 25 cm/s (15°C) was found in H. pellucidula. H. contubernalis showed no change in net-building activity, over this range of current velocity. H. contubernalis can be classified as an euryoecious species with a great potency amplitude in relation to O₂-concentration and current velocity. The high tolerance to O₂-deficiency conditions probably allowed the early recolonization of the Mittel-and Niederrhein when a distinct O₂-deficit (mean 29%, max 56%) could still be measured. H. contubernalis can be considered a bioindicator of O₂-deficiency situations. Undisturbed development of H. pellucidula in this section of the river requires a further improvement in O₂-content.