Can snail herbivory influence the outcome of competition between Elodea species?

Elodea nuttallii is more competitive than Elodea canadensis but is also more palatable to herbivores. We tested if grazing by generalist invertebrate herbivores could modify the competitive abilities of E. nuttallii, and thereby influence the outcome of the competition between the two Elodea species. The influence of snail herbivory on the competitive interactions between the two Elodea species cultivated together in indoor tanks was tested for 2 months. The presence of Lymnaea stagnalis in the tanks reduced significantly the final number of rootings, as well as the final biomass of E. nuttallii, but not the surface colonised by this species, whereas only the final number of rootings of E. canadensis was reduced. However, E. nuttallii remained the dominant species. We conclude that snail herbivory can influence the outcome of competition between Elodea species. However, snails are probably not an effective biological control agent of E. nuttallii.     

Experiments on growth interactions between two invasive macrophyte species

Abstract. The success of invasive species has been attributed to the ability to displace other species by direct competition. We studied growth and possible competition between the two macrophyte species Elodea nuttallii and E. canadensis, because the former has been observed to replace the latter in the field. Additional experiments were conducted in aquaria with mixed plantings of Elodea species. Species growth was measured and competitive abilities of each species determined by applying the reciprocal yield model to mean plant weight and length. In monocultures the growth rates of the two species were similar, while in mixtures the growth rate of E. canadensis was significantly lower than that of E. nuttallii. E. canadensis was more sensitive to intraspecific than to interspecific neighbours, whereas E. nuttallii was indifferent to the presence of neighbours. Differential growth characteristics of Elodea species can explain the displacement of E. canadensis by E. nuttallii under eutrophic field conditions.     

Competition between Invasive and Indigenous Species: Impact of Spatial Pattern and Developmental Stage

Elodea canadensis (indigenous) and Elodea nuttallii (invasive) were grown in experimental tanks in monocultures and mixtures in two spatial patterns (aggregated or mixed) and two developmental stages (small or large plants of E. canadensis, and small plants of E. nuttallii). Competitive interactions between the two species were assessed by monitoring the area colonised by each species, the number of rootings and biomass after 10 weeks. In monocultures the growth of E. canadensis was significantly lower than that of E. nuttallii. In mixtures the number of rootings and biomass of E. canadensis were always significantly less than those of E. nuttallii. The tank surface area colonised by E. canadensis was always significantly less than that occupied by E. nuttallii, but it was higher in the aggregated treatment, where the colonisation of E. nuttallii was lower. Therefore both spatial pattern and developmental stage of an indigenous species (E. canadensis) may influence the outcome of competition with potential invaders (E. nuttallii).     

Spatial pattern of native species Myriophyllum spicatum and invasive alien species Elodea nuttallii after introduction of the latter one into the Drava River (Slovenia)

Invasive alien species Elodea nuttallii was introduced into the Drava River (Slovenia) in 2007. The spatial distribution of native macrophyte species Myriophyllum spicatum and the invasive alien species E. nuttallii was studied in two impoundments, HPP Vuhred and HPP Mariborski otok in the years 2009, 2010 and 2011. The very heterogeneous environment of the Drava River had resulted in dynamic and non-uniform distributions of E. nuttalli and M. spicatum in both impoundments. The distribution of E. nuttalli was shown to be suppressed in river stretches exposed to great turbulence. It had, however, successfully invaded areas with gentle slope of littoral, low exposition to main water flow and muddy sediment and prevailed against the native species M. spicatum. The latter was more abundant than E. nuttallii in stretches exposed to higher flow velocity. Thus, the competitive success of M. spicatum and E. nuttallii depended on environmental conditions. Our results lead to the prediction that E. nuttallii will increase further in the river Drava, causing serious economic problems.     

Present distribution of the genus Elodea in the Alsatian Upper Rhine floodplain (France) with a special focus on the expansion of Elodea nuttallii St. John during recent decades

Three species of Elodea (Elodea canadensis Michaux, E. nuttallii St John and E. ernstiae St John) have colonized Europe from the American continent. All three arrived in the Alsatian Rhine floodplain (north-eastern France) soon after their arrival in Europe, i.e. in the mid-19th century for E. canadensis, and in the mid-20th century for E. nuttallii and E. ernstiae. The paper investigates the present distribution of Elodea spp. in the floodplain by quantifying the species’ respective occurrences and by describing their habitats. The study further focuses on E. nuttallii which is presently colonizing other parts of Europe. It analyses whether it has continued to expand in the Alsatian Rhine floodplain during recent decades, and it checks whether changes in the abundance of E. nuttallii have had an impact on species richness of water plant communities. E.␣nuttallii has been found to be at present one of the most dominant and most frequent aquatic plant species in the study sector, while E. canadensis and E. ernstiae are less abundant. The species’ distributions differ with regard to water chemistry and water temperature: E. canadensis occurs in oligo-mesotrophic, rather stenothermic habitats, whereas E. nuttallii and E. ernstiae can be encountered in meso- to eutrophic sites with little or no arrival of stenothermic ground water. By comparing successive vegetation relevés from the same sites the study revealed further that the distribution of E. nuttallii has been stable in recent decades, despite local fluctuations in abundance. No relationship could be established between those fluctuations and changes in species richness or type of local plant communities. The sum of the results suggests that the expansion of E. nuttallii in the Alsatian Rhine floodplain had been completed prior to the study period. The species’ present distribution in the study sector as well as its position in local plant communities might therefore be considered a model for what can be expected to happen in areas where E.␣nuttallii has only recently arrived.     

Phenotypic plasticity as a clue for invasion success of the submerged aquatic plant Elodea nuttallii

• Two closely related alien submerged aquatic plants were introduced into Europe. The new invader (Elodea nuttallii) gradually displaced E. canadensis even at sites where the latter was well established. The aim of the study was to evaluate the combined effects of environmental factors on several phenotypic characteristics of the two Elodea species, and to relate these phenotypic characteristics to the invasion success of E. nuttallii over E. canadensis.
• In a factorial design, Elodea plants were grown in aquaria containing five different nitrogen concentrations and incubated at five different light intensities. We used six functional traits (apical shoot RGR), total shoot RGR, relative elongation, root length, lateral spread, branching degree) to measure the environmental response of the species. We calculated plasticity indices to express the phenotypic differences between species.
• Light and nitrogen jointly triggered the development of phenotypic characteristics that make E. nuttallii a more successful invader in eutrophic waters than E. canadensis. The stronger invader showed a wider range of phenotypic plasticity. The apical elongation was the main difference between the two species, with E. nuttallii being more than two times longer than E. canadensis. E. canadensis formed dense side shoots even under high shade and low nitrogen levels, whereas E. nuttallii required higher light and nitrogen levels.
• We found that under more eutrophic conditions, E. nuttallii reach the water surface sooner than E. canadensis and through intensive branching outcompetes all other plants including E. canadensis. Our findings support the theory that more successful invaders have wider phenotypic plasticity.

     

Comparative Analyses of Plastid Sequences between Native and Introduced Populations of Aquatic Weeds Elodea canadensis and E. nuttallii

Non-indigenous species (NIS) are species living outside their historic or native range. Invasive NIS often cause severe environmental impacts, and may have large economical and social consequences. Elodea (Hydrocharitaceae) is a New World genus with at least five submerged aquatic angiosperm species living in fresh water environments. Our aim was to survey the geographical distribution of cpDNA haplotypes within the native and introduced ranges of invasive aquatic weeds Elodea canadensis and E. nuttallii and to reconstruct the spreading histories of these invasive species. In order to reveal informative chloroplast (cp) genome regions for phylogeographic analyses, we compared the plastid sequences of native and introduced individuals of E. canadensis. In total, we found 235 variable sites (186 SNPs, 47 indels and two inversions) between the two plastid sequences consisting of 112,193 bp and developed primers flanking the most variable genomic areas. These 29 primer pairs were used to compare the level and pattern of intraspecific variation within E. canadensis to interspecific variation between E. canadensis and E. nuttallii. Nine potentially informative primer pairs were used to analyze the phylogeographic structure of both Elodea species, based on 70 E. canadensis and 25 E. nuttallii individuals covering native and introduced distributions. On the whole, the level of variation between the two Elodea species was 53% higher than that within E. canadensis. In our phylogeographic analysis, only a single haplotype was found in the introduced range in both species. These haplotypes H1 (E. canadensis) and A (E. nuttallii) were also widespread in the native range, covering the majority of native populations analyzed. Therefore, we were not able to identify either the geographic origin of the introduced populations or test the hypothesis of single versus multiple introductions. The divergence between E. canadensis haplotypes was surprisingly high, and future research may clarify mechanisms that structure native E. canadensis populations.     

Allelopathic activity of Elodea canadensis and Elodea nuttallii against epiphytes and phytoplankton

Elodea nuttallii and Elodea canadensis have both been introduced from North America to Europe. They are now common in many water bodies where they often form dominating stands. It was suggested that negative relationships between Elodea and phytoplankton or epiphytic covers exist, probably due to the release of growth inhibiting allelochemicals. This would be an effective strategy to avoid light limitation caused by algae and cyanobacteria. We investigated the allelopathic potential of both E. nuttallii and E. canadensis against different photoautotrophs, focussing on epiphytic algae and cyanobacteria isolated from different submersed macrophytes and culture strains. Methanolic extracts of both species inhibited the growth of most of these organisms. Only a culture strain of Scenedesmus brevispina was stimulated. Further separation of extracts yielded several active fractions, indicating that hydrophilic and slightly lipophilic compounds were responsible for growth reduction. At least some of the activity seems to be related to phenolic substances, but flavonoids in these species are inactive. Since growth declined also in a moderately lipophilic fraction of culture filtrate of E. nuttallii, we assume that active compounds were exuded in the water. Allelopathy might thus be relevant in situ and suppress cyanobacteria and algae. We furthermore found differences in the susceptibility of target organisms, which could (1) at least partly be a result of adaptation to the respective host plants and (2) indicate that allelopathic interference might reduce the abundance of some species, especially cyanobacteria, in epiphytic biofilms.     

Redundancy and niche differentiation among the European invasive <Emphasis Type="Italic">Elodea</Emphasis> species

Community ecologists implicitly assume redundancy when they aggregate species into functional groups. But there have been remarkably few empirical efforts to investigate the accuracy of this concept in situ. The concept of redundancy could be roughly split into two components: the ecological redundancy (similar response to environmental variations involving similar ecological processes) and the functional redundancy (similar biological trait combinations shaping similar functional processes). Both types of redundancy are tested among the 3 invasive European Elodeas. In 11 sites and during two successive years 2004–2005, the cover growth rate of each Elodea species was monthly recorded. To test ecological redundancy, cover growth rates were related to a large suite of environmental variables. To test functional redundancy, 13 biological traits involved in competitive relationships were measured each month. Firstly, the redundancy hypothesis looks problematic for Elodea ernstiae. Indeed, the later possess numerous biological traits involved in light competition and niche overlap with the other Elodeas is very low. Secondly, ecological and functional redundancy can be successfully applied to Elodea canadensis and Elodea nuttallii. They share a large suite of biological traits leading to wide niche overlaps through the growing season. And the measured environmental variables do not differentially influence their growth rates, which are, in turn, controlled by a similar group of biological traits. In this way, the different invasiveness patterns of E. canadensis and E. nuttallii could be solely due to the ecological drift and their ecological dynamic could follow neutral rules.     

Resistance to herbivory of two populations of <Emphasis Type="Italic">Elodea</Emphasis><Emphasis Type="Italic">canadensis</Emphasis> Michaux and <Emphasis Type="Italic">Elodea nuttallii</Emphasis> Planchon St. John

In this article, we compared the resistance of two introduced populations of Elodea nuttallii and Elodea canadensis to two different herbivores. Samples were collected from the River Rhine and River Rhône in eastern France. The two populations of E. nuttallii differed in their introduction history, whereas E. canadensis was introduced at the same time in the two sites. The Daily Food Consumption (DFC) rates of the two macrophyte populations were evaluated in no-choice experiments using the scraper Lymnaea stagnalis and the shredder Gammarus roeseli. At the same time, we assessed four plant traits: dry matter content (DMC), total nitrogen content, carbon/nitrogen ratio and total phenolic content. The two populations of E. canadensis were consumed at low levels by both the herbivores. L. stagnalis showed a higher DFC on the Rhône population of E. nuttallii than on the Rhine population. No significant difference between the two populations was established with G. roeseli, but the level of DFC was high. This result demonstrates that the assessment of plant palatability should be carried out with several generalist herbivores belonging to various feeding groups (e.g. scrapers or shredders). Although the Rhône population of E. nuttallii had higher levels of phenols than the other populations, it was consistently consumed in greater quantities than E. canadensis. Neither the phenolic contents were not effective against these herbivores, nor the levels of phenolics too low to induce an efficient resistance. The higher DMC and the lower DFC of the two populations of E. canadensis suggest that this introduced plant has co-evolved with indigenous enemies in the introduced range.     

Response of invasive macrophyte species to drawdown: The case of Elodea sp.

The response to drawdown of vegetative fragments (whole plants, shoot fragments and turions) of two invasive macrophyte species, Elodea canadensis and Elodea nuttallii, was studied through laboratory experiments. In addition, field observations were made on the colonisation of a wetland by E. nuttallii before and after a natural drawdown. The survival and the growth of vegetative fragments of E. nuttallii were higher than those of E. canadensis after an artificial drawdown of several days. In the field the recolonisation by E. nuttallii of a wetland that was drained for 10 weeks during a summer drawdown was very rapid, the abundance of this macrophyte species being not affected by the drawdown event. We conclude that E. nuttallii possesses a high resilience to desiccation and that a summer drawdown would not be efficient in the control of this invasive species.     

Response of invasive macrophyte species to drawdown: The case of Elodea sp.

The response to drawdown of vegetative fragments (whole plants, shoot fragments and turions) of two invasive macrophyte species, Elodea canadensis and Elodea nuttallii, was studied through laboratory experiments. In addition, field observations were made on the colonisation of a wetland by E. nuttallii before and after a natural drawdown. The survival and the growth of vegetative fragments of E. nuttallii were higher than those of E. canadensis after an artificial drawdown of several days. In the field the recolonisation by E. nuttallii of a wetland that was drained for 10 weeks during a summer drawdown was very rapid, the abundance of this macrophyte species being not affected by the drawdown event. We conclude that E. nuttallii possesses a high resilience to desiccation and that a summer drawdown would not be efficient in the control of this invasive species.     

Friends of mine: An invasive freshwater mussel facilitates growth of invasive macrophytes and mediates their competitive interactions

1. Increasing rates of invasions in ecosystems worldwide necessitate experiments to determine the role of biotic interactions in the success and impact of multiple alien species. Here, we examined competitive and facilitative interactions among various combinations of three widespread and often co-occurring invaders: the zebra mussel Dreissena polymorpha, and the macrophytes Elodea canadensis and Elodea nuttallii.
2. Using a mesocosm-based, factorial experimental design, we assessed the effect of interspecific competition on macrophyte growth rates in the absence and presence at varying biomass of D. polymorpha.
3. Growth rates (wet g/day) of E. canadensis and E. nuttallii were similar when grown in isolation. When grown together, in the absence of D. polymorpha, E. canadensis growth was not significantly reduced in the presence of E. nuttallii and vice versa. In the presence of D. polymorpha (26.0 ± 1 mm), monocultural growth of E. canadensis was largely unaffected, while E. nuttallii growth was strongly enhanced. Low (2.64 g) and medium (3.96 g) mussel biomass led to negative interspecific effects between E. canadensis and E. nuttallii; at high (5.28 g) mussel biomass, the effect of interspecific competition was negated.
4. Overall, D. polymorpha alleviated competitive interactions between the two invasive macrophytes when all three species co-occurred, and substantially enhanced growth of E. nuttallii with increasing mussel biomass, thereby suggesting a possible influence on the relative dominance of these macrophytes in the field.
5. Our study demonstrates how facilitations can cause shifts in dominance among closely related invaders. The consequences of such facilitations for the structure and function of communities remain to be explored generally.

     

Preferences of juveniles and adults of the invasive Ponto-Caspian amphipod <Emphasis Type="Italic">Pontogammarus robustoides</Emphasis> for various species of macrophytes and artificial substrata

We studied preferences of invasive Ponto-Caspian amphipod P. robustoides for various macrophyte species (Myriophyllum spicatum, Ceratophyllum demersum, Potamogeton perfoliatus, Elodea canadensis) and artificial plant-like objects (artificial Christmas tree branches) in laboratory pairwise-choice tests. Juvenile (<7 mm) and adult gammarids exhibited different habitat preferences. Adults did not discriminate between artificial and natural substrata, or among most of the tested species of plants. In contrast, juveniles clearly preferred all tested macrophytes over artificial substrata. Moreover, they particularly preferred plants with the finest leaf elements: M. spicatum and C. demersum over the others and E. canadensis over P. perfoliatus. We found no influence of chironomid larvae, a potential food source for adult gammarids, on their distribution, nor any effect of adults on the habitat choice by juveniles. The habitat partitioning between juvenile and adult P. robustoides may help them survive in a new environment and increase their invasive potential by reducing the intraspecific competition and cannibalism.     

Seasonal variability in the palatability of freshwater macrophytes: a case study

The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation in palatability of three freshwater macrophytes: Potamogeton lucens, Elodea canadensis and E. nuttallii. For each species, 2–5 populations were investigated in spring and in summer. Preliminary results showed that the feeding rate of similarly-aged snails bred under standard conditions was stable over time. In contrast, snail feeding rate on the three macrophyte species decreased from spring to summer, which was therefore interpreted as a decrease in plant palatability. This decrease was probably due to tissue maturation, as suggested by the concomitant increase in the dry matter content of leaves of the three species. The high palatability of the species studied during the spring may prove detrimental in cases of strong herbivore pressure, and could have consequences for macrophyte distribution among aquatic habitats.     

Trade-offs between growth and defence in two phylogenetically close invasive species

The success of an invasive plant species could be explained by trade-off between growth and defence. The aim of this paper was to explore the responses of two non-native aquatic macrophytes Elodea canadensis and Elodea nuttallii to herbivores in their introduced range. We assessed the palatability of the two phylogenetically close aquatic plant species in field and their responses to gammarid consumption in spring, summer and autumn in a microcosm experiment. We measured the variation of functional traits for each season. The traits selected were those judged most closely related to the allocation of resources to growth or to resistance against herbivores. We clearly established that the strategies of the two species were different and that their consumption rate differed in summer. In summer, E. canadensis allocated more of its resources to structural defence (leaf toughness). The increase in leaf thickness reduced the palatability of E. canadensis, whereas E. nuttallii stimulated its growth. Moreover, a decrease in dry matter content in E. nuttallii was found during the growing season in field. In autumn, both plant species accumulated nitrogen and phosphorus in their tissues. We also demonstrated that neither species induced efficient chemical defences against the herbivores. The different strategies of these two Elodea species could be explained by their different resident times in the introduced area and by an adaptation of the naturalised E. canadensis to herbivores.     

Responses of three invasive aquatic macrophytes to nutrient enrichment do not explain their observed field displacements

In some eutrophic inland waters the invasive aquatic macrophyte Elodea canadensis has been displaced by the morphologically similar species Elodea nuttallii and subsequently E. nuttallii by Lagarosiphon major. We investigated whether differences in the responses of these species and their associated epiphytic floras to five nutrient loadings in the range 30–480 μg L⁻¹ P and 0.21–3.36 mg L⁻¹ N could explain their observed field displacements. The mean relative growth rate (RGR) of E. nuttallii (RGR 0.086 d⁻¹) was significantly higher than that of either E. canadensis (RGR 0.066 d⁻¹) or L. major (RGR 0.063 d⁻¹). All three species exhibited a plastic morphological response to increasing nutrient loadings with mean root weights reduced at the highest nutrient loading compared with the lowest loading by 33, 75 and 56% for E. canandensis, E. nuttallii and L. major, respectively. Mean tissue nitrogen concentrations increased significantly with increasing nutrient loading, with concentrations in E. canadensis (1.83–2.10% dry wt.) significantly higher than either E. nuttallii (1.56–2.10% dry wt.) or L. major (1.50–1.90% dry wt.). Tissue phosphorus concentrations likewise increased with increasing nutrient loadings although this trend was not as pronounced. Epiphyte biomass per unit photosynthetic surface area (PSA) was significantly higher on E. canadensis than on either E. nuttallii or L. major, but did not increase significantly with increasing nutrient loadings. We suggest that differences in species responses to nutrient enrichment do not explain the species displacements observed in the field. E. nuttallii's higher RGR may, regardless of nutrient supply, enable this species to shade out neighbouring species and outpace the establishment of algae on its leaves.     

Experimental estimation of the possible use of submersed macrophytes for biotesting bottom sediments of the Yenisei River

The laboratory testing of bottom sediments (BSs) from the Yenisei River containing different concentrations of technogenic radionuclides, heavy metals, and biogenic elements (N and P) based on aquatic such plants as Elodea canadensis (Canadian waterweed) and Myriophyllum spicatum (Eurasian watermilfoil) has revealed a higher sensitivity of roots to the general quality of BSs than shoots: shoot length (9%) < root length (11%) < root number (15%) in M. spicatum; shoot length (22%) < root length (42%) < root number (44%) in E. canadensis. In contrast to M. spicatum, the growth parameters of roots and shoots in E. canadensis have differed in a significant statistical manner between most BS samples. A reverse correlation has been found between the increase in shoot length and the activity of technogenic radionuclides in BSs, which is mostly significant in E. canadensis (r ² = 0.90–0.95, p = 0.05). Since the growth of shoots and roots in E. canadensis has turned out to be more sensitive to changes in the quality of BSs than that in M. spicatum, E. canadensis can be considered more prospective for biotesting BSs.     

High phenolic content fails to deter mesograzer consumption of <Emphasis Type="Italic">Myriophyllum spicatum</Emphasis> (Eurasian watermilfoil) in New England

Eurasian watermilfoil (Myriophyllum spicatum) is often considered one of the most aggressive macrophyte invaders in freshwater habitats throughout the USA. However, conditions leading to successful milfoil invasions are not well understood. This study sought to illuminate the role of herbivores in determining milfoil invasion success via the potential mechanisms of enemy release and biotic resistance. We determined feeding preferences of three herbivores native to the northeastern United States and measured macrophyte phenolic content, which may act as an herbivore feeding deterrent. We found that phenolic content in milfoil was more than two times higher than in the most abundant native macrophytes at our study sites, consistent with enemy release. However, laboratory feeding experiments demonstrated that milfoil phenolics did not deter amphipod (Hyalella azteca), snail (Physella sp.), or weevil (Euhrychiopsis lecontei) herbivory. Furthermore, amphipod consumption rates in our study were an order of magnitude higher than amphipod consumption rates reported in milfoil’s native range, contrary to the predictions of enemy release. Amphipods and snails from habitats invaded by milfoil consumed similar quantities of both milfoil and the low-phenolic native plant Elodea canadensis. In contrast, weevils consumed milfoil but not E. canadensis in choice experiments. Amphipods collected from milfoil-free habitats also readily consumed milfoil, and they consumed 2.5 times more milfoil than E. canadensis in a choice feeding trial. These results suggest that high phenolic levels do not prevent native herbivores from consuming invasive milfoil. Instead, native generalist grazers like amphipods and snails may limit milfoil proliferation and provide a measure of biotic resistance.     

Growth and nutrient uptake by two species of Elodea in experimental conditions and their role in nutrient accumulation in a macrophyte-dominated lake

The capacity of Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx. to remove nitrogen from water was evaluated in laboratory experiment. The growth rate of plants and their effect on the nitrogen level of hypertrophic Lake Zwemlust (the Netherlands) as well as on lake water enriched with nitrogen were investigated. The plants grew best in water enriched with up to 2 mg NH₄-Nl^–1 and 2 mg NH₄-Nl^–1 plus 2 mg NO₃ Nl^–1. During a 14 day experiment, plants absorbed from 75% to 90% of nitrogen. Higher nitrogen concentration than 4 mg l^–1 had a negative effect on growth of both species. Elodea nuttallii and E. canadensis prefer NOinf4/^p+ over NOinf4/^p– when both ions were present in water in equal concentrations.