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.     

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).     

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.     

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.     

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.

     

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.     

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.     

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.     

Morphological and Chemical Changes Induced by Herbivory in Three Common Aquatic Macrophytes

The Dry Matter Content (DMC), the total phenolic content, the production of new branches and the plant fragmentation were compared in three macrophyte species (Elodea canadensis, Elodea nuttallii and Myriophyllum spicatum) exposed or not to snail herbivory. Grazing significantly reduced the DMC of M. spicatum and E. canadensis, but had no effect on the DMC of E. nuttallii. The phenolic contents of Elodea species were not modified by snail herbivory, whereas that of M. spicatum significantly increased when exposed to grazers. The number of new branches produced by M. spicatum and E. canadensis plants, and the fragmentation of E. canadensis also increased in response to herbivory. Chemical defences are therefore probably constitutive in Elodea and induced in M. spicatum, and morphological changes can be related to species growth form and synthesis of phenolic compounds. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.     

Vegetative Propagule Pressure and Water Depth Affect Biomass and Evenness of Submerged Macrophyte Communities

Vegetative propagule pressure may affect the establishment and structure of aquatic plant communities that are commonly dominated by plants capable of clonal growth. We experimentally constructed aquatic communities consisting of four submerged macrophytes (Hydrilla verticillata, Ceratophyllum demersum, Elodea nuttallii and Myriophyllum spicatum) with three levels of vegetative propagule pressure (4, 8 and 16 shoot fragments for communities in each pot) and two levels of water depth (30 cm and 70 cm). Increasing vegetative propagule pressure and decreasing water level significantly increased the growth of the submerged macrophyte communities, suggesting that propagule pressure and water depth should be considered when utilizing vegetative propagules to re-establish submerged macrophyte communities in degraded aquatic ecosystems. However, increasing vegetative propagule pressure and decreasing water level significantly decreased evenness of the submerged macrophyte communities because they markedly increased the dominance of H. verticillata and E. nuttallii, but had little impact on that of C. demersum and M. spicatum. Thus, effects of vegetative propagule pressure and water depth are species-specific and increasing vegetative propagule pressure under lower water level can facilitate the establishment success of submerged macrophyte communities.     

Response of <Emphasis Type="Italic">Elodea Nuttallii</Emphasis> (Planch.) H. St. John to Manual Harvesting in the North-East of France

Elodea nuttallii (Planch). H. St John is an introduced aquatic macrophyte which was first observed in France in the early 1950s. The impact of two frequencies of harvesting on the biomass and regrowth strategy of this invasive species was evaluated by assessment of morphological traits monthly from February to October 2003. The effect of this management on the floristic biodiversity was also analysed. Harvesting caused a drastic reduction of biomass of E. nuttallii. Two harvests caused almost total disappearance of E. nuttallii. Furthermore, no significant difference was observed in the architecture of E. nuttallii between an unharvested site and harvested site. In one year, harvest did not allow the development of native aquatic plants.     

Effects of grazing by fish and waterfowl on the biomass and species composition of submerged macrophytes

Biomanipulation improved water transparency of Lake Zwemlust (The Netherlands) drastically. Before biomanipulation no submerged vegetation was present in the lake, but in summer 1987, directly after the measure, submerged macrophyte stands developed following a clear-water phase caused by high zooplankton grazing in spring. During the summers of 1988 and 1989 Elodea nuttallii was the most dominant species and reached a high biomass, but in the summers of 1990 and 1991 Ceratophyllum demersum became dominant. The total macrophyte biomass decreased in 1990 and 1991. In 1992 and 1993 C. demersum and E. nuttallii were nearly absent and Potamogeton berchtholdii became the dominant species, declining to very low abundance during late summer. Successively algal blooms appeared in autumn of those years reaching chlorophyll-a concentrations between 60–130 µg l^–1. However, in experimental cages placed on the lake bottom, serving as exclosures for larger fish and birds, E. nuttallii still reached a high abundance during 1992 and 1993. Herbivory by coots (Fulica atra) in autumn/winter, and by rudd (Scardinius erythrophthalmus) in summer, most probably caused the decrease in total abundance of macrophytes and the shift in species composition.     

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.     

Species Specific Flavone Glucuronides in Elodea Species

The flavonoid patterns of plants of Elodea canadensis, E. ernstae and E. nuttallii apigenin were investigated. The main flavonoids of E. canadensis are apeginin, luteolin and chrysoeriol 7-O-diglucuronides, of E. nuttalli apigenin and luteolin 7-O- diglucuronides, and of E. ernstae apigenin and luteonin 7-O-monoglucoronides. The qualitative stability of these flavonoid patterns is checked by chromatographic comparison of various populations from a wide area of the three species, it is shown that the flavonoid patterns are valuable criteria for the separation of these species.     

Comparative studies of the photosynthesis of the submerged macrophyte Elodea canadensis and the filamentous algae Cladophora glomerata and Spirogyra sp.

The photosynthetic and respiratory responses of Elodea canadensis Michx., Cladophora glomerata (L.) Kütz. and Spirogyra sp. to oxygen, temperature, HCO₃⁻¹ concentration, pH and irradiance were determined. Photosynthesis was inhibited by O₂ in all three species under all conditions and inhibition was greatert in E. canadensis. This inhibition was not caused solely by an accelerated rate of dark respiration and this suggests that photorespiration may be an important factor controlling productivity, particularly in E. canadensis.Photosynthetic performance was impaired much more in E. canadensis than in either of the filamentous algae conditions of high pH and low CO₂ concentrations. In the field photosynthesis of these algae may increase the pH and reduce the CO₂ content of the water sufficiently to exert deleterious effects on macrophyte photosynthesis. Such a plant-induced change in water quality could give C. glomerata a competitive advantage over E. canadensis and be a factor in the replacement of the vascular plant by the alga in some waters.     

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.