Response of Macrophyte Traits to Herbivory and Neighboring Species: Integration of the Functional Trait Framework in the Context of Ecological Invasions

With the increase in the number of introduced species each year, biological invasions are considered as one of the most important environmental problems for native biodiversity. In invaded habitats, the establishment of exotic plant species depends on the abiotic and biotic environment. Herbivores and neighboring plants (native or exotic) comprise an important part of the latter. Herbivores cause trophic and non-trophic damage to focal plants, which respond to herbivory by varying their different traits quantitatively (e.g., growth rate and biomass changes) and qualitatively (e.g., variation in morphological and chemical defenses strategies affecting plant palatability). Neighboring plant species also affect functional traits and the fitness of focal plant species, thus herbivore effects on a focal plant could also depend indirectly on the palatability and defensive traits of the neighboring species inside the community. Here, in a first step toward the integration of associational susceptibility/resistance theories in the field of ecological invasion, we performed a microcosm experiment to consider the effects of an exotic crayfish on the growth rate, morphological traits and damage level of three macrophytes (two exotic, one native) growing in pairwise combinations. We found that (i) the response to herbivore presence and to neighboring species identity seemed to be species specific, and (ii) crayfish enhance the fragmentation rate of the two exotic macrophytes Ludwigia grandiflora and Egeria densa in the presence of the native macrophyte Myriophyllum spicatum, which could indirectly facilitate their invasion success. Indeed, fragmentation can increase dispersal abilities of the exotic macrophytes considered in this study as they are able to generate new plants from their fragments. However, our results showed that the interaction herbivore-neighbor species was hardly significant. Our paper presents some first results on associational resistance/susceptibility and lays the foundation for developing a general framework that combines plant community ecology and biological invasion ecology to explain invasive species success

Short-term effects of macrophyte removal on aquatic biodiversity in rivers and lakes

Mass development of macrophytes is an increasing problem in many aquatic systems worldwide. Dense mats of macrophytes can negatively affect activities like boating, fishing or hydropower production and one of the management measures often applied is mechanical removal. In this study, we analyzed the effect of mechanical macrophyte removal on phytoplankton, zooplankton, and macroinvertebrate (pelagic and benthic samples) assemblages. Our study covered five sites in four countries in Europe and Africa with highly variable characteristics. In all sites, dense mats of different macrophyte species (Juncus bulbosus in a river in Norway; a mix of native macrophytes in a German river, Elodea nuttallii in a lake in Germany, Ludwigia spp. In a French lake and Pontederia crassipes in a South African lake) are problematic and mechanical removal was applied. In every country, we repeated the same BACI (Before-After-Control-Impact) design, including “before”, “one week after”, and “six weeks after” sampling in a control and an impact section. Repeating the same experimental design at all sites allowed us to disentangle common effects across all sites from site-specific effects. For each taxonomic group, we analyzed three structural and three functional parameters, which we combined in a scoring system. Overall, the removal of macrophytes negatively affected biodiversity, in particular of zooplankton and macroinvertebrate assemblages. In contrast, plant removal had positive effects on the phytoplankton assemblages. Effects were more pronounced one week after removal than six weeks after. Consequently, we suggest a stronger consideration of the effect of plant removal on biodiversity to arrive at more sustainable management practices in the future.acceptedVersio

Experimental Assessment of the Water Quality Influence on the Phosphorus Uptake of an Invasive Aquatic Plant: Biological Responses throughout Its Phenological Stage

International audienceUnderstanding how an invasive plant can colonize a large range of environments is still a great challenge in freshwater ecology. For the first time, we assessed the relative importance of four factors on the phosphorus uptake and growth of an invasive macrophyte Elodea nuttallii (Planch.) St. John. This study provided data on its phenotypic plasticity, which is frequently suggested as an important mechanism but remains poorly investigated. The phosphorus uptake of two Elodea nuttallii subpopulations was experimentally studied under contrasting environmental conditions. Plants were sampled in the Rhine floodplain and in the Northern Vosges mountains, and then maintained in aquaria in hard (Rhine) or soft (Vosges) water. Under these conditions, we tested the influence of two trophic states (eutrophic state, 100 mu g.l(-1) P-PO43- and hypertrophic state, 300 mu g.l(-1) P-PO43-) on the P metabolism of plant subpopulations collected at three seasons (winter, spring and summer). Elodea nuttallii was able to absorb high levels of phosphorus through its shoots and enhance its phosphorus uptake, continually, after an increase of the resource availability (hypertrophic > eutrophic). The lowest efficiency in nutrient use was observed in winter, whereas the highest was recorded in spring, what revealed thus a storage strategy which can be beneficial to new shoots. This experiment provided evidence that generally, the water trophic state is the main factor governing P uptake, and the mineral status (softwater > hardwater) of the stream water is the second main factor. The phenological stage appeared to be a confounding factor to P level in water. Nonetheless, phenology played a role in P turnover in the plant. Finally, phenotypic plasticity allows both subpopulations to adapt to a changing environment

Espèces invasives ambivalentes

National audienc

Effets des perturbations liées à l'eutrophisation et l'acidification des cours d'eau des Vosges du nord sur les phytocénoses aquatiques : approche spatio-temporelle et expérimentale

aIn the Northern Vosges, a study has clearly shown the impact of eutrophication on the macrophyte communities during three years (1993-1995). The first determining factor of the macrophyte distribution is the mineralization. The second discriminant variable of the floristic composition is a trophic gradient. Disturbancefactors include changes in nutrient levels (trout hatcheries, waste water, ponds). The floristic composition, which is characteristic of oligotrophic waters upstreams, changes to that of an eutrophic situation as originally found downstream. The comparison between Northern Vosges, Britain and Lozere has shown that the macrophyte communities present homologous species. A comparison between Northern Vosges and Alsatian Plain has established that the common species colonize different nutrient levels in these two area. An experimental study of nitrogen and of phosphorus of macrophyte reveal the importance of nutrients on macrophyte species distribution. In order to follow acidification status and to research the impacts of acidification on macrophyte communities, 41 sites were studied during two years (1993-1995). It was estblished that several weeckly mineralized streams were acidified and the impact of acidification on aquatic plant species were shown. Macrophyte species present a differential tolerance to acidification. A comparison between northern Vosges and Donon mountain has established that the composition of aquatic plants is different in these two sites (Bryophyte species in Donon, phanerogame species in Northern Vosges)Dans les Vosges du nord, l'effet des perturbations liées à l'eutrophisation a été étudié sur les phytocénoses aquatiques pendant trois années (1993 à 1995). Les groupements de macrophytes se répartissent essentiellement selon un gradient de minéralisation qui croit de l'amont vers l'aval. Secondairement, ils se structurent selon le niveau trophique. Suite aux rejets domestiques, aux piscicultures et étangs, les teneurs en nutriments augmentent et conduisent à des modifications au sein des communautés végétales. Les espèces oligotrophes sont alors remplacées par des espèces plus eutrophes. Une comparaison phytosociologique des cours d'eau des Vosges du nord avec d'autres ruisseaux peu minéralisés du massif armoricain et de Lozère montre de grandes analogies dans la composition floristique. La comparaison des communautés végétales des Vosges du nord et de la plaine d'alsace montre que les hydrophytes phanérogames communs aux deux secteurs y possèdent une amplitude écologique différente vis à vis du niveau trophique. Les espèces communes se développent en général pour des gammes trophiques supérieures dans les eaux peu minéralisées. Une approche expérimentale en laboratoire du rôle potentiellement toxique de l'azote ammoniacal a montré que 3 populations d'Elodea canadensis provenant de Bretagne, Ried alsacien, et Vosges du nord n'ont pas la même sensibilité à l'azote ammoniacal. L'étude des relations entre la teneur de phosphore foliaire, dans le sédiment et dans l'eau a permis de préciser l'influence de cet élément sur la distribution des communautés macrophytiques. Les quantités de phosphore total foliaire de certaines espèces de végétaux évoluent de manière parallèle à celles du compartiment eau ou sédiment. Les ruisseaux situés en tête de bassin versant apparaissent comme très sensibles à l'acidification. Plusieurs cours d'eau acidifiés ou en cours d'acidification se caractérisent par la perte de leur pouvoir tampon, par leur pH acide et par des teneurs en aluminium total élevées. Le suivi sur deux ans du degré d'acidification a mis en évidence une acido-sensibilité ou tolérance variable selon les espèce

Impact of Mechanical Removal on the Regeneration and Colonization Abilities of the Alien Aquatic Macrophyte Egeria densa

International audienceThe development of aquatic plant beds can obstruct boat traffic, hinder the practice water activities, and impact functioning freshwaters. In order to mitigate their effects, mechanical removal is often preferred management solution. objective this study was test, in mesocosms, effect frequency (none, one, two cuts) cutting dates (May and/or July) on regeneration colonization capabilities Egeria densa, an invasive alien species France. date had no E. but cuts significantly reduced plant’s biomass. Removal produced numerous fragments, which exhibited very high survival anchoring rates. However, summer fragments with lower abilities compared from spring cutting. Mechanical only temporarily biomass could promote formation new generated by dispersed flow

Les élodées, de bons modèles d’études des strategies de réponses des végétaux aquatiques invasifs?

National audienc

Effets des perturbations liées à l'eutrophisation et l'acidification des cours d'eau des Vosges du nord sur les phytocénoses aquatiques : approche spatio-temporelle et expérimentale

aIn the Northern Vosges, a study has clearly shown the impact of eutrophication on the macrophyte communities during three years (1993-1995). The first determining factor of the macrophyte distribution is the mineralization. The second discriminant variable of the floristic composition is a trophic gradient. Disturbancefactors include changes in nutrient levels (trout hatcheries, waste water, ponds). The floristic composition, which is characteristic of oligotrophic waters upstreams, changes to that of an eutrophic situation as originally found downstream. The comparison between Northern Vosges, Britain and Lozere has shown that the macrophyte communities present homologous species. A comparison between Northern Vosges and Alsatian Plain has established that the common species colonize different nutrient levels in these two area. An experimental study of nitrogen and of phosphorus of macrophyte reveal the importance of nutrients on macrophyte species distribution. In order to follow acidification status and to research the impacts of acidification on macrophyte communities, 41 sites were studied during two years (1993-1995). It was estblished that several weeckly mineralized streams were acidified and the impact of acidification on aquatic plant species were shown. Macrophyte species present a differential tolerance to acidification. A comparison between northern Vosges and Donon mountain has established that the composition of aquatic plants is different in these two sites (Bryophyte species in Donon, phanerogame species in Northern Vosges)Dans les Vosges du nord, l'effet des perturbations liées à l'eutrophisation a été étudié sur les phytocénoses aquatiques pendant trois années (1993 à 1995). Les groupements de macrophytes se répartissent essentiellement selon un gradient de minéralisation qui croit de l'amont vers l'aval. Secondairement, ils se structurent selon le niveau trophique. Suite aux rejets domestiques, aux piscicultures et étangs, les teneurs en nutriments augmentent et conduisent à des modifications au sein des communautés végétales. Les espèces oligotrophes sont alors remplacées par des espèces plus eutrophes. Une comparaison phytosociologique des cours d'eau des Vosges du nord avec d'autres ruisseaux peu minéralisés du massif armoricain et de Lozère montre de grandes analogies dans la composition floristique. La comparaison des communautés végétales des Vosges du nord et de la plaine d'alsace montre que les hydrophytes phanérogames communs aux deux secteurs y possèdent une amplitude écologique différente vis à vis du niveau trophique. Les espèces communes se développent en général pour des gammes trophiques supérieures dans les eaux peu minéralisées. Une approche expérimentale en laboratoire du rôle potentiellement toxique de l'azote ammoniacal a montré que 3 populations d'Elodea canadensis provenant de Bretagne, Ried alsacien, et Vosges du nord n'ont pas la même sensibilité à l'azote ammoniacal. L'étude des relations entre la teneur de phosphore foliaire, dans le sédiment et dans l'eau a permis de préciser l'influence de cet élément sur la distribution des communautés macrophytiques. Les quantités de phosphore total foliaire de certaines espèces de végétaux évoluent de manière parallèle à celles du compartiment eau ou sédiment. Les ruisseaux situés en tête de bassin versant apparaissent comme très sensibles à l'acidification. Plusieurs cours d'eau acidifiés ou en cours d'acidification se caractérisent par la perte de leur pouvoir tampon, par leur pH acide et par des teneurs en aluminium total élevées. Le suivi sur deux ans du degré d'acidification a mis en évidence une acido-sensibilité ou tolérance variable selon les espèce