Does the invasive plant Elymus athericus modify fish diet in tidal salt marshes?

The invasion of Mont-Saint-Michel Bay salt marshes (France) by a grass species (Elymus athericus) has led to important changes in vegetation cover, which is likely to modify the habitat for many invertebrates. Some of them constitute the main food items for several fish species, such as young sea bass (Dicentrarchus labrax) and sand goby (Pomatoschistus minutus), that feed in salt marsh creeks during high tides. As a result, fish nursery functions of salt marshes could be modified by the E. athericus invasion. In order to test this hypothesis, gut contents of the two most abundant fish species (sea bass and sand goby) were compared before and after E. athericus invasion in the same salt marsh creek and using the same methodology. The accessibility and availability of the main food item, the semi-terrestrial amphipod Orchestia gammarella, were estimated and compared between invaded (dominated by E. athericus) and original areas (dominated by Atriplex portulacoides). Gut content analysis showed a significantly greater percentage of fish leaving with empty guts from E. athericus areas than from A. portulacoides areas. The sea bass diet composition study showed a major shift in the relative importance of the main food items: before E. athericus invasion, diets were dominated by the semi-terrestrial species O. gammarella, whereas after the E. athericus invasion they were dominated by a marine mysid Neomysis integer. The same trend was found for sand gobies, with a shift of the main food item from O. gammarella before invasion to the polychaete Hediste diversicolor after invasion. These trophic changes may be explained by the lower accessibility and availability of O. gammarella in invaded communities than in natural ones. The E. athericus invasion, observed throughout northern Europe, is thus likely to disturb trophic function of natural salt marshes for fish. This preliminary study of the E. athericus invasion is also an illustration that invasive species are an urgent problem in conservation biology

Les habitats rocheux intertidaux sous l'influence d'activités anthropiques (structure, dynamique et enjeux de conservation)

Les activités humaines, de loisir et professionnelles, vont en s intensifiant sur les zones rocheuses intertidales. Le piétinement associé à ces activités, le remaniement des blocs pour rechercher des espèces consommables, ne sont pas sans incidence sur les biocénoses des milieux rocheux. Les conséquences de ces activités sur la structure et la dynamique des roches du médiolittoral et des champs de blocs de bas d estran en particulier, nécessitent d'être caractérisées. A travers l identification des espèces et descripteurs qui répondent le plus aux perturbations "piétinement des platiers rocheux" et "retournement des blocs par les pêcheurs à pied", des indicateurs écologiques simplifiés peuvent être dégagés et faciliter, à terme, le diagnostic de l état de conservation de ces habitats. Les processus écologiques de dégradation et de déstructuration des biocénoses sous l'effet du piétinement passent par des interactions entre espèces très complexes. Les résultats d expérimentations in situ soulignent des phénomènes de seuils très marqués, liés en particulier à la protection jouée par les algues brunes structurantes sur les autres espèces. L'étude du retournement des blocs par les pêcheurs à pied met en évidence une phase de mortalité presque totale des organismes fixés ou peu mobiles des blocs et une faible résilience de l habitat. Le choix de stations de référence appropriées et l utilisation des espèces et descripteurs les plus sensibles au retournement des blocs ont permis la création de deux indicateurs de perturbation. Déclinés à deux échelles spatiales différentes, ils sont applicables en routine par les scientifiques et les gestionnaires d aires marines protégées.Recreational and professional activities are intensifying on the rocky intertidal. The overturning of boulders and the trampling associated with human activities, have consequences on the structure and dynamic of rocky intertidal communities. Through the identification of species and descriptors that respond more to the disturbances trampling of mediolittoral rock platforms and the overturning of low eulittoral boulders by hand-fishermen , SMART indicators (Simple, Measurable, Achievable, Realistic, and Time limited indicator) can be created for the assessment of the conservation status of these habitats. The effects of trampling were tested with control approach. Experiments in situ underlined very complex interactions between species during the ecological processes of degradation of biocenoses. High thresholds of disturbance were also observed. They were particularly related to the protection played by structuring brown algae on other species. The study of boulder s overturning highlighted a step of almost total mortality of fixed or low mobile organisms that live on upper or lower surfaces of boulders. Results also showed a low resilience of the habitat. The use of appropriate reference situations and of the most sensitive species to the boulder s overturning, led to the creation of two ecological indicators. Declined to two different spatial scales, they may be applied routinely by scientists and managers of marine protected areas.BREST-SCD-Bib. electronique (290199901) / SudocSudocFranceF

Connectivité hydrologique et distribution spatiale de l ichtyofaunedans le bassin versant de la Loire (de l assemblage à la population (cas de l anguille européenne))

Dans un contexte d érosion de la biodiversité riveraine, cette thèse s intéresse à l influence de la connectivité hydrologique sur la distribution de l ichtyofaune dans le bassin de la Loire à différentes échelles spatiales (le bassin dans son ensemble et la plaine inondable) et biologiques (l assemblage et la population d anguille européenne). Les résultats montrent que la connectivité hydrologique influence fortement les patrons de distribution des poissons aux deux échelles spatiales considérées. Elle détermine la qualité de l habitat et la composition et la structure des assemblages. Les faibles niveaux de connectivité créent des habitats lentiques favorables aux espèces exotiques et défavorables aux espèces natives vulnérables. Elle détermine par ailleurs les possibilités de déplacements des poissons entre éléments de l hydrosystème comme cela a été illustré par le cas de l anguille.In a context of river biodiversity loss, this work aims at understanding the influence of hydrological connectivity on fish distribution in the Loire catchment at different spatial (the whole catchment and the floodplain) and biological (the assemblage and the European eel s population) scales. The results show that hydrological connectivity strongly drives fish distribution patterns at both spatial scales. It determines the habitat quality and the composition and structure of fish assemblages. Low connectivity levels create lentic habitats that favour exotic species and disfavour threatened native species. Connectivity also determines fish opportunities to move between various elements of the hydrosystem as it has been shown for the European eel.RENNES1-BU Sciences Philo (352382102) / SudocRENNES-Agrocampus-Bibl. Linné (352382308) / SudocSudocFranceF

Bases biologiques d'un modèle pour estimer la biomasse féconde de l'anguille européenne en fonction des recrues fluviales et du contexte de croissance (approche comparative à l'échelle de petits bassins versants)

Nous avons examiné sur une période de 8 années (1996-2003) la dynamique continentale d'une sous population d'anguilles européennes Anguilla anguilla d'un fleuve côtier breton aménagé, le Frémur. Sur cette période, les trois écophases (Recrutement, Stock et Dévalaison) ont été quantifiées et caractérisées (stade, longueur et âge). Le taux de disparition estimé est important (53,2%) probablement en raison des abondances éleve es (0,40 anguilles.m-2) et des habitats dégradés. Chaque année, une prédiction du nombre potentiel de futurs reproducteurs à entreprendre leur migration à partir de la connaissance de la structure des anguilles sédentaires, est réalisée. Le nombre effectif variant en fonction de la gestion hydraulique du système. Finalement, l'influence du contexte de croissance sur la production du nombre et de la qualité des futurs reproducteurs, et les implications de cette étude en matière de conservation de cette espèce à l'échelle européenne, sont discutées.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Integrating multiple scales in rarity assessments of invertebrate taxa

International audienceAim To provide a procedure for measuring the rarity of both invertebrate species and assemblages of species from multiple scales without the need for fineresolution datasets over broad areas. Location The western Palearctic (WP) and western France.Methods On the basis of different datasets from different geographical extents, we applied a multiscale rarity weight to species occurrence from multiple scales. Multiscale rarity weights were then averaged at an assemblage level in a multiscale index of relative rarity (I RR). These rarity weights were calculated using a flexible, scale-dependent method that ensures equitable contributions of each scale to the final index. We provided a simple two-scale example of the application, on spiders of western France, for which we obtained occurrence information from a regional-extent dataset (regional scale) and a western Palearctic-extent dataset (WP scale). Thus, we showed the necessity of a two-scale approach by successively analysing species occurrence, multiscale rarity weights of species and multiscale indices of species assemblage. Finally, we presented a case study within a nature reserve.Results Species occurrences are not predictable from one scale to another, and rarity indices of assemblages are poorly congruent among scales, which supports the necessity of a two-scale approach. Multiscale rarity weights accurately showed information on species rarity from both scales. Multiscale indices of assemblages were congruent but with additional information over each onescale index.Main conclusions The novelty of the multiscale method developed here is to accurately combine different datasets of varying extents and resolutions to provide multiscale rarity weights for species and indices for assemblages. Given the increasing availability of datasets for invertebrate taxa, this method represents a significant improvement for rarity and conservation studies on invertebrates

Évolutions structurales, conservatoires et fonctionnelles de marais salésenvahis par le chiendent (études de communautés d'arthropodes en baie du Mont Saint-Michel)

resumeRENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Functional diversity and conservation value of the habitats in wetlands: Rehabilitation of a managed marsh.

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Impact of cutting and sheep grazing on ground-active spiders and carabids in intertidal salt marshes (western France).

International audienceThe aims of this study were to characterize spider (Araneae) and ground beetle (Coleoptera Carabidae) communities in managed (cutting and sheep grazing) and non–managed salt marshes and to assess the efficiency of management regimes in these particular ecosystems. The two groups were studied during 2002 in salt marshes of the Mont Saint–Michel Bay (NW France) using pitfall traps. By opening soil and vegetation structures cutting and grazing enhanced the abundances of some halophilic species of spiders and ground beetles. Nevertheless, grazing appeared to be too intensive as spider species richness decreased. We discuss the implications of management practices in terms of nature conservation and their application in the particular area of intertidal salt marshes

Impact of cutting and sheep grazing on ground-active spiders and carabids in intertidal salt marshes (western France).

International audienceThe aims of this study were to characterize spider (Araneae) and ground beetle (Coleoptera Carabidae) communities in managed (cutting and sheep grazing) and non–managed salt marshes and to assess the efficiency of management regimes in these particular ecosystems. The two groups were studied during 2002 in salt marshes of the Mont Saint–Michel Bay (NW France) using pitfall traps. By opening soil and vegetation structures cutting and grazing enhanced the abundances of some halophilic species of spiders and ground beetles. Nevertheless, grazing appeared to be too intensive as spider species richness decreased. We discuss the implications of management practices in terms of nature conservation and their application in the particular area of intertidal salt marshes