Isotopic niche is not equal to trophic niche

International audienceThe work of Sheppard et al. (Ecol. Lett., 21, 2018, 1395) relies on the strong assumption that isotopic niche is equal to trophic niche. Here I raise three main concerns showing that classic hypotheses built upon trophic niche cannot be directly interpreted in isotopic space. Future studies should always keep isotopic and trophic niches distinct

Organisation trophique des peuplements piscicoles des cours d'eau, approche par l'analyse des isotopes stables

The study of food webs is necessary to understand ecosystem functioning. However, the potential mechanisms determing foods web (ressource availability, ecosystem stability and size) are still discussed. In this context three new questions concerning stream food webs were addressed using stable isotope analysis. A first study confirms that the upstream-downstream gradient plays a structural role in the functioning of streams and shows a direct and indirect effect of the gradient on fish food web. A second study shows that temperature variability of streams negatively influences the food chain lenght and gives new support to the "ecosystem stability" hypothesis. Last, a third study explores the impacts of land use at two spatial scales, and reveals a complex effect affecting both the ressources diversity and the range of trophic levels.L'étude des réseaux trophiques est un prérequis à la compréhension du fonctionnement des écosystèmes. Cependant, les déterminants potentiels des réseaux trophiques (disponibilité des ressources, stabilité et taille de l'écosystème) restent discutés. Dans ce contexte, 3 nouvelles questions portant sur les réseaux trophiques des cours d'eau sont abordées par l'analyse des isotopes stables. Une première étude confirme le rôle structurant du gradient amont-aval dans le fonctionnement des cours d'eau, en précisant son influence directe et indirecte sur le réseau trophique piscicole. Une deuxième étude montre l'incidence négative de la variabilité du régime thermique des cours d'eau sur la longueur de chaîne trophique et apporte un nouveau soutien au déterminant "stabilité de l'écosystème". Enfin, une troisième étude explore les effets de l'occupation des sols à 2 échelles spatiales, et suggère une influence complexe jouant sur la diversité des ressources et des niveaux trophiques du réseau

Caractérisation des réseaux trophiques en cours d'eau, problématiques, bilan des activités de recherche 2013 et 2014 et projet pour l'année 2015

[Departement_IRSTEA]Eaux [TR1_IRSTEA]QUASARE [Relecteur_IRSTEA]Reyjol, Y.Après un bref rappel des enjeux, le rapport détaille l’objectif et les problématiques abordés par le projet (effet des facteurs environnementaux, influence des pressions anthropiques et développement de métriques isotopiques). Il propose également le bilan des activités menées en 2013 et 2014, en mettant l’accent sur le plan d’expériences obtenu après les campagnes d’échantillonnage, et sur l’état d’avancement du projet. Enfin il expose les activités prévues pour l’année 2015 qui seront essentiellement dédiées aux analyses isotopiques et à l’exploitation/valorisation des résultats

Influence de la végétation riparienne sur le réseau trophique des cours d'eau tempérés : variations saisonnières le long d'un gradient d'ombrage

International audienceRiparian vegetation is of paramount importance for stream ecology but the underlying mechanisms are still poorly understood. For instance, riparian vegetation plays a key role in determining the nature and amount of autochthonous and allochthonous organic matter entering the streams. In temperate regions, both aquatic primary production and terrestrial organic matter are strongly influenced by the seasonal patterns displayed by riparian vegetation: i.e. leaf emergence in spring and leaf fall in late autumn. Still it remains unclear, how these seasonal patterns affect the contributions of autochthonous and allochthonous organic matter to stream food webs. Here, we shed light on the seasonal variations of stream food webs in relation with the processes of leaf emergence and leaf fall. Using stable isotope analysis of nitrogen, carbon and hydrogen, we studied stream food webs in four sites differing in their riparian vegetation (from meadow to forest) at four different dates (before leaf emergence, just after leaf emergence, before leaf fall and just after leaf fall). All sites were located maximum 50 km from each others, so that they display homogenous geological and climatic conditions. Following a quantitative sampling, we estimated the biomasses of macroinvertebrates and fish. Accordingly, we analysed the isotope ratios of the representative taxa (1531 values) from basal resources (algae, macrophytes, biofilm, suspended organic matter and leaf litter) to consumers (macroinvertebrates and fish). In a first step, we described the food web trajectories across seasons, using circular statistics on different food web metrics. In a second step, we estimated the contributions of autochthonous and allochthonous organic matter to the food webs, running isotopic mixing models. Given the riparian vegetation differences between sites, we expected a sharp contrast in terms of food web functioning, with sites mainly influenced by aquatic primary production (meadow sites), and sites mainly influenced by allochthonous organic matters (forested sites). In addition to these differences between sites, we expected to observe a seasonal variation of food web at each site, in relation with light availability and terrestrial inputs of organic matter

Modifications du réseau trophique d'une rivière tempérée le long du gradient amont-aval révélée par les isotopes stables

International audienceThe upstream-downstream gradient (UDG) is a key feature of streams. For instance food webs are assumed to change from upstream to downstream. We tested this hypothesis in a small European river catchment (937 km2), and examined whether food web modifications are related to structural (i.e. food web composition) or functional changes i.e. alteration of linkages within the web). We adopted a double approach at two levels of organisation (assemblage and species levels) using two isotopic metrics (isotopic space area and isotopic niche overlap), and proposed a new hypothesis-testing framework for exploring the dominant feeding strategy within a food web. We confirmed that the UDG influenced stream food webs, and found that food web modifications were related to both structural and functional changes. The structural change was mainly related to an increase in species richness, and induced functional modifications of the web (indirect effect). In addition, the UDG also modified the functional features of the web directly, without changing the web composition. The proposed framework allowed relating the direct effect of the UDG to a diet specialisation of the species, and the indirect effect via the structural changes to a generalist feeding strategy. The framework highlights the benefits of conducting the double approach, and provides a foundation for future studies investigating the dominant feeding strategy that underlies food web modifications

Stable isotopes reveal food web modifications along the upstream-downstream gradient of a temperate stream

The upstream-downstream gradient (UDG) is a key feature of streams. For instance food webs are assumed to change from upstream to downstream. We tested this hypothesis in a small European river catchment (937 km(2)), and examined whether food web modifications are related to structural (i.e. food web composition) or functional changes (i.e. alteration of linkages within the web). We adopted a double approach at two levels of organisation (assemblage and species levels) using two isotopic metrics (isotopic space area and isotopic niche overlap), and proposed a new hypothesis-testing framework for exploring the dominant feeding strategy within a food web. We confirmed that the UDG influenced stream food webs, and found that food web modifications were related to both structural and functional changes. The structural change was mainly related to an increase in species richness, and induced functional modifications of the web (indirect effect). In addition, the UDG also modified the functional features of the web directly, without changing the web composition. The proposed framework allowed relating the direct effect of the UDG to a diet specialisation of the species, and the indirect effect via the structural changes to a generalist feeding strategy. The framework highlights the benefits of conducting the double approach, and provides a foundation for future studies investigating the dominant feeding strategy that underlies food web modifications

How effective are existing solutions to mitigate impacts of onshore wind farms on flying vertebrates and invertebrates? A Rapid Review

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