The Importance of Ecological Networks in Multiple-Stressor Research and Management

Multiple stressors are increasingly affecting organisms and communities, thereby modifying ecosystems' state and functioning. Raising awareness about the threat from multiple stressors has increased the number of experimental and observational studies specifically addressing consequences of stressor interactions on biota. Most studies measure the direct effects of multiple stressors and their interactions on biological endpoints such as abundance, biomass, or diversity of target organisms. This yields invaluable information for the management and restoration of stressed ecosystems. However, as we argue in our perspective paper, this common approach ignores a fundamental characteristic of communities and ecosystems, i.e., that organisms in ecosystems are interlinked by biotic interactions in ecological networks. Examples from the literature show that biotic interactions can modify stressor effects, transfer stressor effects to distant groups of organisms, and create new stressor interactions. These examples also suggest that changes in biotic interactions can have effects of similar or greater magnitude than direct stressor effects. We provide a perspective on how to include network characteristics and biotic interactions into analyses of multiple-stressor effects on ecosystems. Our approach can also make use of biomonitoring data produced with established and intercalibrated methods, and can combine it with novel metrics used to describe the functioning of ecosystems, such as trait information or stable-isotope measurements. The insights on network-mediated effects gained via the approach we propose can substantially increase mechanistic understanding of multiple-stressor effects, and in turn, the efficiency of ecosystem management and restoration

Variable temperature effects between heterotrophic stream processes and organisms

Temperature is known to stimulate metabolism with cascading effects on multiple biological processes. These effects may, however, vary across processes, types of organisms or levels of biological organisation. They can also vary with nutrient availability, with potentially stronger temperature effects when nutrients are not limiting. This context dependence of temperature effects on processes challenges our ability to anticipate their consequences on ecosystems in a changing world. In headwater streams, the decomposition of allochthonous leaf litter, driven by both microbial decomposers and invertebrates, is known to respond to both temperature and nutrient availability. These food webs are highly tractable and a useful model system to investigate the variations of temperature effects on processes across types of organisms (microbes versus invertebrates), resource availability levels (nutrient concentration), and levels of biological organisation (from individual to ecosystem). In a microcosm experiment, we measured the effects of temperature and nitrogen availability (four levels each) on respiration rates of litter-consuming microbes and invertebrates and their decomposition activity in different contexts of food web complexity. The latter included one treatment without invertebrate detritivore (microbial decomposers only), three single invertebrate taxa (Gammarus, Potamophylax, and Sericostoma) treatments, and one mixed invertebrate taxa treatment (three‐species altogether). Microbial processes increased nearly exponentially with temperature (Arrhenius model, activation energy (± 95% confidence interval) = 0.56 ± 0.53 and 1.00 ± 0.23 eV for litter decomposition and respiration), while invertebrate‐driven processes increased (activation energy from 0.47–1.15 eV) up to a maximal value at an intermediate temperature (c. 11–15°C depending on species and process), above which process rates decreased. By contrast, litter consumption in mixed invertebrate species treatments was not significantly influenced by temperature, because of a negative effect of species mixing occurring above 12°C. Nitrogen had a weaker influence, only slightly stimulating litter consumption by mixed‐species invertebrates, which limited the scope for synergies with temperature effects. Our results raise issues about how aquatic litter consumers meet their energy requirements at high temperature and suggest that a general consequence of warming could be loss of carbon through mineralisation in headwater stream food webs. In several aspects, our results deviate from expectations based on universal relationships between temperature and individual metabolism (e.g. metabolic theory of ecology), suggesting that we may need to develop less simplistic assumptions to predict the consequence of warming on ecosystem processes

Effets de la variabilité intraspécifique du phénotype des invertébrés sur la décomposition des litières

Intraspecific variability of individuals' phenotype is at the root of natural selection, but remains poorly evaluated from the ecosystem functioning perspective. My research has focused on measuring individual and population differences in phenotypic traits of invertebrates involved in detrital food webs, as well as evaluating their impact on litter decomposition. First, I showed that intraspecific differences of litter consumption rate is high compared to the interspecific differences measured experimentally on a sample of 10 terrestrial and aquatic detritivore species. Another laboratory study revealed that litter consumption rate differences between individuals of a same population can be explained by behavioral and energetic traits. Oppositely, body size of individuals proved to be less effective in explaining inter-individual variability of litter consumption rates. Traits of predator invertebrates can also act on litter decomposition indirectly through interactions with detritivores. An in situ experiment has shown that predator individuals with different phenotypes can have opposite effects on decomposition. Trophic cascade variability was explained by sex and a lifestyle index, defined by a combination of behavioral and life history traits. Finally, I validated the hypothesis that the origin of predator populations sampled along a latitudinal gradient in Europe modulates the response of litter decomposition to global warming. All my work highlights the underestimated role of intraspecific variability of phenotype in Ecology. Individual, rather than species, should be the basic unit to better understand the biotic control of ecosystem functioning and to predict the ecological consequences of environmental changes.La variabilité intraspécifique du phénotype des individus est à la base de la sélection naturelle, mais est peu évaluée d'un point de vue des conséquences sur le fonctionnement de l'écosystème. Mes recherches se sont attachées à mesurer les différences individuelles et populationnelles de traits phénotypiques chez des invertébrés au sein du réseau trophique détritique et d'en évaluer les conséquences sur le processus de décomposition des litières. Je montre d'abord que la variabilité intraspécifique du taux de consommation de litières est forte par rapport à la variabilité interspécifique mesurée expérimentalement sur un échantillon de 10 espèces de détritivores terrestres et aquatiques. Une autre étude en laboratoire a permis d'établir que les différences de taux de consommation de litières entre individus d'une même population de détritivores aquatiques sont explicables à partir de traits comportementaux et énergétiques. La taille corporelle des individus s'est révélée être moins importante que des traits comportementaux et énergétiques pour expliquer la variabilité inter-individuelle du taux de consommation des litières. Les traits des invertébrés prédateurs peuvent aussi agir sur la décomposition des litières de manière indirecte via des interactions avec les détritivores. Une expérience in situ en ruisseau a permis de montrer que des individus prédateurs arborant différents phénotypes peuvent avoir des effets sur la décomposition qui s'opposent. La variabilité de la cascade trophique était expliquée par les différences de sexe et de rythme de vie, défini par une combinaison de traits de comportement et d'histoire de vie. Finalement, j'ai validé l'hypothèse selon laquelle l'origine de la population de prédateur, le long d'un gradient latitudinal en Europe, pourrait moduler la réponse de la décomposition des litières au réchauffement climatique. L'ensemble de mes travaux mettent en lumière le rôle sous-estimé de la variabilité intraspécifique du phénotype en Ecologie. L'individu, plutôt que l'espèce, devrait constituer l'unité de base pour mieux appréhender le contrôle biotique du fonctionnement des écosystèmes et prédire les conséquences écologiques des changements environnementaux

Effects of intraspecific variability of invertebrates' phenotype on litter decomposition

La variabilité intraspécifique du phénotype des individus est à la base de la sélection naturelle, mais est peu évaluée d'un point de vue des conséquences sur le fonctionnement de l'écosystème. Mes recherches se sont attachées à mesurer les différences individuelles et populationnelles de traits phénotypiques chez des invertébrés au sein du réseau trophique détritique et d'en évaluer les conséquences sur le processus de décomposition des litières. Je montre d'abord que la variabilité intraspécifique du taux de consommation de litières est forte par rapport à la variabilité interspécifique mesurée expérimentalement sur un échantillon de 10 espèces de détritivores terrestres et aquatiques. Une autre étude en laboratoire a permis d'établir que les différences de taux de consommation de litières entre individus d'une même population de détritivores aquatiques sont explicables à partir de traits comportementaux et énergétiques. La taille corporelle des individus s'est révélée être moins importante que des traits comportementaux et énergétiques pour expliquer la variabilité inter-individuelle du taux de consommation des litières. Les traits des invertébrés prédateurs peuvent aussi agir sur la décomposition des litières de manière indirecte via des interactions avec les détritivores. Une expérience in situ en ruisseau a permis de montrer que des individus prédateurs arborant différents phénotypes peuvent avoir des effets sur la décomposition qui s'opposent. La variabilité de la cascade trophique était expliquée par les différences de sexe et de rythme de vie, défini par une combinaison de traits de comportement et d'histoire de vie. Finalement, j'ai validé l'hypothèse selon laquelle l'origine de la population de prédateur, le long d'un gradient latitudinal en Europe, pourrait moduler la réponse de la décomposition des litières au réchauffement climatique. L'ensemble de mes travaux mettent en lumière le rôle sous-estimé de la variabilité intraspécifique du phénotype en Ecologie. L'individu, plutôt que l'espèce, devrait constituer l'unité de base pour mieux appréhender le contrôle biotique du fonctionnement des écosystèmes et prédire les conséquences écologiques des changements environnementaux.Intraspecific variability of individuals' phenotype is at the root of natural selection, but remains poorly evaluated from the ecosystem functioning perspective. My research has focused on measuring individual and population differences in phenotypic traits of invertebrates involved in detrital food webs, as well as evaluating their impact on litter decomposition. First, I showed that intraspecific differences of litter consumption rate is high compared to the interspecific differences measured experimentally on a sample of 10 terrestrial and aquatic detritivore species. Another laboratory study revealed that litter consumption rate differences between individuals of a same population can be explained by behavioral and energetic traits. Oppositely, body size of individuals proved to be less effective in explaining inter-individual variability of litter consumption rates. Traits of predator invertebrates can also act on litter decomposition indirectly through interactions with detritivores. An in situ experiment has shown that predator individuals with different phenotypes can have opposite effects on decomposition. Trophic cascade variability was explained by sex and a lifestyle index, defined by a combination of behavioral and life history traits. Finally, I validated the hypothesis that the origin of predator populations sampled along a latitudinal gradient in Europe modulates the response of litter decomposition to global warming. All my work highlights the underestimated role of intraspecific variability of phenotype in Ecology. Individual, rather than species, should be the basic unit to better understand the biotic control of ecosystem functioning and to predict the ecological consequences of environmental changes

Data from: Phenotypic determinants of inter-individual variability of litter consumption rate in a detritivore population

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Workshop virtuel - 70e anniversaire de la CEDH. Séance 5. CEDH et droit constitutionnel

Workshop virtuelhttps://70ans-cedh.sciencesconf.org/resource/page/id/

Repeatability of the metabolic rate over two measurments

The data set contains rows corresponding to each gammaridae individuals and columns corresponding to the variables including: the identity of individuals (Indiv), the metabolic rate (Respi), the temperature in Celsius degree (Temp), the inverse temperature corresponding to the Boltzman equation (InvTem), the body mass (Masse) and the identity of the measurement/trial (Essai). Please note that metabolic rates were corrected by control measurements which has created some negative values and "NA" values correspond to dead individuals and outliers points that have been removed

Energetic mismatch induced by warming decreases leaf litter decomposition by aquatic detritivores

International audienceThe balance of energetic losses and gains is of paramount importance for understanding and predicting the persistence of populations and ecosystem processes in a rapidly changing world. Previous studies suggested that metabolic rate often increases faster with warming than resource ingestion rate, leading to an energetic mismatch at high temperature. However, little is known about the ecological consequences of this energetic mismatch for population demographyand ecosystem functions.2. Here, we combined laboratory experiments and modelling to investigate the energetic balance of a stream detritivore Gammarus fossarum along a temperaturegradient and the consequences for detritivore populations and organic matterdecomposition.3. We experimentally measured the energetic losses (metabolic rate) and supplies (ingestion rate) of Gammarus and we modelled the impact of rising temperaturesand changes in Gammarus body size induced by warming on population dynamics and benthic organic matter dynamics in freshwater systems.4. Our experimental results indicated an energetic mismatch in a Gammarus population where losses via metabolic rate increase faster than supplies via food ingestion with warming, which translated in a decrease in energetic efficiency withtemperature rising from 5 to 20°C. Moreover, our consumer–resource model predicts a decrease in the biomass of Gammarus population with warming, associated with lower maximum abundances and steeper abundance decreases after biomass annual peaks. These changes resulted in a decrease in leaf litter decomposition rate and thus longer persistence of leaf litter standing stock over years in the simulations. In addition, Gammarus body size reductions led to shorter persistence for both leaf litter and Gammarus biomasses at low temperature and the opposite trend at high temperature, revealing that body size reduction was weakening the effect of temperature on resource and consumer persistence

Data from: Repeatable inter-individual variation in the thermal sensitivity of metabolic rate

Assessing whether trait variations among individuals are consistent over time and among environmental conditions is crucial to understand evolutionary responses to new selective pressures such as climate change. According to the universal thermal dependence hypothesis, thermal sensitivity of metabolic rate should not vary strongly and consistently among organisms, implying limited evolutionary response for metabolic traits under climate change. However, this hypothesis has been rarely tested at an individual level, leaving a gap in our understanding of climate change impacts on metabolic responses and their potential evolution. Using the amphipod Gammarus fossarum, we investigated the variability and repeatability of individual metabolic thermal reaction norms over time. We found large variations in both the thermal sensitivity (i.e. slope) and expression level (i.e. intercept) of individual metabolic reaction norms. Moreover, differences among individuals were consistent over time, and therefore repeatable. Inter-individual variations in body mass resulted in a high repeatability of metabolic expression level but had no significant effect on the repeatability of thermal sensitivity. Overall, our results highlight that inter-individual variability and repeatability of thermal reaction norms can be substantial. We conclude that these consistent differences among individuals should not be overlooked when apprehending the ecological and evolutionary effects of climate change