Metaecosystem dynamics drive community composition in experimental, multi‐layered spatial networks

Cross‐ecosystem subsidies are studied with a focus on resource exchange at local ecosystem boundaries. This perspective ignores regional dynamics that can emerge via constraints imposed by the landscape, potentially leading to spatially‐dependent effects of subsidies and spatial feedbacks. Using miniaturized landscape analogues of river dendritic and terrestrial lattice spatial networks, we manipulated and studied resource exchange between the two whole networks. We found community composition in dendritic networks depended on the resource pulse from the lattice network, with the strength of this effect declining in larger downstream patches. In turn, this spatially‐dependent effect imposed constraints on the lattice network with populations in that network reaching higher densities when connected to more central patches in the dendritic network. Consequently, localized cross‐ecosystem fluxes, and their respective effects on recipient ecosystems, must be studied in a perspective taking into account the explicit spatial configuration of the landscape

Interactions multi-échelles entre ressources abiotiques, réseaux trophiques et propriétés des écosystèmes : nouveaux jalons théoriques pour une écologie intégrative

RÉSUMÉ: Ce travail de thèse s'inscrit dans l'effort actuel de construction d'une écologie intégrative. J'y étudie les mécanismes d'interaction entre ressources abiotiques, réseaux trophiques et propriétés des écosystèmes, au moyen d'une expérience d'évolution, d'un modèle de méta-écosystème et d'un modèle bioénergétique d'assemblage d'écosystèmes. Les organismes modifient la disponibilité des ressources en les prélevant pour leur croissance. Inversement, la disponibilité des ressources influence la diversité et la composition en espèces du réseau trophique, en agissant comme force de sélection sur les traits d'acquisition des ressources (chap. 1,5). Les propriétés de l'écosystème, telles que stabilité et productivité, dérivent des interactions entre la dynamique des ressources et celle du réseau trophique (chap. 2). Enfin,le fonctionnement de l'écosystème rétroagit sur les ressources abiotiques via le recyclage de la biomasse (chap. 2, 5). Ces processus interviennent lors de I'assemblage des réseaux trophiques et structurent le développement des écosystèmes (chap. 3-5). Dans cette thèse, j'analyse ces mécanismes de rétroaction biotique-abiotique sur plusieurs échelles d'organisation, d'espace et de temps. Notamment, les modèles développés ici fournissent des outils novateurs pour étudier les mécanismes de construction des écosystèmes, en mettant en évidence les liens entre métabolisme des espèces, structure du réseau trophique et fonctionnement de l'écosystème, et leur variation au cours du temps. Ce travail ouvre de vastes perspectives de recherche en combinant les derniers progrès d'une écologie intégrative dans une conception mécaniste du développement des écosystèmes. -- Mot(s) clé(s) en français : biodiversité, développement des écosystèmes, assemblage des communautés, méta-écosystèmes, recyclage, nutriments inorganiques, modèle bioénergétique. -- ABSTRACT: This thesis participates to the current effort towards the construction of an integrative ecology. I study the feedback mechanisms between abiotic resources, food webs and ecosystem properties, through an evolution experiment, a model of metaecosystem, and a bioenergetic ecosystem assembly model. Organisms modify resource availability by consuming them for their growth. Conversely, resource availability influences the species diversity and composition of the food web, by acting as a selection pressure on traits for resource acquisition (chap. 1, 5). Ecosystem properties, such as stability and productivity, derive from the interactions between resource and food web dynamics (chap. 2). Finally, ecosystem functioning feeds back on abiotic resources through the recycling of biomass (chap. 2 and 5). These processes occur during the food web assembly and drive the development of ecosystems (chap. 3-5). In this thesis I analyze these biotic-abiotic feedback mechanisms on several scales of organization, space and time. The models developed here provide innovative tools to study the mechanisms of ecosystem construction by pointing out the links between species metabolism, food web structure and ecosystem functioning, and their variation through time. This work opens wide research perspectives, as it combines the most recent progress of an integrative ecology into a mechanistic framework of ecosystem development. -- Mot(s) clé(s) en anglais : biodiversity, ecosystem development, community assembly, metaecosystems, inorganic nutrients, recycling, bioenergetic model

MULTI-SCALE FEEDBACKS BETWEEN ABIOTIC RESSOURCES, FOOD WEBS AND ECOSYSTEM PROPERTIES : New theoretical milestones for an integrative ecology

Ce travail de thèse s'inscrit dans l'effort actuel de construction d'une écologie intégrative. J'y étudie les mécanismes d'interaction entre ressources abiotiques, réseaux trophiques et propriétés des écosystèmes, au moyen d'une expérience d'évolution, d'un modèle de méta-écosystème et d'un modèle bioénergétique d'assemblage d'écosystèmes. Les organismes modifient la disponibilité des ressources en les prélevant pour leur croissance. Inversement, la disponibilité des ressources influence la diversité et la composition en espèces du réseau trophique, en agissant comme force de sélection sur les traits d'acquisition des ressources (chap. 1, 5). Les propriétés de l'écosystème, telles que stabilité et productivité, dérivent des interactions entre la dynamique des ressources et celle du réseau trophique (chap. 2). Enfin, le fonctionnement de l'écosystème rétroagit sur les ressources abiotiques via le recyclage de la biomasse (chap. 2, 5). Ces processus interviennent lors de l'assemblage des réseaux trophiques et structurent le développement des écosystèmes (chap. 3-5). Dans cette thèse j'analyse ces mécanismes de rétroaction biotique-abiotique sur plusieurs échelles d'organisation, d'espace et de temps. Notamment, les modèles développés ici fournissent des outils novateurs pour étudier les mécanismes de construction des écosystèmes, en mettant en évidence les liens entre métabolisme des espèces, structure du réseau trophique et fonctionnement de l'écosystème, et leur variation au cours du temps. Ce travail ouvre de vastes perspectives de recherche en combinant les derniers progrès d'une écologie intégrative dans une conception mécaniste du développement des écosystèmes.This thesis participates to the current effort towards the construction of an integrative ecology. I study the feedback mechanisms between abiotic resources, food webs and ecosystem properties, through an evolution experiment, a model of metaecosystem, and a bioenergetic ecosystem assembly model. Organisms modify resource availability by consuming them for their growth. Conversely, resource availability influences the species diversity and composition of the food web, by acting as a selection pressure on traits for resource acquisition (chap. 1, 5). Ecosystem properties, such as stability and productivity, derive from the interactions between resource and food web dynamics (chap. 2). Finally, ecosystem functioning feeds back on abiotic resources through the recycling of biomass (chap. 2 and 5). These processes occur during the food web assembly and drive the development of ecosystems (chap. 3-5). In this thesis I analyze these biotic-abiotic feedback mechanisms on several scales of organization, space and time. The models developed here provide innovative tools to study the mechanisms of ecosystem construction by pointing out the links between species metabolism, food web structure and ecosystem functioning, and their variation through time. This work opens wide research perspectives, as it combines the most recent progress of an integrative ecology into a mechanistic framework of ecosystem development

How pulse disturbances shape size‐abundance pyramids

Ecological pyramids represent the distribution of abundance and biomass of living organisms across body-sizes. Our understanding of their expected shape relies on the assumption of invariant steady-state conditions. However, most of the world’s ecosystems experience disturbances that keep them far from such a steady state. Here, using the allometric scaling between population growth rate and body-size, we predict the response of size-abundance pyramids within a trophic guild to any combination of disturbance frequency and intensity affecting all species in a similar way. We show that disturbances narrow the base of size-abundance pyramids, lower their height and decrease total community biomass in a nonlinear way. An experimental test using microbial communities demonstrates that the model captures well the effect of disturbances on empirical pyramids. Overall, we demonstrate both theoretically and experimentally how disturbances that are not size-selective can nonetheless have disproportionate impacts on large species

La conservation à la croisée des chemins

Dans le contexte actuel de déclin impor- tant de la biodiversité et d’effritement constant des services écosystémiques qu’elle procure, les approches de conservation classiques semblent en partie inadéquates pour maintenir l’in- tégrité des écosystèmes dont les socié- tés humaines ont pourtant besoin, car les priorités de conservation sont d’abord établies en fonction de la rareté de l’espèce et non de son importance fonctionnelle dans l’écosystème. Dans un contexte de restriction budgétaire globale pour la conservation, cela crée un déséquilibre disproportionné entre les efforts mis en œuvre pour maintenir une espèce dans un écosystème donné et la nécessité de protéger le fonction- nement et la résilience de l’ensemble de l’écosystème. Nous suggérons qu’un changement d’objectif vers la protection des réseaux écologiques pourrait per- mettre, du même souffle, de protéger la biodiversité et l’intégrité écosystémique que celle-ci soutient

Global quantitative synthesis of ecosystem functioning across climatic zones and ecosystem types

Aim Providing a quantitative overview of ecosystem functioning in a three‐dimensional space defined by ecosystem stocks, fluxes and rates, across major ecosystem types and climatic zones. Location Global. Time period 1966–2019. Major taxa studied Ecosystem‐level measurements (all organism types). Methods We conducted a global quantitative synthesis of a wide range of ecosystem variables related to carbon stocks and fluxes. We gathered a total of 4,479 values from 1,223 individual sites (unique geographical coordinates) reported in the literature (604 studies), covering ecosystem variables including biomass and detritus stocks, gross primary production, ecosystem respiration, detritus decomposition and carbon uptake rates, across eight major aquatic and terrestrial ecosystem types and five broad climatic zones (arctic, boreal, temperate, arid and tropical). We analysed the relationships among variables emerging from the comparisons of stocks, fluxes and rates across ecosystem types and climates. Results Within our three‐dimensional functioning space, average ecosystems align along a gradient from fast rates–low fluxes and stocks (freshwater and pelagic marine ecosystems) to low rates–high fluxes and stocks (forests), a gradient that we hypothesize results mainly from variation in primary producer characteristics. Moreover, fluxes and rates decrease from warm to colder climates, consistent with the metabolic theory of ecology. However, the strength of climatic effects differs among variables and ecosystem types, resulting, for instance, in opposing effects on net ecosystem production between terrestrial and freshwater ecosystems (positive versus negative effects). Main conclusions This large‐scale synthesis provides a first quantified cross‐ecosystem and cross‐climate comparison of multivariate ecosystem functioning. This gives a basis for a mechanistic understanding of the interdependency of different aspects of ecosystem functioning and their sensitivity to global change. To anticipate responses to change at the ecosystem level, further work should investigate potential feedbacks between ecosystem variables at finer scales, which involves site‐level quantifications of multivariate functioning and theoretical developments

How pulse disturbances shape size-abundance pyramids

International audienceEcological pyramids represent the distribution of abundance and biomass of living organisms across body‐sizes. Our understanding of their expected shape relies on the assumption of invariant steady‐state conditions. However, most of the world’s ecosystems experience disturbances that keep them far from such a steady state. Here, using the allometric scaling between population growth rate and body‐size, we predict the response of size‐abundance pyramids within a trophic guild to any combination of disturbance frequency and intensity affecting all species in a similar way. We show that disturbances narrow the base of size‐abundance pyramids, lower their height and decrease total community biomass in a nonlinear way. An experimental test using microbial communities demonstrates that the model captures well the effect of disturbances on empirical pyramids. Overall, we demonstrate both theoretically and experimentally how disturbances that are not size‐selective can nonetheless have disproportionate impacts on large species

Data from: Leaf litter diversity and structure of microbial decomposer communities modulate litter decomposition in aquatic systems

1. Leaf litter decomposition is a major ecosystem process that can link aquatic to terrestrial ecosystems by flows of nutrients. Biodiversity and ecosystem functioning research hypothesizes that the global loss of species leads to impaired decomposition rates and thus to slower recycling of nutrients. Especially in aquatic systems an understanding of diversity effects on litter decomposition is still incomplete. 2. Here we conducted an experiment to test two main factors associated with global species loss that might influence leaf litter decomposition. Firstly, we tested whether mixing different leaf species alters litter decomposition rates compared to decomposition of these species in monoculture. Secondly, we tested the effect of the size structure of a lotic decomposer community on decomposition rates. 3. Overall, leaf litter identity strongly affected decomposition rates, and the observed decomposition rates matched measures of metabolic activity and microbial abundances. While we found some evidence of a positive leaf litter diversity effect on decomposition, this effect was not coherent across all litter combinations and the effect was generally additive and not synergistic. 4. Microbial communities, with a reduced functional and trophic complexity, showed a small but significant overall reduction in decomposition rates compared to communities with the naturally complete functional and trophic complexity, highlighting the importance of a complete microbial community on ecosystem functioning. 5. Our results suggest that top-down diversity effects of the decomposer community on litter decomposition in aquatic systems are of comparable importance as bottom-up diversity effects of primary producers

Trait selection during food web assembly: the roles of interactions and temperature

Understanding the processes driving community assembly is a central theme in ecology, yet this topic is marginally studied in food webs. Bioenergetic models have been instrumental in the development of food web theory, using allometric relationships with body mass, temperature, and explicit energy flows. However, despite their popularity, little is known about the constraints they impose on assembly dynamics. In this study, we build on classical consumer–resource theory to analyze the implications of the assembly process on trait selection in food webs. Using bioenergetic models, we investigate the selective pressure on body mass and conversion efficiency and its dependence on trophic structure and temperature.We find that the selection exerted by exploitative competition is highly sensitive to how the energy fluxes are modeled. However, the addition of a trophic level consistently selects for smaller body masses of primary producers. An increase in temperature triggers important cascading changes in food webs via a reduction of producer biomass, which is detrimental to herbivore persistence. This affects the structure of trait distributions, which in turn strengthens the exploitative competition and the selective pressure on traits. Our results suggest that greater attention should be devoted to the effects of food web assembly on trait selection to understand the diversity and the functioning of real food webs, as well as their possible response to ongoing global changes

Data from: Subsidies mediate interactions between communities across space

Most spatial ecology focuses on how species dispersal affects community dynamics and coexistence. Ecosystems, however, are also commonly connected by flows of resources. We experimentally tested how neighbouring communities indirectly influence each other in absence of dispersal, via resource exchanges. Using two-patch microcosm meta-ecosystems, we manipulated community composition and dynamics, by varying separately species key functional traits (autotroph versus heterotroph species and size of consumer species) and trophic structure of aquatic communities (species growing alone, or in presence of competitors or predators). We then analysed the effects of species functional traits and trophic structure on communities connected through spatial subsidies in the absence of actual dispersal. Both functional traits and trophic structure strongly affected dynamics across neighbouring communities. Heterotroph communities connected to autotroph neighbours developed better than with heterotroph neighbours, such that coexistence of competitors was determined by the functional traits of the neighbouring community. Densities in autotroph communities were also strikingly higher when receiving subsidies from heterotroph communities compared to their own subsidies when grown in isolated ecosystems. In contrast, communities connected to predator-dominated ecosystems collapsed, without any direct contact with the predators. Our results demonstrate that because community composition and structure modify the distribution of biomass within a community, they may also affect communities connected through subsidies through quantitative and qualitative changes of detritus flows. This stresses that ecosystem management should account for such interdependencies mediated by spatial subsidies, given that local community alterations cascade across space onto other ecosystems even if species dispersal is completely absent