Climate drives the geography of marine consumption by changing predator communities

Este artículo contiene 7 páginas, 3 figuras, 1 tabla.The global distribution of primary production and consumption by humans (fisheries) is well-documented, but we have no map linking the central ecological process of consumption within food webs to temperature and other ecological drivers. Using standardized assays that span 105° of latitude on four continents, we show that rates of bait consumption by generalist predators in shallow marine ecosystems are tightly linked to both temperature and the composition of consumer assemblages. Unexpectedly, rates of consumption peaked at midlatitudes (25 to 35°) in both Northern and Southern Hemispheres across both seagrass and unvegetated sediment habitats. This pattern contrasts with terrestrial systems, where biotic interactions reportedly weaken away from the equator, but it parallels an emerging pattern of a subtropical peak in marine biodiversity. The higher consumption at midlatitudes was closely related to the type of consumers present, which explained rates of consumption better than consumer density, biomass, species diversity, or habitat. Indeed, the apparent effect of temperature on consumption was mostly driven by temperature-associated turnover in consumer community composition. Our findings reinforce the key influence of climate warming on altered species composition and highlight its implications for the functioning of Earth’s ecosystems.We acknowledge funding from the Smithsonian Institution and the Tula Foundation.Peer reviewe

Effects of multiple disturbances and stresses on a benthic eelgrass community

Many ecosystems are facing environmental change and anthropogenic pressures that may affect communities in terms of both structure and/or function. Disturbances and stresses are commonly co-occurring in nature, however the interaction between them are generally considered as being additive without knowing the true in situ effects. The response of a Zostera marina bed community (1 m2 plots) facing two stresses (sediment enrichment through slow nutrient diffusers; light reduction through shading) and one disturbance (80 % eelgrass density reduction) was studied in the summer of 2015. The major and interactive effects were estimated based on diversity indices and community structure of the associated species, as well as the density and relative eelgrass growth. Density reduction increased the values (per g of Zostera) of the diversity indices and affected community structure over the course of the entire experiment (10 weeks). Sediment enrichment temporarily decreased diversity while shading increased diversity although these effects disappeared after five weeks. Antagonistic effects were observed when two disturbances/stresses were applied to community richness and evenness. Additive and antagonistic effects were observed for eelgrass growth (shading and density reduction) and for plant density (shading and enrichment). The inclusion of multiple disturbances and stresses in field experiments in order to assess their potential interactive effects will help disentangle the mechanisms structuring communities following disturbances

Effets simples et cumulés des perturbations et des stress multiples sur les communautés benthiques intertidales : rôle des macrophytes

Les écosystèmes côtiers sont sujets à de nombreux stress et perturbations (stress ci-après) naturels et anthropiques. Ceux-ci agissent sur la stabilité et le fonctionnement des écosystèmes qui peuvent aller jusqu’à une disparition d’habitats et une perte de biodiversité. Bien que ces milieux soient généralement soumis à plusieurs stress à la fois, peu d’études se sont intéressées à la nature et aux conséquences potentielles des effets cumulatifs. Les études mettant l’accent sur de multiples stress sont importantes pour aider à la compréhension des mécanismes qui façonnent les communautés dans un environnement complexe et changeant. L’objectif principal de cette thèse est d’évaluer les effets des stress et leurs interactions sur les communautés macrobenthiques littorales. Pour répondre à cet objectif, mon projet cible le rôle des macrophytes structurants sur leurs communautés associées lorsque des stress affectant les contrôles descendant et/ou ascendant (« top-down » et « bottom-up ») sont présents. Des expériences in situ ont été mises en place dans deux habitats du littoral de l’estuaire maritime du Saint-Laurent : herbiers de zostères et les macroalgues. Le premier chapitre évalue le rôle des macroalgues en milieu médiolittoral rocheux en combinaison avec un enrichissement de la colonne d’eau et une réduction des gastéropodes brouteurs sur la résilience des communautés macrobenthiques associées. Le second chapitre évalue le rôle de la densité des zostères marines en combinaison avec un enrichissement des sédiments et une réduction de la lumière sur l’épifaune associée et les zostères en soi. Le dernier chapitre porte sur les effets de bordure et la densité de zostères marines sur leur épifaune associée sur cinq sites de l’hémisphère Nord (côte ouest de l’Atlantique, côte est du Pacifique, Québec et France) afin de vérifier si les effets de la complexité de l’habitat et du paysage à petite échelle est le même dans des herbiers de zostères pouvant avoir des caractéristiques différentes. Dans chacun des chapitres, des mesures de diversité univariées (richesse, diversité, équitabilité, abondances) et multivariées (structure et composition) au niveau des invertébrés et des algues ont été évaluées. Les types d’interactions entre stresseurs ont aussi été déterminé dans les chapitres 1 et 2 (addition, dominance, synergisme, antagonisme). Des mesures sur la zostère marine ont été ajoutées pour les chapitres 2 et 3 (densité des plants et masse des épibiontes ; chapitre 2 seulement : élongation relative des plants et glucides non-structuraux). Le dernier chapitre utilise une approche par traits biologiques pour comparer des sites qui ont très peu d’espèces communes. Les effets de l’habitat sont ainsi mesurés sur des traits communs à tous les sites. Cette approche permet de faire des rapprochements entre les communautés et leurs fonctions. Les résultats de ma thèse montrent que les espèces structurantes en présence de stress multiples jouent un rôle primordial pour les communautés des habitats côtiers et confirment leurs rôles structurant et protecteur sur les différentes composantes de la biodiversité. Également, les milieux rocheux dominés par les macroalgues et les herbiers de zostères peuvent présenter de la résistance (chapitre II) et sont résilients (chapitres I et II) selon le type de stress temporairement appliqués. Contrairement aux attentes, les parcelles ayant subi les traitements triples n’ont pas été plus affectées que les traitements simples ou doubles à l’exception du traitement triple dans les macroalgues de milieux rocheux (chapitre I) qui a démontré un taux de récupération plus lent que les autres traitements. Ma thèse démontre que lorsque les stress interagissent, les effets ne sont pas systématiquement additifs ou synergiques tels que fréquemment sous-entendus dans la littérature. Plusieurs des interactions mesurées étaient de type dominant, c’est-à-dire que l’effet d’un stress vient éclipser celui d’un second alors qu’en majorité du temps, il n’y a pas eu d’interaction entre stress. Des interactions synergiques négatives, additives et antagonistes ont aussi été observées. Le dernier chapitre montre que l’effet de la complexité des espèces structurantes et l’effet de bordure peuvent être importants ou pas, ainsi ils ne se généralisent pas entre des sites distants, et ce, même en utilisant des traits biologiques. Les résultats suggèrent que la répartition des espèces et les traits biologiques sont influencés par d’autres aspects que seulement l’effet de bordure ou la complexité des zostères, et qu’aucun de ces deux effets ne domine les effets observés sur les assemblages. Ma thèse met en valeur l’importance des expériences in situ qui utilisent des perturbations et des stress multiples pour déterminer leurs effets cumulatifs. Entre autres, la détermination des types d’interaction entre stress est importante au niveau de la gestion des écosystèmes et qu’une simple additivité des stress ne devrait pas être supposée sans tests in situ. Il est primordial que les gestionnaires reconnaissent que les stress pourront avoir des effets locaux spécifiques et que les interactions entre les stress présents sont imprévisibles. Effectivement, les résultats de cette thèse suggèrent que les stress multiples peuvent interagir différemment sur les indices liés à la biodiversité des communautés, leurs structures et leurs fonctions et que leurs interactions ne peuvent pas être prédites en utilisant des mesures sur des stress simples seulement. Effectivement, il sera important pour les gestionnaires d’inclure plusieurs mesures de la diversité, particulièrement des mesures multivariées et des mesures de fonctionnement, dans le but d’évaluer la santé des écosystèmes. Il devient donc prioritaire de maintenir la présence de macrophytes structurants qui soutiennent directement les capacités de résilience et de résistance des communautés face aux stress. Ma thèse permettra une meilleure gestion des écosystèmes en invitant les différents acteurs à porter une attention particulière aux différents indices de biodiversité, aux interactions imprévisibles des stress présents ou prédits, tout en tenant compte de l’unicité dans les caractéristiques et réponses de certains habitats. Coastal ecosystems are exposed to many natural and anthropogenic stress and disturbances (stress afterwards). These stresses affect the stability and functioning of ecosystems and their effect may lead to a loss in biodiversity and habitat. Although coastal systems are exposed to multiple simultaneous stresses, few studies investigated the interaction type and the cumulative effect of stress. Such studies are important for the understanding of how communities are shaped in a complex and changing environment. The main objective of this thesis is to measure the effects of stresses and their interactions on intertidal macrobenthic communities. To reach this goal, this thesis is centered on the role of habitat-forming macrophytes over their associated communities when they are facing stress affecting top-down and bottom-up controls. In situ experiments were performed in two different habitats: eelgrass meadows and rocky intertidal dominated by fucoids. The first chapter evaluates the role of macroalgae in a rocky intertidal system combined to water column enrichment and a reduction of grazing gastropod on the associated macrobenthic community resilience. The second chapter estimates the role of eelgrass shoot density combined to sediment nutrient enrichment and light reduction on associated epifaunal assemblages and eelgrass itself. The last chapter assesses the effect of edge and eelgrass shoot density on associated epifaunal assemblages on five different sites from the northern hemisphere (West Atlantic Coast, East Pacific Coast, Québec and France) to verify if the effects of habitat complexity and small-scale seascape are the same in different eelgrass meadows. In each chapter, diversity univariate and multivariate invertebrates and algae diversity measures were analyzed: abundance, richness, diversity, evenness, structure, composition. The type of interaction among stressors were determined in chapters 1 and 2 (addition, dominance, synergism, antagonism). Some eelgrass measures were added in chapters 2 (shoot density, shoot relative elongation, non-structural carbohydrates, epibionts biomass) and 3 (shoot density, epibionts biomass). The last chapter uses a biological traits approach in combination to the species approach. The biological trait approach allows to compare the effect of habitat on species among sites that have almost no species in common. My results indicate that habitat-forming species play an important role when communities are facing multiple stresses which confirms their structuring and protecting roles over different biodiversity components. Moreover, rocky systems dominated by macroalgae and eelgrass meadows may present resistance (chapter II) and are resilient (chapters I and II) depending on the temporary stress they are facing. Contrary to expectations, plots that were facing three stresses were not more affected than were single or double stressed plots except for the triple stress in chapter I that had the slowest recovery. My thesis shows that interacting stresses are not systematically additive or synergistic as is regularly assumed in the literature. Indeed, many of the interactions were of the type dominant, that is, the effect of one stressor overshadows the effect of the other stressor, while we mainly measured no interactions. Some negative synergistic, additive and antagonistic interactions were also observed. The last chapter shows that the effect of the complexity of habitat-forming species and edge effect may be or may not be important. Indeed, no common general results were observed on five distant sites even when using biological traits. My thesis highlights the importance of in situ experiments using multiple disturbances and stresses in order to determine the cumulative effects. Determining the interaction type between stresses is essential for system management since additivity of stresses should not be assumed without proper testing. It is important that managers know that stresses can have local and specific effects, and that the interactions among stresses can not easily be predicted. Indeed, the results of this thesis indicate that multiple stresses will not have the same impact depending on the identity of the investigated variables. Moreover, it is impossible to predict the interaction of stresses based only on their single effect. Managers should include complementary diversity measures as well as functioning measures to insure the health of ecosystems. Notably, it is of a great importance to maintain the presence of habitat-forming macrophytes since they promote the resistance and resilience of communities facing stress. My thesis will allow a better management of ecosystems by inviting decision makers to look at various biodiversity indices, to take into account that the interaction of stresses are unpredictable, and that every habitat or system may show unique characteristics that will affect their responses to stressors

Intertidal species abundances facing a multiple stress/disturbance experiment in St. Lawrence Estuary (Sainte-Flavie, QC, Canada) 2012-2013

Abundances (percent cover, counts or biomass) of rocky mid-intertidal macro-species measured through a multi-factorial in situ experiment of 14 months in 2012-2013 in Sainte-Flavie, Québec, Canada (48°37'42.5″ N, 68°11'55.7″ W). Using 0.25-m² plots, we ran an in situ one-pulse experiment (removal of all materials to bare rock and then burning of the surface) followed by a full orthogonal factorial design of three press-type disturbances or stresses: grazer reduction, canopy removal, and nutrient enrichment. We evaluated percentage cover visually. Counts and biomass were obtained destructively at the end of the experiment. For further details, refer to our paper (https://doi.org/10.1002/ecs2.2467)

Macro-species taxonomy measured through a multi-factorial in situ experiment of 10 weeks in a eelgrass bed in St. Lawrence Estuary (Baie-Saint-Ludger, Canada)

We evaluated single and interactive effects of density reduction of Zostera marina L., a habitat-forming species, shading, and sediment nutrient enrichment on the response of intertidal epibenthic macro-species. Epibenthos was collected using mesh bags (mesh size of 500 µm, diameter about 18 cm). This dataset reports all species identified through the experiment and their taxonomic classification

Relative eelgrass leaf elongation and microalgal epiphyte load on eelgrass leaf measured through a multi-factorial in situ experiment of 10 weeks in a eelgrass bed in St. Lawrence Estuary (Baie-Saint-Ludger, Canada)

We evaluated single and interactive effects of density reduction of Zostera marina L., a habitat-forming species, shading, and sediment nutrient enrichment on the response of eelgrass relative leaf elongation (RLE) and of microalgal epiphyte load on leaves. REL was evaluated once, as a proxy for growth, using five shoots per plot that were each marked with a reference hole at the top of the sheath using a pushpin marked at the end of Period T1 (two weeks after reducing eelgrass density and at the same time than adding nutrients sediment enrichment and shades). After 19 days, we collected the shoots and brought them back to the lab where leaf elongation was measured as the displacement of the mark relative to the reference mark on the oldest nongrowing leaf (Olesen & Sand-Jensen 1994). Total leaf elongation was then divided by sheath length and the number of days of elongation. We estimated the epiphyte (microalgae) load on eelgrass by scraping the leaves of one randomly selected shoot; we then filtered the water containing the epiphytes on GF/F filters that were then kept wrapped in aluminum foil at -80 °C until analysis. Epiphyte load was assessed using chlorophyll extraction with 90% acetone, following Parsons et al. (1984). Calculated chlorophyll weight was then divided by shoot dry weight

Eelgrass shoot density measured through a multi-factorial in situ experiment of 10 weeks in a eelgrass bed in St. Lawrence Estuary (Baie-Saint-Ludger, Canada)

We evaluated single and interactive effects of shading and sediment nutrient enrichment on the response of Zostera marina L. shoot density. Shoot density was evaluated using counts from three hoops of 20-cm diameter. Shoot density was measured before starting the experiment (T0), five weeks after the start of the experiment (T2) and at the end of the experiment after ten weeks (T3)

Eelgrass biomass (dry weight) distribution measured through a multi-factorial in situ experiment of 10 weeks in a eelgrass bed in St. Lawrence Estuary (Baie-Saint-Ludger, Canada)

We evaluated single and interactive effects of density reduction of Zostera marina L., a habitat-forming species, shading, and sediment nutrient enrichment on the response of Z. marina and its associated epifauna over a 10-week experiment during the summer of 2015. Before the experiment was started (Period T0), above-ground eelgrass shoot biomass was evaluated using the shoots collected from 10 cells of 10 x 10 cm while applying the Z. marina density reduction