Benchmark solution for a three-dimensional mixed convection flow - Part 1: reference solutions

International audienceAbstract A solution to a benchmark problem for a three-dimensional mixed convection flow in a horizontal rectangular channel heated from below and cooled from above (Poiseuille-Rayleigh-Bénard flow) is proposed. This flow is a steady thermoconvective longitudinal roll flow in a large aspect ratio channel at moderate Reynolds and Rayleigh numbers (Re=50, Ra=5000) and Prandtl number Pr=0.7. The model is based on the Navier-Stokes equations with Boussinesq approximation. We propose reference solutions resulting from computations on large grids, Richardson extrapolation (RE) and cubic spline interpolations. The solutions obtained with one finite difference, one finite volume and two finite element codes are in good agreement and reference values for the flow and thermal fields and the heat and momentum fluxes are given with 4 to 5 significant digits

The plant-ant Camponotus schmitzi helps its carnivorous host-plant Nepenthes bicalcarata to catch its prey

The Bornean climber, Nepenthes bicalcarata, is unique among plants because it is both carnivorous and myrmecophytic, bearing pitcher-shaped leaves and the ant Camponotus schmitzi within tendrils. We explored, in the peat swamp forests of Brunei, the hypothesis that these ants contribute to plant nutrition by catching and digesting its prey.Wefirst tested whether ants increasedplant's capture rate.Wefound that unlikemost plant-ants, C. schmitzidonot exhibit dissuasive leaf-patrolling behaviour (zero patrol on 67 pitchers of 10 plants) but lie concealed under pitcher rim (13 ± 6 ants per pitcher) allowing numerous insect visits. However, 47 out of 50 individuals of the largest visitor dropped into the pitchers of five plants were attacked by ants and the capture rate of the same pitchers deprived of their ambush hunting ants decreased three-fold.We then tested whether ants participated in plant's digestion.We showed in a 15-d long experiment that ants fed on prey and returned it in pieces in seven out of eight pitchers. The 40 prey deposited in ant-deprived pitchers remained intact indicating a weak digestive power of the fluid confirmed to be only weakly acidic (pH ?5, n = 67). The analysis of 10 pitcher contents revealed that prey, mainly ants and termites, was very numerous (?400 per pitcher per plant) and highly fragmented. Altogether, these data suggest a positive effect of C. schmitzi on both prey intake and breakdown. This ant-plant interaction could thus be a nutritional mutualism involving the unusual association of carnivory and myrmecotrophy

Electromagnetic induced transparency and slow light in interacting quantum degenerate atomic gases

We systematically develop the full quantum theory for the electromagnetic induced transparency (EIT) and slow light properties in ultracold Bose and Fermi gases. It shows a very different property from the classical theory which assumes frozen atomic motion. For example, the speed of light inside the atomic gases can be changed dramatically near the Bose-Einstein condensation temperature, while the presence of the Fermi sea can destroy the EIT effect even at zero temperature. From experimental point of view, such quantum EIT property is mostly manifested in the counter-propagating excitation schemes in either the low-lying Rydberg transition with a narrow line width or in the D2 transitions with a very weak coupling field. We further investigate the interaction effects on the EIT for a weakly interacting Bose-Einstein condensate, showing an inhomogeneous broadening of the EIT profile and nontrivial change of the light speed due to the quantum many-body effects beyond mean field energy shifts.Comment: 7 figure

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

Seasonal dynamics of detritus flows and decomposition across ecosystem boundaries

Material fluxes are ubiquitous in nature within and across ecosystems, connecting habitats with vastly different characteristics, like forests to rivers and lakes.1–3 Although individual fluxes and their cascading effects are well known,4–6 very few studies address the intra-annual phenology of ecosystem processes, despite the pronounced seasonality of fluxes. Here, we empirically quantified and resolved fluxes of recalcitrant and labile types of leaf litter in temperate riparian forests and streams across a year, representing one of the most emblematic examples of seasonal systems. We quantified intra-annual variation in litter inputs from terrestrial plants to forest floors and streams and estimated aquatic and terrestrial decomposition rates across the year at 6-week intervals. Our data show that the autumn pulse of leaf litter is complemented by smaller magnitude but more constant-through-the-year lateral flows to the stream ecosystems. Decomposition of labile litter fluctuated seasonally, on a different phenology, with generally higher rates in summer, but rates of recalcitrant litter decomposition remained largely constant. Microorganisms were the main contributors to the decomposition process in both forests and streams. Overall, our work highlights the asynchronous and seasonally variable changes in decomposition rates between recalcitrant and labile detritus despite their initial synchronized availability and suggests that the dominating presence of recalcitrant litter buffers ecosystem responses to the concentrated temporal distribution of litter resources.7,8 Investigating such ecological processes both across ecosystem borders and at fine intra-annual resolutions is imperative to understand complex system responses in the context of species’ shifts in phenologies and resource quality.9–1

Generation and application of river network analogues for use in ecology and evolution

Several key processes in freshwater ecology are governed by the connectivity inherent to dendritic river networks. These have extensively been analyzed from a geomorphological and hydrological viewpoint, yet structures classically used in ecological modeling have been poorly representative of the structure of real river basins, often failing to capture well-known scaling features of natural rivers. Pioneering work identified optimal channel networks (OCNs) as spanning trees reproducing all scaling features characteristic of natural stream networks worldwide. While OCNs have been used to create landscapes for studies on metapopulations, biodiversity, and epidemiology, their generation has not been generally accessible. Given the increasing interest in dendritic riverine networks by ecologists and evolutionary biologists, we here present a method to generate OCNs and, to facilitate its application, we provide the R-package OCNet. Owing to the stochastic process generating OCNs, multiple network replicas spanning the same surface can be built; this allows performing computational experiments whose results are irrespective of the particular shape of a single river network. The OCN construct also enables the generation of elevational gradients derived from the optimal network configuration, which can constitute three-dimensional landscapes for spatial studies in both terrestrial and freshwater realms. Moreover, the package provides functions that aggregate OCNs into an arbitrary number of nodes, calculate several descriptors of river networks, and draw relevant network features. We describe the main functionalities of the package and its integration with other R-packages commonly used in spatial ecology. Moreover, we exemplify the generation of OCNs and discuss an application to a metapopulation model for an invasive riverine species. In conclusion, OCNet provides a powerful tool to generate realistic river network analogues for various applications. It thereby allows the design of spatially realistic studies in increasingly impacted ecosystems and enhances our knowledge on spatial processes in freshwater ecology in general

Deforestación y agroindustria en Bolivia: el comercio internacional como motor del “extractivismo” en la Ecología-Mundo

La Amazonía es una región de gran importancia socioecológica, además de un territorio de disputa política y discursiva. Los complejos procesos de la región demandan un análisis interdisciplinario. El presente trabajo se centra en la dinámica de la deforestación en Bolivia, identificando los motores próximos y ulteriores. La principal causa próxima de la deforestación en Bolivia es el cultivo agroindustrial de soya y la ganadería extensiva. En el bioma amazónico el principal motor posterior de la deforestación se relaciona con la demanda de mercancías. Dado que existe una fuerte dependencia del régimen de lluvias en la evapotranspiración de los árboles en la Amazonía boliviana, la conversión del uso de la tierra causada por este sector puede afectar profundamente el clima regional, a través de procesos de retroalimentación. La caracterización de la producción de soya en Bolivia como “extractivismo agrario” es insuficiente. Este concepto por sí solo, y sin ser vinculado a la Ecología-mundo, dificulta una crítica necesaria al capitalismo, así como abordar adecuadamente los problemas socioecológicos, frecuentemente originados en asimetrías históricas. Por lo tanto, es necesario considerar el “extractivismo” como una agregación de relaciones de intercambio desigual

Natural acidification changes the timing and rate of succession, alters community structure, and increases homogeneity in marine biofouling communities

Ocean acidification may have far-reaching consequences for marine community and ecosystem dynamics, but its full impacts remain poorly understood due to the difficulty of manipulating pCO2 at the ecosystem level to mimic realistic fluctuations that occur on a number of different timescales. It is especially unclear how quickly communities at various stages of development respond to intermediate-scale pCO2 change and, if high pCO2 is relieved mid-succession, whether past acidification effects persist, are reversed by alleviation of pCO2 stress, or are worsened by departures from prior high pCO2 conditions to which organisms had acclimatized. Here, we used reciprocal transplant experiments along a shallow water volcanic pCO2 gradient to assess the importance of the timing and duration of high pCO2 exposure (i.e., discrete events at different stages of successional development vs. continuous exposure) on patterns of colonization and succession in a benthic fouling community. We show that succession at the acidified site was initially delayed (less community change by 8\uc2 weeks) but then caught up over the next 4\uc2 weeks. These changes in succession led to homogenization of communities maintained in or transplanted to acidified conditions, and altered community structure in ways that reflected both short- and longer-term acidification history. These community shifts are likely a result of interspecific variability in response to increased pCO2 and changes in species interactions. High pCO2 altered biofilm development, allowing serpulids to do best at the acidified site by the end of the experiment, although early (pretransplant) negative effects of pCO2 on recruitment of these worms were still detectable. The ascidians Diplosoma sp. and Botryllus sp. settled later and were more tolerant to acidification. Overall, transient and persistent acidification-driven changes in the biofouling community, via both past and more recent exposure, could have important implications for ecosystem function and food web dynamics

Towards an integrative, eco-evolutionary understanding of ecological novelty: studying and communicating interlinked effects of global change

Global change has complex eco-evolutionary consequences for organisms and ecosystems, but related concepts (e.g., novel ecosystems) do not cover their full range. Here we propose an umbrella concept of “ecological novelty” comprising (1) a site-specific and (2) an organism-centered, eco-evolutionary perspective. Under this umbrella, complementary options for studying and communicating effects of global change on organisms, ecosystems, and landscapes can be included in a toolbox. This allows researchers to address ecological novelty from different perspectives, e.g., by defining it based on (a) categorical or continuous measures, (b) reference conditions related to sites or organisms, and (c) types of human activities. We suggest striving for a descriptive, non-normative usage of the term “ecological novelty” in science. Normative evaluations and decisions about conservation policies or management are important, but require additional societal processes and engagement with multiple stakeholders