Isabelle Gounand

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. We first tested whether ants increased plants capture rate. We found that unlike most plant-ants, C. schmitzi do not 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 plants 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.

Bridging ecology and conservation: from ecological networks to ecosystem function

Summary 1. Current approaches to conservation may be inadequate to maintain ecosystem integrity because they are mostly based on rarity status of organisms rather than functional signifi- cance. Alternatively, approaches focusing on the protection of ecological networks lead to more appropriate conservation targets to maintain ecosystem integrity. 2. We propose that a shift in focus from species to interaction networks is necessary to achieve pressing conservation management and restoration ecology goals of conserving biodi- versity, ecosystem processes and ultimately landscape-scale delivery of ecosystem services. 3. Using topical examples from the literature, we discuss historical and conceptual advances, current challenges and ways to move forward. We also propose a road map to ecological net- work conservation, providing a novel ready to use approach to identify clear conservation targets with flexible data requirements. 4. Synthesis and applications. Integration of how environmental and spatial constraints affect the nature and strength of local interaction networks will improve our ability to predict their response to change and to conserve them. This will better protect species, ecosystem pro- cesses, and the resulting ecosystem services we depend on.

The Paradox of Enrichment in Metaecosystems

The paradox of enrichment has been studied almost exclusively within communities or metacommunities, without explicit nutrient dynamics. Yet local recycling of materials from enriched ecosystems may affect the stability of connected ecosystems. Here we study the effect of nutrient, detritus, producer, and consumer spatial flows—combined with changes in regional enrichment—on the stability of a metaecosystem model. We considered both spatially homogeneous and heterogeneous enrichment. We found that nutrient and detritus spatial flows are destabilizing, whereas producer or consumer spatial flows are either neutral or stabilizing. We noticed that detritus spatial flows have only a weak impact on stability. Our study reveals that heterogeneity no longer stabilizes well-connected systems when accounting for explicit representation of nutrient dynamics. We also found that intermediate consumer diffusion could lead to multiple equilibria in strongly enriched metaecosystems. Stability can emerge from a top-down control allowing the storage of materials into inorganic form, a mechanism never documented before. In conclusion, local enrichment can be stabilized if spatial flows are strong enough to efficiently redistribute the local excess of enrichment to unfertile ecosystems. However, high regional enrichment can be dampened only by intermediate consumer diffusion rates.

Spatially cascading effect of perturbations in experimental meta-ecosystems

Ecosystems are linked to neighbouring ecosystems not only by dispersal, but also by the movement of subsidy. Such subsidy couplings between ecosystems have important landscape-scale implications because perturbations in one ecosystem may affect community structure and functioning in neighbouring ecosystems via increased/decreased subsidies. Here, we combine a general theoretical approach based on harvesting theory and a two-patch protist meta-ecosystem experiment to test the effect of regional perturbations on local community dynamics. We first characterized the relationship between the perturbation regime and local population demography on detritus production using a mathematical model. We then experimentally simulated a perturbation gradient affecting connected ecosystems simultaneously, thus altering cross-ecosystem subsidy exchanges. We demonstrate that the perturbation regime can interact with local population dynamics to trigger unexpected temporal variations in subsidy pulses from one ecosystem to another. High perturbation intensity initially led to the highest level of subsidy flows; however, the level of perturbation interacted with population dynamics to generate a crash in subsidy exchange over time. Both theoretical and experimental results show that a perturbation regime interacting with local community dynamics can induce a collapse in population levels for recipient ecosystems. These results call for integrative management of human-altered landscapes that takes into account regional dynamics of both species and resource flows.

Meta-Ecosystems 2.0: Rooting the Theory into the Field

The meta-ecosystem framework demonstrates the significance of among-ecosystem spatial flows for ecosystem dynamics and has fostered a rich body of theory. The high level of abstraction of the models, however, impedes applications to empirical systems. We argue that further understanding of spatial dynamics in natural systems strongly depends on dense exchanges between field and theory. From empiricists, more and specific quantifications of spatial flows are needed, defined by the major categories of organismal movement (dispersal, foraging, life-cycle, and migration). In parallel, the theoretical framework must account for the distinct spatial scales at which these naturally common spatial flows occur. Integrating all levels of spatial connections among landscape elements will upgrade and unify landscape and meta-ecosystem ecology into a single framework for spatial ecology.

Size evolution in microorganisms masks trade-offs predicted by the growth rate hypothesis

Adaptation to local resource availability depends on responses in growth rate and nutrient acquisition. The growth rate hypothesis (GRH) suggests that growing fast should impair competitive abilities for phosphorus and nitrogen due to high demand for biosynthesis. However, in microorganisms, size influences both growth and uptake rates, which may mask trade-offs and instead generate a positive relationship between these traits (size hypothesis, SH). Here, we evolved a gradient of maximum growth rate (μmax) from a single bacterium ancestor to test the relationship among μmax, competitive ability for nutrients and cell size, while controlling for evolutionary history. We found a strong positive correlation between μmax and competitive ability for phosphorus, associated with a trade-off between μmax and cell size: strains selected for high μmax were smaller and better competitors for phosphorus. Our results strongly support the SH, while the trade-offs expected under GRH were not apparent. Beyond plasticity, unicellular populations can respond rapidly to selection pressure through joint evolution of their size and maximum growth rate. Our study stresses that physiological links between these traits tightly shape the evolution of competitive strategies.

Worldwide cross-ecosystem carbon subsidies and their contribution to ecosystem functioning

Ecosystems are widely inter-connected by spatial flows of resources1,2, yet primarily studied in a local context. Meta-ecosystem models suggest that cross-ecosystem subsidies can play an essential role in ecosystem functioning, notably by controlling local availability of resources for biological communities3–6. The general contribution of these resource connections to ecosystem functioning, however, remains unclear in natural systems, due to the heterogeneity and dispersion of data across the ecological literature. Here we provide the first quantitative synthesis on spatial flows of carbon connecting ecosystems worldwide. These cross-ecosystem subsidies range over eight orders of magnitude, between 10−3 and 105 gC m−2 yr−1, and are highly diverse in their provenance. We found that spatial carbon flows and local carbon fluxes are of the same order of magnitudes in freshwater and benthic ecosystems, suggesting an underlying dependency of these systems on resources provided by connected terrestrial and pelagic ecosystems respectively. By contrast, in terrestrial systems, cross-ecosystem subsidies were two to three orders of magnitude lower than local production (grasslands and forests), indicating a weaker quantitative influence on functioning. Those subsidies may still be qualitatively important, however, as some have high nutrient content7,8. We also find important gaps in carbon flow quantification, notably of cross-ecosystem subsidies driven by animal movements, which likely leads to general underestimations of the magnitude and direction of cross-ecosystem linkages9. Overall, we demonstrate strong ecosystem couplings, suggesting that ecosystems can be vulnerable to alterations of these flows and pointing to an urgent need to re-think ecosystem functioning in a spatial perspective.

Upstream trophic structure modulates downstream community dynamics via resource subsidies

Abstract In many natural systems, the physical structure of the landscape dictates the flow of resources. Despite mounting evidence that communities’ dynamics can be indirectly coupled by reciprocal among ecosystem resource flows, our understanding of how directional resource flows might indirectly link biological communities is limited. We here propose that differences in community structure upstream should lead to different downstream dynamics, even in the absence of dispersal of organisms. We report an experimental test of the effect of upstream community structure on downstream community dynamics in a simplified but highly controlled setting, using protist microcosms. We implemented directional flows of resources, without dispersal, from a standard resource pool into upstream communities of contrasting interaction structure and then to further downstream communities of either one or two trophic levels. Our results demonstrate that different types of species interactions in upstream habitats may lead to different population sizes and levels of biomass in these upstream habitats. This, in turn, leads to varying levels of detritus transfer (dead biomass) to the downstream communities, thus influencing their population densities and trophic interactions in predictable ways. Our results suggest that the structure of species interactions in directionally structured ecosystems can be a key mediator of alterations to downstream habitats. Alterations to upstream habitats can thus cascade down to downstream communities, even without dispersal.

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

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. 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. 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. 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. 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. This article is protected by copyright. All rights reserved.

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.