Trophic groups and modules: two levels of group detection in food webs

Within food webs, species can be partitioned into groups according to various criteria. Two notions have received particular attention: trophic groups, which have been used for decades in the ecological literature, and more recently, modules. The relationship between these two group definitions remains unknown in empirical food webs because they have so far been studied separately. While recent developments in network theory have led to efficient methods for detecting modules in food webs, the determination of trophic groups (sets of species that are functionally similar) is based on subjective expert knowledge. Here, we develop a novel algorithm for trophic group detection. We apply this method to several well-resolved empirical food webs, and show that aggregation into trophic groups allows the simplification of food webs while preserving their information content. Furthermore, we reveal a 2-level hierarchical structure where modules partition food webs into large bottom-top trophic pathways whereas trophic groups further partition these pathways into sets of species with similar trophic connections. Bringing together trophic groups and modules provides new perspectives to the study of dynamical and functional consequences of food-web structure, bridging topological analysis and dynamical systems. Trophic groups have a clear ecological meaning in terms of trophic similarity, and are found to provide a trade-off between network complexity and information loss

How plants connect pollination and herbivory networks and their contribution to community stability

Pollination and herbivory networks have mainly been studied separately, highlighting their distinct structural characteristics and the related processes and dynamics. However, most plants interact with both pollinators and herbivores, and there is evidence that both types of interaction affect each other. Here we investigated the way plants connect these mutualistic and antagonistic networks together, and the consequences for community stability. Using an empirical data set, we show that the way plants connect pollination and herbivory networks is not random and promotes community stability. Analyses of the structure of binary and quantitative networks show different results: the plants’ generalism with regard to pollinators is positively correlated to their generalism with regard to herbivores when considering binary interactions, but not when considering quantitative interactions. We also show that plants that share the same pollinators do not share the same herbivores. However, the way plants connect pollination and herbivory networks promotes stability for both binary and quantitative networks. Our results highlight the relevance of considering the diversity of interaction types in ecological communities, and stress the need to better quantify the costs and benefits of interactions, as well as to develop new metrics characterizing the way different interaction types are combined within ecological networks

Predicting effects of multiple interacting global change drivers across trophic levels

International audienceGlobal change encompasses many co-occurring anthropogenic drivers, which can act synergistically or antagonistically on ecological systems. Predicting how different global change drivers simultaneously contribute to observed biodiversity change is a key challenge for ecology and conservation. However, we lack the mechanistic understanding of how multiple global change drivers influence the vital rates of multiple interacting species. We propose that reaction norms, the relationships between a driver and vital rates like growth, mortality, and consumption, provide insights to the underlying mechanisms of community responses to multiple drivers. Understanding how multiple drivers interact to affect demographic rates using a reaction-norm perspective can improve our ability to make predictions of interactions at higher levels of organization-that is, community and food web. Building on the framework of consumer-resource interactions and widely studied thermal performance curves, we illustrate how joint driver impacts can be scaled up from the population to the community level. A simple proof-of-concept model demonstrates how reaction norms of vital rates predict the prevalence of driver interactions at the community level. A literature search suggests that our proposed approach is not yet used in multiple driver research. We outline how realistic response surfaces (i.e., multidimensional reaction norms) can be inferred by parametric and nonparametric approaches. Response surfaces have the potential to strengthen our understanding of how multiple drivers affect communities as well as improve our ability to predict when interactive effects emerge, two of the major challenges of ecology today

Comparing the conservatism of ecological interactions in plant-pollinator and plant-herbivore networks

International audienceConservatism in species interaction, meaning that related species tend to interact with similar partners, is an important feature of ecological interactions. Studies at community scale highlight variations in conservatism strength depending on the characteristics of the ecological interaction studied. However, the heterogeneity of datasets and methods used prevent to compare results between mutualistic and antagonistic networks. Here we perform such a comparison by taking plant-insect communities as a study case, with data on plant-herbivore and plant-pollinator networks. Our analysis reveals that plants acting as resources for herbivores exhibit the strongest conservatism in species interaction among the four interacting groups. Conservatism levels are similar for insect pollinators, insect herbivores and plants as interacting partners of pollinators, although insect pollinators tend to have a slightly higher conservatism than the two others. Our results thus clearly support the current view that within antagonistic networks, conservatism is stronger for species as resources than for species as consumer. Although the pattern tends to be opposite for plant-pollinator networks, our results suggest that asymmetry in conservatism is much less pronounced between the pollinators and the plant they interact with. We discuss these differences in conservatism strength in relation with the processes structuring plant-insect communities

Emerging niche clustering results from both competition and predation

Abstract Understanding species coexistence has been a central question in ecology for decades, and the notion that competing species need to differ in their ecological niche for stable coexistence has dominated. Recent theoretical and empirical work suggests differently. Species can also escape competitive exclusion by being similar, leading to clusters of species with similar traits. This theory has so far only been explored under competition. By combining mathematical and numerical analyses, we reveal that competition and predation are equally capable to promote clusters of similar species in prey–predator communities, their relative importance being modulated by resource availability. We further show that predation has a stabilizing effect on clustering patterns, making the clusters more diverse. Our results merge different ecological theories and bring new light to the emergent neutrality theory by adding the perspective of trophic interactions. These results open new perspectives to the study of trait distributions in ecological interaction networks

Relative effects of anthropogenic pressures, climate, and sampling design on the structure of pollination networks at the global scale

International audiencePollinators provide crucial ecosystem services that underpin to wild plant reproduction and yields of insect-pollinated crops. Understanding the relative impacts of anthropogenic pressures and climate on the structure of plant–pollinator interaction networks is vital considering ongoing global change and pollinator decline. Our ability to predict the consequences of global change for pollinator assemblages worldwide requires global syntheses, but these analytical approaches may be hindered by variable methods among studies that either invalidate comparisons or mask biological phenomena. Here we conducted a synthetic analysis that assesses the relative impact of anthropogenic pressures and climatic variability, and accounts for heterogeneity in sampling methodology to reveal network responses at the global scale. We analyzed an extensive dataset, comprising 295 networks over 123 locations all over the world, and reporting over 50,000 interactions between flowering plant species and theirinsect visitors. Our study revealed that anthropogenic pressures correlate with an increase in generalism in pollination networks while pollinator richness and taxonomic composition are more related to climatic variables with an increase in dipteran pollinator richness associated with cooler temperatures. The contrasting response of species richness and generalism of the plant–pollinator networks stresses the importance ofconsidering interaction network structure alongside diversity in ecological monitoring. In addition, differences in sampling design explained more variation than anthropogenic pressures or climate on both pollination networks richness and generalism, highlighting the crucial need to report and incorporate sampling design in macroecological comparative studies of pollination networks. As a whole, our study reveals a potential human impact on pollination networks at a global scale. However, further research is needed to evaluate potential consequences of loss of specialist species and their unique ecological interactions and evolutionary pathways on the ecosystem pollination function at a global scale

Are insect pollinators more generalist than insect herbivores?

Recent community-level studies have acknowledged that generalist species are more widespread than previously thought and highlighted their preponderant impact on community functioning and evolution. It is suggested that the type of interaction, trophic versus mutualistic, should affect species generalization level; however, no direct comparison has been made yet. Here, we performed such a comparison using 44 plant–insect networks describing either pollination or herbivory communities. Our analysis shows that the type of interaction does indeed have an impact on various aspects of species generalism, from the distribution of generalism in the community to the phylogenetic diversity of the plants with which a given insect species interacts. However, the amplitude of the observed differences depends on the aspect of species generalism studied. While the non-quantitative and quantitative measures of generalism suggest that pollinators interact with more plant species and more evenly than herbivores, phylogenetic measures clearly show that herbivores interact with plant species far more closely related to each other than pollinators. This comparative approach offers a promising perspective to better understand the functioning and evolution of multispecies assemblages by pointing out some fundamental singularities of communities depending on the type of interaction considered

Stability of a diamond-shaped module with multiple interaction types

International audienceIndirect interactions among species emerge from the complexity of ecological networks and can strongly affect the response of communities to disturbances. To determine these indirect interactions and understand better community dynamics, ecologists focused on the interactions within small sets of species or modules. Thanks to their analytical tractability, modules bring insights on the mechanisms occurring in complex interaction networks. So far, most studies have considered modules with a single type of interaction although numerous species are involved in mutualistic and antagonistic interactions simultaneously. In this study, we analyse the dynamics of a diamond-shaped module with multiple interaction types: two resource species sharing a mutualist and a consumer. We describe the different types of indirect interaction occurring between the resource species and the conditions for a stable coexistence of all species. We show that the nature of indirect interactions between resource species (i.e. apparent facilitation, competition or antagonism), as well as stable coexistence, depend on the species generalism and asymmetry of interactions, or in other words, on the distribution of interaction strengths among species. We further unveil that a balance between mutualistic and antagonistic interactions at the level of resource species favours stable coexistence, and that species are more likely to coexist stably if there is apparent facilitation between the two resource species rather than apparent competition. Our results echo existing knowledge on the trophic diamond-shaped module, and confirm that our understanding of communities combining different interaction types can gain from module analyses

Phenological traits foster persistence of mutualistic networks by promoting facilitation

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