Intransitive competition and its effects on community functional diversity

Can competitive interactions be inferred from the analysis of community functional diversity patterns? Originally, at the scale of a community, competitive interactions were supposed to generate trait overdispersion patterns due to limiting similarity process. More recently, by highlighting the importance of competitive hierarchies, it has been shown that when only one resource limits species coexistence, competition can also lead to patterns of trait clustering. However, these two expectations (overdispersion and clustering) ignore potential multi‐species indirect competitive interactions, and especially intransitive competition. Indeed, little is yet known about intransitive competition and its influence on community's functional diversity. Here I propose a brief appraisal of empirical evidence for intransitive competition in nature, and an overview of the current understanding of this mechanism and its properties. I then demonstrate with a theoretical model that intransitive competitive interactions can actually generate random‐like functional diversity patterns. The variety of diversity patterns (overdispersion, clustering, randomness) that can emerge from diverse types of competitive interactions makes it difficult to identify the presence of competition in nature, potentially leading to an underestimation of its importance as a structuring force. New methodologies able to capture both simple and complex competition mechanisms are thus urgently needed.     

Reduction of species coexistence through mixing in a spatial competition model

Many ecological systems exhibit self-organized spatial patterns due to local interactions. Such patterns can promote species diversity and therefore serve as an important mechanism for biodiversity maintenance. Previous work has shown that when species interactions occurred at local spatial scales, species diversity was greatest when robust mosaic spatial patterns formed. Also, intransitive interactions led to the emergence of spiral patterns, frequently resulting in multispecies coexistence. In some instances, intransitive interactions reduced species diversity as the consequence of competitive hierarchies. Here, we extend and broaden this line of investigation and examine the role of global competition along a continuum ranging from spatial mosaics to spiral patterns. While previous models have predicted that species diversity is reduced when interactions occur over larger spatial scales, our model considers the effects of various levels of mixing on species diversity, in the context of various network structures as measured by the covariance of row and column sums of the competition matrix. First, we compare local competition (unmixed system) versus global competition (mixed systems) and show that greater species diversity is maintained under a positive covariance. Second, we show that under various levels of mixing, species diversity declines more rapidly under a negative covariance. Lastly, we demonstrate that time to extinction in our model occurs much more rapidly under a negative covariance.     

生物间高阶相互作用研究进展

生物间的相互作用是物种共存和生物多样性维持的关键。传统的物种共存研究主要关注配对物种之间的直接相互作用, 而忽略了更为复杂的间接相互作用。本文首先介绍了两种间接相互作用: 链式相互作用(本质上仍是两两物种之间的相互作用)和高阶相互作用。在此基础上, 我们回顾了高阶相互作用定义的演变历史(包括狭义的高阶相互作用和广义的高阶相互作用)及其检验方法, 并介绍了高阶相互作用在多营养级之间和同一营养级内的研究概况。目前, 生态学家主要对多营养级之间(如食物网)的高阶相互作用的特征、发生机制、作用途径及实验证据等方面进行了详尽的研究。近年来, 同一营养级内的高阶相互作用也开始受到关注, 因此我们进一步介绍了同一营养级内个体水平高阶相互作用的重要意义和度量方法。从个体水平上研究高阶相互作用, 既能统一狭义和广义高阶相互作用在定义上的争议, 又可以将个体间的差异(如个体大小、个体的空间分布等信息)考虑进来。最后, 本文对高阶相互作用一些可能的重要研究方向进行了展望: 在自然群落中(尤其同一营养级内)检验高阶相互作用的普遍性与相对重要性, 探讨高阶相互作用的发生机制以及如何将高阶相互作用整合到现有的理论体系中等。高阶相互作用的研究有助于我们全面深刻地理解物种共存和生物多样性的维持机制, 丰富和完善群落生态学的理论框架, 为人类世背景下的生物多样性保护和生态系统功能维持与提升提供基础。     

The effects of intransitive competition on coexistence

Coexistence theory has been developed with an almost exclusive focus on interactions between two species, often ignoring more complex and indirect interactions, such as intransitive loops, that can emerge in competition networks. In fact, intransitive competition has typically been studied in isolation from other pairwise stabilising processes, and thus little is known about how intransitivity interacts with more traditional drivers of species coexistence such as niche partitioning. To integrate intransitivity into traditional coexistence theory, we developed a metric of growth rate when rare, , to identify and quantify the impact of intransitive competition against a backdrop of pairwise stabilising niche differences. Using this index with simulations of community dynamics, we demonstrate that intransitive loops can both stabilise or destabilise species coexistence, but the strength and importance of intransitive interactions are significantly affected by the length and the topology of these loops. We conclude by showing how can be used to evaluate effects of intransitivity in empirical studies. Our results emphasise the need to integrate complex mechanisms emerging from diverse interactions into our understanding of species coexistence.     

PRODUCTIVITY AND DIVERSITY IN A CROSS‐FEEDING POPULATION OF ARTIFICIAL ORGANISMS

Cross‐feeding interactions are a common feature of many microbial systems, such as colonies of Escherichia coli grown on a single limiting resource, and microbial consortia cooperatively degrading complex compounds. We have studied this phenomenon from an abstract point of view by considering artificial organisms that metabolize binary strings from a shared environment. The organisms are represented as simple cellular automaton rules and the analog of energy in the system is an approximation of the Shannon entropy of the binary strings. Only organisms that increase the entropy of the transformed strings are allowed to replicate. This system exhibits a large degree of species diversity, which increases when the flow of binary strings into the system is reduced. Investigating the relation between ecosystem productivity and diversity we find that diversity is negatively correlated with biomass production and energy uptake, while it correlates positively with energy‐uptake efficiency. By performing invasion experiments, we show that the source of diversity is negative frequency‐dependent selection acting among the different species, and that some of these interactions are intransitive, another mechanism known to promote diversity.     

Species interactions and random dispersal rather than habitat filtering drive community assembly during early plant succession

Theory on plant succession predicts a temporal increase in the complexity of spatial community structure and of competitive interactions: initially random occurrences of early colonising species shift towards spatially and competitively structured plant associations in later successional stages. Here we use long‐term data on early plant succession in a German post mining area to disentangle the importance of random colonisation, habitat filtering, and competition on the temporal and spatial development of plant community structure. We used species co‐occurrence analysis and a recently developed method for assessing competitive strength and hierarchies (transitive versus intransitive competitive orders) in multispecies communities. We found that species turnover decreased through time within interaction neighbourhoods, but increased through time outside interaction neighbourhoods. Successional change did not lead to modular community structure. After accounting for species richness effects, the strength of competitive interactions and the proportion of transitive competitive hierarchies increased through time. Although effects of habitat filtering were weak, random colonization and subsequent competitive interactions had strong effects on community structure. Because competitive strength and transitivity were poorly correlated with soil characteristics, there was little evidence for context dependent competitive strength associated with intransitive competitive hierarchies.     

Indirect plant-mediated interactions among parasitoid larvae

Communities are riddled with indirect species interactions and these interactions can be modified by organisms that are parasitic or symbiotic with one of the indirectly interacting species. By inducing plant responses, herbivores are well known to alter the plant quality for subsequent feeders. The reduced performance of herbivores on induced plants cascades into effects on the performance of higher trophic level organisms such as parasitoids that develop inside herbivores. Parasitoids themselves may also, indirectly, interact with the host plant by affecting the behaviour and physiology of their herbivorous host. Here, we show that, through their herbivorous host, larvae of two parasitoid species differentially affect plant phenotypes leading to asymmetric interactions among parasitoid larvae developing in different hosts that feed on the same plant. Our results show that temporally separated parasitoid larvae are involved in indirect plant-mediated interactions by a network of trophic and non-trophic relationships.     

EXPERIMENTAL STUDIES OF COMMUNITY EVOLUTION I: THE RESPONSE TO SELECTION AT THE COMMUNITY LEVEL

Coevolution generally refers to the process of two or more organisms adapting to each other as a result of individual selection. Another possibility, however, is that coevolution may result from selection acting directly at the community level. Certain types of multispecies associations, such as lichens, which are a symbiotic association between an alga and a fungus, are examples of simple two species communities that may be units of selection. The study presented here uses two species communities of Tribolium castaneum and T. confusum in an investigation of selection acting at the community level. Selection at the community level is performed on one trait measured in one species and correlated responses in other traits measured both within species and among species are monitored. I demonstrate that community selection, defined as the differential survival and or reproduction of communities, can result in significant changes in the phenotype of a community. The observed changes in the phenotype of a community as a result of community selection included changes in the trait under selection (direct effects of selection), as well as changes in traits that are not under selection (correlated responses to selection). Furthermore, two types of correlated responses to selection were observed. The first, within-species correlated responses to selection, are changes in a trait measured in one species as a result of community selection acting on another trait measured in the same species. The second, between-species correlated responses to selection, are changes in a trait measured in one species as a result of community selection acting on a trait measured in another species. Between species correlated responses to selection are of particular interest because they cannot be mediated by pathways of gene action that are internal to an individual, rather they can be mediated only through ecological pathways. In other words, between-species correlated responses to selection suggest that genetically based interactions among individuals are contributing to the response to community selection. These among species ecological pathways of gene action cannot contribute to a response to selection at a lower level; thus community selection may be able to bring about a response to selection that is qualitatively different from the response selection that would occur as a result of selection acting at a lower level.     

Dissecting the role of transitivity and intransitivity on coexistence in competing species networks

It is well established that intransitively assembled interaction networks can support the coexistence of competing species, while transitively assembled (hierarchical) networks are prone to species loss through competitive exclusion. However, as the number of species grows, the complexity of ecological interaction networks grows disproportionately, and species can get involved simultaneously in transitive and intransitive groups of interactions. In such complex networks, the effects of intransitivity on species persistence are not straightforward. Dissecting networks into intransitive/transitive components can help us to understand the complex role that intransitivity may play in supporting species diversity. We show through simulations that those species participating in the largest group of intransitive interactions (the core of the network) have high probabilities of persisting in the long term. However, participation in a group of intransitive interactions other than the core does not always improve persistence. Likewise, participating in transitive interactions does not always decrease persistence because certain species (the satellites) transitively linked to the core have also a high persistence probability. Therefore, when networks contain transitive and intransitive structures, as it can be expected in real ecological networks, the existence of a large intransitive core of species can have a disproportionate positive effect on species richness.     

Herbivore community promotes trait evolution in a leaf beetle via induced plant response

Several recent studies have emphasised that community composition alters species trait evolution. Here, we demonstrate that differences in composition of local herbivore communities lead to divergent trait evolution of the leaf beetle Plagiodera versicolora through plant‐mediated indirect interactions. Our field surveys, genetic analyses and community‐manipulation experiments show that herbivore community composition determines the degree of herbivore‐induced regrowth of willows (Salicaceae), which in turn, promotes the divergent evolution of feeding preference in the leaf beetle from exclusive preference for new leaves to a lack of preference among leaf‐age types. Regrowth intensity depends both on the differential response of willows to different herbivore species and the integration of those herbivore species in the community. Because herbivore‐induced regrowth involves phenological changes in new leaf production, leaf beetle populations develop divergent feeding preferences according to local regrowth intensity. Therefore, herbivore community composition shapes the selection regime for leaf beetle evolution through trait‐mediated indirect interactions.     

The consequences of direct versus indirect species interactions to selection on traits: pollination and nectar robbing in Ipomopsis aggregata

Organisms experience a complex suite of species interactions. Although the ecological consequences of direct versus indirect species interactions have received attention, their evolutionary implications are not well understood. I examined selection on floral traits through direct versus indirect pathways of species interactions using the plant Ipomopsis aggregata and its pollinators and nectar robber. Using path analysis and structural equation modeling, I tested competing hypotheses comparing the relative importance of direct (pollinator-mediated) versus indirect (robber-mediated) interactions to trait selection through female plant function in 2 years. The hypothesis that provided the best fit to the observed data included robbing and pollination, suggesting that both interactors are important in driving selection on some traits; however, the direction and intensity of selection through robbing versus pollination varied between years. I then increased my scope of inference by assessing traits and species interactions across more years. I found that the potential for temporal variation in the direction and intensity of selection was pronounced. Taken together, results suggest that assessing the broader context in which organisms evolve, including both direct and indirect interactions and across multiple years, can provide increased mechanistic understanding of the diversity of ways that animals shape floral and plant evolution.     

拉萨河中下游纤毛虫群落时空分布模式及其驱动机制

为了探究拉萨河中下游纤毛虫群落的组成模式、时空多样性格局及其维持机制, 本文于2015年5月和8月以及2016年10月在拉萨河中下游17个样点进行采样, 采用活体观察、鲁哥氏碘液固定染色以及Wilbert蛋白银法相结合的物种鉴定方法, 对纤毛虫群落结构进行了研究。通过Shannon多样性指数、Margalef丰富度指数、物种数分析群落结构时空上的差异性; 通过共现网络分析纤毛虫类群之间的相互作用; 通过冗余分析(redundancy analysis, RDA)探讨水体理化因子对纤毛虫群落结构的影响。结果表明, Shannon多样性指数在季节和河段间没有显著性差异; Margalef丰富度指数、物种数在河段间存在极显著性差异; 中游和下游河段共现网络节点间的相关关系均以正相关为主; 溶解氧(DO)、总氮(TN)、总磷(TP)、总溶解盐(TDS)是影响纤毛虫群落结构的关键因子。综上所述, 拉萨河中下游纤毛虫群落结构在季节间没有显著差异, 在空间上具有显著差异; 纤毛虫在纲级水平上类群间的相互作用以协同作用为主导, 不同类群间存在复杂的相互作用, 整体上互作关系在春季较为复杂、夏季较为简单; 影响拉萨河中下游纤毛虫群落结构是多个环境因子共同作用的结果。     

How competitive intransitivity and niche overlap affect spatial coexistence

Competitive intransitivity is mostly considered outside the main body of coexistence theories that rely primarily on the role of niche overlap and differentiation. How the interplay of competitive intransitivity and niche overlap jointly affects species coexistence has received little attention. Here, we consider a rock–paper–scissors competition system where interactions between species can represent the full spectra of transitive–intransitive continuum and niche overlap/differentiation under different levels of competition asymmetry. By comparing results from pair approximation that only considers interference competition between neighbouring cells in spatial lattices, with those under the mean-field assumption, we show that 1) species coexistence under transitive competition is only possible at high niche differentiation; 2) in communities with partial or pure intransitive interactions, high levels of niche overlap are not necessary to beget species extinction; and 3) strong spatial clustering can widen the condition for intransitive loops to facilitate species coexistence. The two mechanisms, competitive intransitivity and niche differentiation, can support species persistence and coexistence, either separately or in combination. Finally, the contribution of intransitive loops to species coexistence can be enhanced by strong local spatial correlations, modulated and maximised by moderate competition asymmetry. Our study, therefore, provides a bridge to link intransitive competition to other generic ecological theories of species coexistence.     

Spatial dynamics in model plant communities: what do we really know?

A variety of models have shown that spatial dynamics and small-scale endogenous heterogeneity (e.g., forest gaps or local resource depletion zones) can change the rate and outcome of competition in communities of plants or other sessile organisms. However, the theory appears complicated and hard to connect to real systems. We synthesize results from three different kinds of models: interacting particle systems, moment equations for spatial point processes, and metapopulation or patch models. Studies using all three frameworks agree that spatial dynamics need not enhance coexistence nor slow down dynamics; their effects depend on the underlying competitive interactions in the community. When similar species would coexist in a nonspatial habitat, endogenous spatial structure inhibits coexistence and slows dynamics. When a dominant species disperses poorly and the weaker species has higher fecundity or better dispersal, competition-colonization trade-offs enhance coexistence. Even when species have equal dispersal and per-generation fecundity, spatial successional niches where the weaker and faster-growing species can rapidly exploit ephemeral local resources can enhance coexistence. When interspecific competition is strong, spatial dynamics reduce founder control at large scales and short dispersal becomes advantageous. We describe a series of empirical tests to detect and distinguish among the suggested scenarios.     

Resource selection by tropical frugivorous birds: integrating multiple interactions

Summary Resource selection is a function of interactions of organisms (competition, predation) as well as characteristics of the resource and organisms. I provide a quantitative model that integrates these factors. I use the model to predict profitability of fruits to tropical birds, but the model and its predictions are applicable to a wider array of systems and organisms. Profitability of a fruit is determined by rewards provided by the pericarp (mass and caloric yields) relative to costs (metabolic requirements, handling time, search time, behavioral interference, predator avoidance) associated with finding and eating that fruit (Fig. 1). Fruits increase in profitability with increases in fruit size until increases in handling time offset increases in pericarp mass. The fruit size at which increases in handling time offset increases in pericarp mass varies among bird species due to differences in bill and body size. Decreases in feeding rate due to decreasing numbers of fruits and increasing search time causes reduced profitability and this effect becomes more severe with decreasing fruit size and/or increasing frugivore size. Consequently, as fruit size decreases relative to frugivore size, fruit abundance becomes increasingly important to fruit selection by frugivores. However, while profitability of resources is a function of characteristics of the resources and the organisms, biological interactions can change profitability rankings; resources that may be more profitable in the absence of behavioral interference, exploitation competition, or predation risk can become less profitable in the face of these interactions. The proposed model integrates these interactions to provide predictions of resource selection and these predictions are supported by published studies.     

The ectomycorrhizal community structure in high mountain Norway spruce stands

The species composition of ectomycorrhizal (ECM) fungal communities can be strongly influenced by abiotic and biotic factors, which determine interactions among the species such as resource partitioning, disturbance, competition, or relationships with other organisms. To verify whether ectomycorrhization of the root tips and composition of the ECM community in Norway spruce vary according to site features and if ECM species peculiar to these environmental variables can be detected, ten comparable stands differing in bedrock pH and exposure were selected and studied. The results demonstrated that tips vitality and ectomycorrhization degree do not change significantly either on the same tree, or among trees growing in the same stand, whereas they differ greatly with bedrock pH and exposure, even if no spatial or temporal trend were found. ECM species composition revealed instead a significant connection with the two environmental features, with a few species significantly associated to them. The results suggest that pH/exposure patterns play a primary role in the adaptive selection of ECM species constituting the consortium.     

Are Trade-Offs Among Species’ Ecological Interactions Scale Dependent? A Test Using Pitcher-Plant Inquiline Species

Trade-offs among species' ecological interactions is a pervasive explanation for species coexistence. The traits associated with trade-offs are typically measured to mechanistically explain species coexistence at a single spatial scale. However, species potentially interact at multiple scales and this may be reflected in the traits among coexisting species. I quantified species' ecological traits associated with the trade-offs expected at both local (competitive ability and predator tolerance) and regional (competitive ability and colonization rate) community scales. The most common species (four protozoa and a rotifer) from the middle trophic level of a pitcher plant (Sarracenia purpurea) inquiline community were used to link species traits to previously observed patterns of species diversity and abundance. Traits associated with trade-offs (competitive ability, predator tolerance, and colonization rate) and other ecological traits (size, growth rate, and carrying capacity) were measured for each of the focal species. Traits were correlated with one another with a negative relationship indicative of a trade-off. Protozoan and rotifer species exhibited a negative relationship between competitive ability and predator tolerance, indicative of coexistence at the local community scale. There was no relationship between competitive ability and colonization rate. Size, growth rate, and carrying capacity were correlated with each other and the trade-off traits: Size was related to both competitive ability and predator tolerance, but growth rate and carrying capacity were correlated with predator tolerance. When partial correlations were conducted controlling for size, growth rate and carrying capacity, the trade-offs largely disappeared. These results imply that body size is the trait that provides the basis for ecological interactions and trade-offs. Altogether, this study showed that the examination of species' traits in the context of coexistence at different scales can contribute to our understanding of the mechanisms underlying community structure.     

Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms

The impact of exotic species on native organisms is widely acknowledged, but poorly understood. Very few studies have empirically investigated how invading plants may alter delicate ecological interactions among resident species in the invaded range. We present novel evidence that antifungal phytochemistry of the invasive plant, Alliaria petiolata, a European invader of North American forests, suppresses native plant growth by disrupting mutualistic associations between native canopy tree seedlings and belowground arbuscular mycorrhizal fungi. Our results elucidate an indirect mechanism by which invasive plants can impact native flora, and may help explain how this plant successfully invades relatively undisturbed forest habitat.     

Predicting rates of interspecific interaction from phylogenetic trees

Integrating phylogenetic information can potentially improve our ability to explain species' traits, patterns of community assembly, the network structure of communities, and ecosystem function. In this study, we use mathematical models to explore the ecological and evolutionary factors that modulate the explanatory power of phylogenetic information for communities of species that interact within a single trophic level. We find that phylogenetic relationships among species can influence trait evolution and rates of interaction among species, but only under particular models of species interaction. For example, when interactions within communities are mediated by a mechanism of phenotype matching, phylogenetic trees make specific predictions about trait evolution and rates of interaction. In contrast, if interactions within a community depend on a mechanism of phenotype differences, phylogenetic information has little, if any, predictive power for trait evolution and interaction rate. Together, these results make clear and testable predictions for when and how evolutionary history is expected to influence contemporary rates of species interaction.     

Complexity of multitrophic interactions in a grassland ecosystem depends on plant species diversity

1.?We studied the theoretical prediction that a loss of plant species richness has a strong impact on community interactions among all trophic levels and tested whether decreased plant species diversity results in a less complex structure and reduced interactions in ecological networks. 2.?Using plant species-specific biomass and arthropod abundance data from experimental grassland plots (Jena Experiment), we constructed multitrophic functional group interaction webs to compare communities based on 4 and 16 plant species. 427 insect and spider species were classified into 13 functional groups. These functional groups represent the nodes of ecological networks. Direct and indirect interactions among them were assessed using partial Mantel tests. Interaction web complexity was quantified using three measures of network structure: connectance, interaction diversity and interaction strength. 3.?Compared with high plant diversity plots, interaction webs based on low plant diversity plots showed reduced complexity in terms of total connectance, interaction diversity and mean interaction strength. Plant diversity effects obviously cascade up the food web and modify interactions across all trophic levels. The strongest effects occurred in interactions between adjacent trophic levels (i.e. predominantly trophic interactions), while significant interactions among plant and carnivore functional groups, as well as horizontal interactions (i.e. interactions between functional groups of the same trophic level), showed rather inconsistent responses and were generally rarer. 4.?Reduced interaction diversity has the potential to decrease and destabilize ecosystem processes. Therefore, we conclude that the loss of basal producer species leads to more simple structured, less and more loosely connected species assemblages, which in turn are very likely to decrease ecosystem functioning, community robustness and tolerance to disturbance. Our results suggest that the functioning of the entire ecological community is critically linked to the diversity of its component plants species.