Species richness and trophic diversity increase decomposition in a co-evolved food web

Ecological communities show great variation in species richness, composition and food web structure across similar and diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the maintenance of diversity and the potential effects of species losses and gains on ecosystems. While research often focuses on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can provide further insight into the relationship between diversity and ecosystem functioning and elucidate potential mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity affect decomposition rates in a complete aquatic food web: the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea. We identified a trophic cascade in which top-predators--larvae of the pitcher-plant mosquito--indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occur in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of trophic diversity and species interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species richness does not give a complete picture as to how ecosystems may change with the loss or gain of species.     

Integrating ecosystem engineering and food webs

Ecosystem engineering, the physical modification of the environment by organisms, is a common and often influential process whose significance to food web structure and dynamics is largely unknown. In the light of recent calls to expand food web studies to include non‐trophic interactions, we explore how we might best integrate ecosystem engineering and food webs. We provide rationales justifying their integration and present a provisional framework identifying how ecosystem engineering can affect the nodes and links of food webs and overall organization; how trophic interactions with the engineer can affect the engineering; and how feedbacks between engineering and trophic interactions can affect food web structure and dynamics. We use a simple integrative food chain model to illustrate how feedbacks between the engineer and the food web can alter 1) engineering effects on food web dynamics, and 2) food web responses to extrinsic environmental perturbations. We identify four general challenges to integration that we argue can readily be met, and call for studies that can achieve this integration and help pave the way to a more general understanding of interaction webs in nature. Synthesis All species are affected by their physical environment. Because ecosystem engineering species modify the physical environment and belong to food webs, such species are potentially one of the most important bridges between the trophic and non‐trophic. We examine how to integrate the so far, largely independent research areas of ecosystem engineering and food webs. We present a conceptual framework for understanding how engineering can affect food webs and vice versa, and how feedbacks between the two alter ecosystem dynamics. With appropriate empirical studies and models, integration is achievable, paving the way to a more general understanding of interaction webs in nature.     

Forecasting fine‐scale changes in the food‐web structure of coastal marine communities under climate change

Climate change is inducing deep modifications in local communities worldwide as a consequence of individualistic species range shifts. Understanding how complex interaction networks will be reorganized under climate change represents a major challenge in the fields of ecology and biogeography. However, forecasting the potential effects of climate change on local communities, and more particularly on food‐web structure, requires the consideration of highly structuring processes, such as trophic interactions. A major breakthrough is therefore expected by combining predictive models integrating habitat selection processes, the physiological limits of marine species and their trophic interactions. In this study, we forecasted the potential impacts of climate change on the local food‐web structure of the highly threatened Gulf of Gabes ecosystem located in the south of the Mediterranean Sea. We coupled the climatic envelope and habitat models to an allometric niche food web model, hence taking into account the different processes acting at regional (climate) and local scales (habitat selection and trophic interactions). Our projections under the A2 climate change scenario showed that future food webs would be composed of smaller species with fewer links, resulting in a decrease of connectance, generality, vulnerability and mean trophic level of communities and an increase of the average path length, which may have large consequences on ecosystem functioning. The unified framework presented here, by connecting food‐web ecology, biogeography and seascape ecology, allows the exploration of spatial aspects of interspecific interactions under climate change and improves our current understanding of climate change impacts on local marine food webs.     

Scaling from individuals to networks in food webs

1.  Food webs, the set of predator–prey interactions in an ecosystem, are a prototypical complex system. Much research to date has concentrated on the use of models to identify and explain the key structural features which characterize food webs.
2.  These models often fall into two general categories: (i) phenomenological models which are built upon a set of heuristic rules in order to explain some empirical observation and (ii) population-level models in which interactions between individuals result in emergent properties for the food web. Both types of models have helped to uncover how food-web structure is a product of factors such as foraging behaviour, prey selection and species' body sizes.
3.  Historically, the two types of models have followed rather different approaches to the problem. Despite the apparent differences, the overlap between the two styles of models is substantial. Examples are highlighted here.
4.  By paying greater attention to both the similarities and differences between the two, we will be better able to demonstrate the ecological insights offered by phenomenological models. This will help us, for example, design experiments which could validate or refute underlying assumptions of the models. By linking models to data, scaling from individuals to networks, we will be closer to understanding the true origins of food-web structure.     

Species loss and the structure and functioning of multitrophic aquatic systems

Experiments and theory in single trophic level systems dominate biodiversity and ecosystem functioning research and recent debates. All natural ecosystems contain communities with multiple trophic levels, however, and this can have important effects on ecosystem structure and functioning. Furthermore, many experiments compare assembled communities, rather than examining loss of species directly. We identify three questions around which to organise an investigation of how species loss affects the structure and functioning of multitrophic systems. 1) What is the distribution of species richness among trophic levels; 2) from which trophic levels are species most often lost; and 3) does loss of species from different trophic levels influence ecosystem functioning differently? Our analyses show that: 1) Relatively few high‐quality data are available concerning the distribution of species richness among trophic levels. A new data‐set provides evidence of a decrease in species richness as trophic height increases. 2) Multiple lines of evidence indicate that species are lost from higher trophic levels more frequently than lower trophic levels. 3) A theoretical model suggests that both the structure of food webs (occurrence of omnivory and the distribution of species richness among trophic levels) and the trophic level from which species are lost determines the impact of species loss on ecosystem functioning, which can even vary in the sign of the effect. These results indicate that, at least for aquatic systems, models of single trophic level ecosystems are insufficient for understanding the functional consequences of extinctions. Knowledge is required of food web structure, which species are likely to be lost, and also whether cascading extinctions will occur.     

Defining and measuring trophic role similarity in food webs using regular equivalence

We present a graph theoretic model of analysing food web structure called regular equivalence. Regular equivalence is a method for partitioning the species in a food web into "isotrophic classes" that play the same structural roles, even if they are not directly consuming the same prey or if they do not share the same predators. We contrast regular equivalence models, in which two species are members of the same trophic group if they have trophic links to the same set of other trophic groups, with structural equivalence models, in which species are equivalent if they are connected to the exact same other species. Here, the regular equivalence approach is applied to two published food webs: (1) a topological web (Malaysian pitcher plant insect food web) and (2) a carbon-flow web (St. Marks, Florida seagrass ecosystem food web). Regular equivalence produced a more satisfactory set of classes than did the structural approach, grouping basal taxa with other basal taxa and not with top predators. Regular equivalence models provide a way to mathematically formalize trophic position, trophic group and trophic niche. These models are part of a family of models that includes structural models used extensively by ecologists now. Regular equivalence models uncover similarities in trophic roles at a higher level of organization than do the structural models. The approach outlined is useful for measuring the trophic roles of species in food web models, measuring similarity in trophic relations of two or more species, comparing food webs over time and across geographic regions, and aggregating taxa into trophic groups that reduce the complexity of ecosystem feeding relations without obscuring network relationships. In addition, we hope the approach will prove useful in predicting the outcome of predator-prey interactions in experimental studies.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

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Interplay between the paradox of enrichment and nutrient cycling in food webs

Nutrient cycling is fundamental to ecosystem functioning. Despite recent major advances in the understanding of complex food web dynamics, food web models have so far generally ignored nutrient cycling. However, nutrient cycling is expected to strongly impact food web stability and functioning. To make up for this gap, we built an allometric and size structured food web model including nutrient cycling. By releasing mineral nutrients, recycling increases the availability of limiting resources for primary producers and links each trophic level to the bottom of food webs. We found that nutrient cycling can provide a significant part of the total nutrient supply of the food web, leading to a strong enrichment effect that promotes species persistence in nutrient poor ecosystems but leads to a paradox of enrichment at high nutrient inputs. The presence of recycling loops linking each trophic level to the basal resources weakly affects species biomass temporal variability in the food web. Recycling loops tend to slightly dampen the destabilising effect of nutrient enrichment on consumer temporal variability while they have opposite effects for primary producers. By considering nutrient cycling, this new model improves our understanding of the response of food webs to nutrient availability and opens perspectives to better link studies on food web dynamics and ecosystem functioning.     

Climate change undermines the global functioning of marine food webs

Sea water temperature affects all biological and ecological processes that ultimately impact ecosystem functioning. In this study, we examine the influence of temperature on global biomass transfers from marine secondary production to fish stocks. By combining fisheries catches in all coastal ocean areas and life‐history traits of exploited marine species, we provide global estimates of two trophic transfer parameters which determine biomass flows in coastal marine food web: the trophic transfer efficiency (TTE) and the biomass residence time (BRT) in the food web. We find that biomass transfers in tropical ecosystems are less efficient and faster than in areas with cooler waters. In contrast, biomass transfers through the food web became faster and more efficient between 1950 and 2010. Using simulated changes in sea water temperature from three Earth system models, we project that the mean TTE in coastal waters would decrease from 7.7% to 7.2% between 2010 and 2100 under the ‘no effective mitigation’ representative concentration pathway (RCP8.5), while BRT between trophic levels 2 and 4 is projected to decrease from 2.7 to 2.3 years on average. Beyond the global trends, we show that the TTEs and BRTs may vary substantially among ecosystem types and that the polar ecosystems may be the most impacted ecosystems. The detected and projected changes in mean TTE and BRT will undermine food web functioning. Our study provides quantitative understanding of temperature effects on trophodynamic of marine ecosystems under climate change.     

Unravelling the complex structure of forest soil food webs: higher omnivory and more trophic levels

Food web topologies depict the community structure as distributions of feeding interactions across populations. Although the soil ecosystem provides important functions for aboveground ecosystems, data on complex soil food webs is notoriously scarce, most likely due to the difficulty of sampling and characterizing the system. To fill this gap we assembled the complex food webs of 48 forest soil communities. The food webs comprise 89 to 168 taxa and 729 to 3344 feeding interactions. The feeding links were established by combining several molecular methods (stable isotope, fatty acid and molecular gut content analyses) with feeding trials and literature data. First, we addressed whether soil food webs (n = 48) differ significantly from those of other ecosystem types (aquatic and terrestrial aboveground, n = 77) by comparing 22 food web parameters. We found that our soil food webs are characterized by many omnivorous and cannibalistic species, more trophic chains and intraguild‐predation motifs than other food webs and high average and maximum trophic levels. Despite this, we also found that soil food webs have a similar connectance as other ecosystems, but interestingly a higher link density and clustering coefficient. These differences in network structure to other ecosystem types may be a result of ecosystem specific constraints on hunting and feeding characteristics of the species that emerge as network parameters at the food‐web level. In a second analysis of land‐use effects, we found significant but only small differences of soil food web structure between different beech and coniferous forest types, which may be explained by generally strong selection effects of the soil that are independent of human land use. Overall, our study has unravelled some systematic structures of soil food‐webs, which extends our mechanistic understanding how environmental characteristics of the soil ecosystem determine patterns at the community level.     

A bioenergetic framework for aboveground terrestrial food webs

Bioenergetic approaches have been greatly influential for understanding community functioning and stability and predicting effects of environmental changes on biodiversity. These approaches use allometric relationships to establish species’ trophic interactions and consumption rates and have been successfully applied to aquatic ecosystems. Terrestrial ecosystems, where body mass is less predictive of plant–consumer interactions, present inherent challenges that these models have yet to meet. Here, we discuss the processes governing terrestrial plant–consumer interactions and develop a bioenergetic framework integrating those processes. Our framework integrates bioenergetics specific to terrestrial plants and their consumers within a food web approach while also considering mutualistic interactions. Such a framework is poised to advance our understanding of terrestrial food webs and to predict their responses to environmental changes.     

Biodiversity and ecosystem functioning in food webs: the vertical diversity hypothesis

One challenge in merging community and ecosystem ecology is to integrate the complexity of natural multitrophic communities into concepts of ecosystem functioning. Here, we combine food‐web and allometry theories to demonstrate that primary production, as measured by the total nutrient uptake of the multitrophic community, is determined by vertical diversity (i.e. food web's maximum trophic level) and structure (i.e. distributions of species and their abundances and metabolic rates across trophic levels). In natural ecosystems, the community size distribution determines all these vertical patterns and thus the total nutrient uptake. Our model suggests a vertical diversity hypothesis (VDH) for ecosystem functioning in complex food webs. It predicts that, under a given nutrient supply, the total nutrient uptake increases exponentially with the maximum trophic level in the food web and it increases with its maximum body size according to a power law. The VDH highlights the effect of top–down regulation on plant nutrient uptake, which complements traditional paradigms that emphasised the bottom–up effect of nutrient supply on vertical diversity. We conclude that the VDH contributes to a synthetic framework for understanding the relationship between vertical diversity and ecosystem functioning in food webs and predicting the impacts of global changes on multitrophic ecosystems.     

稻田节肢动物群落的营养联系

根据田间调查和室内饲养观察的资料,研究了稻田节肢动物群落的营养结构及类型。在稻田生态系统中,物种之间由于取食与被取食、寄生与被寄生、捕食与被捕食的营养联系,形成了复杂的食物链和食物网。依据物种在食物网中的位置和功能,可将福州市郊区稻田节肢动物群落的营养结构分为3种类型:1)食物网中尚未发现有重寄生环节;2)食物网中有重寄生环节;3)食物网中有兼寄生环节。为了探讨定量研究生物群落营养联系的可能性,本文运用图论的知识把食物网的结构描述为标向图、集合或邻接矩阵,同时用图论的运算法则解决了各种类型的食物网的合并问题,为研究复杂群落的营养关系提供了一种新方法。     

Nontrophic interactions, biodiversity, and ecosystem functioning: an interaction web model

Research into the relationship between biodiversity and ecosystem functioning has mainly focused on the effects of species diversity on ecosystem properties in plant communities and, more recently, in food webs. Although there is growing recognition of the significance of nontrophic interactions in ecology, these interactions are still poorly studied theoretically, and their impact on biodiversity and ecosystem functioning is largely unknown. Existing models of mutualism usually consider only one type of species interaction and do not satisfy mass balance constraints. Here, we present a model of an interaction web that includes both trophic and nontrophic interactions and that respects the principle of mass conservation. Nontrophic interactions are represented in the form of interaction modifications. We use this model to study the relationship between biodiversity and ecosystem properties that emerges from the assembly of entire interaction webs. We show that ecosystem properties such as biomass and production depend not only on species diversity but also on species interactions, in particular on the connectance and magnitude of nontrophic interactions, and that the nature, prevalence, and strength of species interactions in turn depend on species diversity. Nontrophic interactions alter the shape of the relationship between biodiversity and biomass and can profoundly influence ecosystem processes.     

Herbivore-induced indirect interaction webs on terrestrial plants: the importance of non-trophic, indirect, and facilitative interactions

One of the most important issues in ecology is understanding the causal mechanisms that shape the structure of ecological communities through trophic interactions. The focus on direct, trophic interactions in much of the research to date means that the potential significance of non-trophic, indirect, and facilitative interactions has been largely ignored in traditional food webs. There is a growing appreciation of the community consequences of such non-trophic effects, and the need to start including them in food web research. This review highlights how non-trophic, indirect, and facilitative interactions play an important role in organizing the structure of plant-centered arthropod communities. I argue that herbivore-induced plant responses, insect ecosystem engineers, and mutualisms involving ant–honeydew-producing insects all generate interaction linkages among insect herbivores, thereby producing complex indirect interaction webs on terrestrial plants. These interactions are all very common and widespread on terrestrial plants, in fact they are almost ubiquitous, but these interactions have rarely been included in traditional food webs. Finally, I will emphasize that because the important community consequences of these non-trophic and indirect interactions have been largely unexplored, it is critical that indirect interaction webs should be the focus of future research.     

Predicting food‐web structure with metacommunity models

Synthesis Metacommunity theory aims to elucidate the relative influence of local and regional‐scale processes in generating diversity patterns across the landscape. Metacommunity research has focused largely on assemblages of competing organisms within a single trophic level. Here, we test the ability of metacommunity models to predict the network structure of the aquatic food web found in the leaves of the northern pitcher plant Sarracenia purpurea. The species‐sorting and patch‐dynamics models most accurately reproduced nine food web properties, suggesting that local‐scale interactions play an important role in structuring Sarracenia food webs. Our approach can be applied to any well‐resolved food web for which data are available from multiple locations. The metacommunity framework explores the relative influence of local and regional‐scale processes in generating diversity patterns across the landscape. Metacommunity models and empirical studies have focused mostly on assemblages of competing organisms within a single trophic level. Studies of multi‐trophic metacommunities are predominantly restricted to simplified trophic motifs and rarely consider entire food webs. We tested the ability of the patch‐dynamics, species‐sorting, mass‐effects, and neutral metacommunity models, as well as three hybrid models, to reproduce empirical patterns of food web structure and composition in the complex aquatic food web found in the northern pitcher plant Sarracenia purpurea. We used empirical data to determine regional species pools and estimate dispersal probabilities, simulated local food‐web dynamics, dispersed species from regional pools into local food webs at rates based on the assumptions of each metacommunity model, and tested their relative fits to empirical data on food‐web structure. The species‐sorting and patch‐dynamics models most accurately reproduced nine food web properties, suggesting that local‐scale interactions were important in structuring Sarracenia food webs. However, differences in dispersal abilities were also important in models that accurately reproduced empirical food web properties. Although the models were tested using pitcher‐plant food webs, the approach we have developed can be applied to any well‐resolved food web for which data are available from multiple locations.     

Detritus, trophic dynamics and biodiversity

Traditional approaches to the study of food webs emphasize the transfer of local primary productivity in the form of living plant organic matter across trophic levels. However, dead organic matter, or detritus, a common feature of most ecosystems plays a frequently overlooked role as a dynamic heterogeneous resource and habitat for many species. We develop an integrative framework for understanding the impact of detritus that emphasizes the ontogeny and heterogeneity of detritus and the various ways that explicit inclusion of detrital dynamics alters generalizations about the structure and functioning of food webs. Through its influences on food web composition and dynamics, detritus often increases system stability and persistence, having substantial effects on trophic structure and biodiversity. Inclusion of detrital heterogeneity in models of food web dynamics is an essential new direction for ecological research.     

Energetics of microbial food webs

The energetic demand of microorganisms in natural waters and the flux of energy between microorganisms and metazoans has been evaluated by empirical measurements in nature, in microcosms and mesocosms, and by simulation models. Microorganisms in temperate and tropical waters often use half or more of the energy fixed by photosynthesis. Most simulations and some experimental results suggest significant energy transfer to metazoans, but empirical evidence is mixed. Considerations of the range of growth yields of microorganisms and the number of trophic transfers among them indicate major energy losses within microbial food webs. Our ability to verify and quantify these processes is limited by the variability of assimilation efficiency and uncertainty about the structure of microbial food webs. However, even a two-step microbial chain is a major energy sink. As an energetic link to metazoans, the detritus food web is inefficient, and its significance may have been overstated. There is not enough bacterial biomass associated with detritus to support metazoan detritivores. Much detritus is digestible by metazoans directly. Thus, metazoans and bacteria may to a considerable degree compete for a common resource. Microorganisms, together with metazoans, are important to the stability of planktonic communities through their roles as rapid mineralizers of organic matter, releasing inorganic nutrients. The competition for organic matter and the resultant rapid mineralization help maintain stable populations of phytoplankton in the absence of advective nutrient supply. At temperatures near O °C, bacterial metabolism is suppressed more than is the rate of photosynthesis. As a result, the products of the spring phytoplankton bloom in high-temperate latitudes are not utilized rapidly by bacteria. At temperatures below 0°C microbial food webs are neither energy sinks or links: they are suppressed. Because the underlying mechanism of low-temperature inhibition is not known, we cannot yet generalize about this as a control of food web processes. Microorganisms may operate on several trophic levels simultaneously. Therefore, the realism of the trophic level concept and the reality of the use of ecological efficiency calculations in ecosystem models is questionable.     

The form of direct interspecific competition modifies secondary extinction patterns in multi‐trophic food webs

Forcibly removing species from ecosystems has important consequences for the remaining assemblage, leading to changes in community structure, ecosystem functioning and secondary (cascading) extinctions. One key question that has arisen from single‐ and multi‐trophic ecosystem models is whether the secondary extinctions that occur within competitive communities (guilds) are also important in multi‐trophic ecosystems? The loss of consumer–resource links obviously causes secondary extinction of specialist consumers (topological extinctions), but the importance of secondary extinctions in multi‐trophic food webs driven by direct competitive exclusion remains unknown. Here I disentangle the effects of extinctions driven by basal competitive exclusion from those caused by trophic interactions in a multi‐trophic ecosystem (basal producers, intermediate and top consumers). I compared food webs where basal species either show diffuse (all species compete with each other identically: no within guild extinctions following primary extinction) or asymmetric competition (unequal interspecific competition: within guild extinctions are possible). Basal competitive exclusion drives extra extinction cascades across all trophic levels, with the effect amplified in larger ecosystems, though varying connectance has little impact on results. Secondary extinction patterns based on the relative abundance of the species lost in the primary extinction differ qualitatively between diffuse and asymmetric competition. Removing asymmetric basal species with low (high) abundance triggers fewer (more) secondary extinctions throughout the whole food web than removing diffuse basal species. Rare asymmetric competitors experience less pressure from consumers compared to rare diffuse competitors. Simulations revealed that diffuse basal species are never involved in extinction cascades, regardless of the trophic level of a primary extinction, while asymmetric competitors were. This work highlights important qualitative differences in extinction patterns that arise when different assumptions are made about the form of direct competition in multi‐trophic food webs.