Uncovering trophic positions and food resources of soil animals using bulk natural stable isotope composition

Despite the major importance of soil biota in nutrient and energy fluxes, interactions in soil food webs are poorly understood. Here we provide an overview of recent advances in uncovering the trophic structure of soil food webs using natural variations in stable isotope ratios. We discuss approaches of application, normalization and interpretation of stable isotope ratios along with methodological pitfalls. Analysis of published data from temperate forest ecosystems is used to outline emerging concepts and perspectives in soil food web research. In contrast to aboveground and aquatic food webs, trophic fractionation at the basal level of detrital food webs is large for carbon and small for nitrogen stable isotopes. Virtually all soil animals are enriched in ¹³C as compared to plant litter. This ‘detrital shift’ likely reflects preferential uptake of ¹³C‐enriched microbial biomass and underlines the importance of microorganisms, in contrast to dead plant material, as a major food resource for the soil animal community. Soil organic matter is enriched in ¹⁵N and ¹³C relative to leaf litter. Decomposers inhabiting mineral soil layers therefore might be enriched in ¹⁵N resulting in overlap in isotope ratios between soil‐dwelling detritivores and litter‐dwelling predators. By contrast, ¹³C content varies little between detritivores in upper litter and in mineral soil, suggesting that they rely on similar basal resources, i.e. little decomposed organic matter. Comparing vertical isotope gradients in animals and in basal resources can be a valuable tool to assess trophic interactions and dynamics of organic matter in soil. As indicated by stable isotope composition, direct feeding on living plant material as well as on mycorrhizal fungi is likely rare among soil invertebrates. Plant carbon is taken up predominantly by saprotrophic microorganisms and channelled to higher trophic levels of the soil food web. However, feeding on photoautotrophic microorganisms and non‐vascular plants may play an important role in fuelling soil food webs. The trophic niche of most high‐rank animal taxa spans at least two trophic levels, implying the use of a wide range of resources. Therefore, to identify trophic species and links in food webs, low‐rank taxonomic identification is required. Despite overlap in feeding strategies, stable isotope composition of the high‐rank taxonomic groups reflects differences in trophic level and in the use of basal resources. Different taxonomic groups of predators and decomposers are likely linked to different pools of organic matter in soil, suggesting different functional roles and indicating that trophic niches in soil animal communities are phylogenetically structured. During last two decades studies using stable isotope analysis have elucidated the trophic structure of soil communities, clarified basal food resources of the soil food web and revealed links between above‐ and belowground ecosystem compartments. Extending the use of stable isotope analysis to a wider range of soil‐dwelling organisms, including microfauna, and a larger array of ecosystems provides the perspective of a comprehensive understanding of the structure and functioning of soil food webs.     

Catchment land cover influences macroinvertebrate food‐web structure and energy flow pathways in mountain streams

相似文献   

Trickle-down effects of aboveground trophic cascades on the soil food web

Trophic cascades are increasingly being regarded as important features of aboveground and belowground food webs, but the effects of aboveground cascades on soil food webs, and vice versa, remains essentially unexplored. We conducted an experiment consisting of model synthesised communities containing grassland plant and invertebrate species, in which treatments included soil only, soil+plants, soil+plants+aphids, and soil+plants+aphids+predators; predator treatments consisted of the lacewing Micromus tasmaniae and ladybird beetle Coccinella undecimpunctata added either singly or in combination. Addition of Micromus largely reversed the negative effects of aphids on plant biomass, while both of the predator species caused large changes in the relative abundances of dominant plant species. Predators of aphids also affected several components of the belowground subsystem. Micromus had positive indirect effects on the primary consumer of the soil decomposer food web (microflora), probably through promoting greater input of basal resources to the decomposer subsystem. Predator treatments also influenced densities of the tertiary consumers of the soil food web (top predatory nematodes), most likely through inducing changes in plant community composition and therefore the quality of resource input to the soil. The secondary consumers of the soil food web (microbe‐feeding nematodes) were, however, unresponsive. The fact that some trophic levels of the soil food web but not others responded to aboveground manipulations is explicable in terms of top‐down and bottom‐up forces differentially regulating different belowground trophic levels. Addition of aphids also influenced microbial community structure, promoted soil bacteria at the expense of fungi, and enhanced the diversity of herbivorous nematodes; in all cases these effects were at least partially reversed by addition of Micromus. These results in tandem point to upper level consumers in aboveground food webs as a potential driver of the belowground subsystem, and provide evidence that predator‐induced trophic cascades aboveground can have effects that trickle through soil food webs.     

Communities at the extreme: Aquatic food webs in desert landscapes

Studying food webs across contrasting abiotic conditions is an important tool in understanding how environmental variability impacts community structure and ecosystem dynamics. The study of extreme environments provides insight into community‐wide level responses to environmental pressures with relevance to the future management of aquatic ecosystems. In the western Lake Eyre Basin of arid Australia, there are two characteristic and contrasting aquatic habitats: springs and rivers. Permanent isolated Great Artesian Basin springs represent hydrologically persistent environments in an arid desert landscape. In contrast, hydrologically variable river waterholes are ephemeral in space and time. We comprehensively sampled aquatic assemblages in contrasting ecosystem types to assess patterns in community composition and to quantify food web attributes with stable isotopes. Springs and rivers were found to have markedly different invertebrate communities, with rivers dominated by more dispersive species and springs associated with species that show high local endemism. Qualitative assessment of basal resources shows autochthonous carbon appears to be a key basal resource in both types of habitat, although the particular sources differed between habitats. Food‐web variables such as trophic length, trophic breadth, and community isotopic niche size were relatively similar in the two habitat types. The basis for the similarity in food‐web structure despite differences in community composition appears to be broader isotopic niches for predatory invertebrates and fish in springs as compared with rivers. In contrast to published theory, our findings suggest that the food webs of the hydrologically variable river sites may show less dietary generalization and more compact food‐web modules than in springs.     

Intensive agriculture reduces soil biodiversity across Europe

Soil biodiversity plays a key role in regulating the processes that underpin the delivery of ecosystem goods and services in terrestrial ecosystems. Agricultural intensification is known to change the diversity of individual groups of soil biota, but less is known about how intensification affects biodiversity of the soil food web as a whole, and whether or not these effects may be generalized across regions. We examined biodiversity in soil food webs from grasslands, extensive, and intensive rotations in four agricultural regions across Europe: in Sweden, the UK, the Czech Republic and Greece. Effects of land‐use intensity were quantified based on structure and diversity among functional groups in the soil food web, as well as on community‐weighted mean body mass of soil fauna. We also elucidate land‐use intensity effects on diversity of taxonomic units within taxonomic groups of soil fauna. We found that between regions soil food web diversity measures were variable, but that increasing land‐use intensity caused highly consistent responses. In particular, land‐use intensification reduced the complexity in the soil food webs, as well as the community‐weighted mean body mass of soil fauna. In all regions across Europe, species richness of earthworms, Collembolans, and oribatid mites was negatively affected by increased land‐use intensity. The taxonomic distinctness, which is a measure of taxonomic relatedness of species in a community that is independent of species richness, was also reduced by land‐use intensification. We conclude that intensive agriculture reduces soil biodiversity, making soil food webs less diverse and composed of smaller bodied organisms. Land‐use intensification results in fewer functional groups of soil biota with fewer and taxonomically more closely related species. We discuss how these changes in soil biodiversity due to land‐use intensification may threaten the functioning of soil in agricultural production systems.     

Highly resolved early Eocene food webs show development of modern trophic structure after the end-Cretaceous extinction

Generalities of food web structure have been identified for extant ecosystems. However, the trophic organization of ancient ecosystems is unresolved, as prior studies of fossil webs have been limited by low-resolution, high-uncertainty data. We compiled highly resolved, well-documented feeding interaction data for 700 taxa from the 48 million-year-old latest early Eocene Messel Shale, which contains a species assemblage that developed after an interval of protracted environmental and biotal change during and following the end-Cretaceous extinction. We compared the network structure of Messel lake and forest food webs to extant webs using analyses that account for scale dependence of structure with diversity and complexity. The Messel lake web, with 94 taxa, displays unambiguous similarities in structure to extant webs. While the Messel forest web, with 630 taxa, displays differences compared to extant webs, they appear to result from high diversity and resolution of insect–plant interactions, rather than substantive differences in structure. The evidence presented here suggests that modern trophic organization developed along with the modern Messel biota during an 18 Myr interval of dramatic post-extinction change. Our study also has methodological implications, as the Messel forest web analysis highlights limitations of current food web data and models.     

Linking soil food web structure to above‐ and belowground ecosystem processes: a meta‐analysis

Soil fauna can be an important regulator of community parameters and ecosystem processes, but there have been few quantitative syntheses of the role of soil fauna in terrestrial soil communities and ecosystems. Here, we conducted a meta‐analysis to investigate the impacts of invertebrate soil micro‐ and mesofauna (grazers and predators) on plant productivity and microbial biomass. Overall our results indicate that an increase in the biomass of soil fauna increased aboveground plant productivity across ecosystems by 35% and decreased microbial biomass by 8%. In addition, we found no evidence for trophic cascades in terrestrial soil food webs, but the bacterivorous component of soil fauna influenced plant productivity and microbial biomass more than did the fungivorous component. Furthermore, changes in the biomass of soil fauna differentially affected plant productivity among plant functional groups: a higher biomass of soil fauna increased aboveground productivity by 70% in coniferous systems. However, in ecosystems dominated by legumes, a functional group with lower inorganic nitrogen requirements, there was no response of aboveground productivity to increases in the biomass of soil fauna. In sum, the results of this meta‐analysis indicate that soil fauna help to regulate ecosystem production, especially in nutrient‐limited ecosystems.     

Body sizes,cumulative and allometric degree distributions across natural food webs

The distributions of body masses and degrees (i.e. the number of trophic links) across species are key determinants of food‐web structure and dynamics. In particular, allometric degree distributions combining both aspects in the relationship between degrees and body masses are of critical importance for the stability of these complex ecological networks. They describe decreases in vulnerability (i.e. the number of predators) and increases in generality (i.e. the number of prey) with increasing species’ body masses. We used an entirely new global body‐mass database containing 94 food webs from four different ecosystem types (17 terrestrial, 7 marine, 54 lake, 16 stream ecosystems) to analyze (1) body mass distributions, (2) cumulative degree distributions (vulnerability, generality, linkedness), and (3) allometric degree distributions (e.g. generality – body mass relationships) for significant differences among ecosystem types. Our results demonstrate some general patterns across ecosystems: (1) the body masses are often roughly log‐normally (terrestrial and stream ecosystems) or multi‐modally (lake and marine ecosystems) distributed, and (2) most networks exhibit exponential cumulative degree distributions except stream networks that most often possess uniform degree distributions. Additionally, with increasing species body masses we found significant decreases in vulnerability in 70% of the food webs and significant increases in generality in 80% of the food webs. Surprisingly, the slopes of these allometric degree distributions were roughly three times steeper in streams than in the other ecosystem types, which implies that streams exhibit a more pronounced body mass structure. Overall, our analyses documented some striking generalities in the body‐mass (allometric degree distributions of generality and vulnerability) and degree structure (exponential degree distributions) across ecosystem types as well as surprising exceptions (uniform degree distributions in stream ecosystems). This suggests general constraints of body masses on the link structure of natural food webs irrespective of ecosystem characteristics.     

Aboveground mammal and invertebrate exclusions cause consistent changes in soil food webs of two subalpine grassland types,but mechanisms are system‐specific

Ungulates, smaller mammals, and invertebrates can each affect soil biota through their influence on vegetation and soil characteristics. However, direct and indirect effects of the aboveground biota on soil food webs remain to be unraveled. We assessed effects of progressively excluding aboveground large‐, medium‐ and small‐sized mammals as well as invertebrates on soil nematode diversity and feeding type abundances in two subalpine grassland types: short‐ and tall‐grass vegetation. We explored pathways that link exclusions of aboveground biota to nematode feeding type abundances via changes in plants, soil environment, soil microbial biomass, and soil nutrients. In both vegetation types, exclusions caused a similar shift toward higher abundance of all nematode feeding types, except plant feeders, lower Shannon diversity, and lower evenness. These effects were strongest when small mammals, or both small mammals and invertebrates were excluded in addition to excluding larger mammals. Exclusions resulted in a changed abiotic soil environment that only affected nematodes in the short‐grass vegetation. In each vegetation type, exclusion effects on nematode abundances were mediated by different drivers related to plant quantity and quality. In the short‐grass vegetation, not all exclusion effects on omni–carnivorous nematodes were mediated by the abundance of lower trophic level nematodes, suggesting that omni–carnivores also depended on other prey than nematodes. We conclude that small aboveground herbivores have major impacts on the soil food web of subalpine short‐ and tall‐grass ecosystems. Excluding aboveground animals caused similar shifts in soil nematode assemblages in both subalpine vegetation types, however, mechanisms turned out to be system‐specific.     

Species traits as drivers of food web structure

The use of functional traits to describe community structure is a promising approach to reveal generalities across organisms and ecosystems. Plant ecologists have demonstrated the importance of traits in explaining community structure, competitive interactions as well as ecosystem functioning. The application of trait‐based methods to more complex communities such as food webs is however more challenging owing to the diversity of animal characteristics and of interactions. The objective of this study was to determine how functional structure is related to food web structure. We consider that food web structure is the result of 1) the match between consumer and resource traits, which determine the occurence of a trophic interaction between them, and 2) the distribution of functional traits in the community. We implemented a statistical approach to assess whether or not 35 466 pairwise interactions between soil organisms are constrained by trait‐matching and then used a Procrustes analysis to investigate correlations between functional indices and network properties across 48 sites. We found that the occurrence of trophic interactions is well predicted by matching the traits of the resource with those of the consumer. Taxonomy and body mass of both species were the most important traits for the determination of an interaction. As a consequence, functional evenness and the variance of certain traits in the community were correlated to trophic complementarity between species, while trait identity, more than diversity, was related to network topology. The analysis was however limited by trait data availability, and a coarse resolution of certain taxonomic groups in our dataset. These limitations explain the importance of taxonomy, as well as the complexity of the statistical model needed. Our results outline the important implications of trait composition on ecological networks, opening promising avenues of research into the relationship between functional diversity and ecosystem functioning in multi‐trophic systems.     

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.     

Pyramids of species richness: the determinants and distribution of species diversity across trophic levels

How species richness is distributed across trophic levels determines several dimensions of ecosystem functioning, including herbivory, predation, and decomposition rates. We perform a meta‐analysis of 72 large published food webs to investigate their trophic diversity structure and possible endogenous, exogenous, and methodological causal variables. Consistent with classic theory, we found that published food webs can generally be described as ‘pyramids of species richness’. The food webs were more predator‐poor, prey‐rich and hierarchical than is expected by chance or by the niche or cascade models. The trophic species richness distribution also depended on centrality, latitude, ecosystem‐type and methodological bias. Although trophic diversity structure is generally pyramidal, under many conditions the structure is consistently uniform or inverse‐pyramidal. Our meta‐analysis adds nuance to classic assumptions about food web structure: diversity decreases with trophic level, but not under all conditions, and the decrease may be scale‐dependent. Synthesis The distribution of species richness across trophic levels has not been evaluated in recent decades, despite improvement in food web resolution and the relevance of biodiversity distribution to ecosystem function. Our meta‐analysis of 72 large, recent food webs, illustrates that published food webs can generally be described as basal‐rich, top‐poor ‘pyramids of species richness’, consistent with classic theory. Although trophic diversity structure is generally pyramidal, under some environmental and ecological conditions the structure is uniform or inverse‐pyramidal. Our meta‐analysis confirms classic theory about food web structure, while adding nuance by describing conditions under which classic pyramid structure is not observed.     

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。     

Divergence of feeding channels within the soil food web determined by ecosystem type

Understanding trophic linkages within the soil food web (SFW) is hampered by its opacity, diversity, and limited niche adaptation. We need to expand our insight between the feeding guilds of fauna and not just count biodiversity. The soil fauna drive nutrient cycling and play a pivotal, but little understood role within both the carbon (C) and nitrogen (N) cycles that may be ecosystem dependent. Here, we define the structure of the SFW in two habitats (grassland and woodland) on the same soil type and test the hypothesis that land management would alter the SFW in these habitats. To do this, we census the community structure and use stable isotope analysis to establish the pathway of C and N through each trophic level within the ecosystems. Stable isotope ratios of C and N from all invertebrates were used as a proxy for trophic niche, and community‐wide metrics were obtained. Our empirically derived C/N ratios differed from those previously reported, diverging from model predictions of global C and N cycling, which was unexpected. An assessment of the relative response of the different functional groups to the change from agricultural grassland to woodland was performed. This showed that abundance of herbivores, microbivores, and micropredators were stimulated, while omnivores and macropredators were inhibited in the grassland. Differences between stable isotope ratios and community‐wide metrics, highlighted habitats with similar taxa had different SFWs, using different basal resources, either driven by root or litter derived resources. Overall, we conclude that plant type can act as a top‐down driver of community functioning and that differing land management can impact on the whole SFW.     

Invasive plants have different effects on trophic structure of green and brown food webs in terrestrial ecosystems: a meta‐analysis

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta‐analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.     

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.     

土壤微食物网结构与生态功能

土壤微食物网是碎屑食物网中与土壤生态过程密切相关的一部分,通过取食资源基质直接或间接地参与养分循环过程,影响陆地生态系统功能.本文从土壤微食物网的组成、结构和生态功能等方面综述了近年来土壤微食物网的研究进展.通过对土壤微食物网的能量通道及营养级联效应的介绍,阐述了土壤微食物网在碳(C)、氮(N)转化、凋落物分解和植物生长等方面的重要作用.针对目前的研究现状,提出未来土壤生态学研究应与高通量测序及稳定同位素技术相结合;通过构建模型进一步加强对土壤食物网结构和功能的研究,从而深入揭示地下生态过程及其对地上植物生长的反馈作用机理.     

Predator and detritivore niche width helps to explain biocomplexity of experimental detritus-based food webs in four aquatic and terrestrial ecosystems

In the study of food webs, the existence and explanation of recurring patterns, such as the scale invariance of linkage density, predator–prey ratios and mean chain length, constitute long-standing issues. Our study focused on litter-associated food webs and explored the influence of detritivore and predator niche width (as δ¹³C range) on web topological structure. To compare patterns within and between aquatic and terrestrial ecosystems and take account of intra-habitat variability, we constructed 42 macroinvertebrate patch-scale webs in four different habitats (lake, lagoon, beech forest and cornfield), using an experimental approach with litterbags. The results suggest that although web differences exist between ecosystems, patterns are more similar within than between aquatic and terrestrial web types. In accordance with optimal foraging theory, we found that the niche width of predators and prey increased with the number of predators and prey taxa as a proportion of total taxa in the community. The tendency was more marked in terrestrial ecosystems and can be explained by a lower per capita food level than in aquatic ecosystems, particularly evident for predators. In accordance with these results, the number of links increased with the number of species but with a significantly sharper regression slope for terrestrial ecosystems. As a consequence, linkage density, which was found to be directly correlated to niche width, increased with the total number of species in terrestrial webs, whereas it did not change significantly in aquatic ones, where connectance scaled negatively with the total number of species. In both types of ecosystem, web robustness to rare species removal increased with connectance and the niche width of predators. In conclusion, although limited to litter-associated macroinvertebrate assemblages, this study highlights structural differences and similarities between aquatic and terrestrial detrital webs, providing field evidence of the central role of niche width in determining the structure of detritus-based food webs and posing foraging optimisation constraints as a general mechanistic explanation of food web complexity differences within and between ecosystem types.     

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.     

Spatial guilds in the Serengeti food web revealed by a Bayesian group model

Food webs, networks of feeding relationships in an ecosystem, provide fundamental insights into mechanisms that determine ecosystem stability and persistence. A standard approach in food-web analysis, and network analysis in general, has been to identify compartments, or modules, defined by many links within compartments and few links between them. This approach can identify large habitat boundaries in the network but may fail to identify other important structures. Empirical analyses of food webs have been further limited by low-resolution data for primary producers. In this paper, we present a Bayesian computational method for identifying group structure using a flexible definition that can describe both functional trophic roles and standard compartments. We apply this method to a newly compiled plant-mammal food web from the Serengeti ecosystem that includes high taxonomic resolution at the plant level, allowing a simultaneous examination of the signature of both habitat and trophic roles in network structure. We find that groups at the plant level reflect habitat structure, coupled at higher trophic levels by groups of herbivores, which are in turn coupled by carnivore groups. Thus the group structure of the Serengeti web represents a mixture of trophic guild structure and spatial pattern, in contrast to the standard compartments typically identified. The network topology supports recent ideas on spatial coupling and energy channels in ecosystems that have been proposed as important for persistence. Furthermore, our Bayesian approach provides a powerful, flexible framework for the study of network structure, and we believe it will prove instrumental in a variety of biological contexts.