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.     

Structure of tropical river food webs revealed by stable isotope ratios

Fish assemblages in tropical river food webs are characterized by high taxonomic diversity, diverse foraging modes, omnivory, and an abundance of detritivores. Feeding links are complex and modified by hydrologic seasonality and system productivity. These properties make it difficult to generalize about feeding relationships and to identify dominant linkages of energy flow. We analyzed the stable carbon and nitrogen isotope ratios of 276 fishes and other food web components living in four Venezuelan rivers that differed in basal food resources to determine 1) whether fish trophic guilds integrated food resources in a predictable fashion, thereby providing similar trophic resolution as individual species, 2) whether food chain length differed with system productivity, and 3) how omnivory and detritivory influenced trophic structure within these food webs. Fishes were grouped into four trophic guilds (herbivores, detritivores/algivores, omnivores, piscivores) based on literature reports and external morphological characteristics. Results of discriminant function analyses showed that isotope data were effective at reclassifying individual fish into their pre-identified trophic category. Nutrient-poor, black-water rivers showed greater compartmentalization in isotope values than more productive rivers, leading to greater reclassification success. In three out of four food webs, omnivores were more often misclassified than other trophic groups, reflecting the diverse food sources they assimilated. When fish δ¹⁵N values were used to estimate species position in the trophic hierarchy, top piscivores in nutrient-poor rivers had higher trophic positions than those in more productive rivers. This was in contrast to our expectation that productive systems would promote longer food chains. Although isotope ratios could not resolve species-level feeding pathways, they did reveal how top consumers integrate isotopic variability occurring lower in the food web. Top piscivores, regardless of species, had carbon and nitrogen profiles less variable than other trophic groups.     

Stable isotope analysis of aquatic invertebrate communities in irrigated rice fields cultivated under different management regimes

In this study we have used stable isotope analysis to identify major food resources driving food webs in commercial rice agroecosystems and to examine the effects of agricultural management practices on the trophic structure of these food webs. Potential carbon sources and aquatic macroinvertebrate consumers were collected from large-scale rice farms in south-eastern Australia cultivated under three different crop management regimes conventional-aerial (agrochemicals applied, aerially sown), conventional-sod (agrochemicals applied, directly sown) and organic-sod (agrochemical-free, directly sown). Evidence from stable isotope analysis demonstrated the importance of food sources, such as biofilm and detritus, as the principal energy sources driving aquatic food webs in rice agroecosystems. Despite the greater diversity of potential food sources collected from the organic-sod regime across all sampling occasions, the range of food resources directly assimilated by macroinvertebrate consumers did not differ substantially across management regimes. Trophic complexity of aquatic food webs, as evidenced by the number of trophic levels identified using δ¹⁵N data, differed across management regimes at the early season sampling. Sites with low or no agrochemical applications contained more than two trophic levels, but at the site with the highest pesticide application no primary or secondary consumers were found. Our data demonstrates that the choice of agricultural management regime has a season-long influence on aquatic food webs in rice crops, and highlights the importance of conserving non-rice food resources that drive these trophic networks.     

Use of compound-specific nitrogen isotope analysis of amino acids in trophic ecology: assumptions,applications, and implications

Knowledge of the diet source and trophic position of organisms is fundamental in food web science. Since the 1980s, stable isotopes of light elements such as ¹³C and ¹⁵N have provided unique information on the food web structure in a variety of ecosystems. More recently, novel isotope tools such as stable isotopes of heavy elements, radioisotopes, and compound-specific isotope analysis, have been examined by researchers. Here I reviewed the use of compound-specific nitrogen isotope analysis of amino acids (CSIA-AA) as a useful dietary tracer in food web ecology. Its three key features—(1) offsetting against isotopic variation; (2) universality of the trophic discrimination factor; and (3) sensitivity to source mixing—were compared with conventional isotope analysis for the bulk tissue of organisms. These three advantages of CSIA-AA will allow future researchers to (1) estimate the integrated trophic position (iTP) of animal communities; (2) infer trophic transfer efficiency (TTE) in food webs; and (3) quantify contributions from different resources to animals, all of which are crucial for understanding the relationship between biodiversity and multi-trophic ecosystem functioning. Further development of trophic ecology will be facilitated by both methodological refinement of CSIA-AA and its application to a wider range of organisms, food webs, and ecosystems.     

Stable nitrogen isotopic composition of amino acids reveals food web structure in stream ecosystems

The stable N isotopic composition of individual amino acids (SIAA) has recently been used to estimate trophic positions (TPs) of animals in several simple food chain systems. However, it is unknown whether the SIAA is applicable to more complex food web analysis. In this study we measured the SIAA of stream macroinvertebrates, fishes, and their potential food sources (periphyton and leaf litter of terrestrial C3 plants) collected from upper and lower sites in two streams having contrasting riparian landscapes. The stable N isotope ratios of glutamic acid and phenylalanine confirmed that for primary producers (periphyton and C3 litter) the TP was 1, and for primary consumers (e.g., mayfly and caddisfly larvae) it was 2. We built a two-source mixing model to estimate the relative contributions of aquatic and terrestrial sources to secondary and higher consumers (e.g., stonefly larva and fishes) prior to the TP calculation. The estimated TPs (2.3–3.5) roughly corresponded to their omnivorous and carnivorous feeding habits, respectively. We found that the SIAA method offers substantial advantages over traditional bulk method for food web analysis because it defines the food web structure based on the metabolic pathway of amino groups, and can be used to estimate food web structure under conditions where the bulk method cannot be used. Our result provides evidence that the SIAA method is applicable to the analysis of complex food webs, where heterogeneous resources are mixed.     

Parasites as prey in aquatic food webs: implications for predator infection and parasite transmission

While the recent inclusion of parasites into food‐web studies has highlighted the role of parasites as consumers, there is accumulating evidence that parasites can also serve as prey for predators. Here we investigated empirical patterns of predation on parasites and their relationships with parasite transmission in eight topological food webs representing marine and freshwater ecosystems. Within each food web, we examined links in the typical predator–prey sub web as well as the predator–parasite sub web, i.e. the quadrant of the food web indicating which predators eat parasites. Most predator– parasite links represented ‘concomitant predation’ (consumption and death of a parasite along with the prey/host; 58–72%), followed by ‘trophic transmission’ (predator feeds on infected prey and becomes infected; 8–32%) and predation on free‐living parasite life‐cycle stages (4–30%). Parasite life‐cycle stages had, on average, between 4.2 and 14.2 predators. Among the food webs, as predator richness increased, the number of links exploited by trophically transmitted parasites increased at about the same rate as did the number of links where these stages serve as prey. On the whole, our analyses suggest that predation on parasites has important consequences for both predators and parasites, and food web structure. Because our analysis is solely based on topological webs, determining the strength of these interactions is a promising avenue for future research.     

Food Web Topology in High Mountain Lakes

Although diversity and limnology of alpine lake systems are well studied, their food web structure and properties have rarely been addressed. Here, the topological food webs of three high mountain lakes in Central Spain were examined. We first addressed the pelagic networks of the lakes, and then we explored how food web topology changed when benthic biota was included to establish complete trophic networks. We conducted a literature search to compare our alpine lacustrine food webs and their structural metrics with those of 18 published lentic webs using a meta-analytic approach. The comparison revealed that the food webs in alpine lakes are relatively simple, in terms of structural network properties (linkage density and connectance), in comparison with lowland lakes, but no great differences were found among pelagic networks. The studied high mountain food webs were dominated by a high proportion of omnivores and species at intermediate trophic levels. Omnivores can exploit resources at multiple trophic levels, and this characteristic might reduce competition among interacting species. Accordingly, the trophic overlap, measured as trophic similarity, was very low in all three systems. Thus, these alpine networks are characterized by many omnivorous consumers with numerous prey species and few consumers with a single or few prey and with low competitive interactions among species. The present study emphasizes the ecological significance of omnivores in high mountain lakes as promoters of network stability and as central players in energy flow pathways via food partitioning and enabling energy mobility among trophic levels.     

Effects of partitioning allochthonous and autochthonous resources on food web stability

The flux of energetic and nutrient resources across habitat boundaries can exert major impacts on the dynamics of the recipient food web. Competition for these resources can be a key factor structuring many ecological communities. Competition theory suggests that competing species should exhibit some partitioning to minimize competitive interactions. Species should partition both in situ (autochthonous) resources and (allochthonous) resources that enter the food web from outside sources. Allochthonous resources are important sources of energy and nutrients in many low productivity systems and can significantly influence community structure. The focus of this paper is on: (i) the influence of resource partitioning on food web stability, but concurrently we examine the compound effects of; (ii) the trophic level(s) that has access to allochthonous resources; (iii) the amount of allochthonous resource input; and (iv) the strength of the consumer–resource interactions. We start with a three trophic level food chain model (resource–consumer–predator) and separate the higher two trophic levels into two trophospecies. In the model, allochthonous resources are either one type available to both consumers and predators or two distinct types, one for consumers and one for predators. The feeding preferences of the consumer and predator trophospecies were varied so that they could either be generalists or specialists on allochthonous and/or autochthonous resources. The degree of specialization influenced system persistence by altering the structure and, therefore, the indirect effects of the food web. With regard to the trophic level(s) that has access to allochthonous resources, we found that a single allochthonous resource available to both consumers and predators is more unstable than two allochthonous resources. The results demonstrate that species populating food webs that experience low to moderate allochthonous resources are more persistent. The results also support the notion that strong links destabilize food web dynamics, but that weak to moderate strength links stabilize food web dynamics. These results are consistent with the idea that the particular structure, resource availability, and relative strength of links of food webs (such as degree of specialization) can influence the stability of communities. Given that allochthonous resources are important resources in many ecosystems, we argue that the influence of such resources on species and community persistence needs to be examined more thoroughly to provide a clearer understanding of food web dynamics.     

基于稳定同位素的湿地食物源判定和食物网构建研究进展

湿地生物营养动力学是湿地生态系统结构和功能评价研究的基础.碳、氮稳定同位素作为识别营养关系的方法,已在湿地生态系统食物来源、组成和食物链传递研究中得到广泛运用.本文系统综述了稳定同位素食物贡献度计算模型和营养级确定的基本方法和理论;讨论了动物营养分馏值和基线的选择依据;概括了湿地生态系统典型食物源及其稳定同位素变化特征;总结了草食、杂食和肉食等不同营养级动物的食物来源.指出了稳定同位素在湿地食物源溯源和食物网研究中的不足;基于国内外研究现状和发展趋势及需求,展望了未来同位素技术在湿地食物网生态学研究中的运用前景和研究重点,提出需要加强稳定同位素营养分馏和基线的影响因素、样品处理和保存方式研究以及胃含物、分子标记物和多元素同位素结合分析.     

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.     

Applying stable isotopes to examine food‐web structure: an overview of analytical tools

Stable isotope analysis has emerged as one of the primary means for examining the structure and dynamics of food webs, and numerous analytical approaches are now commonly used in the field. Techniques range from simple, qualitative inferences based on the isotopic niche, to Bayesian mixing models that can be used to characterize food‐web structure at multiple hierarchical levels. We provide a comprehensive review of these techniques, and thus a single reference source to help identify the most useful approaches to apply to a given data set. We structure the review around four general questions: (1) what is the trophic position of an organism in a food web?; (2) which resource pools support consumers?; (3) what additional information does relative position of consumers in isotopic space reveal about food‐web structure?; and (4) what is the degree of trophic variability at the intrapopulation level? For each general question, we detail different approaches that have been applied, discussing the strengths and weaknesses of each. We conclude with a set of suggestions that transcend individual analytical approaches, and provide guidance for future applications in the field.     

Habitat size influences food web structure in drying streams

Biodiversity in running waters is threatened by an increased severity and incidence of low‐flow extremes resulting from global climate change and a growing human demand for freshwater resources. Although it is unknown how and to what extent riverine communities will change in the face of these threats, considerable insight will be gained from efforts aimed at quantifying habitat size‐related controls on the trophic relationships among taxa in streams experiencing extreme flow loss. Here we report on a detailed space‐for‐time survey of replicate stream food webs sampled along the perennial‐ to‐drying continuum in each of fourteen different intermittent South Island, New Zealand streams. We quantified several structural attributes of food webs at fifty‐eight sites, including two taxonomically‐based metrics (web size, predator:prey ratio) and three stable isotope‐based metrics (food chain length [FCL], trophic area, δ¹³C range); we also quantified habitat size‐, disturbance‐, and resource‐related covariates at each site. Food web structure varied widely across sample sites within and across study streams and much of this variation was explained by habitat size. Consistent with our predictions, we found that food webs became smaller (ca 30 to ca 15 taxa, ca 20‐fold reduction in stable isotope‐based trophic area) and shorter (maximum trophic position [FCL] from 4.1 to 2.0, 25% reduction in predator:prey ratio) as we moved from the largest to smaller habitats. These results, and a comparison of our findings with those from a similar assessment conducted in perennial streams, suggest that there are perturbation thresholds which may trigger food web collapse when exceeded, and further imply that food webs may ultimately be ‘sized’ to minimum flows rather than average flow conditions. Our work provides a basis for making general predictions about how habitat contraction, and flow loss in particular, may affect communities and additionally provides insight on mechanisms warranting further attention.     

High‐resolution food webs based on nitrogen isotopic composition of amino acids

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well‐understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher‐order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free‐roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.     

Multiple sources of isotopic variation in a terrestrial arthropod community: challenges for disentangling food webs

Documenting trophic links in a food web has traditionally required complex exclusion experiments coupled with extraordinarily labor-intensive direct observations of predator foraging. Newer techniques such as stable isotope analysis (SIA) may facilitate relatively quick and accurate assessments of consumer feeding behavior. Ratios of N and C isotopes are thought to be useful for determining species' trophic position (e.g., 1 degrees consumer, 2 degrees consumer, or omnivore) and their original carbon source (e.g., C3 or C4 plants; terrestrial or marine nutrients). Thus far, however, applications of stable isotopes to terrestrial arthropod food webs have suggested that high taxon-specific variation may undermine the effectiveness of this method. We applied stable isotope analysis to a pear orchard food web, in which biological control of a dominant pest, pear psylla (Cacopsylla pyricola), involves primarily generalist arthropod predators with a high frequency of omnivory. We found multiple sources of isotopic variation in this food web, including differences among plant tissues; time, stage, and taxon-specific differences among herbivores (despite similar feeding modes); and high taxon-specific variation among predators (with no clear evidence of omnivory). Collectively, these multiple sources of isotopic variation blur our view of the structure of this food web. Idiosyncrasies in consumer trophic shifts make ad hoc application of SIA to even moderately complex food webs intractable. SIA may not be a generally applicable "quick and dirty" method for delineating terrestrial food web structure-not without calibration of specific consumer food trophic shifts.     

Food Web Structure and Basal Resource Utilization along a Tropical Island Stream Continuum, Puerto Rico

Tropical stream food webs are thought to be based primarily on terrestrial resources (leaf litter) in small forested headwater streams and algal resources in larger, wider streams. In tropical island streams, the dominant consumers are often omnivorous freshwater shrimps that consume algae, leaf litter, insects, and other shrimps. We used stable isotope analysis to examine (1) the relative importance of terrestrial and algal‐based food resources to shrimps and other consumers and determine (2) if the relative importance of these food resources changed along the stream continuum. We examined δ¹⁵N and δ¹³C signatures of leaves, algae, macrophytes, biofilm, insects, snails, fishes, and shrimps at three sites (300, 90, and 10 m elev.) along the Río Espíritu Santo, which drains the Caribbean National Forest, Puerto Rico. Isotope signatures of basal resources were distinct at all sites. Results of two‐source δ¹³C mixing models suggest that shrimps relied more on algal‐based carbon resources than terrestrially derived resources at all three sites along the continuum. This study supports other recent findings in tropical streams, demonstrating that algal‐based resources are very important to stream consumers, even in small forested headwater streams. This study also demonstrates the importance of doing assimilation‐based analysis (i.e., stable isotope or trophic basis of production) when studying food webs.     

Benthos as the basis for arctic lake food webs

Plankton have traditionally been viewed as the basis for limnetic food webs, with zooplankton acting as a gateway for energy passing between phytoplanktonic primary producers and fish. Often, benthic production has been considered to be important primarily in shallow systems or as a subsidy to planktonic food web pathways. Stable isotope food web analyses of two arctic lakes (NE14 and I minus) in the Toolik Lake region of Alaska indicate that benthos are the primary source of carbon for adults of all species of benthic and pelagic fish present. We found no effect of turbidity, which may suppress benthic algae by shading, on food web structure. Even though Secchi transparency varied from 10.2 m in NE14 to 0.55–2.6 m in I minus, food webs in both lakes were based upon benthos, had four trophic levels, and culminated with omnivorous lake trout. We suggest that the importance of benthos in the food webs of these lakes is due to their extreme oligotrophy, resulting in planktonic resources that are insufficient for the support of planktivorous consumers.     

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.     

Flow intermittency influences the trophic base,but not the overall diversity of alpine stream food webs

Alpine streams can exhibit naturally high levels of flow intermittency. However, how flow intermittency in alpine streams affects ecosystem functions such as food web trophic structure is virtually unknown. Here, we characterized the trophic diversity of aquatic food webs in 28 headwater streams of the Val Roseg, a glacierized alpine catchment. We compared stable isotope (δ¹³C and δ¹⁵N) trophic indices to high temporal resolution data on flow intermittency. Overall trophic diversity, food chain length and diversity of basal resource use did not differ to a large extent across streams. In contrast, gradient and mixing model analysis indicated that primary consumers assimilated proportionally more periphyton and less allochthonous organic matter in more intermittent streams. Higher coarse particulate organic matter (CPOM) C:N ratios were an additional driver of changes in macroinvertebrate diets. These results indicate that the trophic base of stream food webs shifts away from terrestrial organic matter to autochthonous organic matter as flow intermittency increases, most likely due to reduced CPOM conditioning in dry streams. This study highlights the significant, yet gradual shifts in ecosystem function that occur as streamflow becomes more intermittent in alpine streams. As alpine streams become more intermittent, identifying which functional changes occur via gradual as opposed to threshold responses is likely to be vitally important to their management and conservation.