Trophic levels in community food webs

Summary Four concepts are considered for the trophic level of a species in a community food web. The long-way-up-level (or LU-level) of species A is the length of the longest simple food chain from a basal species (one with no prey in the web) to A. (A simple chain is a chain that does not pass through any given species more than once.) The short-way-up-level (SU-level) of species A is the length of the shortest chain from a basal species to A. The long-way-down-level (LD-level) of species A is the length of the longest simple chain from species A to a top species (one with no consumers in the web). The short-way-down-level (SD-level) of species A is the length of the shortest chain from species A to a top species. The stratigraphy of a web is the analogue for species of the pyramid of numbers for individuals: it is the frequency distribution of species according to level. The LU-, SU-, LD-, and SD-stratigraphies of the seven webs in the Briand-Cohen collection with 30 or more trophic species reveal no species with LU-level or LD-level more than 6, no species with SU-level more than 3, and no species with SD-level more than 2. In all seven webs, SD-levels are stochastically less than SU-levels: species tend to be closer to a top predator than to a basal species. Two stochastic models of food web structure (the cascade model and the homogeneous superlinear model) correctly predict that 95% or more of species should have LU-level and LD-level in the range 0–6. The models also correctly predict some details of the distribution of species in the SU- and SD-stratigraphies, particularly the fraction of species in level 1. The models do not, in general, correctly predict the distribution of species within the range 0–6 of LU-levels and LD-levels.     

Trophic cascades and trophic trickles in pelagic food webs

Prey-dependent models, with the predation rate (per predator) a function of prey numbers alone, predict the existence of a trophic cascade. In a trophic cascade, the addition of a top predator to a two-level food chain to make a three-level food chain will lead to increases in the population size of the primary producers, and the addition of nutrients to three-level chains will lead to increases in the population numbers at only the first and third trophic levels. In contrast, ratio-dependent models, with the predation rate (per predator) dependent on the ratio of predator numbers to prey, predict that additions of top predators will not increase the population sizes of the primary producers, and that the addition of nutrients to a three-level food chain will lead to increases in population numbers at all trophic levels. Surprisingly, recent meta-analyses show that freshwater pelagic food web patterns match neither prey-dependent models (in pelagic webs, ''prey'' are phytoplankton, and ''predators'' are zooplankton), nor ratio-dependent models. In this paper we use a modification of the prey-dependent model, incorporating strong interference within the zooplankton trophic level, that does yield patterns matching those found in nature. This zooplankton interference model corresponds to a more reticulate food web than in the linear, prey-dependent model, which lacks zooplankton interference. We thus reconcile data with a new model, and make the testable prediction that the strength of trophic cascades will depend on the degree of heterogeneity in the zooplankton level of the food chain.     

Emergence of complexity in evolving niche-model food webs

We have analysed mechanisms that promote the emergence of complex structures in evolving model food webs. The niche model is used to determine predator-prey relationships. Complexity is measured by species richness as well as trophic level structure and link density. Adaptive dynamics that allow predators to concentrate on the prey species they are best adapted to lead to a strong increase in species number but have only a small effect on the number and relative occupancy of trophic levels. The density of active links also remains small but a high number of potential links allows the network to adjust to changes in the species composition (emergence and extinction of species). Incorporating effects of body size on individual metabolism leads to a more complex trophic level structure: both the maximum and the average trophic level increase. So does the density of active links. Taking body size effects into consideration does not have a measurable influence on species richness. If species are allowed to adjust their foraging behaviour, the complexity of the evolving networks can also be influenced by the size of the external resources. The larger the resources, the larger and more complex is the food web it can sustain. Body size effects and increasing resources do not change size and the simple structure of the evolving networks if adaptive foraging is prohibited. This leads to the conclusion that in the framework of the niche model adaptive foraging is a necessary but not sufficient condition for the emergence of complex networks. It is found that despite the stabilising effect of foraging adaptation the system displays elements of self-organised critical behaviour.     

Trophic interactions in changing landscapes: responses of soil food webs

Soil communities in landscapes that are rapidly changing due to a range of anthropogenic processes can be regarded as highly transient systems where interactions between competing species or trophic levels may be seriously disrupted. In disturbed communities dispersal in space and time has a role in ensuring continuity of community function. Stable communities, in undisturbed systems, are more dependent on competition and other biotic interactions between species. We predicted how food web components would respond to disturbance, based on their dispersal and colonizing abilities. During decomposition, flows of energy and nutrients generally follow either a bacterial-based path, with bacteria as the primary decomposer and bacterial-feeding fauna and their predators forming the associated food web, or a fungal-based channel. Trophic links that were generally resistant to change were the organisms of the bacterial pathway that have high abilities to disperse in time and passively disperse in space. Organisms in the fungal pathway were less resistant to disturbance. Resource inputs to the soil system are derived from plants, either through root exudation and root turnover during active growth or from dead plant material following senescence or agricultural tillage. Disturbances to the soil system can arise as a direct action on the soil, or indirectly from effects on the above-ground plant community. Disturbance-induced changes in plant community composition will change the soil food web composition. Organisms involved in direct interactions with plants (e.g. AM-mycorrhizal fungi) were also predicted to be vulnerable to disturbance.

Zusammenfassung

Bodengemeinschaften in Landschaften, die sich aufgrund einer Reihe von anthropogenen Prozessen schnellstens verändern, können als sehr kurzlebige Systeme angesehen werden, in denen Interaktionen zwischen konkurrierenden Arten oder trophischen Leveln nachhaltig unterbrochen sind. In gestörten Gemeinschaften hat die Ausbreitung in Raum und Zeit eine Rolle bei der Wahrung der Kontinuität von Gemeinschaftsfunktionen. Stabile Gemeinschaften, in ungestörten Systemen, sind stärker von Konkurrenz und anderen biotischen Interaktionen zwischen den Arten abhängig. Wir sagten voraus, wie Nahrungsnetzkomponenten auf Störung antworten würden, basierend auf ihrer Ausbreitungs- und Kolonisationsfähigkeit. Während der Zersetzung folgen die Flüsse von Energie und Nährstoffen im Allgemeinen entweder einem Weg, der auf Bakterien basiert, mit Bakterien als den primären Zersetzern und bacterienfressender Fauna und ihre Predatoren, die das assoziierte Nahrungsnetz bilden, oder sie folgen einem Kanal, der auf Pilzen basiert. Trophische Verknüpfungen, die im Allgemeinen resistent gegen Veränderungen waren, waren die Organismen des bakteriellen Weges, die große Möglichkeiten haben sich in Zeit und passiv im Raum auszubreiten. Organismen des pilzbasierten Weges waren weniger widerstandsfähig gegenüber Störungen. Die Ressourcenzufuhr in das Bodensystem stammte von Pflanzen, entweder über Wurzelausscheidungen und/oder Wurzelturnover während des aktiven Wachstums oder von totem Pflanzenmaterial aufgrund von Seneszenz oder landwirtschatlicher Bodenbearbeitung. Störungen des Bodensystems können durch direkte Einwirkungen auf den Boden oder indirekt durch Effekte der oberirdischen Pflanzemeinschaft entstehen. Störungsinduzierte Veränderungen in der Zusammensetzung der Pflanzengemeinschaft werden die Zusammensetzung des Bodennahrungsnetzes verändern. Für Organismen, die an direkten Interaktionen mit Pflanzen beteiligt sind (beispielsweise AM-Mykorrhizapilze), wurde ebenfalls vorhergesagt, dass sie anfällig für Störungen sind.     

水生生态系统食物网复杂性与多样性的关系

探索食物网的复杂结构是生态学的中心问题之一。基于构建的黄河口海草床食物网并耦合实际食物网的数据集,整理了包含河口、湖泊、海洋和河流四种水生生态系统类型的48个实际食物网案例。以食物网的节点数反映食物网多样性,物种之间的营养链接数、链接密度和连通度来表示食物网的复杂性,采用营养缩尺模型描述水生生态系统食物网的复杂性特征与节点数的普适性规律。结果表明：所涉及的48个水生生态系统食物网的多样性和复杂性跨度较大,其中,节点数的分布范围为4-124,链接数为3-1830,链接密度为0.75-15.71,连通度为0.06-0.25。不同类型水生生态系统间的连通度存在显著性差异（P=0.01）,节点数、链接数、链接密度不存在显著性差异。各类型生态系统的食物网链接数、链接密度均随节点数的增加而增加（R²=0.92,P<0.001和R²=0.82,P<0.001）。湖泊生态系统的连通度随节点数的变化不明显,围绕在0.20附近;而其他3种类型生态系统的食物网连通度随节点数的增加而降低（R²=0.06-0.41,P<0.001）。对全球尺度的水生食物网多样性和复杂性的定量化研究对于提升对食物网的复杂结构的科学认识,从系统尺度探究多样性和复杂性的关系提供数据支撑。     

Trophic complementarity drives the biodiversity–ecosystem functioning relationship in food webs

The biodiversity–ecosystem functioning (BEF) relationship is central in community ecology. Its drivers in competitive systems (sampling effect and functional complementarity) are intuitive and elegant, but we lack an integrative understanding of these drivers in complex ecosystems. Because networks encompass two key components of the BEF relationship (species richness and biomass flow), they provide a key to identify these drivers, assuming that we have a meaningful measure of functional complementarity. In a network, diversity can be defined by species richness, the number of trophic levels, but perhaps more importantly, the diversity of interactions. In this paper, we define the concept of trophic complementarity (TC), which emerges through exploitative and apparent competition processes, and study its contribution to ecosystem functioning. Using a model of trophic community dynamics, we show that TC predicts various measures of ecosystem functioning, and generate a range of testable predictions. We find that, in addition to the number of species, the structure of their interactions needs to be accounted for to predict ecosystem productivity.     

Relation between complexity and stability in food webs with adaptive behavior

We investigate the influence of functional responses (Lotka-Volterra or Holling type), initial topological web structure (randomly connected or niche model), adaptive behavior (adaptive foraging and predator avoidance) and the type of constraints on the adaptive behavior (linear or nonlinear) on the stability and structure of food webs. Two kinds of stability are considered: one is the network robustness (i.e., the proportion of species surviving after population dynamics) and the other is the species deletion stability. When evaluating the network structure, we consider link density as well as the trophic level structure. We show that the types of functional responses and initial web structure do not have a large effect on the stability of food webs, but foraging behavior has a large stabilizing effect. It leads to a positive complexity-stability relationship whenever higher "complexity" implies more potential prey per species. The other type of adaptive behavior, predator avoidance behavior, makes food webs only slightly more stable. The observed link density after population dynamics depends strongly on the presence or absence of adaptive foraging, and on the type of constraints used. We also show that the trophic level structure is preserved under population dynamics with adaptive foraging.     

Methane carbon supports aquatic food webs to the fish level

Large amounts of the greenhouse gas methane (CH(4)) are produced by anaerobic mineralization of organic matter in lakes. In spite of extensive freshwater CH(4) emissions, most of the CH(4) is typically oxidized by methane oxidizing bacteria (MOB) before it can reach the lake surface and be emitted to the atmosphere. In turn, it has been shown that the CH(4)-derived biomass of MOB can provide the energy and carbon for zooplankton and macroinvertebrates. In this study, we demonstrate the presence of specific fatty acids synthesized by MOB in fish tissues having low carbon stable isotope ratios. Fish species, zooplankton, macroinvertebrates and the water hyacinth Eichhornia crassipes were collected from a shallow lake in Brazil and analyzed for fatty acids (FA) and carbon stable isotope ratios (δ(13)C). The fatty acids 16∶1ω8c, 16∶1ω8t, 16∶1ω6c, 16∶1ω5t, 18∶1ω8c and 18∶1ω8t were used as signature for MOB. The δ(13)C ratios varied from -27.7‰ to -42.0‰ and the contribution of MOB FA ranged from 0.05% to 0.84% of total FA. Organisms with higher total content of MOB FAs presented lower δ(13)C values (i.e. they were more depleted in (13)C), while organisms with lower content of MOB signature FAs showed higher δ(13)C values. An UPGMA cluster analysis was carried out to distinguish grouping of organisms in relation to their MOB FA contents. This combination of stable isotope and fatty acid tracers provides new evidence that assimilation of methane-derived carbon can be an important carbon source for the whole aquatic food web, up to the fish level.     

Trophic interactions affect the population dynamics and risk of extinction of basal species in food webs

This paper addresses effects of trophic complexity on basal species, in a Lotka–Volterra model with stochasticity. We use simple food web modules, with three trophic levels, and expose every species to random environmental stochasticity and analyze (1) the effect of the position of strong trophic interactions on temporal fluctuations in basal species’ abundances and (2) the relationship between fluctuation patterns and extinction risk. First, the numerical simulations showed that basal species do not simply track the environment, i.e. species dynamics do not simply mirror the characteristics of the applied environmental stochasticity. Second, the extinction risk of species was related to the fluctuation patterns of the species.More specifically, we show (i) that despite being forced by random stochasticity without temporal autocorrelation (i.e. white noise), there is significant temporal autocorrelation in the time series of all basal species’ abundances (i.e. the spectra of basal species are red-shifted), (ii) the degree of temporal autocorrelation in basal species time series is affected by food web structure and (iii) the degree of temporal autocorrelation tend to be correlated to the extinction risks of basal species.Our results emphasize the role of food web structure and species interactions in modifying the response of species to environmental variability. To shed some light on the mechanisms we compare the observed pattern in abundances of basal species with analytically predicted patterns and show that the change in the predicted pattern due to the addition of strong trophic interactions is correlated to the extinction risk of the basal species. We conclude that much remain to be understood about the mechanisms behind the interaction among environmental variability, species interactions, population dynamics and vulnerability before we quantitatively can predict, for example, effects of climate change on species and ecological communities. Here, however, we point out a new possible approach for identifying species that are vulnerable to environmental stochasticity by checking the degree of temporal autocorrelation in the time series of species. Increased autocorrelation in population fluctuations can be an indication of increased extinction risk.     

Effects of seasonality and fish movement on tropical river food webs

Tropical rivers and their associated floodplain habitats are dynamic habitat mosaics to which fishes are challenged to respond in an adaptive manner. Migratory fishes create linkages among food webs that are partitioned along a nested hierarchy of spatial scales. Such linkages are examined across a hierarchy of spatio-temporal scales, ranging from small streams to entire drainage basins, for rivers in South America and Africa. Migratory herbivorous fishes originating from eutrophic, productive ecosystems may subsidize resident predators of oligotrophic river ecosystems, which may result in cascading direct and indirect Effects on other species in local food webs. Successful management of many of the most important stocks of tropical river fishes requires conceptual models of how fish movement influences food web structure and dynamics.     

Trophic interactions in rockpool food webs: regulation of zooplankton and phytoplankton by Notonecta and Daphnia

We studied trophic interactions in experimental rockpools with three different food web structures: phytoplankton and small-bodied zooplankton; phytoplankton, small-bodied zooplankton and Daphnia ; and phytoplankton, small-bodied zooplankton, Daphnia and Notonecta . Nutrients, primary productivity, chlorophyll a and zooplankton species composition and biomass were measured over eight weeks.
2. Daphnia had a negative impact on other zooplankton and reduced the phytoplankton biomass and primary productivity. In the absence of Daphnia , small-bodied zooplankton species were abundant, in particular cyclopoid copepods. Concentrations of dissolved nutrients were lower and the standing crop of primary producers was higher when Daphnia was absent.
3. The presence of the invertebrate predator Notonecta produced a top-down effect which was similar to that reported for planktivorous fish, i.e. a selective reduction of daphnids followed by an increase of small-bodied zooplankton species and phytoplankton biomass.
4. The study showed that consumer regulation of Daphnia by Notonecta and of algae by Daphnia are important, but also demonstrated that trophic level biomasses were controlled by a combination of predation and resource limitation.     

Trophic diversity in deep-sea fish

The general composition and diversity of the diets of the 43 most commonly caught pelagic and demersal fish of the Rockall Trough, north-eastern Atlantic Ocean, are assessed. The fish are divided into three Groups. The 8 species in Group I consist of both pelagic and demersal species feeding on relatively few prey-classes and having a diet of low diversity and few items per meal. Group II contains 22 pelagic and demersal species with more diverse diets, less restricted dietary composition, but still consuming relatively few items per meal. Group III is the 12 demersal macrourid species with the most diverse diets, a variable dietary composition and the greatest mean number of items per meal. One species, Maurolicus muelleri , had too many unidentified components in its diet to allow classification in terms of Groups I, II or III. All diets contained dominant items, the diversity within diets offish in Groups II and III arising from the inclusion of subdominants and rare items. The diets of species in Groups I and II can be defined in terms of ecological constitution, trophic diversity and prey-species composition. Those of the Group III macrourids differ in that their definition is liable to be a compromise between the situation where ecological constitution and trophic diversity are adequately defined but not species composition.     

Threshold extinction in food webs

Food web response to species loss has been investigated in several ways in the previous years. In binary food webs, species go secondarily extinct if no resource item remains to be exploited. In this work, we considered that species can go extinct before the complete loss of their resources and we introduced thresholds of minimum energy requirement for species survival. According to this approach, extinction of a node occurs whenever an initial extinction event eliminates its incoming links so it is left with an overall energy intake lower than the threshold value. We tested the robustness of 18 real food webs by removing species from most to least connected and considering different scenarios defined by increasing the extinction threshold. Increasing energy requirement threshold negatively affects food web robustness. We found that a very small increase of the energy requirement substantially increases system fragility. In addition, above a certain value of energy requirement threshold we found no relationship between the robustness and the connectance of the web. Further, food webs with more species showed higher fragility with increasing energy threshold. This suggests that the shape of the robustness–complexity relationship of a food web depends on the sensitivity of consumers to loss of prey.     

Making connections in food webs

Patterns in food web structure have provided an important, though contentious, testing ground for ideas about the population dynamics and energetics of multispecies systems. One of the most debated of these patterns is the apparent decrease in food web connectance as the number of species in a web Increases. Several contrasting mechanisms that might determine food web connectance have been suggested. These mechanisms, in combination with new, food web data, suggest that the conventional pattern, and explanations for it, may well be open to dispute. The true nature of the relationship between connectance and species number has implications for the explanation of other web patterns and for theories of food web structure, but a general explanation remains elusive.     

Microbial food chains and food webs

Mathematical models for simple microbial food chains and food webs in continuous culture are developed and analyzed. A model for competition of two microbial species for a single scarce resource is also presented as a degenerate case of the food web model. Two models for food chains are developed. The first is based on a model of microbial growth (Monod's) that is widely mentioned and used at the present time. The second is based on a generalization of that model that recent experimental results on microbial food chains seem to require. Experimental data for microbial food webs are almost entirely lacking but a tentative model having what are felt to be the right properties is developed and analyzed. The results obtained from these models seem to be consistent in most circumstances with current ecological thinking on community dynamics.     

Parasites entangled in food webs

Food webs are a fundamental concept in ecology in which parasites have been virtually ignored. In a recent article, Lafferty et al. address this imbalance, finding that the inclusion of parasites in food webs could be of greater importance to ecosystem stability than was previously thought. Furthermore, the bottom of the food chain is perhaps no longer the most dangerous place to be.