Food Web Architecture Research Articles

Modeling Food Webs: Exploring Unexplained Structure Using Latent Traits

Several stochastic models have tried to capture the architecture of food webs. This approach is interesting, but it is limited by the fact that different assumptions can yield similar results. To overcome this limitation, we develop a purely statistical approach. Body size in terms of an optimal ratio between prey and predator is used as explanatory variable. In 12 observed food webs, this model predicts, on average, 20% of interactions. To analyze the unexplained part, we introduce a latent term: each species is described by two latent traits, foraging and vulnerability, that represent nonmeasured characteristics of species once the optimal body size has been accounted for. The model now correctly predicts an average of 73% of links. The key features of our approach are that latent traits quantify the structure that is left unexplained by the explanatory variable and that this quantification allows a test of whether independent biological information, such as microhabitat use, camouflage, or phylogeny, explains this structure. We illustrate this method with phylogeny and find that it is linked to one or both latent traits in nine of 12 food webs. Our approach opens the door to the formulation of more complex models that can be applied to any kind of biological network.

Coupled predator–prey oscillations in a chaotic food web

Coupling of several predator-prey oscillations can generate intriguing patterns of synchronization and chaos. Theory predicts that prey species will fluctuate in phase if predator-prey cycles are coupled through generalist predators, whereas they will fluctuate in anti-phase if predator-prey cycles are coupled through competition between prey species. Here, we investigate predator-prey oscillations in a long-term experiment with a marine plankton community. Wavelet analysis of the species fluctuations reveals two predator-prey cycles that fluctuate largely in anti-phase. The phase angles point at strong competition between the phytoplankton species, but relatively little prey overlap among the zooplankton species. This food web architecture is consistent with the size structure of the plankton community, and generates highly dynamic food webs. Continued alternations in species dominance enable coexistence of the prey species through a non-equilibrium 'killing-the-winner' mechanism, as the system shifts back and forth between the two predator-prey cycles in a chaotic fashion.

Predator foraging behaviour drives food‐web topological structure

1. The structure and dynamics of prey populations are shaped by the foraging behaviours of their predators. Yet, there is still little documentation on how distinct predator foraging types control biodiversity, food-web architecture and ecosystem functioning. 2. We experimentally compared the effects of model fish species of two major foraging types of lake planktivores: a size-selective visual feeder (bluegill), and a filter feeder (gizzard shad). The visual feeder forages on individually captured consumer prey, whereas the filter feeder forages on various prey simultaneously, not only consumers but also primary producers. We ran a 1-month mesocosm experiment cross-classifying a biomass gradient of each predator type. We analysed the effect of each fish on food-web architecture by computing major topological descriptors over time (connectance, link density, omnivory index, etc.). These descriptors were computed from 80 predator-prey binary matrices, using taxa mostly identified at the species level. 3. We found that the visual feeder induced more trophic cul-de-sac (inedible) primary-producer species, lower link density and connectance, and lower levels of food-web omnivory and generalism than the filter feeder. Yet, predator biomass did not affect food-web topology. 4. Our results highlight that top-predator foraging behaviour is a key functional trait that can drive food-web topology and ultimately ecosystem functioning.

Benthic respiration in a seep habitat dominated by dense beds of ampharetid polychaetes at the Hikurangi Margin (New Zealand)

Many biological seep studies focused on the distribution, structure, nutrition and food web architecture of seep communities as well as on their interaction with the seep geochemistry. However, overall respiration at cold seeps received only little attention. We conducted in-situ oxygen flux measurements in combination with ex-situ oxygen micro-profiles, respiration measurements, as well as rate determinations of microbial methane and sulfate turnover to assess respiration pathways as well as carbon turnover at a seep habitat that was recently discovered alongside the Hikurangi Margin offshore northern New Zealand. This habitat is dominated by dense beds of tube-building, heterotrophic ampharetid polychaetes. Average total oxygen uptake (TOU) from this habitat was very high (83.7 mmol m − 2 day − 1 ). TOU at a non-seep reference site ranged between 2.7 and 5.8 mmol m − 2 day − 1 . About 37% (30.8 mmol m − 2 day − 1 ) of the average TOU was consumed by ampharetids. Considering mean diffusive oxygen uptake (8.5 mmol m − 2 day − 1 ) the remaining fraction of ~ 53% of the TOU (44.4 mmol m − 2 day − 1 ) might be explained by respiration of epibenthic organisms as well as aerobic methane and sulfide oxidation at the sediment–water interface. The strongly negative carbon isotopic signatures (− 52.9 ± 5‰ VPDB) of the ampharetid tissues indicate a methane derived diet. However, carbon production via anaerobic oxidation of methane (AOM) was too low (0.1 mmol C m − 2 day − 1 ) to cover the mean carbon demand of the ampharetid communities (21 mmol C m − 2 day − 1 ). Likely, organic carbon generated via aerobic methane oxidation represents their major carbon source. This is in contrast to other seep habitats, where energy bound in methane is partly transferred to sulfide via AOM and finally consumed by sulfide-oxidizing chemoautotrophs providing carbon that subsequently enters the benthic food web.

A landscape theory for food web architecture

Ecologists have long searched for structures and processes that impart stability in nature. In particular, food web ecology has held promise in tackling this issue. Empirical patterns in food webs have consistently shown that the distributions of species and interactions in nature are more likely to be stable than randomly constructed systems with the same number of species and interactions. Food web ecology still faces two fundamental challenges, however. First, the quantity and quality of food web data required to document both the species richness and the interaction strengths among all species within food webs is largely prohibitive. Second, where food webs have been well documented, spatial and temporal variation in food web structure has been ignored. Conversely, research that has addressed spatial and temporal variation in ecosystems has generally ignored the full complexity of food web architecture. Here, we incorporate empirical patterns, largely from macroecology and behavioural ecology, into a spatially implicit food web structure to construct a simple landscape theory of food web architecture. Such an approach both captures important architectural features of food webs and allows for an exploration of food web structure across a range of spatial scales. Finally, we demonstrated that food webs are hierarchically organized along the spatial and temporal niche axes of species and their utilization of food resources in ways that stabilize ecosystems.

Ecological consequences of evolution in plant defenses in a metacommunity

Dispersal can affect the assembly of local communities in a metacommunity as well as evolution of local populations in a metapopulation. These two processes may also affect each other in ways that have not yet been well studied and that may have novel effects on community structure. Here, we illustrate the interaction of these two processes on community structure with a model of adaptive evolutionary dynamics of plant defenses in a metacommunity food web involving multiple patches along a productivity gradient. We find an enhanced suite of adaptive plant types in our metacommunity model than is predicted in the absence of dispersal. We also find that this, and the movement of nutrients among patches via dispersal, alters patterns of food web architecture, trophic structure and diversity along the productivity gradient. Overall, our model illustrates that evolutionary and metacommunity dynamics may influence communities in complex interactive ways that may not be predicted by models that ignore either of these types of processes.

Niche width collapse in a resilient top predator following ecosystem fragmentation.

Much research has focused on identifying species that are susceptible to extinction following ecosystem fragmentation, yet even those species that persist in fragmented habitats may have fundamentally different ecological roles than conspecifics in unimpacted areas. Shifts in trophic role induced by fragmentation, especially of abundant top predators, could have transcendent impacts on food web architecture and stability, as well as ecosystem function. Here we use a novel measure of trophic niche width, based on stable isotope ratios, to assess effects of aquatic ecosystem fragmentation on trophic ecology of a resilient, dominant, top predator. We demonstrate collapse in trophic niche width of the predator in fragmented systems, a phenomenon related to significant reductions in diversity of potential prey taxa. Collapsed niche width reflects a homogenization of energy flow pathways to top predators, likely serving to destabilize remnant food webs and render apparently resilient top predators more susceptible to extinction through time.

Effects of dynamics on ecological networks

Ecological food webs define the feeding patterns of interacting species. The architecture of such networks may be affected by dynamical processes operating within them, ultimately influencing the capacity of the networks to persist. As yet relatively little is known about these effects. We compared the architecture of ecological networks with a fixed number of species, constructed in four contrasting ways: (I) topological networks, which required only that species had prey to eat; (II) persistent networks, in which species had also to persist under a simple model of population dynamics; (III) assembled networks, built up by sequential addition of species with dynamical persistence at each step in the sequence; (IV) evolved networks where, in addition to dynamical persistence, body size of species was determined by a simple mutation-selection process. Dynamics had fundamental effects on architecture, the networks of classes II, III and IV being restricted to a small number of trophic levels, in contrast to the non-dynamic, topological class I networks. Class III assembled networks tended to have fewer trophic levels and a more pyramidal biomass distribution than networks of classes II and IV. In evolved class IV networks, the smallest consumers converged to similar body sizes, whereas larger consumers evolved more slowly and did not show such convergence. The results indicate that dynamics affect the architecture of food webs, and that assumptions about simultaneous arrival, sequential arrival and evolution lead to different outcomes. Sequential assembly was shown to have a special property of finding rare sets of persistent species in a small number of steps, suggesting that the rarity of stable communities is not a serious problem in the development of complex communities.

Seasonal variation in food web composition and structure in a temperate tidal estuary

Seasonal variation in aquatic food web structure at Mad Island Marsh, Matagorda Bay, Texas, was examined using dietary information obtained from the analysis of gut contents from large samples of fish and crustacean specimens. Unique aspects of this study include the use of large samples of consumer gut contents (n=6,452), long-term sampling (bimonthly surveys over 18 mo), and standard methods of data collection and analysis facilitating comparisons with other aquatic food webs. Dietary data were partitioned for analysis into warm (summer) and cold (winter) seasons. Most consumers fed low in the food web, with trophic levels ranging from about 2 to 3.5 during both summer and winter. Vegetative detritus was more important in macroconsumer diets than live algae and macrophytes. Low trophic levels of consumers reflected the important role of abundant detritivores (e.g., striped mulletMugil cephalus, Gulf menhadenBrevortia patronum, and macroinvertebrates) in linking detritus to top predators via short food chains, a finding consistent with many other estuarine food web studies. Despite changes in community composition and population size structure of certain species, most food web properties revealed comparatively little seasonal variation. The summer food web had more nodes (86), more links (562), a higher density of links as indicated by connectance (0.08), and a slightly higher predator: prey ratio (0.51) compared to the winter food web (75 nodes, 394 links, connectance = 0.07, predator: prey ratio = 0.47). Proportions of top (0.06–0.07), intermediate (0.75–0.76), and basal (0.19) species did not vary significantly between seasons, but mean trophic level was higher during summer. Addition of feeding links based on information from the literature increased connectance to 0.13 during both seasons; other web parameters had values similar to those obtained for our directly estimated food webs. Seasonal variation in food web structure was influenced by changes in community composition (e.g., influxes of postlarval estuarine-dependent marine fishes during winter), availability of resources (e.g., more submerged macrophytes and amphipods during summer), and size structure and ontogenetic diet shifts of dominant consumer taxa. Our findings suggest that some basic properties of estuarine food web are resilient to seasonal changes in population and community structures and food web architecture.

Universal scaling in food-web structure?

Arising from: D. Garlaschelli, D. Caldarelli & L. Pietronero Nature 423, 165–168 (2003); Garlaschelli, Caldarelli & Pietronero reply . The statistical analysis of empirical food webs seeks to discover patterns in their structure. Garlaschelli et al.1 describe food webs as transportation networks and show that the empirical webs used in their study have universal scaling exponents. Here we analyse 17 of the most comprehensive food webs — including the nine used by the authors1 — but find no evidence for this universality. We also argue that the exponents that are observed are not a signature of food-web architecture but are a general property of networks that have few trophic levels, irrespective of their structure. We conclude that the short range of empirical exponents occurs because food webs contain only a few trophic levels and therefore that it does not add to our understanding of food-web topology.

Stoichiometrically explicit competition between grazers: species replacement, coexistence, and priority effects along resource supply gradients.

Assuming key trade-offs among interactors, several models (resource ratio, keystone predation, intraguild predation) predict changes in species composition over resource supply gradients. Ecological stoichiometry could also predict compositional shifts of grazers over gradients of nutrient and light supply through a mechanism involving (mis)matches between elemental body composition of grazers and plants. This hypothesis is explored here using a suite of two-grazer, one-plant models that incorporate three key components: plant production depends on light and nutrients, nutrient content of plants can vary, and homeostatic grazers can be carbon or nutrient limited. The results from this suite closely resemble the classical resource ratio model describing plant competition for two resources. Here, the models predict shifts of grazer composition along resource supply gradients if species trade off competitive abilities for plant carbon and nutrients. Given this trade-off, superior nutrient competitors should dominate low nutrient environments, and superior carbon competitors should dominate high nutrient environments. At intermediate nutrient supply, species can coexist at a stable equilibrium, or alternative stable states emerge, depending on how grazers impact their resources. These results depend on food web architecture, however. For instance, predators can alter or reduce possibilities for stoichiometry-mediated coexistence of grazers.

Food web structure and climate effects on the dynamics of small mammals and owls in semi‐arid Chile

Population dynamics of small mammals and predators in semi‐arid Chile is positively correlated with rainfall associated with incursions of El Niño (El Niño Southern Oscillation: ENSO). However, the causal relationships between small mammal fluctuations, predator oscillations, and climatic disturbances are poorly understood. Here, we report time series models for three species of small mammal prey and two species of owl predators. The large differences in population fluctuations between the three small mammal species are related to differences in their respective feedback structures. The analyses reveal that per capita growth rate of the leaf‐eared mouse is a decreasing function of log density and of log barn owl abundance together with a positive rainfall effect. In turn, per capita population growth rate (R‐function) of the barn owl is a negative function of log barn owl abundance and a positive function of leaf‐eared mouse abundance, suggesting a predator–prey interaction. The dramatic population fluctuations exhibited by leaf‐eared mouse (Phyllotis darwini) are caused by climate effects coupled with a complex food web architecture.

Sensitivity of Primary Production to Changes in the Architecture of Belowground Food Webs

The proposed mechanisms for the species diversity-function relationship in plant communities stress the recognition of functional properties of species, and interactions between plants and soil processes. As resource availability to plants is influenced by the architecture of decomposer food webs, it has been hypothesised that the diversity of decomposers can also control ecosystem processes, including primary production. We manipulated the complexity of soil animal communities in a miniecosysten experiment in which a boreal forest floor with birch seedlings infected with mycorrhizal fungi was created. The soil animal diversity ranged from zero to typical species richness of soil fauna (approximately 50 taxa) in coniferous forests. Between these extremes was a nested factorial design with manually assembled communities consisting of two sets of one-species, and two sets of five-species animal communities within fungivorous and microbi-detritivorous trophic groups. To investigate the role of predators in system functioning, the miniecosystems with fungivorous and microbidetritivorous fauna were established either with or without mesostigmatid mites as top predators. The miniecosystems were incubated in a climate chamber with varying illumination and temperature regimes for 40 weeks. Our experiment provides evidence that primary productivity is generally insensitive to variation at the species level or even at the level of trophic groups. Although top predators generally reduced prey population size, no effect was found on systen functioning. However, the removal of microbe- or detritus-feeding fauna, especially the microbi-detritivore Cognettia sphagnetorum reduced plant N uptake and accumulation of plant biomass. The functional importance of soil fauna was inversely related to the trophic position of the group. Our results suggest that ecosystem functioning is robust against species extinctions in belowground food webs, and that primary production is predominantly controlled by organisms at low trophic positions in the decomposer food web.

Restoration Ecology and the Ecosystem Perspective

AbstractThe ecosystem perspective provides a framework within which most other aspects of the ecology of restoration can be incorporated. By considering the ecosystem functions of a restoration project, the restorationist is forced to consider the placement of the project in the landscape—its boundaries, its connections or lack thereof to adjoining ecosystems, and its receipts and losses of materials and energy from its physical surroundings. These characteristics may set limits on the kind(s) of biotic communities that can be created on the site. The ecosystem perspective also gives restorationists conceptual tools for structuring and evaluating restorations. These include the mass balance approach to nutrient, pollutant, and energy budgets; subsidy/stress effects of inputs; food web architecture; feedback among ecosystem components; efficiency of nutrient transfers, primary productivity and decomposition as system‐determining rates; and disturbance regimes. However, there are many uncertainties concerning these concepts, their relation to each other, and their relationships to population‐ and community‐level phenomena. The nature of restoration projects provides a unique opportunity for research on these problems; the large spatial scale of restorations and the freedom to manipulate species, soil, water, and even the landscape could allow ecosystem‐level experiments to be conducted that could not be performed otherwise.

Enclosing decomposer food web: implications for community structure and function

We designed a field experiment to evaluate how restriction of soil faunal movements affects decomposer community structure, food web architecture, and decomposition of organic matter. Intact soil cores (3cm thick, diameter 16cm) were placed either in “open” (mesh size 1mm, allowing all meso- and microfauna to move through) or “closed” (27μm, animal movement prevented except for the smallest microfauna) mesh bags in early May. Before being buried in the forest floor of a mixed spruce stand, hay litter was placed in the mesh bags in separate litter bags. The samplings took place 2 and 6 months after establishing the experiment. Additional “field samples” were taken from the adjacent soil to determine possible side effects of the mesh-bags. Physicochemical conditions, decomposition rate of hay litter, and total respiration of soil cores were identical in the two bag treatments. Enchytraeids increased significantly in the closed treatment, while macrofauna, such Coleoptera larvae and dipteran larvae, went close to extinction in the closed bags. The elevated enchytraeid number is in accordance with the findings of closed microcosm studies, and is best explained by reduced predation by macrofauna. Although a set of 14 mite taxa was found to distinctively reflect the degree of isolation, neither the total number of individuals nor the number of microarthropod taxa differed between the bag treatments, or between the bags and the field samples. It is concluded that in the time-span of one growing season, reduction in the spatial scale does not necessarily reduce the diversity of fauna but can significantly change the decomposer food-web architecture.

Omnivory as a Stabilizing Feature of Natural Communities

Omnivory-defined broadly as feeding on more than one trophic level-occupies a prominent position in discussions of food web architecture and dynamics, due in large part to an enduring conflict regarding omnivory's role in community dynamics. According to classical results from mathematical food web theory, omnivory destabilizes ecological communities, whereas more recent conceptual syntheses suggest that omnivory should be a strongly stabilizing factor in food webs. Working with an arthropod assemblage at Mount Saint Helens, I experimentally addressed this controversy using a two-way factorial design that crossed a manipulation of the degree of omnivory with another "disturbance" manipulation that targeted a specific component of the assemblage. In this statistical design, significant interaction effects (i.e., how the community impacts of the disturbance varied with the degree of omnivory) identified key stabilizing or destabilizing influences of omnivory. Overall, my experimental results indicated that increasing the degree of omnivory stabilized community dynamics, in keeping with recent conceptual syntheses.

Food Web Architecture and Population Dynamics in Laboratory Microcosms of Protists

In theory, food chain length and omnivory are pivotal elements of food web structure that can affect the population dynamics of species within the web. Long food chains are thought to be less stable than shorter food chains, and omnivores are thought to destabilize food webs, although populations of omnivores may be more stable than populations of nonomnivores. In three of four simple food webs assembled from bacteria and protists in laboratory microcosms, the abundance of bacterivorous protists varied more over time when the species occurred in longer versus shorter food chains. The abundance of protists attacked by omnivorous top predators was either more or less temporally variable than in webs where top predators fed only at one adjacent trophic level, depending on the particular combination of interacting species. The abundance of omnivorous top predators varied less over time than the abundance of top predators restricted to feeding only at an adjacent trophic level. Observations of increased temporal variation in prey abundance in longer food chains and low temporal variation in omnivore abundance agree broadly with several predictions of food web theory. The observation that different species in similar trophic positions can exhibit very different dynamics suggests that stability may depend on complex interactions between species-specific life-history traits and general patterns of food web architecture.