Effects of habitat configuration on host–parasitoid food web structure

The dispersal of organisms among patches affects community structure in spatially heterogeneous habitats. The enhancement of dispersal frequency among patches can be expected to increase potential interaction between organisms in food webs. However, it has been difficult to fairly evaluate the effects of dispersal on the food web structure because the quantification of actual dispersal is difficult. In this study, in order to manipulate the dispersal frequency, two oak plantations (each with 100 oak trees) were established as high-patch connectivity (1-m interval) and low-patch connectivity (3-m interval) plots. Quantitative food webs of herbivores and their parasitoids were constructed for the high- and low-connectivity plots, and quantitative measures of food web metrics as indices of structure were calculated for both webs to examine dispersal effects on food web complexity. In the entire web, 86 herbivore species (Lepidoptera and Coleoptera) were attacked by 50 parasitoid species (Hymenoptera and Diptera). As a result, although we found no significant difference in herbivore abundance between high- and low-connectivity plots, a higher parasitism rate and greater complexity in web structure were observed in many food web metrics for the high-connectivity plot. Furthermore, the parasitoid overlap diagram showed a higher potential for indirect interactions among herbivore species in the high-connectivity plot. These results imply that the increase in dispersal frequency among habitat patches facilitates food web complexity, and the role of dispersal as a determinant of food web structure should be considered in food web ecology.     

Effects of evolutionary changes in prey use on the relationship between food web complexity and stability

The relationship between food web complexity and stability has been the subject of a long-standing debate in ecology. Although rapid changes in the food web structure through adaptive foraging behavior can confer stability to complex food webs, as reported by Kondoh (Science 299:1388–1391, 2003), the exact mechanisms behind this adaptation have not been specified in previous studies; thus, the applicability of such predictions to real ecosystems remains unclear. One mechanism of adaptive foraging is evolutionary change in genetically determined prey use. We constructed individual-based models of evolution of prey use by predators assuming explicit population genetics processes, and examined how this evolution affects the stability (i.e., the proportion of species that persist) of the food web and whether the complexity of the food web increased the stability of the prey–predator system. The analysis showed that the stability of food webs decreased with increasing complexity regardless of evolution of prey use by predators. The effects of evolution on stability differed depending on the assumptions made regarding genetic control of prey use. The probabilities of species extinctions were associated with the establishment or loss of trophic interactions via evolution of the predator, indicating a clear link between structural changes in the food web and community stability.     

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.     

Persistence increases with diversity and connectance in trophic metacommunities

Background

We are interested in understanding if metacommunity dynamics contribute to the persistence of complex spatial food webs subject to colonization-extinction dynamics. We study persistence as a measure of stability of communities within discrete patches, and ask how do species diversity, connectance, and topology influence it in spatially structured food webs.

Methodology/Principal Findings

We answer this question first by identifying two general mechanisms linking topology of simple food web modules and persistence at the regional scale. We then assess the robustness of these mechanisms to more complex food webs with simulations based on randomly created and empirical webs found in the literature. We find that linkage proximity to primary producers and food web diversity generate a positive relationship between complexity and persistence in spatial food webs. The comparison between empirical and randomly created food webs reveal that the most important element for food web persistence under spatial colonization-extinction dynamics is the degree distribution: the number of prey species per consumer is more important than their identity.

Conclusions/Significance

With a simple set of rules governing patch colonization and extinction, we have predicted that diversity and connectance promote persistence at the regional scale. The strength of our approach is that it reconciles the effect of complexity on stability at the local and the regional scale. Even if complex food webs are locally prone to extinction, we have shown their complexity could also promote their persistence through regional dynamics. The framework we presented here offers a novel and simple approach to understand the complexity of spatial food webs.     

Examining the potential effects of species aggregation on the network structure of food webs

One of the key measures that have been used to describe the topological properties of complex networks is the “degree distribution”, which is a measure that describes the frequency distribution of number of links per node. Food webs are complex ecological networks that describe the trophic relationships among species in a community, and the topological properties of empirical food webs, including degree distributions, have been examined previously. Previously, the “niche model” has been shown to accurately predict degree distributions of empirical food webs, however, the niche model-generated food webs were referenced against empirical food webs that had their species grouped together based on their taxonomic and/or trophic relationships (aggregated food webs). Here, we explore the effects of species aggregation on the ability of the niche model to predict the total- (sum of prey and predator links per node), in- (number of predator links per node), and out- (number of prey links per node) degree distributions of empirical food webs by examining two food webs that can be aggregated at different levels of resolution. The results showed that (1) the cumulative total- and out-degree distributions were consistent with the niche model predictions when the species were aggregated, (2) when the species were disaggregated (i.e., higher resolution), there were mixed conclusions with regards to the niche model's ability to predict total- and out-degree distributions, (3) the model's ability to predict the in-degree distributions of the two food webs was generally inadequate. Although it has been argued that universal functional form based on the niche model could describe the degree distribution patterns of empirical food webs, we believe there are some limitations to the model's ability to accurately predict the structural properties of food webs.     

Evidence for the existence of a robust pattern of prey selection in food webs

Food webs aim to provide a thorough representation of the trophic interactions found in an ecosystem. The complexity of empirical food webs, however, is leading many ecologists to focus dynamic ecosystem studies on smaller microcosm or mesocosm studies based upon community modules, which comprise three to five species and the interactions likely to have ecological relevance. We provide here a structural counterpart to community modules. We investigate food-web 'motifs' which are n-species connected subgraphs found within the food web. Remarkably, we find that the over- and under-representation of three-species motifs in empirical food webs can be understood through comparison to a static food-web model, the niche model. Our result conclusively demonstrates that predation upon species with some 'characteristic' niche value is the prey selection mechanism consistent with the structural properties of empirical food webs.     

Food webs in river networks

Food webs and river drainages are both hierarchical networks and complex adaptive systems. How does living within the second affect the first? Longitudinal gradients in productivity, disturbance regimes and habitat structure down rivers have long interested ecologists, but their effects on food web structure and dynamics are just beginning to be explored. Even less is known about how network structure per se influences river and riparian food webs and their members. We offer some preliminary observations and hypotheses about these interactions, emphasizing observations on upstream–downstream changes in food web structure and controls, and introducing some ideas and predictions about the unexplored question of food web responses to some of the network properties of river drainages.     

Connectance determines invasion success via trophic interactions in model food webs

Human induced global change has greatly altered the structure and composition of food webs through the invasion of non‐native species and the extinction of native species. Much attention has been paid to the effects of species deletions on food web structure and stability. However, recent empirical evidence suggests that for most taxa local species richness has increased as successful invasions outpace extinctions at this scale. This pattern suggests that food webs, which represent feeding interactions at the local scale, may be increasing in species richness. Knowledge of how food web structure relates to invasive species establishment and the effect of successful invaders on subsequent food web structure remains an unknown but potentially important aspect of global change. Here we explore the effect of food web topology on invasion success in model food webs to develop hypotheses about how the distribution of biodiversity across trophic levels affects the success of invasion at each trophic level. Our results suggest a connectance (C) based framework for predicting invasion success in food webs due to the way that C constrains the number of species at each trophic level and thus the number of potential predators and prey for an invader at a given trophic level. We use the relationship between C and the proportion of species at each trophic level in 14 well studied food webs to make the following predictions; 1) the success of basal invaders will increase as C increases due to the decrease in herbivores in high C webs, 2) herbivore invasion success will decrease as C increases due to the decrease in the proportion of basal species and increase in intermediate species and omnivores in high C webs. 3) Top predator invasion success will increase as C increases due to the increase in intermediate prey species. However, it is not clear how the relative influence of trophic structure compares to empirically known predictors of invasion success such as invader traits, propagule pressure, and resource availability.     

Evaluating the effects of trophic complexity on a keystone predator by disassembling a partial intraguild predation food web

1. Many taxa can be found in food webs that differ in trophic complexity, but it is unclear how trophic complexity affects the performance of particular taxa. In pond food webs, larvae of the salamander Ambystoma opacum occupy the intermediate predator trophic position in a partial intraguild predation (IGP) food web and can function as keystone predators. Larval A. opacum are also found in simpler food webs lacking either top predators or shared prey. 2. We conducted an experiment where a partial IGP food web was simplified, and we measured the growth and survival of larval A. opacum in each set of food webs. Partial IGP food webs that had either a low abundance or high abundance of total prey were also simplified by independently removing top predators and/or shared prey. 3. Removing top predators always increased A. opacum survival, but removal of shared prey had no effect on A. opacum survival, regardless of total prey abundance. 4. Surprisingly, food web simplification had no effect on the growth of A. opacum when present in food webs with a low abundance of prey but had important effects on A. opacum growth in food webs with a high abundance of prey. Simplifying a partial IGP food web with a high abundance of prey reduced A. opacum growth when either top predators or shared prey were removed from the food web and the loss of top predators and shared prey influenced A. opacum growth in a non-additive fashion. 5. The non-additive response in A. opacum growth appears to be the result of supplemental prey availability augmenting the beneficial effects of top predators. Top predators had a beneficial effect on A. opacum populations by reducing the abundance of A. opacum present and thereby reducing the intensity of intraspecific competition. 6. Our study indicates that the effects of food web simplification on the performance of A. opacum are complex and depend on both how a partial IGP food web is simplified and how abundant prey are in the food web. These findings are important because they demonstrate how trophic complexity can create variation in the performance of intermediate predators that play important roles in temporary pond food webs.     

Food web structure and the strength of transient indirect effects

The relative importance of direct and indirect effects in ecological communities remains unresolved. Indirect effects may diminish as they propagate through highly reticulate food webs. We tested this hypothesis by assembling replicate food webs of different complexity in laboratory microcosms, and comparing the transmission of indirect effects through these webs. By providing the top predator ( Didinium ) with either one ( Paramecium ) or two ( Paramecium and Colpidium ) species of protists as prey, we created linear or reticulate food webs where we could examine the transient response of predators to an indirect effect. Addition of Chlamydomonas , a small alga consumed by Paramecium , but not by Colpidium , perturbed the system and generated an indirect effect on Didinium . We expected the proportional response of Didinium to Chlamydomonas addition would be smaller in the reticulate web containing alternative, unperturbed prey ( Colpidium ). We measured predator response as predator yield, the maximum number of predators produced prior to overexploitation of prey and subsequent predator decline. The ratio of yield in perturbed bottles to yield in unperturbed bottles measures the proportional response of Didinium to Chlamydomonas addition. We expected this ratio to be smaller with Colpidium present. Contrary to expectations, alternative prey enhanced rather than diminished predator response to the perturbation. This resulted from competition between the prey species, a factor ignored in some simple verbal arguments. Food web complexity may have unanticipated consequences for the strength of indirect effects.     

Prey preference of top predators manipulates the functioning and stability of multi-trophic ecosystems

The relationship between biodiversity and ecosystem functioning, and the mechanisms underpinning the food web stability, have been intensively investigated in ecological research. The ubiquities of generalists in natural food webs and its important role in dictating these ecosystem properties have been generally recognized. However, how competition between multiple top predators shape these ecosystem properties and determine the success of invasive predators remain largely unexplored. Here, we use a well-developed food web model to investigate the effects of prey preference of top predators on ecosystem functioning and food web stability in both local and invasive conditions. We design several modeling scenarios to mimic combinations of different types of top predators (specialist/generalist) and their origins (local/invasive). Our model theoretically shows that lower exploitation competition for prey between top predators (with distinct prey preferences featured by higher attack rates) would be beneficial for the ecosystem functioning and food web stability. We also demonstrate that the success of top predator invasion depends on the prey preference of both local and invasive top predators. Sensitivity analysis on the model further supports our findings. Our results highlight the importance of prey preference of multiple top predators in manipulating the properties of multi-trophic ecosystems. Our findings may have important implications because the current ongoing global changes profoundly change the phenology of many biological systems and create trophic mismatch, which may manipulate prey preference of top predators and in turn deteriorate ecosystem functioning and food web stability.     

Fish–prey interactions in,and associated data from,a temperate stream of the Yura River,Kyoto, Japan

Although quantitative data on interspecific interactions within complex food webs are essential for evaluation of assumptions, hypotheses, and predictions of ecological theories; empirical studies yielding quantitative data on complex food webs are very limited. Ecological information on body size, habitat use, and seasonality of the component species of food webs aids in determining the mechanisms of food web structures. Ideally, ecological information on component species should be obtained contemporaneously when used to describe quantitative food webs, but such observations and sampling strategies are labor intensive and thus have been rarely described. We conducted year-round samplings of, and performed observations on, a temperate stream: the upper reaches of the Yura River, Kyoto, Japan. We derived quantitative data on the abundance, biomass, body mass, microhabitat use, and those seasonality of 7 fish species and 167 invertebrate taxa of the temperate stream food web. In addition, we estimated the per mass consumption rates of 7 predatory fish species, consuming 183 prey invertebrates, and the ratios between the per mass consumption rates of the 7 predatory fish species and the production rates of 78 prey invertebrates in each trophic link. All fishes and aquatic invertebrates were identified to species or lowest possible taxon. Our data may contribute to the construction of mathematical models explaining the behavior of stream communities/ecosystems.     

Effects of global environmental changes on parasitoid–host food webs and biological control

Global environmental changes threaten biodiversity and the interactions between species, and food-web approaches are being used increasingly to measure their community-wide impacts. Here we review how parasitoid–host food webs affect biological control, and how their structure responds to environmental change. We find that land-use intensification tends to produce webs with low complexity and uneven interaction strengths. Dispersal, spatial arrangement of habitats, the species pool and community differences across habitats have all been found to determine how webs respond to landscape structure, though clear effects of landscape complexity on web structure remain elusive. The invasibility of web structures and response of food webs to invasion have been the subject of theoretical and empirical work respectively, and nutrient enrichment has been widely studied in the food-web literature, potentially driving dynamic instability and altering biomass ratios of different trophic levels. Combined with food-web changes observed under climate change, these responses of food webs could signal changes to biological control, though there have been surprisingly few studies linking food-web structure to pest control, and these have produced mixed results. However, there is strong potential for food-web approaches to add value to biological control research, as parasitoid–host webs have been used to predict indirect effects among hosts that share enemies, to study non-target effects of biological control agents and to quantify the use of alternative prey resources by enemies. Future work is needed to link food-web interactions with evolutionary responses to the environment and predator–prey interactions, while incorporating recent advances in predator biodiversity research. This holistic understanding of agroecosystem responses and functioning, made possible by food-web approaches, may hold the key to better management of biological control in changing environments.     

Modelling Size Structured Food Webs Using a Modified Niche Model with Two Predator Traits

The structure of food webs is frequently described using phenomenological stochastic models. A prominent example, the niche model, was found to produce artificial food webs resembling real food webs according to a range of summary statistics. However, the size structure of food webs generated by the niche model and real food webs has not yet been rigorously compared. To fill this void, I use a body mass based version of the niche model and compare prey-predator body mass allometry and predator-prey body mass ratios predicted by the model to empirical data. The results show that the model predicts weaker size structure than observed in many real food webs. I introduce a modified version of the niche model which allows to control the strength of size-dependence of predator-prey links. In this model, optimal prey body mass depends allometrically on predator body mass and on a second trait, such as foraging mode. These empirically motivated extensions of the model allow to represent size structure of real food webs realistically and can be used to generate artificial food webs varying in several aspects of size structure in a controlled way. Hence, by explicitly including the role of species traits, this model provides new opportunities for simulating the consequences of size structure for food web dynamics and stability.     

Habitat selection in a large orb‐weaving spider: vegetational complexity determines site selection and distribution

1. The distribution of the large orb‐weaving spider Argiope trifasciata in old field habitats of North America and the habitat selection process this species used was studied for 2 years. 2. Because web spiders have limited dispersal abilities and an energetically costly prey capture device, they do not have the ability to sample potential foraging sites. Structural complexity of the vegetation to which the web must be attached is relatively easy to assess. The hypothesis that the structural complexity is a primary factor in determining initial web site selection was tested both by relating the natural distribution of the spiders across habitats to vegetational complexity and by manipulating the complexity of the habitats in a series of experiments. 3. Argiope trifasciata was not distributed evenly among three old field vegetation types. Habitat complexity was related to spider density in both years although no measure of insect activity, prey capture, or prey consumption was correlated with spider distribution. 4. Three experimental manipulations were conducted to test the impact of habitat structure on spider establishment: (1) the amount of natural vegetation was reduced, (2) structures were added to a simple habitat, and (3) the complexity of the structures added was varied. In each case, spiders were introduced and establishment of webs was monitored. In all manipulations, spider establishment was related to the complexity of the substrate available. 5. These results are important for understanding the cues that influence foraging site selection and therefore provide insight into the distribution of species with limited dispersal abilities and high site investment requirements.     

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.     

Correlation between interaction strengths drives stability in large ecological networks

Food webs have markedly non‐random network structure. Ecologists maintain that this non‐random structure is key for stability, since large random ecological networks would invariably be unstable and thus should not be observed empirically. Here we show that a simple yet overlooked feature of natural food webs, the correlation between the effects of consumers on resources and those of resources on consumers, substantially accounts for their stability. Remarkably, random food webs built by preserving just the distribution and correlation of interaction strengths have stability properties similar to those of the corresponding empirical systems. Surprisingly, we find that the effect of topological network structure on stability, which has been the focus of countless studies, is small compared to that of correlation. Hence, any study of the effects of network structure on stability must first take into account the distribution and correlation of interaction strengths.     

Anti-predator defence and the complexity-stability relationship of food webs

The mechanism for maintaining complex food webs has been a central issue in ecology because theory often predicts that complexity (higher the species richness, more the interactions) destabilizes food webs. Although it has been proposed that prey anti-predator defence may affect the stability of prey-predator dynamics, such studies assumed a limited and relatively simpler variation in the food-web structure. Here, using mathematical models, I report that food-web flexibility arising from prey anti-predator defence enhances community-level stability (community persistence and robustness) in more complex systems and even changes the complexity-stability relationship. The model analysis shows that adaptive predator-specific defence enhances community-level stability under a wide range of food-web complexity levels and topologies, while generalized defence does not. Furthermore, while increasing food-web complexity has minor or negative effects on community-level stability in the absence of defence adaptation, or in the presence of generalized defence, in the presence of predator-specific defence, the connectance-stability relationship may become unimodal. Increasing species richness, in contrast, always lowers community-level stability. The emergence of a positive connectance-stability relationship however necessitates food-web compartmentalization, high defence efficiency and low defence cost, suggesting that it only occurs under a restricted condition.     

Current food web models cannot explain the overall topological structure of observed food webs

Topological food webs illustrating “who eats whom” in different systems exhibit similar, non‐random, structures suggesting that general rules govern food web structure. Current food web models correctly predict many measures of food web topology from knowledge of species richness and connectance (fraction of possible predator–prey links that actually occur), together with assumptions about the ecological rules governing “who eats whom”. However, current measures are relatively insensitive to small changes in topology. Improvement of, and discrimination among, current models requires development of new measures of food web structure. Here I examine whether current food web models (cascade, niche, and nested hierarchy models, plus a random null model) can predict a new measure of food web structure, structural stability. Structural stability complements other measures of food web topology because it is sensitive to changes in topology that other measures often miss. The cascade and null models respectively over‐ and underpredict structural stability for a set of 17 high‐quality food webs. While the niche and nested hierarchy models provide unbiased predictions on average, their 95% confidence intervals frequently fail to include the observed data. Observed structural stabilities for all models are overdispersed compared to model predictions, and predicted and observed structural stabilities are uncorrelated, indicating that important sources of variation in structural stability are not captured by the models. Crucially, poor model performance arises because observed variation in structural stability is unrelated to variation in species richness and connectance. In contrast, almost all other measures of food web topology vary with species richness and connectance in natural webs. No model that takes species richness and connectance as the only input parameters can reproduce observed variation in structural stability. Further progress in predicting and explaining food web topology will require fundamentally new models based on different input parameters.     

Effects of trophic skewing of species richness on ecosystem functioning in a diverse marine community

Widespread overharvesting of top consumers of the world's ecosystems has "skewed" food webs, in terms of biomass and species richness, towards a generally greater domination at lower trophic levels. This skewing is exacerbated in locations where exotic species are predominantly low-trophic level consumers such as benthic macrophytes, detritivores, and filter feeders. However, in some systems where numerous exotic predators have been added, sometimes purposefully as in many freshwater systems, food webs are skewed in the opposite direction toward consumer dominance. Little is known about how such modifications to food web topology, e.g., changes in the ratio of predator to prey species richness, affect ecosystem functioning. We experimentally measured the effects of trophic skew on production in an estuarine food web by manipulating ratios of species richness across three trophic levels in experimental mesocosms. After 24 days, increasing macroalgal richness promoted both plant biomass and grazer abundance, although the positive effect on plant biomass disappeared in the presence of grazers. The strongest trophic cascade on the experimentally stocked macroalgae emerged in communities with a greater ratio of prey to predator richness (bottom-rich food webs), while stronger cascades on the accumulation of naturally colonizing algae (primarily microalgae with some early successional macroalgae that recruited and grew in the mesocosms) generally emerged in communities with greater predator to prey richness (the more top-rich food webs). These results suggest that trophic skewing of species richness and overall changes in food web topology can influence marine community structure and food web dynamics in complex ways, emphasizing the need for multitrophic approaches to understand the consequences of marine extinctions and invasions.