Estimating morphologically determined connectance and structure for food webs of freshwater invertebrates

1. Connectance is a parameter of central importance in determining food-web structure, but the processes determining its value remain unclear. In evaluating possible explanations it is useful to know what patterns, and values, of connectance occur in food webs assembled at random from a set of species in a regional species pool; i.e. where the number of links is determined by the morphological features of the species present, not by the immediate effects of energetics or stability on the particular web. 2. This study examines, by means of laboratory experiments, the occurrence of potential feeding interactions among a set of freshwater invertebrate species randomly selected from different freshwater sites in a geographical region. The results from pairwise feeding trials are used to construct two ‘theoretical’ food webs, in which the patterns and values of connectance are examined. 3. Analyses of these webs indicate that their structure is consistent with the observed values in previously documented ‘real’ webs. Directed connectance values of 0.12–0.16 (or less) suggest that the assembled webs are no more connected than many freshwater webs from natural systems. The number of links per species increases curvilinearly with the number of species, during web assembly, consistent with recent hypotheses. 4. These results also indicate that quantifying, and understanding the determinants of, trophic generalism or specialism does have implications for understanding how connectance is constrained in real webs. Freshwater invertebrates seem to be relatively generalist, and freshwater food webs perhaps correspondingly highly connected. Such arguments have implications for interpreting other aspects of food-web structure in these systems, and for parameterizing models that are based on connectance.     

Global synthesis suggests that food web connectance correlates to invasion resistance

Biological invasions are a key component of global change, and understanding the drivers of global invasion patterns will aid in assessing and mitigating the impact of invasive species. While invasive species are most often studied in the context of one or two trophic levels, in reality species invade communities comprised of complex food webs. The complexity and integrity of the native food web may be a more important determinant of invasion success than the strength of interactions between a small subset of species within a larger food web. Previous efforts to understand the relationship between food web properties and species invasions have been primarily theoretical and have yielded mixed results. Here, we present a synthesis of empirical information on food web connectance and species invasion success gathered from different sources (estimates of food web connectance from the primary literature and estimates of invasion success from the Global Invasive Species Database as well as the primary literature). Our results suggest that higher‐connectance food webs tend to host fewer invaders and exert stronger biotic resistance compared to low‐connectance webs. We argue that while these correlations cannot be used to infer a causal link between food web connectance and habitat invasibility, the promising findings beg for further empirical research that deliberately tests for relationships between food web connectance and invasion.     

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

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

Food‐web structure varies along environmental gradients in a high‐latitude marine ecosystem

Large‐scale patterns in species diversity and community composition are associated with environmental gradients, but the implications of these patterns for food‐web structure are still unclear. Here, we investigated how spatial patterns in food‐web structure are associated with environmental gradients in the Barents Sea, a highly productive shelf sea of the Arctic Ocean. We compared food webs from 25 subregions in the Barents Sea and examined spatial correlations among food‐web metrics, and between metrics and spatial variability in seawater temperature, bottom depth and number of days with ice cover. Several food‐web metrics were positively associated with seawater temperature: connectance, level of omnivory, clustering, cannibalism, and high variability in generalism, while other food‐web metrics such as modularity and vulnerability were positively associated with sea ice and negatively with temperature. Food‐web metrics positively associated with habitat heterogeneity were: number of species, link density, omnivory, path length, and trophic level. This finding suggests that habitat heterogeneity promotes food‐web complexity in terms of number of species and link density. Our analyses reveal that spatial variation in food‐web structure along the environmental gradients is partly related to species turnover. However, the higher interaction turnover compared to species turnover along these gradients indicates a consistent modification of food‐web structure, implying that interacting species may co‐vary in space. In conclusion, our study shows how environmental heterogeneity, via environmental filtering, influences not only turnover in species composition, but also the structure of food webs over large spatial scales.     

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。     

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.     

The Serengeti food web: empirical quantification and analysis of topological changes under increasing human impact

1.?To address effects of land use and human overexploitation on wildlife populations, it is essential to better understand how human activities have changed species composition, diversity and functioning. Theoretical studies modelled how network properties change under human-induced, non-random species loss. However, we lack data on realistic species-loss sequences in threatened, real-world food webs to parameterize these models. 2.?Here, we present a first size-structured topological food web of one of the most pristine terrestrial ecosystems in the world, the Serengeti ecosystem (Tanzania). The food web consists of 95 grouped nodes and includes both invertebrates and vertebrates ranging from body masses between 10(-7) and 10(4) kg. 3.?We study the topological changes in this food web that result from the simulated IUCN-based species-loss sequence representing current species vulnerability to human disturbances in and around this savanna ecosystem. We then compare this realistic extinction scenario with other extinction sequences based on body size and connectance and perform an analysis of robustness of this savanna food web. 4.?We demonstrate that real-world species loss in this case starts with the biggest (mega) herbivores and top predators, causing higher predator-prey mass ratios. However, unlike theoretically modelled linear species deletion sequences, this causes poor-connected species to be lost first, while more highly connected species become lost as human impact progresses. This food web shows high robustness to decreasing body size and increasing connectance deletion sequences compared with a high sensitivity to the decreasing connectance deletion scenario. 5.?Furthermore, based on the current knowledge of the Serengeti ecosystem, we discuss how the focus on food web topology alone, disregarding nontrophic interactions, may lead to an underestimation of human impacts on wildlife communities, with the number of trophic links affected by a factor of two. 6.?This study underlines the importance of integrative efforts between the development of food web theory and basic field work approaches in the quantification of the structure of interaction networks to sustain natural ecosystems in a changing world.     

Major food web properties of two sandy beaches with contrasting morphodynamics, and effects on the stability

We determined major structural properties influencing the food webs of two sandy beaches with contrasting morphodynamics in the Atlantic coast of Uruguay: reflective (narrow and steep) and dissipative beaches (wide and flat). Furthermore, we evaluated how these characteristics could influence the stability of the local food webs. To this end, we examined the correlation of several food web properties with different ecosystem types (including freshwater habitats, estuary, marine, and terrestrial environments) using a principal components analysis. Sandy beach food web components included detritus, phytoplankton, zooplankton, benthic invertebrates, fishes, and seabirds. Our results revealed that the dissipative beach presented higher trophic levels, a higher number of trophic species, more links per species, as well as a higher proportion of intermediate trophic species, but lower connectance and proportion of omnivorous species than the reflective beach. The variation in the food web properties was explained by two principal components. Sandy beach food webs contribute mainly to one dimension of the principal components analysis that was determined by the number of trophic species, links per species, the trophic similarity, and the characteristic path length. We suggest that species and link characteristics, such as predominance of scavengers and detritivorous, the relatively high connectance and the short path length are drivers in the food web structure and may play a role in the community dynamic.     

Relative ascendency predicts food web robustness

Threats to ecosystems globally from anthropogenic disturbance and climate change requires us to urgently identify the most sensitive biological communities to ensure they are effectively preserved. It is for this reason that understanding and predicting food web stability has been topical within ecology. Food web stability is a multi-faceted concept that represents the ability of a food web to maintain its integrity following disturbance, it includes resistance, resilience and fragility. In this study, we examine the ability of four food web metrics to predict the fragility to random species extinctions in 120 qualitative food webs. We show that three information-based indices out performed food web connectance in predicting fragility, with relative ascendency having the strongest relationship. Relative ascendency was a much stronger predictor of fragility than MacArthur’s stability metric, Average Mutual Information and connectance as it accounted for both the distribution and number of links between species. We also find that most qualitative food webs persist around a central tendency of relative ascendency.     

Predicting the effects of temperature on food web connectance

Few models concern how environmental variables such as temperature affect community structure. Here, we develop a model of how temperature affects food web connectance, a powerful driver of population dynamics and community structure. We use the Arrhenius equation to add temperature dependence of foraging traits to an existing model of food web structure. The model predicts potentially large temperature effects on connectance. Temperature-sensitive food webs exhibit slopes of up to 0.01 units of connectance per 1°C change in temperature. This corresponds to changes in diet breadth of one resource item per 2°C (assuming a food web containing 50 species). Less sensitive food webs exhibit slopes down to 0.0005, which corresponds to about one resource item per 40°C. Relative sizes of the activation energies of attack rate and handling time determine whether warming increases or decreases connectance. Differences in temperature sensitivity are explained by differences between empirical food webs in the body size distributions of organisms. We conclude that models of temperature effects on community structure and dynamics urgently require considerable development, and also more and better empirical data to parameterize and test them.     

Connectance and parasite diet breadth in flea-mammal webs

The number of links in webs of species interactions, which lies at the heart of the biodiversity-stability debate, has given rise to controversy during the last 20 yr. Studies exploring these web properties have mainly focused on symmetric webs where each species can potentially feed on any other species; asymmetric webs such as host-parasite webs, where one set of species feed on another set of species, have been overlooked. However, food webs are incomplete without parasites and the study of parasite-host sub-web properties deserves attention. Here, using a large database involving 33 regional interaction webs between mammals and their flea parasites, we found a negative relationship between species richness and host-parasite connectance. We suggest that some phylogenetic constraints on flea diet may explain our observed patterns because we found that parasite diet breadth, measured as host taxonomic diversity, was invariant along our host richness gradient. We found that the slope of the logarithmic relationship between the number of realized links and species richness is lower than slope values reported for food webs. We suggest that connectance may not respond to increasing species richness as rapidly in host-parasite webs as in predator-prey food webs due to stronger coevolutionary requirements.     

How habitat-modifying organisms structure the food web of two coastal ecosystems

The diversity and structure of ecosystems has been found to depend both on trophic interactions in food webs and on other species interactions such as habitat modification and mutualism that form non-trophic interaction networks. However, quantification of the dependencies between these two main interaction networks has remained elusive. In this study, we assessed how habitat-modifying organisms affect basic food web properties by conducting in-depth empirical investigations of two ecosystems: North American temperate fringing marshes and West African tropical seagrass meadows. Results reveal that habitat-modifying species, through non-trophic facilitation rather than their trophic role, enhance species richness across multiple trophic levels, increase the number of interactions per species (link density), but decrease the realized fraction of all possible links within the food web (connectance). Compared to the trophic role of the most highly connected species, we found this non-trophic effects to be more important for species richness and of more or similar importance for link density and connectance. Our findings demonstrate that food webs can be fundamentally shaped by interactions outside the trophic network, yet intrinsic to the species participating in it. Better integration of non-trophic interactions in food web analyses may therefore strongly contribute to their explanatory and predictive capacity.     

The structure of food webs along river networks

Do changes in the species composition of riverine fish assemblages along river networks lead to predictable changes in food‐web structure? We assembled empirical “fish‐centered” river food webs for three rivers located along a latitudinal gradient in the South Saskatchewan River Basin (SSRB) that differ in land‐use impacts and geomorphology but flow through similar mountain, foothill, and prairie physiographic regions. We then calculated 17 food‐web properties to determine whether the nine river food webs differed according to physiographic region or river sub‐basin. There were no statistically significant differences in the 17 food‐web properties calculated among the rivers. In contrast, fish species richness, connectance, the proportion of herbivores, and the proportion of cannibals changed longitudinally along the river network. Our results suggest that regional changes in river geomorphology and physicochemistry play an important role in determining longitudinal variation in food‐web properties such as fish species richness and connectance. In contrast, the overall structure of river food webs may be relatively similar and insensitive to regional influences such as zoogeography. Further explorations of river and other food webs would greatly illuminate this suggestion.     

Indirect Energy Flows in Niche Model Food Webs: Effects of Size and Connectance

Indirect interactions between species have long been of interest to ecologists. One such interaction type takes place when energy or materials flow via one or more intermediate species between two species with a direct predator-prey relationship. Previous work has shown that, although each such flow is small, their great number makes them important in ecosystems. A new network analysis method, dynamic environ approximation, was used to quantify the fraction of energy flowing from prey to predator over paths of length greater than 1 (flow indirectness or FI) in a commonly studied food web model. Web structure was created using the niche model and dynamics followed the Yodzis-Innes model. The effect of food web size (10 to 40 species) and connectance (0.1 to 0.48) on FI was examined. For each of 250 model realizations run for each pair of size and connectance values, the FI of every predator-prey interaction in the model was computed and then averaged over the whole network. A classification and regression tree (CART) analysis was then used to find the best predictors of FI. The mean FI of the model food webs is 0.092, with a standard deviation of 0.0279. It tends to increase with system size but peaks at intermediate connectance levels. Of 27 potential predictor variables, only five (mean path length, dominant eigenvalue of the adjacency matrix, connectance, mean trophic level and fraction of species belonging to intermediate trophic levels) were selected by the CART algorithm as best accounting for variation in the data; mean path length and the dominant eigenvalue of the adjacency matrix were dominant.     

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.     

Food web complexity and stability across habitat connectivity gradients

The effects of habitat connectivity on food webs have been studied both empirically and theoretically, yet the question of whether empirical results support theoretical predictions for any food web metric other than species richness has received little attention. Our synthesis brings together theory and empirical evidence for how habitat connectivity affects both food web stability and complexity. Food web stability is often predicted to be greatest at intermediate levels of connectivity, representing a compromise between the stabilizing effects of dispersal via rescue effects and prey switching, and the destabilizing effects of dispersal via regional synchronization of population dynamics. Empirical studies of food web stability generally support both this pattern and underlying mechanisms. Food chain length has been predicted to have both increasing and unimodal relationships with connectivity as a result of predators being constrained by the patch occupancy of their prey. Although both patterns have been documented empirically, the underlying mechanisms may differ from those predicted by models. In terms of other measures of food web complexity, habitat connectivity has been empirically found to generally increase link density but either reduce or have no effect on connectance, whereas a unimodal relationship is expected. In general, there is growing concordance between empirical patterns and theoretical predictions for some effects of habitat connectivity on food webs, but many predictions remain to be tested over a full connectivity gradient, and empirical metrics of complexity are rarely modeled. Closing these gaps will allow a deeper understanding of how natural and anthropogenic changes in connectivity can affect real food webs.     

Resource tracking in marine parasites: going with the flow?

Understanding how diversity interacts with energy supply is of broad ecological interest. Most studies to date have investigated patterns within trophic levels, reflecting a lack of food webs which include information on energy flow. We added parasites to a published marine energy‐flow food web, to explore whether parasite diversity is correlated with energy flow to host taxa. Parasite diversity was high with 36 parasite taxa affecting 40 of the 51 animal taxa. Adding parasites increased the number of trophic links per species, trophic link strength, connectance, and food chain lengths. There was evidence of an asymptotic relationship between energy flowing through a food chain and parasite diversity, although there were clear outliers. High parasite diversity was associated with host taxa which were highly connected within the food web. This suggests that energy flow through a taxon may favour parasite diversity, up to a maximal value. The evolutionary and energetic basis for that limitation is of key interest in understanding the basis for parasite diversity in natural food webs and thus their role in food web dynamics.     

Will a large complex system be productive?

While the relationship between food web complexity and stability has been well documented, how complexity affects productivity remains elusive. In this study, we combine food web theory and a data set of 149 aquatic food webs to investigate the effect of complexity (i.e. species richness, connectance, and average interaction strength) on ecosystem productivity. We find that more complex ecosystems tend to be more productive, although different facets of complexity have contrasting effects. A higher species richness and/or average interaction strength increases productivity, whereas a higher connectance often decreases it. These patterns hold not only between realized complexity and productivity, but also characterize responses of productivity to simulated declines of complexity. Our model also predicts a negative association between productivity and stability along gradients of complexity. Empirical analyses support our predictions on positive complexity-productivity relationships and negative productivity-stability relationships. Our study provides a step forward towards reconciling ecosystem complexity, productivity and stability.     

Food web structure in a tropical stream ecosystem

Abstract This study investigated the structure and properties of a tropical stream food web in a small spatial scale, characterizing its planktonic, epiphytic and benthic compartments. The study was carried out in the Potreirinho Creek, a second‐order stream located in the south‐east of Brazil. Some attributes of the three subwebs and of the conglomerate food web, composed by the trophic links of the three compartments plus the fish species, were determined. Among compartments, the food webs showed considerable variation in structure. The epiphytic food web was consistently more complex than the planktonic and benthic webs. The values of number of species, number of links and maximum food chain length were significantly higher in the epiphytic compartment than in the other two. Otherwise, the connectance was significantly lower in epiphyton. The significant differences of most food web parameters were determined by the increase in the number of trophic species, represented mainly by basal and intermediate species. High species richness, detritus‐based system and high degree of omnivory characterized the stream food web studied. The aquatic macrophytes probably provide a substratum more stable and structurally complex than the sediment. We suggest that the greater species richness and trophic complexity in the epiphytic subweb might be due to the higher degree of habitat complexity supported by macrophyte substrate. Despite differences observed in the structure of the three subwebs, they are highly connected by trophic interactions, mainly by fishes. The high degree of fish omnivory associated with their movements at different spatial scales suggests that these animals have a significant role in the food web dynamic of Potreirinho Creek. This interface between macrophytes and the interconnections resultant from fish foraging, diluted the compartmentalization of the Potreirinho food web.     

Topological properties of food webs: from real data to community assembly models

We explore patterns of trophic connections between species in the largest and highest-quality empirical food webs to date, introducing a new topological property called the link distribution frequency (i.e. degree distribution), defined as the frequency of species S _L with L links. Non-trivial differences are shown in link distribution frequencies between species-rich and species-poor communities, which might have important consequences for the responses of ecosystems to disturbances. Coarse-grained topological properties observed, as species richness-connectance and number of links-species richness relationships, provide no support for the theory of links-species scaling law or constant connectance across empirical food webs investigated. We further explore these observations by means of simulated food webs resulting from multitrophic assembly models using different functional responses between species. Species richness-connectance and links-species richness relationships of empirical food webs are reproduced by our models, but degree distributions are not properly predicted, suggesting the need of new theoretical approximations to food web assembly. The best agreement between empirical and simulated webs occurs for low values of interaction strength between species, corroborating previous empirical and theoretical findings where weak interactions govern food web dynamics.