Food Web Statistics Research Articles

A topological similarity-based bootstrapping method for inferring food web parameters

A food web is a representation of trophic interactions in an ecosystem. Food webs are often generated by a single dataset aggregated from one or several surveys. Point estimates of food web parameters can be calculated from the data, but it remains an open question as to how their corresponding interval estimates can be quantified. Although conventional methods such as bootstrapping represent potential solutions, they tend to underestimate several network parameters. Here, we propose a simple bootstrap-based resampling procedure for inferring food web parameters. First, for a particular food web parameter, we obtain its point estimate by calculating the corresponding statistics from the original food web. Second, we generate a resampled food web by sampling with replacement the same number of species from the original food web, and for each resampled species we record how many prey items it consumes in the original food web. Third, a resampled species is allowed to consume its original prey species if such a species is also present; if not present, it instead consumes the resampled species that is most topologically similar to its original prey species. Many resampled food webs can be generated in this manner, and we calculate particular food web statistics for each of them. These form a sampling distribution from which interval estimates of the true food web parameter can be determined. We demonstrate our methodology on two different food web datasets and discuss its application in comparing food webs of various sizes and connectance.

Parasitism and Food Web Structure in Defoliating Lepidoptera - Parasitoid Communities on Soybean.

Food webs are usually regarded as snapshots of community feeding interactions. Here, we describe the yearly and cumulative structure of parasitoid-caterpillar food webs on soybean in central Argentina, analyzing parasitism rates and their variability in relation to parasitoid diversity and food web vulnerability in the system. Lepidoptera larvae were collected along four seasons from soybean crops and reared in laboratory to obtain and identify adults and parasitoids. Eleven species of defoliating Lepidoptera and ten parasitoid species were recorded. Food web statistics showed rather low annual variability, with most variation coefficients in the order of 0.20 and generality showing the most stable values. Parasitism showed the highest variability, which was independent of parasitoid diversity and food web vulnerability, although parasitism rates were negatively related to parasitoid richness. Our study highlights the need to consider food web structure and variability in order to understand the functioning of ecological communities in general and in extensive agricultural ecosystems in particular.

Loop: An R package for performing decomposition of weighted directed graphs, food web analysis and flexible network plotting

Demographic loop analysis is one of the basic methods applied in life cycle analysis in population ecology. Here, we developed an R package called “loop” to implement the algorithmic approach of loop analysis developed by a previous work. Additionally, the package can provide flexible network plotting and food web analysis as well. In this paper we illustrated the loop decomposition analysis using the life-cycle graph of a tropical tree species Vouacapoua americana; and performed food web statistics for the two real food webs for illustrating food web plotting and detecting key species in securing food web persistence. The package, including source code and binary versions, is available at the following URL: http://cran.r-project.org/web/packages/loop/.

The food web of parasitoid wasps and their non-phytophagous fly hosts in birds' nests (Hymenoptera: Chalcidoidea, and Diptera: Cyclorrhapha)

We present a food-web study of the parasitoid wasps and their fly hosts found in the nests of cavity-nesting songbirds. This birds' nest parasitoid web is different from all parasitoid webs studied comprising exclusively non-phytophagous host species. A total of 490 nests were examined for host occurrence, abundance and parasitism rates. Thirty-two species of potential hosts and 10 species of parasitoid wasps were recorded in nests from 25 sites in northern Germany and from two additional sites in south-western Germany. Most of these species were rare and unspecialized. The only very common parasitoid was Nasonia vitripennis (Chalcidoidea: Pteromalidae). The records of 14 regular host species from northern Germany, which are connected to all 10 parasitoid species, were used for further calculation of food-web statistics. The parasitoid communities in birds' nests were compared with other known parasitoid communities. The birds' nests exhibited a lower species richness compared with communities of phytophagous hosts, but showed similarities with communities of necrophagous or coprophagous hosts, or of parasitoid hosts. The parasitism rate was comparable in all host–parasitoid-systems, supporting the idea that the parasitism rate is independent of species richness.

Food webs

Food webs

Food web structure of three guilds of natural enemies: predators, parasitoids and pathogens of aphids

1. Most communities of insect herbivores are unlikely to be structured by resource competition, but they may be structured by apparent competition mediated by shared natural enemies. 2. The potential of three guilds of natural enemies (parasitoids, fungal entomopathogens and predators) to influence aphid community structure through indirect interactions is assessed. Based on the biology, we predicted that the scope for apparent competition would be greatest for the predator and least for the parasitoid guilds. 3. Separate fully quantitative food webs were constructed for 3 years for the parasitoid guild, 2 years for the pathogen guild and for a single year for the predator guild. The webs were analysed using standard food web statistics designed for binary data, and using information-theory-based metrics that make use of the full quantitative data. 4. A total of 29 aphid, 24 parasitoid, five entomopathogenic fungi and 13 aphid specialist predator species were recorded in the study. Aphid density varied among years, and two species of aphid were particularly common in different years. Omitting these species, aphid diversity was similar among years. 5. The parasitoid web showed the lowest connectance while standard food web statistics suggested the pathogen and predator webs had similar levels of connectance. However, when a measure based on quantitative data was used the pathogen web was intermediate between the other two guilds. 6. There is evidence that a single aphid species had a particularly large effect on the structure of the pathogen food web. 7. The predator and pathogen webs were not compartmentalized, and the vast majority of parasitoids were connected in a single large compartment. 8. It was concluded that indirect effects are most likely to be mediated by predators, a prediction supported by the available experimental evidence.

Parasites dominate food web links.

Parasitism is the most common animal lifestyle, yet food webs rarely include parasites. The few earlier studies have indicated that including parasites leads to obvious increases in species richness, number of links, and food chain length. A less obvious result was that adding parasites slightly reduced connectance, a key metric considered to affect food web stability. However, reported reductions in connectance after the addition of parasites resulted from an inappropriate calculation. Two alternative corrective approaches applied to four published studies yield an opposite result: parasites increase connectance, sometimes dramatically. In addition, we find that parasites can greatly affect other food web statistics, such as nestedness (asymmetry of interactions), chain length, and linkage density. Furthermore, whereas most food webs find that top trophic levels are least vulnerable to natural enemies, the inclusion of parasites revealed that mid-trophic levels, not low trophic levels, suffered the highest vulnerability to natural enemies. These results show that food webs are very incomplete without parasites. Most notably, recognition of parasite links may have important consequences for ecosystem stability because they can increase connectance and nestedness.

Influence of life history and seasonal hydrology on lipid storage in three neotropical fish species

The seasonal dynamics of energy (lipid) storage in three neotropical fish species with differing life histories were evaluated. Lipid content was substantially greater in the liver than in dorsal musculature of all three species. Two piscivores (Cichla temensis and Serrasalmus manueli) showed large, statistically significant seasonal fluctuations in liver lipid content. Liver lipid content increased during high water, through the falling water, and into the early dry season for both piscivores. Seasonal variation in dorsal muscle lipid content was large and statistically significant for C. temensis, but was small and non‐significant for S. manueli. Cichla temensis appeared to ‘finance’ costs associated with reproduction by accumulating lipids during the falling‐water period when migratory prey allowed the species to subsidize their energetic dynamics. Semaprochilodus kneri, a migratory algivore and detritivore, showed no significant seasonal variation in dorsal muscle lipid content and minimally significant seasonal variation in liver lipid content. Statistically significant effects of lipid content on δ13C was observed when tissue lipid content varied by >12%, while biological interpretation of food web statistics based on δ13C values appears robust to minor variations in lipid content. Nonetheless, when lipid content varied by larger amounts (e.g. >35% for C. temensis and S. manueli liver tissue) lipids appeared to have a large potential effect on δ13C and food web statistics calculated from such measurements may have been biased. Surprisingly, even large variation in tissue lipid content did not affect δ15N.

Structure of a diverse tropical forest insect–parasitoid community

SummaryQuantitative host–parasitoid food webs are descriptions of community structure that include data on the abundance of hosts and parasitoids, and the frequency of links between them, all expressed in the same units.Quantitative host–parasitoid food webs were constructed describing the community of leaf‐mining insects (Diptera, Coleoptera and Lepidoptera) and their parasitoids (Hymenoptera) in an 8500‐m2area of moist tropical forest in Belize, Central America, over a 1‐year period.The summary food web, containing data for the whole year, is we believe the most diverse quantitative host–parasitoid web yet described. It contains 93 species of leaf‐miner, 84 species of parasitoid and 196 links between hosts and parasitoids.Most parasitoids were generalists, with individual parasitoid species recorded as parasitizing up to 21 host species. In contrast, most leaf‐miners were specialists, with 114 links documented between leaf‐miners and their host plants.A robustness analysis was used to reveal the effects of different sampling intensities on food web statistics. The results suggest that the sampling had revealed most of the species of host and parasitoid in the community, but further interactions among species would continue to be detected with additional sampling. Measures of the ratio of hosts to parasitoids and of realized connectance were relatively insensitive to sampling intensity.Three seasonal webs were constructed, revealing temporal changes in the structure of the community. There was greater turnover in host species composition than parasitoid species composition among seasons, but most web statistics remained relatively constant across seasons.Both the summary web and the seasonal webs show low levels of compartmentalization, suggesting that the host–parasitoid community is not divided into relatively discrete subwebs with largely independent dynamics.The extent of potential indirect interactions between pairs of hosts was assessed by constructing quantitative parasitoid overlap graphs. These suggest that abundant species are likely to have greater indirect effects on less abundant species thanvice versa, and that species in the same taxonomic order are more likely to interact indirectly. The graphs do not support the hypothesis that species sharing the same host plant are more likely to interact via shared parasitoids.

Spatio‐temporal variation of avian foraging in the rocky intertidal food web

Abstract While birds are top predators in most rocky intertidal communities, the relationship between foraging pattern and variability in food web structure has not been studied. This study examined the spatio‐temporal variation of both avian foraging and food web structure at an intertidal rocky shore in northern Japan over a 1‐year period. Seventeen bird species foraged on the intertidal rocky shore. Crows and gulls were dominant, and their major prey was sea urchins that migrated from the sub‐tidal to intertidal habitat. Interspecific interactions (i.e. stealing of food, utilization of food waste by other species) occurred between crows and gulls especially when feeding on sea urchins. The prey of the birds showed spatial and temporal partitioning. The number of prey items consumed per day by the dominant birds varied with various factors. Factors strongly affecting the foraging pattern of crows were waves, tides, humans and gulls, and those strongly affecting the foraging pattern of gulls were tides, humans, sea urchins and crows. In the rocky intertidal food web, most of the top predators were birds, and most of the birds were omnivores. The birds consumed many more species than did other consumers. Food‐resource partitioning caused spatio‐temporal compartmentation among subwebs in which the top predators were dominant birds. Analysis of food web statistics (i.e. web size, numbers of links, linkage density, chain lengths) revealed that the presence/absence of birds did not change the relationships between web size and the other statistics. The food web statistics depended on web size, and the web size was positively related with time spent emersed and temperature when birds were both present and not present. Birds often foraged across habitat boundaries, and the main food resource of top predators was the prey species from the subtidal habitat. Therefore, the spatial scale of the Hiura rocky intertidal food web temporally varied with birds foraging across habitat boundaries.

Trophic relationships: integrating meiofauna into a realistic benthic food web

1 This paper summarises the most important contributions on trophic relationships of lotic meiofauna. In contrast to marine research, the few quantitative studies of the freshwater meiobenthos have shown that these invertebrates not only take up particulate/fine organic matter, but also dissolved organic substances attached to organic particles. In lotic ecosystems, further estimates of grazing rate and bacterial/algal ingestion rate are needed, particularly in situ measurements. 2 The effects of macroinvertebrate predators upon meiofauna are still under debate. Depending on the type of experiments (laboratory vs. field) it seems that macrofauna may or may not affect meiofauna. Field samples and analyses of gut contents of larval tanypod chironomids have shown that the impact upon meiofauna was low and larvae were nonselective predators. Predation amounted to 2.2% of the combined prey density and prey consumption averaged 1.3 individuals per predator individual per year. 3 Adding taxonomic resolution by including the meiofaunal component within lotic food webs distinctly increases the number of total species and, as a consequence, changes food web statistics. Webs that included meiofauna revealed that these metazoans contributed substantially to the percentage of intermediate species (species with predators and prey). The resolution of dietary analyses of major consumers of macro- and meiobenthos showed that many stream invertebrates feed on meiofauna.

A re‐examination of geographical variation in Nepenthes food webs

The metazoan food webs of Nepenthes pitcher plants exhibit marked geographical variation In previous studies this was attributed lo a number of biogeographical factors including island area. local arthropod biodiversity and degrees of spatial and temporal variation of the host Nepenthes species. Recent investigations showed that some assumptions made in compiling these webs are erroneous and that predation is more widespread in these communities than previously assumed. In this study the new data are used to revise several webs from the geographical studies for comparison with the original versions. Conneetance values and predator:prey ratios increased significantly after revision (though all other food web statistics did not), providing evidence for widespread predation. This is presumed to be a result of some metazoan species adopting variable feeding strategies, depending on which food sources are available in their piteher. Despite this, the geographical patterns are still evident alter revision of the webs. Whether or not these are actually due to geographical Victors is discussed, h is suggested that the structure of Nepenthes webs is influenced by tour major factors‐ “geographical, deterministic, stochastic and host pitcher (the latter was discounted in previous studies), but that the relative importance of each factor is highly variable and cannot be quantified at present.

Predation and food web structure along a habitat duration gradient.

Food web statistics showed a complex relationship with measures of habitat variability in temporary ponds. Connectance was highest in short-duration, highly variable habitats, and lowest in habitats of intermediate duration and variability. The number of links and links/taxon increased with increasing duration. Much of the variation in the food web statistics could be explained by a strong linear relationship between number of taxa and number of links/taxon and a quadratic relationship of taxa number with the number of links. However, after accounting for this variation, there remained a relationship of duration with links and links/taxon. The relationship between the food web statistics and duration corresponded to experimental evaluations of predation in these habitats that showed an increasing importance of predation in long-duration habitats. The food web statistics, however, missed threshold effects in the relationship between predation and habitat duration. Differences in food web statistics before and after a regional drought could be explained by a decrease in taxa number after the drought. Connectance was the most robust statistic in relation to taxa number, but was also the least sensitive to changes in habitat characteristics.

Food web dynamics of irrigated rice fields at five elevations in Luzon, Philippines

AbstractThe above-water food webs of arthropod communities in irrigated rice fields on Luzon Island, Philippines, were studied over the growing season at five sites (Los Baños, Cabanatuan, Bayombong, Kiangan, Banaue) ranging in elevation from 22 m to 1524 m. Arthropod populations were vacuum-sampled at roughly weekly intervals from the date after seedlings were transplanted to flowering at each site. Site- and time-specific webs were constructed from a 687-taxa cumulative Philippines web and time-series of species present. Taxonomic composition, food web structure, and arthropod phenology were broadly similar across different sites. Arthropod abundance was inversely associated with altitude across the five sites, but numbers of taxa and links and six food web statistics showed no obvious increasing or decreasing trend with altitude. The rise of taxa, links and mean food chain length over the growing season at each site reflected an increase in plant size with age and, at some sites, an orderly accumulation of newly arriving herbivore, predator, parasitoid and omnivore species. At each site, herbivores built up faster than predators and parasitoids, and predators arrived faster than parasitoids; the difference between the latest and earliest sampling dates of first arrivals, averaged over the five sites, was 38, 63 and 73 days for herbivores, predators and parasitoids, respectively. Site-to-site consistencies in food web properties and first arrivals suggest that such patterns may be influenced more by crop age than by geography or altitude. Sampled predator, parasitoid and omnivore taxa potentially encountered only a subset of their lifetime prey and predator species at any particular time in the rice field. Prey lists cumulated over time may underestimate the temporal specificity of predation by potential biological control agents. Research opportunities linking rice food webs and integrated pest management with East Indies biogeography are proposed.

Scale-Dependence of Food Web Structures: Tropical Ponds as Paradigm

Two sets of time-specific food webs were constructed using all known details of trophic biology of the 87 component species found in two tropical freshwater pond systems. Analysis of the webs showed that several food web statistics were scale-dependent. To examine the robustness of this finding, 11 500 computerized analog webs were built upon the specified rules of non-random trophic links (L) between the organisms found in the ponds. The structural properties of the analog webs, like the recently published species-rich webs, disagreed with several generalizations made by food web theorists over the last two decades. The magnitude of L and the frequency of long food chains increased with S. Proportions of top, intermediate and basal species showed strong nonlinear variation with S. The mean number of links between different trophic levels seemed to be scale-dependent, though apparently scale-invariant for the pond webs. However, the relative proportions of link fractions appeared to be scale-invariant, with the intermediate-basal links most numerous. The mean predator-prey ratio appeared to be the most scale-invariant of 'all the web properties, but the scale invariance of the ratio may not be directly derived from species fractions. The study represents the largest tropical aquatic food web database, and indicates that (a) as many details of inter-specific interactions as possible within a web are necessary for unraveling the actual complexity of real ecosystems, and (b) computerized analogs of real webs, incorporating all necessary details of interactions, may be fruitfully used to ensure robustness of conclusions about the web features

Spatial and Temporal Variation in the Structure of an Intermittent‐Stream Food Web

Food webs from the Lerderderg River, an intermittent stream in Victoria, Australia, were compiled with the aim of examining changes in food web structure in a highly variable habitat. Emphasis was placed on a high degree of taxonomic precision. Spatial and temporal variation in the food webs was assessed by partitioning the study area into three sites, located ≈1.5 km apart along the river. Sites differed in overall stream width and the length of the low streamflow period during summer. Three separate webs for each site were compiled for four different times of the year. Relatively little spatial variation in community structure was observed. In contrast, temporal variation was considerable, with species composition and the number of species in the community changing considerably over the year. The number of species increased dramatically as the period of constant streamflow lengthened. Detritivores dominated the community, both in terms of species and individual numbers. The proportion of predators in the community increased slightly by the end of the year, suggesting that recolonization of the community by predators lags behind that of detritivores. The increase in the number of predator species also resulted in an increase in the mean food chain length through the year. Patterns observed in the food webs tended to fall within the range of values reported from several previous studies, suggesting that underlying constraints may structure certain aspects of food webs. However, the constancy of certain food web statistics was attributable either to methodological decisions made during compilation, or to an inherent property of the statistic itself. The potential sensitivity of several food web statistics to the methodology used to compile a food web render between—web comparisons difficult due to the confounding effects of methodology. This suggests that comparisons between food webs should be restricted to webs derived from similar habitats using a comparable methodology.

Multiple definitions of food web statistics: An unnecessary problem for food web research

Three of the statistics that are regularly used to describe food web structure have been defined in two different ways. The use of these statistics by several authors illustrates some of the problems caused by the existence of two different definitions for what is, at least in name, the same statistic.

Spatial and Temporal Variation in Tropical Fish Trophic Networks

Observed properties of natural food webs have both important theoretical and important management implications. Four lowland aquatic food webs were investigated over the course of two years: a large swamp and a small stream in Costa Rica, and a similar swamp and stream in the Venezuelan llanos. Each local ecosystem differed from the three others with respect to environmental changes associated with seasonal rainfall. Phylogenetic composition and diversity of biotas also varied among systems. Volumetric proportional utilization coefficients from fish gut contents were used as estimates of the intensity of predator—prey interactions. An annual and two or more seasonal food webs were constructed for each local community. Aquatic communities were defined operationally using common fish species as consumers, and using the sink subweb associated with the top predator of each system. A computer calculated a variety of food—web statistics and plotted food—web diagrams containing either (a) all observed trophic links (predator—prey interactions), or (b) subsets with weak links eliminated at prescribed thresholds. Individual community food webs contained from 58 (stream, Costa Rica) to 104 (swamp, Venezuela) interactive taxonomic units and from 208 to 1243 total trophic links. Food—web parameters were very sensitive to changes in level of link threshold. Web connectance and related parameters converged near link threshold 0.04 (utilization coefficients <0.04 eliminated) in a variety of inter—web comparisons. Despite large differences in assemblage composition and attributes of the physical environment, distributions of trophic levels calculated according to a trophic continuum algorithm were very similar among study systems. Herbivores, detritivores, and their direct predators formed the largest proportions of fishes in each assemblage, followed by omnivores and secondary carnivores. Fishes that fed at more than one trophic interval were extremely common in all food webs. Analysis of covariance was used to compare structural features of different webs across a range of link thresholds. Extensive among—site variation in food—web parameters was associated with differences in species richness and environmental differences associated with rainfall patterns, physiography, and gross primary production. Seasons generally influenced food—web parameters less than did site differences. Relative importance of detritus, aquatic primary production, and terrestrial production in aquatic food webs varied seasonally in each system. Detritus, derived primarily from aquatic macrophytes, was an important pathway in both tropical swamp ecosystems. Aquatic primary productivity comprised the largest fraction of fish diets during the wet season in the Venezuelan swamp, but it formed the major component of fish diets during the dry season at all other sites. Based on comparisons using 13 webs, two—thirds of the pairings among six food—web parameters used (number of nodes, compartmentation, connectance, average number of prey per node, average number of predators per node, ratio of consumer nodes to total nodes) were positively intercorrelated. Several food—web relationships previously described as constant (e.g., connectance x species richness constancy, species scaling law, link—species scaling law) were not confirmed by my data. These earlier food—web trends are extremely sensitive to methodological biases, especially decisions regarding the degree of taxonomic lumping of species into trophic units. Although food webs have unique emergent properties and spinoff a number of potentially informative macrodescriptors, empirical studies must achieve greater precision and uniformity before analyses can be performed across different systems. Several problems and potential resolutions are discussed.

The ecology of water‐filled treeholes in Australian rainforests: Food web reassembly as a measure of community recovery after disturbance

AbstractTen water‐filled tree holes in a subtropical rainforest were emptied out thereby imposing a major perturbation on the community of organisms which occupied these habitat units. During the process of food web reassembly following this disturbance, the average number of predator species in the web increased as did the average number of prey species. Predator‐prey ratios increased in magnitude with the number of days after the disturbance. Furthermore, the characteristic mean predator‐prey ratio of the original community was gradually restored as the food web reassembled. The number of prey species was found to be a better predictor of the number of predator species during community recovery than vice versa. The increasing number of trophic links in the food web over the period of reassembly, due mainly to the increase in the number of predacious species in the web, provides a good overall measure of community recovery. The paper discusses the use of relationships among the food web statistics described within a model of food web reassembly. Such a model can be used to assess the rate of recovery of a community after a disturbance of the food web at the regional level of resolution. Further, the question of whether or not the rate of community recovery can be used to estimate ecosystem recovery with the aid of food web statistics is examined.

Scale invariance in food web properties.

The robustness of five common food web properties is examined by varying the resolution of the data through aggregation of trophic groupings. A surprising constancy in each of these properties is revealed as webs are collapsed down to approximately half their original size. This analysis of 60 invertebrate-dominated community food webs confirms the existence of all but one of these properties in such webs and addresses a common concern held by critics of food web theory that observed food web properties may be sensitive to trophic aggregation. The food web statistics (chain length; predator/prey ratio; fraction of top, intermediate, and bottom species; and rigid circuits) are scaling in the sense that they remain roughly invariant over a wide range of data resolution. As such, within present standards of reporting food web data, these statistics may be used to compare systems whose trophic data are resolved differently within a factor of 2.