Apparent competition leaves no detectable imprint on patterns of community composition: observations from a natural experiment

相似文献   

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.     

Experimental warming transforms multiple predator effects in a grassland food web

This experimental study tests new theory for multiple predator effects on communities by using warming to alter predator habitat use and hence direct and indirect interactions in a grassland food web containing two dominant spider predator species, a dominant grasshopper herbivore and grass and herb plants. Experimental warming further offers insight into how climate change might alter direct and indirect effects. Under ambient environmental conditions, spiders used habitat in spatially complementary locations. Consistent with predictions, the multiple predator effect on grasshoppers and on plants was the average of the individual predator effects. Warming strengthened the single predator effects. It also caused the spider species to overlap lower in the vegetation canopy. Consistent with predictions, the system was transformed into an intraguild predation system with the consequent extinction of one spider species. The results portend climate caused loss of predator diversity with important consequences for food web structure and function.     

Spatial refuge from intraguild predation: implications for prey suppression and trophic cascades

The ability of predators to elicit a trophic cascade with positive impacts on primary productivity may depend on the complexity of the habitat where the players interact. In structurally-simple habitats, trophic interactions among predators, such as intraguild predation, can diminish the cascading effects of a predator community on herbivore suppression and plant biomass. However, complex habitats may provide a spatial refuge for predators from intraguild predation, enhance the collective ability of multiple predator species to limit herbivore populations, and thus increase the overall strength of a trophic cascade on plant productivity. Using the community of terrestrial arthropods inhabiting Atlantic coastal salt marshes, this study examined the impact of predation by an assemblage of predators containing Pardosa wolf spiders, Grammonota web-building spiders, and Tytthus mirid bugs on herbivore populations (Prokelisia planthoppers) and on the biomass of Spartina cordgrass in simple (thatch-free) and complex (thatch-rich) vegetation. We found that complex-structured habitats enhanced planthopper suppression by the predator assemblage because habitats with thatch provided a refuge for predators from intraguild predation including cannibalism. The ultimate result of reduced antagonistic interactions among predator species and increased prey suppression was enhanced conductance of predator effects through the food web to positively impact primary producers. Behavioral observations in the laboratory confirmed that intraguild predation occurred in the simple, thatch-free habitat, and that the encounter and capture rates of intraguild prey by intraguild predators was diminished in the presence of thatch. On the other hand, there was no effect of thatch on the encounter and capture rates of herbivores by predators. The differential impact of thatch on the susceptibility of intraguild and herbivorous prey resulted in enhanced top-down effects in the thatch-rich habitat. Therefore, changes in habitat complexity can enhance trophic cascades by predator communities and positively impact productivity by moderating negative interactions among predators.     

Intraguild interactions among generalist predator functional groups drive impact on herbivore and decomposer prey

Different functional groups of generalist predators may complement each other in controlling prey populations; but intraguild interactions, common among generalist predators, may also reduce the strength of top–down control. In natural communities greater alterations to ecosystem function are expected if a whole functional group declines in abundance or is lost. Therefore studying functional group diversity is important for predicting effects of predator loss. We studied the top–down impact of web‐building spiders, hunting spiders and ants, which are highly abundant generalist predators in most terrestrial ecosystems, on prey from the herbivore and decomposer system of a grassland food web. The density of the three predator groups was manipulated by continuous removal in a three‐factorial designed field experiment, which was carried out for two years. We found no positive effect of increasing predator functional group richness on prey control. However there was evidence for strong composition effects between the functional groups. The presence of ants in predator assemblages reduced the prey suppression through mostly trait‐mediated intraguild interactions, while hunting and web‐building spiders contributed additively to prey suppression and reduced the density of herbivore and decomposer prey by 50–60%. A trophic cascade on plant biomass triggered by web‐builders and hunting spiders was diminished at levels of higher predator group diversity. In conclusion, our experiments showed that intraguild interactions strongly influence the strength of top–down control by generalist predators. Among spiders there was evidence for a positive relation between functional group richness and prey suppression but the overall outcome strongly depended on the occurrence of interference, driven by trait‐mediated indirect interactions.     

Noncrop flowering plants restore top‐down herbivore control in agricultural fields

Herbivore populations are regulated by bottom‐up control through food availability and quality and by top‐down control through natural enemies. Intensive agricultural monocultures provide abundant food to specialized herbivores and at the same time negatively impact natural enemies because monocultures are depauperate in carbohydrate food sources required by many natural enemies. As a consequence, herbivores are released from both types of control. Diversifying intensive cropping systems with flowering plants that provide nutritional resources to natural enemies may enhance top‐down control and contribute to natural herbivore regulation. We analyzed how noncrop flowering plants planted as “companion plants” inside cabbage (Brassica oleracea) fields and as margins along the fields affect the plant–herbivore–parasitoid–predator food web. We combined molecular analyses quantifying parasitism of herbivore eggs and larvae with molecular predator gut content analysis and a comprehensive predator community assessment. Planting cornflowers (Centaurea cynanus), which have been shown to attract and selectively benefit Microplitis mediator, a larval parasitoid of the cabbage moth Mamestra brassicae, between the cabbage heads shifted the balance between trophic levels. Companion plants significantly increased parasitism of herbivores by larval parasitoids and predation on herbivore eggs. They furthermore significantly affected predator species richness. These effects were present despite the different treatments being close relative to the parasitoids’ mobility. These findings demonstrate that habitat manipulation can restore top‐down herbivore control in intensive crops if the right resources are added. This is important because increased natural control reduces the need for pesticide input in intensive agricultural settings, with cascading positive effects on general biodiversity and the environment. Companion plants thus increase biodiversity both directly, by introducing new habitats and resources for other species, and indirectly by reducing mortality of nontarget species due to pesticides.     

Herbivore functional traits and macroinvertebrate food webs have different responses to leaf chemical compounds of two macrophyte species in a tropical lake’s littoral zone

This research addressed the question of whether invertebrate food web structure varied between a native and an invasive macrophyte leaf species in the littoral zone of a tropical reservoir. We compared macroinvertebrate herbivore functional trait diversity composition with food web structure on the two macrophyte leaves, the invasive white ginger lily (Hedichium coronarium—Zingiberaceae) and the native pickerelweed (Pontederia cordata—Pontederiaceae). We predicted that the herbivore macroinvertebrate trait indices would decrease with macrophyte leaf species due to a lower resource quality with the flow-on effects in the food web structure. We calculated the number of functionally singular species (sing.sp) and herbivore functional trait richness (FRic) indices. For the macroinvertebrate food webs, we calculated the total number of trophic links (L), link density (L/S), connectance (C) and predator–prey ratios using a predator–prey matrix. We analysed the relationship between chemical traits of the macrophyte species’ leaves herbivore traits and food web indices using multivariate regression and Pearson’s correlation. Hedichium coronarium leaves had higher biomass and higher nitrogen content than the native P. cordata, which had higher phosphorus and carbohydrate content. Pontederia cordata leaves were associated with specialist macroinvertebrate species which primarily feed on biofilms (e.g. Ulmeritrus and Scirtidae) and plant leaves (e.g. Beardius). Food webs on P. cordata had lower numbers of trophic links (L), links per species (L/S) and predator–prey ratios. Connectance, which represents food web complexity, was similar between macroinvertebrate assemblages on the two leaf types. Our study suggests that chemical compounds of macrophyte leaves quality may have potential flow-on effects on food web structure.     

Structure of a herbivore–parasitoid community: Are parasitoids shared by different herbivore guilds?

Plant–herbivore–parasitoid interactions are a common occurrence in terrestrial food webs. Few parasitoids are thought to be shared by host insects of different feeding guilds because different parasitism strategies are required to use hosts of different feeding types. However, this assumption has rarely been tested using data from nature. To clarify whether parasitoids are shared among host guilds, I examined the structure of parasitoid communities on herbivore guilds associated with two Rhododendron species (Ericaceae) in a temperate secondary forest in central Japan. Leaf- and flower-feeding insects were collected from Rhododendron reticulatum and Rhododendron macrosepalum shrubs and reared in the laboratory for 3 years from April 1999 to March 2002. In total, 79 species of holometabolous herbivores (Lepidoptera, Diptera, Coleoptera, and Hymenoptera) were recorded, with 62 species on R. reticulatum and 51 species on R. macrosepalum. A total of 81 parasitoid species (Hymenoptera and Diptera) was recorded from the sampled herbivores, with 48 species from those on R. reticulatum and 50 species from those on R. macrosepalum. In total, 36 herbivore species were parasitised by 1–18 parasitoid species per host species, although the number of parasitoid species was strongly affected by sample size. Parasitoids that had two or more host species frequently attacked herbivore species from different families or on different host plants, whereas they did not attack species from different herbivore guilds; no parasitoids were shared between external feeders and rollers. Therefore, my results support the hypothesis that few parasitoids are shared among herbivores of different feeding guilds.     

The potential for indirect effects between a weed, one of its biocontrol agents and native herbivores: A food web approach

The safety of biological control is a contentious issue. We suggest that constructing and analyzing food webs may be a valuable addition to standard biological control research techniques, as they offer a means of assessing the post-release safety of control agents. Using preliminary data to demonstrate the value of food webs in biocontrol programs, we quantified the extent to which a key agent has infiltrated natural communities in Australia and, potentially, impacted on non-target species. Using these data, we also demonstrate how food webs can be used to generate testable hypotheses regarding indirect interactions between introduced agents and non-target species. We developed food webs in communities invaded to varying degrees by an exotic weed, bitou bush, Chrysanthemoides monilifera ssp. rotundata, and a key biocontrol agent for this weed in Australia, the tephritid fly, Mesoclanis polana. Three food webs were constructed during springtime showing the interactions between plants, seed-feeding insects and their parasitoids. One food web was constructed in a plot of native Australian vegetation that was free of bitou bush (‘bitou-free’), another in a plot of Australian vegetation surrounded by an invasion of bitou bush (‘bitou-threatened’) and a third from a plot infested with a monoculture of bitou bush (‘bitou-infested’). The bitou-free web contained 36 species, the bitou-threatened plot 9 species and the bitou-infested web contained 6 species. One native Australian herbivore attacked the seeds of bitou bush. M. polana, a seed-feeding fly, was heavily attacked by native parasitoids, these being more abundant than the parasitoids feeding on the native seed feeders. A surprising result is that none of the three species of native parasitoids reared from M. polana were reared from any of the native herbivores. The food webs revealed how a highly host-specific biocontrol agent, such as M. polana has the potential to change community structure by increasing the abundance of native parasitoids. The webs also suggest that indirect interactions between M. polana and native non-target species are possible, these been mediated by shared parasitoids. The experiments necessary to determine the presence of these interactions are outlined.     

Impact of flower-dwelling crab spiders on plant-pollinator mutualisms

Indirect effects in interactions occur when a species influences a third species by modifying the behaviour of a second one. It has been suggested that indirect effects of crab spiders (Thomisidae) on pollinator behaviour can cascade down the food web and negatively affect plant fitness. However, it is poorly understood how different pollinator groups react to crab spiders and, thus, when a reduction in plant fitness is likely to occur. Using continuous video surveillance, we recorded the behaviour of pollinators on two flower species and the pollinators’ responses to three crab spider treatments: inflorescences (1) with a pinned dried spider, (2) with a spider model made of paper, and (3) without spiders (control). We found that pollinators avoided inflorescences with dried spiders only on one plant species (Anthemis tinctoria). Pollinators showed no significant avoidance of paper spiders. Honeybees and bumblebees did not react to dried spiders, but solitary bees and syrphid flies showed a strong avoidance. Finally, we found no evidence that inflorescences with dried spiders suffered from a decrease in fitness in terms of a reduced seed set. We hypothesise that top-down effects of predators on plants via pollinators depend on the degree of specialisation of pollinators and their tendency to avoid spiders.     

Different groups of ground-dwelling spiders share similar trophic niches in temperate forests

1. Generalistic interactions between predator and prey may vary with ecosystem type, predator traits, and prey traits, but the interplay of these factors has not been assessed in ground food webs.
2. We investigated trophic interactions of ground-dwelling spiders across eight forests in European Russia associated with body size, hunting strategy, microhabitat specialization, potential prey type, potential prey population density, and forest type (coniferous vs. broadleaved). We analyzed 128 individual spiders, including juveniles, all identified to the family level with two complementary methods: molecular gut content analysis, and stable isotope analysis of carbon and nitrogen.
3. The results suggest that feeding frequency of spiders is affected by predator body size and by selection of certain prey type. Stable isotope analysis showed similar trophic niches among spider families, varying moderately with forest type. Larger spiders had higher Δ¹³C values than smaller ones, but similar Δ¹⁵N values, suggesting that different size classes of spiders belong to different food chains. Results based on stable isotope and molecular gut content analyses were weakly linked, indicating them targeting different trophic niche dimensions.
4. At least for the group-level interactions, family identity and hunting strategy of predator has little predictive power while predator body size and prey traits affected trophic niche dimensions calling for future studies in this direction. Large spiders feed more and rely on different basal resources than small spiders, suggesting that including small species and juveniles provides a more comprehensive picture of food web organization.

     

松嫩草原不同演替阶段大型土壤动物功能类群特征

大型土壤动物处于整个土壤食物网的最顶端,其各功能类群控制着其他动物所需资源的有效性,是土壤生态系统的重要组成部分。为了查明松嫩草原大型土壤动物的功能类群特征,在2006年5—10月期间,逐月对松嫩草原羊草、羊草+虎尾草、虎尾草、碱茅、碱蓬和光碱斑6个演替阶段大型土壤动物的功能类群组成、结构、多样性等特征进行研究。依据其食性将该区土壤动物划分为杂食性、植食性、捕食性和腐食性4个功能类群其中,杂食性土壤动物个体密度所占比例最多为39.16%,植食性土壤动物的类群数所占比例最多为50.00%,腐食性土壤动物个体密度和类群数所占比例均最小,分别为8.09%和12.82%。各功能类群土壤动物个体密度和类群数的相关性不显著(P0.05)。从水平结构来看,总体上各功能类群土壤动物在羊草群落和羊草+虎尾草群落个体密度和类群数较多,在无植被的光碱斑生境土壤动物的个体密度和类群数较少,植食性土壤动物的个体密度和类群数、杂食性土壤动物个体密度、腐食性土壤动物个体密度和类群数随着群落演替发生显著的变化(P0.01)。植食性和腐食性土壤动物个体密度和类群数相关性显著(P0.05)。垂直结构上,0—10 cm土层和20—30 cm土层除捕食性土壤动物个体密度以外,其它各功能类群土壤动物个体密度随着群落演替发生显著的变化(P0.05或P0.01);10—20 cm土层,除腐食性土壤动物个体密度以外,其它各功能类群土壤动物个体密度随着群落演替发生显著的变化(P0.05或P0.01)。0—10cm土层,植食性和杂食性土壤动物个体密度(P0.05)相关性显著;10—20 cm土层,植食性和腐食性土壤动物个体密度(P0.05)相关性显著。不同演替阶段对各功能类群土壤动物的多样性影响程度有所不同。4种功能类群土壤动物在羊草群落和光碱斑之间相似性指数较低,个体数量组成在演替初期的羊草群落和演替后期的光碱斑差异比较大。以上研究结果表明,松嫩草原不同退化演替阶段能够降低大型土壤动物功能类群组成和结构复杂性。     

Caught in the web: Spider web architecture affects prey specialization and spider–prey stoichiometric relationships

Quantitative approaches to predator–prey interactions are central to understanding the structure of food webs and their dynamics. Different predatory strategies may influence the occurrence and strength of trophic interactions likely affecting the rates and magnitudes of energy and nutrient transfer between trophic levels and stoichiometry of predator–prey interactions. Here, we used spider–prey interactions as a model system to investigate whether different spider web architectures—orb, tangle, and sheet‐tangle—affect the composition and diet breadth of spiders and whether these, in turn, influence stoichiometric relationships between spiders and their prey. Our results showed that web architecture partially affects the richness and composition of the prey captured by spiders. Tangle‐web spiders were specialists, capturing a restricted subset of the prey community (primarily Diptera), whereas orb and sheet‐tangle web spiders were generalists, capturing a broader range of prey types. We also observed elemental imbalances between spiders and their prey. In general, spiders had higher requirements for both nitrogen (N) and phosphorus (P) than those provided by their prey even after accounting for prey biomass. Larger P imbalances for tangle‐web spiders than for orb and sheet‐tangle web spiders suggest that trophic specialization may impose strong elemental constraints for these predators unless they display behavioral or physiological mechanisms to cope with nutrient limitation. Our findings suggest that integrating quantitative analysis of species interactions with elemental stoichiometry can help to better understand the occurrence of stoichiometric imbalances in predator–prey interactions.     

Trophic structure of the spider community of a Mediterranean citrus grove: A stable isotope analysis

Spiders are dominant terrestrial predators that consume a large variety of prey and engage in intraguild predation. Although the feeding habits of certain species are well known, the trophic structure of spider assemblages still needs to be investigated. Stable isotope analysis enables characterisation of trophic relationships between organisms because it tracks the energy flow in food webs and indicates the average number of trophic transfers between a given species and the base of the web, thus being a useful tool to estimate the magnitude of intraguild predation in food webs. Using this technique, we studied the trophic groups of spiders and their links within the arthropod food web of a Mediterranean organic citrus grove. We assessed the trophic positions of the 25 most common spider species relative to other arthropod predators and potential prey in the four seasons of the year, both in the canopy and on the ground. The analyses showed great seasonal variation in the isotopic signatures of some arthropod species, as well as the existence of various trophic groups and a wide range of trophic levels among spiders, even in species belonging to the same family. Differences in δ¹⁵N between spiders and the most abundant prey in the grove usually spanned two trophic levels or more. Our findings provide field evidence of widespread intraguild predation in the food web and caution against using spider families or guilds instead of individual species when studying spider trophic interactions.     

Flowers for better pest control? Ground cover plants enhance apple orchard spiders (Araneae), but not necessarily their impact on pests

Effects of ground cover upon arboreal spiders and their ability to control pests were studied in an apple orchard over six years. The ground cover treatments were: (1) annual and perennial flowering herbs (FLOWER), (2) regularly mowed grass (GRASS) and (3) weed-free bare ground (BAREgr) in the alleys. Spider abundance and species richness increased significantly in FLOWER compared to BAREgr, with GRASS being intermediate between the other two treatments. The effects of ground cover vegetation varied across spider guilds and individual species. The number of stalkers (Salticidae) increased sharply with the amount of vegetation in the alleys, while the increase was much less steep in ambushers (mainly Thomisidae and Philodromidae), and we found no significant difference between the treatments in space web builders (Theridiidae) and orb web builders (mainly Araneidae). The spider community was dominated by the intraguild predator Carrhotus xanthogramma (Salticidae), which showed a 3.5-fold increase in FLOWER compared to BAREgr and represented 40, 54 and 63% of the total spider abundance in BAREgr, GRASS and FLOWER treatments, respectively. The other species were less numerous and their response to FLOWER compared to BAREgr treatments ranged from a ninefold increase to a sixfold decrease. Within potential prey groups, hymenopteran parasitoids, dipterans and Auchenorrhyncha were the best predictors of spider abundance. In contrast, the abundance of apple pests was independent of spider density and showed no difference between treatments, indicating that spiders in the studied orchard had little impact on the size of the pest populations. This study demonstrates that functional traits and abundances of spider guilds and species, rather than the total spider abundance per se, might explain the success or failure of spiders in conservation biological control.     

A food web perspective on large herbivore community limitation

The exceptional diversity of large mammals in African savannas provides an ideal opportunity to explore the relative importance of top‐down and bottom‐up controls of large terrestrial herbivore communities. Recent work has emphasized the role of herbivore and carnivore body size in shaping these trophic relationships. However, the lack of across‐ecosystem comparisons using a common methodology prohibits general conclusions. Here we used published data on primary production, herbivore and carnivore densities and diets to estimate the consumption fluxes between three trophic levels in four African savanna ecosystems. Our food web approach suggests that the body size distribution within and across trophic levels has a strong influence on the strength of top‐down control of herbivores by carnivores and on consumption fluxes within ecosystems, as predicted by theoretical food web models. We generalize findings from the Serengeti ecosystem that suggest herbivore species below 150 kg are more likely to be limited by predation. We also emphasize the key functional role played by the largest species at each trophic level. The abundance of the largest herbivore species largely governs the consumption of primary production in resident communities. Similarly, predator guilds in which the largest carnivore species represent a larger share of carnivore biomass are likely to exert a stronger top‐down impact on herbivores. Our study shows how a food web approach allows integrating current knowledge and offers a powerful framework to better understand the functioning of ecosystems.     

Hyperparasitism and seasonal patterns of parasitism as potential causes of low top-down control in Euproctis chrysorrhoea L. (Lymantriidae)

Pest suppression is an important ecosystem service provided by biodiversity, though antagonistic interactions may jeopardize its impact on pest suppression. Hyperparasitoids may release herbivore populations from natural enemy pressure and lead to outbreaks directly due to parasitism as well as indirect through behavioural interference. In a previous study we reported that in native populations of Euproctis chrysorrhoea L. (Lymantriidae) primary parasitism was very low and outbreaks were more likely in coastal habitats than inland. Here we hypothesise that hyperparasitoids are the underlying cause of such patterns by reporting data on direct hyperparasitism rates as well as seasonal patterns of parasitoid attack. Of the 17 primary parasitoids attacking E. chrysorrhoea, three were found to be hyperparasitized. Hyperparasitoids attack the most important E. chrysorrhoea primary parasitoids which may explain the pattern of moth density in some habitats. Seasonal patterns of parasitoids attack and flight also help to understand antagonistic interactions among E. chrysorrhoea parasitoids. We discuss the implications of our work in the context of pest control in diverse ecosystems.     

Effects of alternative food on cannibalism and herbivore suppression by carabid larvae

1. Predator and alternative food density are important factors influencing herbivore suppression by generalist predators. Herbivore suppression can be reduced if predators forage preferentially on alternative foods. Cannibalism can increase at high predator densities, further reducing herbivore suppression. However, complex interactions are possible, as alternative food can increase predator abundance and survival restoring top‐down effects on herbivores. 2. In two species of carabid larvae (Poecilus chalcites and Anisodactylus ovularis), we studied how alternative foods (fly pupae and grass seeds) and predator density affect predation of black cutworm larvae and how alternative foods affect cannibalism among carabid larvae. 3. Adding alternative food to microcosms generally reduced total predation of cutworms. However, the strength of this effect was dependent on carabid species, larval density, and food type. 4. Increasing larval density from one to three per microcosm reduced per‐capita predation by both species irrespective of alternative food treatment. 5. Alternative food reduced cannibalism in both carabid species and increased survival of carabid larvae in field plots, such that twice as many were captured in plots subsidised with pupae than plots with no alternative food. 6. These results provide new insight into the complex interactions that influence predator survival and herbivore suppression in resource diverse habitats by demonstrating the primacy of intraguild interactions among carabid larvae.     

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.