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。     

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.     

Comparing the fish assemblages and food‐web structures of large floodplain rivers

相似文献   

Structure of tropical river food webs revealed by stable isotope ratios

Fish assemblages in tropical river food webs are characterized by high taxonomic diversity, diverse foraging modes, omnivory, and an abundance of detritivores. Feeding links are complex and modified by hydrologic seasonality and system productivity. These properties make it difficult to generalize about feeding relationships and to identify dominant linkages of energy flow. We analyzed the stable carbon and nitrogen isotope ratios of 276 fishes and other food web components living in four Venezuelan rivers that differed in basal food resources to determine 1) whether fish trophic guilds integrated food resources in a predictable fashion, thereby providing similar trophic resolution as individual species, 2) whether food chain length differed with system productivity, and 3) how omnivory and detritivory influenced trophic structure within these food webs. Fishes were grouped into four trophic guilds (herbivores, detritivores/algivores, omnivores, piscivores) based on literature reports and external morphological characteristics. Results of discriminant function analyses showed that isotope data were effective at reclassifying individual fish into their pre-identified trophic category. Nutrient-poor, black-water rivers showed greater compartmentalization in isotope values than more productive rivers, leading to greater reclassification success. In three out of four food webs, omnivores were more often misclassified than other trophic groups, reflecting the diverse food sources they assimilated. When fish δ¹⁵N values were used to estimate species position in the trophic hierarchy, top piscivores in nutrient-poor rivers had higher trophic positions than those in more productive rivers. This was in contrast to our expectation that productive systems would promote longer food chains. Although isotope ratios could not resolve species-level feeding pathways, they did reveal how top consumers integrate isotopic variability occurring lower in the food web. Top piscivores, regardless of species, had carbon and nitrogen profiles less variable than other trophic groups.     

Quantitative food webs of lepidopteran leafminers and their parasitoids in a Japanese deciduous forest

Quantitative food webs were constructed to explore the community structure of leaf-mining moths in the family Gracillariidae and their parasitoid wasps in a deciduous forest in Hokkaido, Japan. A whole food web was constructed from data collected from June to October 2001. In the web, 16 leafminer species on seven tree species were attacked by 58 species of hymenopteran parasitoid; 376 links between leafminers and parasitoids were observed. Leafminers were specialist herbivores, but most parasitoids were generalists. Five webs were constructed for the seasonal prevalence of leafminers over the one-year period to reveal the temporal dynamics in community structure. Among the seasonal webs, the first web in June was distinctive because two tree species, Japanese umbrella tree Magnolia obovata and Japanese magnolia M. kobus, supported the community. Second to fourth webs from July to September were dominated by the leafminer species on Japanese oak Quercus crispula, and the fifth web was marked by that on Carpinus cordata. The extent of potential apparent competition among leafminers was evaluated using quantitative parasitoid overlap diagrams. These diagrams suggested that abundant host species are likely to have large indirect effects on less abundant species. Moreover, the potential for apparent competition between leafminer species inhabiting different host tree species can occur, although leafminers sharing the same tree species are prone to interact via shared parasitoids. In this system, particular leafminer species, acting as potential sources of apparent competition, can affect other species as sinks, and control whole-community dynamics. Directed apparent competition may potentially occur around oak trees.     

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.     

Structure of the food web including the endangered lycaenid butterfly Shijimiaeoides divinus asonis (Lepidoptera: Lycaenidae)

The structure of the food web including the endangered lycaenid butterfly Shijimiaeoides divinus asonis (Matsumura) was analyzed to identify species contributing most to maintaining the equilibrium of the food web. Twenty‐seven species belonging to 17 families fed on Sophora flavescens Aiton, the host‐plant of S. divinus asonis: 15 species were leaf and stem feeders, seven (including S. divinus asonis) fed on flower buds, four were flower feeders and one fed on the seeds of So. flavescens. Of these 27 species, four were omnivores. The natural enemies of S. divinus asonis comprised six insect species, 11 spider species and one entomopathogenic fungus species, including six new predator records. The linkage density, total number of trophic links, connectance, average chain length and predator–prey ratio were 1.617, 97, 0.0548, 2.267 and 0.694, respectively. Exclusion of any of the 15 species with four or more trophic links reduced the connectance of the food web. These 15 species included facultative mutualistic attendant ants and predators of S. divinus asonis, herbivores to So. flavescens, an omnivore feeding on S. divinus asonis and So. flavescens, and prey insects. Therefore, future studies should monitor these 15 species.     

Geographic variation in network structure of a nearctic aquatic food web

Aim The network structure of food webs plays an important role in the maintenance of diversity and ecosystem functioning in ecological communities. Previous research has found that ecosystem size, resource availability, assembly history and biotic interactions can potentially drive food web structure. However, the relative influence of climatic variables that drive broad‐scale biogeographic patterns of species richness and composition has not been explored for food web structure. In this study, we assess the influence of broad‐scale climatic variables in addition to known drivers of food web structure on replicate observations of a single aquatic food web, sampled from the leaves of the pitcher plant (Sarracenia purpurea), at different geographic sites across a broad latitudinal and climatic range. Location Using standardized sampling methods, we conducted an extensive ‘snapshot’ survey of 780 replicated aquatic food webs collected from the leaves of the pitcher plant S. purpurea at 39 sites from northern Florida to Newfoundland and westward to eastern British Columbia. Methods We examined correlations of 15 measures of food web structure at the pitcher and site scales with geographic variation in temperature and precipitation, concentrations of nutrients from atmospheric nitrogen deposition, resource availability, ecosystem size and the abundance of the pitcher plant mosquito (Wyeomyia smithii), a potential keystone species. Results At the scale of a single pitcher plant leaf, linkage density, species richness, measures of chain length and the proportion of omnivores in a web all increased with pitcher volume. Linkage density and species richness were greater at high‐latitude sites, which experience low mean temperatures and precipitation and high annual variation in both of these variables. At the site scale, variation in 8 of the 15 food web metrics decreased at higher latitudes, and variation in measures of chain length increased with the abundance of mosquitoes. Main conclusions Ecosystem size and climatic variables related to latitude were most strongly correlated with network structure of the Sarracenia food web. However, in spite of large sample sizes, thorough standardized sampling and the large geographic extent of the survey, even the best‐fitting models explained less than 40% of the variation in food web structure. In contrast to biogeographic patterns of species richness, food web structure was largely independent of broad‐scale climatic variables. The large proportion of unexplained variance in our analyses suggests that stochastic assembly may be an important determinant of local food web structure.     

High‐resolution food webs based on nitrogen isotopic composition of amino acids

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well‐understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher‐order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free‐roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.     

Parasites alter the topology of a stream food web across seasons

Relatively few published food webs have included parasites, and in this study we examined the animal community in a stream across eight contiguous seasons to test how inclusion of helminth parasites alters the topology or structure of the food web. Food webs constructed for each season and analyzed using common binary matrix measures show that species richness, linkage density, and the number of observed and possible links increased when parasites were included as individual species nodes. With parasite–parasite and predator–parasite links omitted, measures of community complexity, such as connectance (C), generally increased over multiple seasons. However, relative nestedness (n*) decreased when parasites were included, which may be a result of low resolution of basal resources inflating specialist-to-specialist links. Overall, adding parasites resulted in moderate changes in food web measures when compared to those of four other published food webs representing different ecosystems. In addition, including parasites in the food web revealed consistent pathways of energy flow, and the association of parasite life histories along these pathways suggest stable evolutionary groups of interacting species within the community. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Predicting food‐web structure with metacommunity models

Synthesis Metacommunity theory aims to elucidate the relative influence of local and regional‐scale processes in generating diversity patterns across the landscape. Metacommunity research has focused largely on assemblages of competing organisms within a single trophic level. Here, we test the ability of metacommunity models to predict the network structure of the aquatic food web found in the leaves of the northern pitcher plant Sarracenia purpurea. The species‐sorting and patch‐dynamics models most accurately reproduced nine food web properties, suggesting that local‐scale interactions play an important role in structuring Sarracenia food webs. Our approach can be applied to any well‐resolved food web for which data are available from multiple locations. The metacommunity framework explores the relative influence of local and regional‐scale processes in generating diversity patterns across the landscape. Metacommunity models and empirical studies have focused mostly on assemblages of competing organisms within a single trophic level. Studies of multi‐trophic metacommunities are predominantly restricted to simplified trophic motifs and rarely consider entire food webs. We tested the ability of the patch‐dynamics, species‐sorting, mass‐effects, and neutral metacommunity models, as well as three hybrid models, to reproduce empirical patterns of food web structure and composition in the complex aquatic food web found in the northern pitcher plant Sarracenia purpurea. We used empirical data to determine regional species pools and estimate dispersal probabilities, simulated local food‐web dynamics, dispersed species from regional pools into local food webs at rates based on the assumptions of each metacommunity model, and tested their relative fits to empirical data on food‐web structure. The species‐sorting and patch‐dynamics models most accurately reproduced nine food web properties, suggesting that local‐scale interactions were important in structuring Sarracenia food webs. However, differences in dispersal abilities were also important in models that accurately reproduced empirical food web properties. Although the models were tested using pitcher‐plant food webs, the approach we have developed can be applied to any well‐resolved food web for which data are available from multiple locations.     

The structure of assembled communities

A set of rules is formulated which expresses the random assembly of ecological communities by sequentially arriving species, subject to energetic constraints. It is shown that these “assembled communities” provide a reasonable model for 35 out of the 40 real food webs recently compiled by Briand (1981), on the basis of the statistics: species richness, proportion of herbivores, ratio of prey to predators, proportion of dietary specialists, number of trophic links, number of potential competitive links, connectance, and average maximal food chain length. However, the observed frequency of intervality among Briand's food webs deviates significantly from the value expected on the basis of random sampling from the mathematical universe of assembled webs. Finally, there are indications in this work that the process of community genesis may be fundamentally different in fluctuating and in constant environments.     

Parasites as prey in aquatic food webs: implications for predator infection and parasite transmission

While the recent inclusion of parasites into food‐web studies has highlighted the role of parasites as consumers, there is accumulating evidence that parasites can also serve as prey for predators. Here we investigated empirical patterns of predation on parasites and their relationships with parasite transmission in eight topological food webs representing marine and freshwater ecosystems. Within each food web, we examined links in the typical predator–prey sub web as well as the predator–parasite sub web, i.e. the quadrant of the food web indicating which predators eat parasites. Most predator– parasite links represented ‘concomitant predation’ (consumption and death of a parasite along with the prey/host; 58–72%), followed by ‘trophic transmission’ (predator feeds on infected prey and becomes infected; 8–32%) and predation on free‐living parasite life‐cycle stages (4–30%). Parasite life‐cycle stages had, on average, between 4.2 and 14.2 predators. Among the food webs, as predator richness increased, the number of links exploited by trophically transmitted parasites increased at about the same rate as did the number of links where these stages serve as prey. On the whole, our analyses suggest that predation on parasites has important consequences for both predators and parasites, and food web structure. Because our analysis is solely based on topological webs, determining the strength of these interactions is a promising avenue for future research.     

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.     

Aquatic food‐web dynamics following incorporation of nutrients derived from Atlantic anadromous fishes

Changes in the isotopic composition (δ¹³C and δ¹⁵N) in biofilm, macro‐invertebrates and resident salmonids were used to characterize temporal dynamics of marine derived nutrients (MDNs) incorporation between stream reaches with and without MDN inputs. Five Atlantic rivers were chosen to represent contrasting MDN subsidies: four rivers with considerable numbers of anadromous fishes; one river with little MDN input. Rainbow smelt Osmerus mordax, alewife Alosa pseudoharengus, sea lamprey Petromyzon marinus and Atlantic salmon Salmo salar, were the primary anadromous species for the sampled rivers. Regardless of the spatial resolution or the pathway of incorporation, annual nutrient pulses from spawning anadromous fishes had a positive effect on isotopic enrichment at all trophic levels (biofilm, 1·2–5·4‰; macro‐invertebrates, 0·0–6·8‰; fish, 1·2–2·6‰). Community‐wide niche space shifted toward the marine‐nutrient source, but the total ecological niche space did not always increase with MDN inputs. The time‐integrated marine‐nutrient resource contribution to the diet of S. salar parr and brook trout Salvelinus fontinalis ranged between 16·3 and 36·0% during anadromous fish‐spawning periods. The high degree of spatio‐temporal heterogeneity in marine‐nutrient subsidies from anadromous fishes lead to both direct and indirect pathways of MDN incorporation into stream food webs. This suggests that organisms at many trophic levels derive a substantial proportion of their energy from marine resources when present. The current trend of declining anadromous fish populations means fewer nutrient‐rich marine subsidies being delivered to rivers, diminishing the ability to sustain elevated riverine productivity.     

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.     

Food web complexity and higher-level ecosystem services

Studies mostly focused on communities of primary producers have shown that species richness provides and promotes fundamental ecosystem services. However, we know very little about the factors influencing ecosystem services provided by higher trophic levels in natural food webs. Here we present evidence that differences in food web structure and the richness of herbivores in 19 plant‐herbivore‐parasitoid food webs influence the service supplied by natural enemies, namely, the parasitism rates on hosts. Specifically, we find that parasitoids function better in simple food webs than in complex ones, a result relevant to biological control practice. More generally, we show that species richness per se only contributes partially to the understanding of higher‐level ecosystem services in multitrophic communities, and that changes in food web complexity should also be taken into account when predicting the effects of human‐driven disturbances in natural communities.     

Effects of genetically modified maize events expressing Cry34Ab1, Cry35Ab1, Cry1F,and CP4 EPSPS proteins on arthropod complex food webs

Four genetically modified (GM) maize (Zea mays L.) hybrids (coleopteran resistant, coleopteran and lepidopteran resistant, lepidopteran resistant and herbicide tolerant, coleopteran and herbicide tolerant) and its non‐GM control maize stands were tested to compare the functional diversity of arthropods and to determine whether genetic modifications alter the structure of arthropods food webs. A total number of 399,239 arthropod individuals were used for analyses. The trophic groups’ number and the links between them indicated that neither the higher magnitude of Bt toxins (included resistance against insect, and against both insects and glyphosate) nor the extra glyphosate treatment changed the structure of food webs. However, differences in the average trophic links/trophic groups were detected between GM and non‐GM food webs for herbivore groups and plants. Also, differences in characteristic path lengths between GM and non‐GM food webs for herbivores were observed. Food webs parameterized based on 2‐year in‐field assessments, and their properties can be considered a useful and simple tool to evaluate the effects of Bt toxins on non‐target organisms.     

Ecosystem consequences of fish parasites*

In most aquatic ecosystems, fishes are hosts to parasites and, sometimes, these parasites can affect fish biology. Some of the most dramatic cases occur when fishes are intermediate hosts for larval parasites. For example, fishes in southern California estuaries are host to many parasites. The most common of these parasites, Euhaplorchis californiensis, infects the brain of the killifish Fundulus parvipinnis and alters its behaviour, making the fish 10–30 times more susceptible to predation by the birds that serve as its definitive host. Parasites like E. californiensis are embedded in food webs because they require trophic transmission. In the Carpinteria Salt Marsh estuarine food web, parasites dominate the links and comprise substantial amount of biomass. Adding parasites to food webs alters important network statistics such as connectance and nestedness. Furthermore, some free‐living stages of parasites are food items for free‐living species. For instance, fishes feed on trematode cercariae. Being embedded in food webs makes parasites sensitive to changes in the environment. In particular, fishing and environmental disturbance, by reducing fish populations, may reduce parasite populations. Indirect evidence suggests a decrease in parasites in commercially fished species over the past three decades. In addition, environmental degradation can affect fish parasites. For these reasons, parasites in fishes may serve as indicators of environmental impacts.