Variation In Food Web Structure Research Articles

Geographic differences in body size distributions underlie food web connectance of tropical forest mammals

Understanding variation in food web structure over large spatial scales is an emerging research agenda in food web ecology. The density of predator–prey links in a food web (i.e., connectance) is a key measure of network complexity that describes the mean proportional dietary breadth of species within a food web. Connectance is a critical component of food web robustness to species loss: food webs with lower connectance have been shown to be more susceptible to secondary extinctions. Identifying geographic variation in food web connectance and its drivers may provide insight into community robustness to species loss. We investigated the food web connectance of ground-dwelling tropical forest mammal communities in multiple biogeographic regions to test for differences among regions in food web connectance and to test three potential drivers: primary productivity, contemporary anthropogenic pressure, and variation in mammal body mass distributions reflective of historical extinctions. Mammal communities from fifteen protected forests throughout the Neo-, Afro-, and Asian tropics were identified from systematic camera trap arrays. Predator–prey interaction data were collected from published literature, and we calculated connectance for each community as the number of observed predator–prey links relative to the number of possible predator–prey links. We used generalized linear models to test for differences among regions and to identify the site level characteristics that best predicted connectance. We found that mammal food web connectance varied significantly among continents and that body size range was the only significant predictor. More possible predator–prey links were observed in communities with smaller ranges in body size and therefore sites with smaller body size ranges had higher mean proportional dietary breadth. Specifically, mammal communities in the Neotropics and in Madagascar had significantly higher connectance than mammal communities in Africa. This geographic variation in contemporary mammalian food web structure may be the product of historical extinctions in the Late Quaternary, which led to greater losses of large-bodied species in the Neotropics and Madagascar thus contributing to higher average proportional dietary breadth among the remaining smaller bodied species in these regions.

Seasonal variation in trophic structure and community niche dynamics of an Arctic coastal community of marine vertebrates

Temporal variation in food web structure is widespread among highly seasonal environments, such as the Arctic, and is driven by changes in resource availability. Variation in resource availability can lead to species differences in diet composition, isotopic niche width, and trophic position (TP) across seasons. Here, we used tissue samples that represent two distinct turnover rates for diet (liver = shorter term, muscle = longer term) from 18 fish and three marine mammal species to investigate seasonal (i) variation in TPs within the Southampton Island marine ecosystem of Hudson Bay, (ii) variation in consumer isotopic niche width within this part of the food web, and (iii) variation in community niche dynamics among a fish and marine mammal community using stable carbon (δ13C) and nitrogen (δ15N) isotope analyses. Many zooplanktivores and piscivores increased in TP in summer (i.e., shorter-term turnover period), whereas benthic feeders dependent on ice-obligate prey decreased in TP. Most isotopic niche widths and community metrics (δ15N range, total ellipse area, mean centroid distance) were higher in liver than muscle. Our findings demonstrate seasonal changes in TPs, which suggests that Arctic communities may be differentially affected by longer ice-free periods and earlier onset of primary production due to accelerated climate change.

Effects of urbanization on the trophic niche of the brown anole, a widespread invasive lizard

Urbanization is one of the most dramatic forms of environmental change with the potential to alter trophic relationships among organisms in cities. In this study, we combine gut-content and stable isotope analyses of brown anole diets with field sampling of arthropods to assess whether and how urbanization-induced variation of basal resources influence trophic niche variation of anole populations inhabiting urban, suburban, and natural habitats. We found clear evidence for trophic niche variation across the urbanization gradient. Stable isotope analyses revealed that urban and suburban anoles had strongly elevated δ13C levels and slightly lower δ15N levels compared to anoles from natural forests— results consistent with increased reliance on grasses across the urbanization gradient. Isotopic niche widths were also greater in urban habitats. However, our analysis of anole diets revealed no difference in the diversity of prey consumed—despite urban habitats having the highest diversity of available prey compared to suburban and natural forest habitats. Overall, our results suggest that much of the observed variation in food web structure results from a shift in the diversity of basal resource pools across the urbanization gradient. The broader effects of a shift towards grass-dominated food webs in urban habitats remains to be understood.

Spatial variation in food web structure in a recovering marine ecosystem.

Spatial heterogeneity in food web structure and interactions may reconcile spatial variation in population and community dynamics in large marine ecosystems. In order to assess food web contributions to the different community recovery dynamics along the Newfoundland and Labrador shelf ecosystem, we quantified species interactions using stable isotope mixing models and food web metrics within three sub-regions. Representative samples of each species caught in trawls and plankton tows were analyzed for stomach contents and stable isotope ratios (δ15N and δ13C) to parameterize isotope mixing models. Regional variation, highlighted by the diets of three economically important species, was observed such that the southern region demonstrated a variety of trophic pathways of nutrient flow into the higher food web while the diets of fish in the northern regions were typically dominated by one or two pathways via dominant prey species, specifically shrimp (Pandalus sp.) and hyperiids. Food web metrics indicated that the low-diversity northern regions had higher connectance and shorter food chain lengths. This observed regional variation contributes to our understanding of the role of specific forage species to the ecosystem which is an essential contribution towards ecosystem-based management decisions.

Ecological Networks in the Scotia Sea: Structural Changes Across Latitude and Depth

The Scotia Sea is a productive pelagic ecosystem in the Southern Ocean, which is rapidly changing as a consequence of global warming. Species range shifts are particularly evident, as sub-Antarctic species expand their range from North to South, potentially rearranging the structure of this ecosystem. Thus, studies are needed to determine the current extent of variation in food web structure between these two biogeographic regions of the Scotia Sea and to investigate whether the observed patterns are consistent among depth zones. We compiled a database of 10,888 feeding interactions among 228 pelagic taxa, underpinned by surveys and dietary studies conducted in the Scotia Sea. Network analysis indicated that the Northern Scotia Sea (NSS), relative to the Southern Scotia Sea (SSS), is more complex: with higher species richness (more nodes) and trophic interactions (more links) is more connected overall (greater connectance and linkage density). Moreover, the NSS is characterised by more groups of strongly interacting organisms (greater node clustering) than the SSS, suggesting a higher trophic specialisation of Antarctic compared to sub-Antarctic species. Depth also played a key role in structuring these networks, with higher mean trophic position and more dietary generalism in the mesopelagic and bathypelagic zones relative to the epipelagic zones. This suggests that direct access to primary producers is a key factor influencing the trophic structure of these communities. Our results suggest that under current levels of warming the SSS ecosystem will likely become more connected and less modular, resembling the current structure of the NSS.

Geophysical templates modulate the structure of stream food webs dominated by omnivory

AbstractFood webs show the architecture of trophic relationships, revealing the biodiversity and species interactions in an ecosystem. Understanding which factors modulate the structure of food webs offers us the ability to predict how they will change when influential factors are altered. To date, most of the research about food webs has focused on species interactions whereas the influences of surrounding environments have been overlooked. Here, using network analysis, we identified how the structure of aquatic food webs varied across a range of geophysical conditions within a whole stream system. Within a headwater basin in the Cascade Mountain Range, Oregon, USA, macroinvertebrate and vertebrate composition was investigated at 18 sites. Predator–prey interactions were compiled based on existing literature and dietary analysis. Several structural network metrics were calculated for each food web. We show that the structure of food webs was predictable based on geophysical features at both local (i.e., slope) and broader (i.e., basin size) spatial extents. Increased omnivory, greater connectance, shorter path lengths, and ultimately greater complexity and resilience existed downstream compared to upstream in the stream network. Surprisingly, the variation in food web structure was not associated with geographic proximity. Structural metric values and abundance of omnivory suggest high levels of stability for these food webs. There is a predictable variation in the structure of food webs across the network that is influenced by both longitudinal position within streams and patchy discontinuities in habitat. Hence, findings illustrate that the slightly differing perspectives from the River Continuum Concept, Discontinuity Patch Dynamics, and Process Domains can be integrated and unified using food web networks. Our analyses extend ecologists’ understanding of the stability of food webs and are a vital step toward predicting how webs and communities may respond to both natural disturbances and current global environmental change.

Climate and food web effects on the spring clear‐water phase in two north‐temperate eutrophic lakes

AbstractAlthough climate change has shifted the phenological timing of plankton in lakes, few studies have explicitly addressed the relative contributions of climate change and other factors, including planktivory and nutrient availability. The spring clear‐water phase is a period of marked reduction in algal biomass and increased water transparency observed in many lakes. Here, we quantified the phenological patterns in the start date, maximum date, duration, and magnitude of the clear‐water phase over 38 yr in Lakes Mendota and Monona, and examined the effects of water temperature, total phosphorus, and food web structure (proportion of large‐bodied Daphnia pulicaria and density of invasive Bythotrephes) and interactions between temperature and other predictors on these clear‐water phase metrics. We found that climate and food web structure affected the clear‐water phase, but the effects differed among the metrics. Higher water temperature led to earlier clear‐water phase start dates and maximum dates in both lakes. The proportion of D. pulicaria affected all clear‐water phase metrics in both lakes. When D. pulicaria proportion was higher, the clear‐water phase occurred earlier, lasted longer, and the water was clearer. Moreover, high Bythotrephes density delayed clear‐water phase start dates (both lakes), and decreased clear‐water phase duration (Lake Mendota) in the following year. These results suggest that variation in food web structure changes the full phenological dynamics of the clear‐water phase, while variation in climate condition affects clear‐water phase timing only. Our findings highlight the importance of large‐bodied grazers for managing water quality under climate change.

Regional environmental variation and local species interactions influence biogeographic structure on oyster reefs.

Although species interactions are often assumed to be strongest at small spatial scales, they can interact with regional environmental factors to modify food web dynamics across biogeographic scales. The eastern oyster (Crassostrea virginica) is a widespread foundational species of both ecological and economic importance. The oyster and its associated assemblage of fish and macroinvertebrates is an ideal system to investigate how regional differences in environmental variables influence trophic interactions and food web structure. We quantified multiple environmental factors, oyster reef properties, associated species, and trophic guilds on intertidal oyster reefs within 10 estuaries along 900km of the southeastern United States. Geographical gradients in fall water temperature and mean water depth likely influenced regional (i.e., thenorthern, central and southern sections of the SAB)variation in oyster reef food web structure. Variation in the biomass of mud crabs, an intermediate predator, was mostly (84.1%) explained by reefs within each site, and did not differ substantially among regions; however, regional variation in the biomass of top predators and of juvenile oysters also contributed to biogeographic variation in food web structure. In particular, region explained almost half (40.2%) of the variation in biomass of predators of blue crab, a top predator that was prevalent only in the central region where water depth was greater. Field experiments revealed that oyster mortality due to predation was greatest in the central region, suggesting spatial variation in the importance of trophic cascades. However, high oyster recruitment in the middle region probably compensates for this enhanced predation, potentially explaining why relatively less variation (17.9%) in oyster cluster biomass was explained by region. Region also explained over half of the variation in biomass of mud crab predators (55.2%), with the southern region containing almost an order of magnitude more biomass than the other two regions. In this region, higher water temperatures in the fall corresponded with higher biomass of fish that consume mud crabs and of fish that consumejuvenile and forage fish, whereas biomas oftheir prey (mud crabs and juvenile and forage fish, respectively)was generally low in the southern region. Collectively, these results show how environmental gradients interact with trophic cascades to structure food webs associated with foundation species across biogeographic regions.

Spatial analyses of multi‐trophic terrestrial vertebrate assemblages in Europe

AbstractAimAlthough much has been said on the spatial distribution of taxonomic and phylogenetic diversity of vertebrates, how this diversity interacts in food webs and how these interactions change across space are largely unknown. Here, we analysed the spatial distribution of tetrapod food webs and asked whether the variation in local food web structure is driven by random processes or by natural and anthropogenic factors.LocationEurope.Time periodPresent.Major taxa studiedTetrapods.MethodsWe combined an expert‐based food web (1,140 species and 70,601 links) of all European tetrapods with their respective spatial distributions. We mapped 17 different food web metrics representing complexity, chain length, vertical diversity and diet strategy across Europe and tested whether their distribution reflects the spatial structure of species richness using a null model of food web structure. To avoid multicollinearity issues, we defined composite descriptors of food web structure that we related to a set of environmental layers summarizing both natural and anthropogenic influences and tested their relative importance in explaining the spatial distribution of European terrestrial vertebrate food webs.ResultsOf the 17 metrics, 10 showed a non‐random spatial distribution across Europe and could be summarized along two major axes of variation in food web structure. The first was related to species richness, mean trophic level and the proportion of intermediate species, whereas the second was related to the connectance and proximity of species within the web. Both descriptors varied with latitudinal gradient. The best descriptors of food web structure were mean annual temperature and seasonality (negatively correlated with the first axis), and human footprint (positively correlated with the second axis).Main conclusionsWe demonstrate the importance of climate and anthropogenic pressure in shaping the spatial structure of European tetrapod food webs.

Seasonal Variation in Food Web Structure and Fish Community Composition in the East/Japan Sea

The aim of this study was to assess the temporal variability in the trophic structure of a fish community and the trophic levels of dominant fish species off the eastern coast of Korea. We analyzed the community composition and the stable carbon and nitrogen isotope ratios (δ13C and δ15N) for fish species in Hupo coastal waters over four seasons. Fish assemblages were mainly dominated by resident benthic and benthopelagic fish. In contrast, there were low numbers of pelagic fish, including migrating and occasional species, which varied significantly by season. The fish assemblages generally had relatively wide ranges of δ13C and δ15N (− 21.1 to − 16.3‰ and 10.0 to 14.2‰, respectively), reflecting a clear distinction between resident (mostly benthic or benthopelagic) vs migratory (mostly pelagic) fish. Resident species showed no seasonal differences in their isotopic signatures, whereas pelagic species showed temporal variability in δ13C and δ15N values. The isotopic niche of fish assemblages showed distinctly different patterns between spring–summer and winter–fall with the seasonal changes in the fish community. These patterns might be associated with seasonal variation in the fish community caused by seasonal changes in environmental conditions (e.g., hydrographic parameters and food availability). Our results can offer baseline information/trophic data for ecosystem models that can be used to evaluate the effects of climate on marine ecosystems.

Effects ofPhragmitesManagement on the Ecology of a Wetland

Effective management of wetlands often involves the suppression of invasive species, and understanding the ecological consequences of management efforts is an important goal. A non-native strain of Phragmites australis (Common Reed; hereafter Phragmites) is aggressively invading North American wetlands, and the species has been shown to alter hydrology and impact aquatic organisms. Clear Creek Wildlife Management Area, in Kentucky, is heavily impacted by Phragmites invasion. Portions of the Wildlife Management Area with Phragmites were treated with herbicide to restore wetland flora, and in this study, we evaluated the effects of both the presence and management of Phragmites on wildlife populations and ecosystem function. We selected 3 locations (treated Phragmites, untreated Phragmites, and Phragmites-free) and surveyed each for water chemistry, wildlife populations, and stable-isotope signatures over a 2-y period. Water chemistry varied with the presence or absence of Phragmites, suggesting differences in nutrient cycling. Fish diversity did not differ among sites, but individual species varied in distribution and abundance between the Phragmites sites and the Phragmites-free site. Turtles showed significant differences in both diversity and body size based on the presence or absence of Phragmites, but not herbicide treatment. We detected no significant differences in frog diversity across treatments. We recorded 8 Kentucky Species of Greatest Conservation Need, but there were few differences in the distribution of these species across sites. Stable-isotope analysis revealed variation in food-web structure based on the presence of Phragmites. These results indicate that herbicides had little effect on fish and herpetofaunal communities in the short term, but potentially significant ecological changes may occur if Phragmites were eradicated. Our conclusions highlight the importance of monitoring habitat restoration to guide future management. A holistic, ecosystem-level approach is necessary to understand the impacts of both invasive species and their management.

Spatial and temporal variation in food web structure of an impounded river in Anatolia

Dams interrupt the longitudinal connectivity of rivers by impeding the movement of water, sediments and organisms, which, in turn, could affect aquatic biodiversity and food web ecology. Using stable isotope analysis, we examined spatiotemporal variation in food web structure at four sites in the upper Yeşilırmak River, Anatolia Peninsula, Turkey, in relation to environmental parameters and a dam. It was apparent that the dam created discontinuity in the longitudinal fluvial gradient of fish species richness, with more species observed at upstream sites. Fish assemblages from different sites and seasons occupied distinct areas of isotopic space. Isotopic niche space, trophic diversity, variation in δ13C of basal resources and assemblage redundancy were all higher for the fish assemblage at the site downstream from the dam compared with the site above the dam, a possible indication of greater interspecific dietary variation. Food chain length (the range in δ15N) was lower at the downstream site, possibly resulting from a greater tendency towards omnivory. The findings strongly suggest that the dam affects not only environmental conditions and fish diversity, but also trophic ecology. The results of the present study emphasise the need for research to assess potential effects of new dams in Anatolia on aquatic communities and ecosystem dynamics in rivers.

Dam regulation and riverine food-web structure in a Mediterranean river

Flow regimes are a major driver of community composition and structure in riverine ecosystems, and flow regulation by dams often induces artificially-stable flow regimes downstream. This represents a major source of hydrological alteration, particularly in regions where biota is adapted to strong seasonal and interannual flow variability. We hypothesized that dam-induced hydrological stability should increase the availability of autochthonous resources at the base of the food web. This, in turn, should favour herbivorous over detritivorous strategies, increasing the diversity of primary consumers, and the food-web width and length. We tested this hypothesis by studying the longitudinal variation in food-web structure in a highly-seasonal Mediterranean river affected by an irrigation dam. We compared an unregulated reach to several reaches downstream of the dam. Hydrological and sedimentological stability increased downstream of the dam, and altered the type and quantity of available resources downstream, prompting a change from a detritus-based to an algae-based food web. The fraction of links between top and intermediate species also increased, and the food web became longer and wider at the intermediate trophic levels. Food-web structure did not recover 14km downstream of the dam, despite a partial restitution of the flow regime. Our results advance the notion that hydrologic alteration affects riverine food webs via additions/deletions of taxa and variation in the strength and distribution of food-web interactions. Thus, flow regulation by dams may not only impact individual facets of biodiversity, but also food-web level properties across river networks.

Trophic isotopic carbon variation increases with pond\u2019s hydroperiod: Evidence from an Austral ephemeral ecosystem

Trophic variation in food web structure occurs among and within ecosystems. The magnitude of variation, however, differs from system to system. In ephemeral pond ecosystems, temporal dynamics are relatively more important than in many systems given that hydroperiod is the ultimate factor determining the presence of an aquatic state. Here, using stable isotopes we tested for changes in trophic chain length and shape over time in these dynamic aquatic ecosystems. We found that lower and intermediate trophic level structure increased over time. We discuss these findings within the context of temporal environmental stability. The dynamic nature of these ephemeral systems seems to be conducive to greater levels of intermediate and lower trophic level diversity, with omnivorous traits likely being advantageous.

Fish Biomass in Tropical Estuaries: Substantial Variation in Food Web Structure, Sources of Nutrition and Ecosystem-Supporting Processes

Quantification of key pathways sustaining ecosystem function is critical for underpinning informed decisions on development approvals, zoning and offsets, ecosystem restoration and for meaningful environmental assessments and monitoring. To develop a more quantitative understanding of the importance and variation in food webs and nutrient flows in tropical estuaries, we investigated the spatio-temporal distribution of biomass of fish across 28 mangrove-lined estuaries in tropical Australia. We evaluated the extent to which nekton biomass in tropical estuaries responded to spatial and temporal factors and to trophic identity. Biomass was dominated by two trophic groups, planktivores and macrobenthos feeders. Contributions by other trophic groups, such as detritivores and microbenthos feeders, were more variable. Total biomass and the biomass of all major trophic groups were concentrated in downstream reaches of estuaries. The consistent concentration of biomass downstream, and spatio-temporal differences in the contributions by the different trophic groups, indicates substantial differences in food web structure, differences in the contributions from different sources of nutrition and probably unequal flow of productivity into higher levels of the food web in different parts of the estuary. In turn, this suggests substantial qualitative and quantitative differences in ecosystem-supporting processes in different estuary reaches.

Progression of a Gulf of Mexico food web supporting Atlantis ecosystem model development

This article develops a marine food web matrix for the Gulf of Mexico (GOM) based on local stomach sampling and online diet information. Working at the level of functional groups, we fit diet information to a statistical model based on the Dirichlet distribution. This allows us to quantify likely contributions of prey to predators’ diets. Error ranges on these values reflect diet variability and data quality, and help in identifying functional groups that would benefit from additional sampling. We perform hierarchical cluster analysis to determine functional groups that have similar prey requirements, then produce a food web diagram representing the interactions between predators and prey. A meta-analysis using principle coordinate analysis allows us to compare this study’s diet matrix with ten other published GOM food webs and determine where variation in food web structure exists. We also compare our new food web to the diet matrix used by the Ainsworth et al. (2015) Atlantis ecosystem model, a strategic tool developed to assess ecosystem dynamics in the GOM. A hindcast from 1980 to 2010 using Atlantis shows an improved fit to observational data and reduced error in biomass projections using the revised diet information.

Intraspecific differences in plant chemotype determine the structure of arthropod food webs.

It is becoming increasingly appreciated that the structure and functioning of ecological food webs are controlled by the nature and level of plant chemicals. It is hypothesized that intraspecific variation in plant chemical resistance, in which individuals of a host-plant population exhibit genetic differences in their chemical contents (called 'plant chemotypes'), may be an important determinant of variation in food web structure and functioning. We evaluated this hypothesis using field assessments and plant chemical assays in the tansy plant Tanacetum vulgare L. (Asteraceae). We examined food webs in which chemotypes of tansy plants are the resource for two specialized aphids, their predators and mutualistic ants. The density of the ant-tended aphid Metopeurum fuscoviride was significantly higher on particular chemotypes (borneol) than others. Clear chemotype preferences between predators were also detected. Aphid specialist seven-spotted ladybird beetles (Coccinella septempunctata) were more often found on camphor plants, while significantly higher numbers of the polyphagous nursery web spider (Pisaura mirabilis) were observed on borneol plants. The analysis of plant chemotype effects on the arthropod community clearly demonstrates a range of possible outcomes between plant-aphid-predator networks. The findings help to offer a deeper insight into how one important factor--plant chemical content--influences which species coexist within a food web on a particular host plant and the nature of their trophic linkages.

Continuous, pulsed and disrupted nutrient subsidy effects on ecosystem productivity, stability, and energy flow

Resource pulses and subsidies can supply ecosystems with an important source of nutrients that supports additional productivity at multiple trophic levels. Common carp Cyprinus carpio provide ecosystems with a continuous nutrient subsidy through bioturbation and excretion but may also initiate a nutrient pulse through carcass decomposition. We examined how continuous (common carp foraging and excretion), pulsed (carcass decomposition) and disrupted (carp introduced and then removed) nutrient subsidies differed in their ability to alter nutrient availability, ecosystem productivity and stability and energy flow. Nitrogen and phosphorus availability and primary production were highest in pulsed, intermediate in continuous and lowest for disrupted and control systems. Continuous, and to a lesser extent pulsed, systems were associated with decreased water clarity and macrophyte coverage. Nutrient pulses ascended to higher trophic levels and supported greater densities of consumers (i.e., zooplankton and macroinvertebrates). Biotic and abiotic responses in disrupted systems quickly returned to control levels and water quality improved. Light penetration and Daphnia spp. dynamics were more stable in systems with pulses whereas stability of other variables was similar across treatments. Biotic materials collected from pulsed and continuous subsidy systems were typically enriched in δ15N suggesting common carp‐derived nutrients supported increased productivity whereas δ13C signatures were depleted suggesting a transition to more pelagic energy pathways, likely due to enhanced phytoplankton production. Our results suggest that continuous and pulsed nutrient subsidies vary in their ability to support and sustain ecosystem productivity with resulting variation in food web structure and ecosystem stability.

Geographic variation in network structure of a nearctic aquatic food web

ABSTRACTAim 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.

Food Web Structure and Habitat Connectivity in Fish Estuarine Nurseries—Impact of River Flow

River flow variability is known to influence estuarine production, yet knowledge on its effect upon estuarine food webs dynamics is still scarce. Stable carbon and nitrogen isotopes were used to assess the effect of river flow in the connectivity and food web interactions between the two main fish nursery areas of the Tagus estuary. The aims of the present work were to investigate the seasonal variation in food web structure and the exchange rate of individuals of marine juvenile fish among estuarine nurseries, to compare the spring of a rainy year (2001) with that of an average year (2000), and to investigate the impact of the winter floods of 2001. A low level of connectivity was observed for the fish species that use these areas as nurseries. In low river flow conditions, two isotopically distinct food webs were established in each nursery area. These food webs were very sensitive to small variations in the freshwater input. Winter floods seem to disrupt the localized food webs that are established in low river flow periods, leading to the re-establishment of a wider food web. While in rainy years this wide food web is maintained until spring, in average years the food web undergoes fragmentation into two localized and isotopically distinctive food webs. The increase in frequency of droughts due to climate change should lower the connectivity of the estuarine fish nurseries food webs, causing habitat fragmentation and consequent loss in complexity and resilience.