Links In Food Webs Research Articles

Trophic decline and distribution of barium in a freshwater ecosystem

Barium is generally considered a nonessential element with low toxicity, and with uptake from food sources balanced by excretion. Investigations into the dynamics of Ba in freshwater ecosystems have been limited. I show a 3-fold, trophic level-related decline in Ba in a freshwater ecosystem from a high of 60.9 µg/g for crayfish (Crustacea Orconectes virilis) to 0.438 µg/g for the largest individuals of a top predator (walleye Sander vitreus >3 kg). Barium concentrations were significantly higher in organisms (invertebrates and fish) associated with benthic substrates compared with those associated with pelagic environments. Thus, the concentration of Ba in muscle of 12 fish species was generally aligned along a pelagic to benthic food source gradient. These patterns may be related to relatively low concentration of Ba in water (77 µg/l) compared to the concentration of Ba in lake sediment (679 µg/g). Species level Ba assessments may provide a practical tool to assist with the identification of food web links and species specific routes of energy transfer in freshwater ecosystems.

Structure, dynamics and stability of a Mediterranean river food web

We present the results of a study of the food web of a Mediterranean river in the four seasons of the year. A high-resolution taxonomic description has been produced to characterise the different community components. We have also determined the trophic relationships among organisms by analysing their gut contents. From the network topology, we extracted several descriptors of structural complexity of the comunity in terms of number of nodes and links. We found a positive relationship between connectance and diversity (both biological and functional). Moreover, we developed and applied a quantitative approach to estimate the link strength, which showed that not all links constituting the network are equally important. In the present paper, we show that the strength of the food-web links vary over time, but there is a natural tendency to keep a small set of strong links throughout the year. However, the existence of two consecutive strong links is not common, meaning that trophic cascades are not promoted.

Diet compositions and trophic guild structure of the eastern Chukchi Sea demersal fish community

Fishes are an important link in Arctic marine food webs, connecting production of lower trophic levels to apex predators. We analyzed 1773 stomach samples from 39 fish species collected during a bottom trawl survey of the eastern Chukchi Sea in the summer of 2012. We used hierarchical cluster analysis of diet dissimilarities on 21 of the most well sampled species to identify four distinct trophic guilds: gammarid amphipod consumers, benthic invertebrate generalists, fish and shrimp consumers, and zooplankton consumers. The trophic guilds reflect dominant prey types in predator diets. We used constrained analysis of principal coordinates (CAP) to determine if variation within the composite guild diets could be explained by a suite of non-diet variables. All CAP models explained a significant proportion of the variance in the diet matrices, ranging from 7% to 25% of the total variation. Explanatory variables tested included latitude, longitude, predator length, depth, and water mass. These results indicate a trophic guild structure is present amongst the demersal fish community during summer in the eastern Chukchi Sea. Regular monitoring of the food habits of the demersal fish community will be required to improve our understanding of the spatial, temporal, and interannual variation in diet composition, and to improve our ability to identify and predict the impacts of climate change and commercial development on the structure and functioning of the Chukchi Sea ecosystem.

Sentinel responses to droughts, wildfires, and floods: effects of UV radiation on lakes and their ecosystem services

Environmental drivers such as climate change are responsible for extreme events that are critically altering freshwater resources across the planet. In the continental US, these events range from increases in the frequency and duration of droughts and wildfires in the West, to increasing precipitation and floods that are turning lakes and reservoirs brown in the East. Such events transform and transport organic carbon in ways that affect the exposure of ecosystems to ultraviolet (UV) radiation and visible light, with important implications for ecosystem services. Organic matter dissolved in storm runoff or released as black carbon in smoke selectively reduces UV radiation exposure. In contrast, droughts generally increase water transparency, so that UV radiation and visible light penetrate to greater depths. These shifts in water transparency alter the potential for solar disinfection of waterborne parasites, the production of carcinogenic disinfection byproducts in drinking water, and the vertical distribution of zooplankton that are a critical link in aquatic food webs.

The influence of cosmetic microbeads on the sorptive behavior of cadmium and lead within intertidal sediments: A laboratory study

Concentrations of microplastics within two geographically distinct urban locations within Burrard Inlet, British Columbia (BC), and the influence of facial scrub microbeads on lead and cadmium sorption within intertidal sediments were determined. Bulk intertidal sediment sampled from Cates Park (CP) located within the protected part of the inlet contained greater concentrations of microplastics (5560/kg wet sediment) as compared to Horseshoe Bay (HSB) (3120/kg wet sediment) located on the exposed open part of the inlet. Of the recovered microplastics ca. 75% were characterized as microbeads. Laboratory controlled microcosm experiments in which microbeads separated from a commercial facial scrub were added to bulk sediments collected from CP at ambient and 10-fold ambient (high) concentrations indicated that the microbeads acted as sorption sites. At ambient concentrations, less lead was recovered from pore water and surface water of treatment as compared to control microcosms. At high concentrations, the microbeads acted as a contaminant source to the microcosms, notably cadmium. Sorption of lead to microbeads has important implications for the potential role of microplastics, in this case microbeads acting as a yet quantified link in aquatic food webs.

Spliced leader RNA trans-splicing discovered in copepods

Copepods are one of the most abundant metazoans in the marine ecosystem, constituting a critical link in aquatic food webs and contributing significantly to the global carbon budget, yet molecular mechanisms of their gene expression are not well understood. Here we report the detection of spliced leader (SL) trans-splicing in calanoid copepods. We have examined nine species of wild-caught copepods from Jiaozhou Bay, China that represent the major families of the calanoids. All these species contained a common 46-nt SL (CopepodSL). We further determined the size of CopepodSL precursor RNA (slRNA; 108-158 nt) through genomic analysis and 3′-RACE technique, which was confirmed by RNA blot analysis. Structure modeling showed that the copepod slRNA folded into typical slRNA secondary structures. Using a CopepodSL-based primer set, we selectively enriched and sequenced copepod full-length cDNAs, which led to the characterization of copepod transcripts and the cataloging of the complete set of 79 eukaryotic cytoplasmic ribosomal proteins (cRPs) for a single copepod species. We uncovered the SL trans-splicing in copepod natural populations, and demonstrated that CopepodSL was a sensitive and specific tool for copepod transcriptomic studies at both the individual and population levels and that it would be useful for metatranscriptomic analysis of copepods.

It takes guts to locate elusive crustacean prey

Mobile crustacean prey, i.e. crangonid, euphausiid, mysid, and pandalid shrimp, are vital links in marine food webs. Their intermediate sizes and characteristic caridoid escape responses lead to chronic underestimation when sampling at large spa- tial scales with either plankton nets or large trawl nets. Here, as discrete sampling units, we utilized in- dividual fish diets (i.e. fish biosamplers) collected by the US National Marine Fisheries Service and North- east Fisheries Science Center to examine abundance and location of these prey families over large spatial and temporal scales in the northeastern US shelf large ecosystem. We found these prey families to be impor- tant to a wide variety of both juvenile and adult dem- ersal fishes from Cape Hatteras to the Scotian Shelf. Fish biosamplers further revealed significant spatial shifts in prey in early spring. Distributions of mysids and crangonids in fish diets shoaled significantly from February to March. Distributions of euphausiids and pandalids in fish diets shifted northward during March. Of multiple hypotheses for these shifts, prey migration is most strongly supported. Rather than only the classic ontogenetic shift from feeding on shrimp to piscivory, of the 25 identified diet shifts in fish predators, 12 shifts were toward increased shrimp feeding frequency with increasing body length.

Mutualistic and antagonistic trophic interactions in canola: The role of aphids in shaping pest and predator populations

Aphids have important effects on the abundance and occurrence of tending ants, predators, and pests in agronomic systems, and DNA-based gut content analysis can aid in establishing predator–prey interactions. The purpose of this study was to determine how the presence of aphids, ants, and pest individuals interact within canola (syn. oilseed rape) fields. Using seasonal data from canola fields, the relationships among ants, aphids and pest individuals were determined, along with the use of PCR techniques in order to amplify aphid DNA and confirm food web links on predators who consume aphids. We determined that aphid presence positively influences the number of ants and predators in a community, and diminishing aphid populations over the growing season were associated with declines in both ants and predators. These reduced populations of predators and aphids may have provided the opportunity for a key pest, Pieris rapae to build populations as the season ensued. This research suggests that complex interactions among herbivores and shared predators contribute to pest outbreaks.

Using structural equation modeling to test established theory and develop novel hypotheses for the structuring forces in soil food webs

Abstract Understanding the structuring elements of soil food webs is a major challenge for ecologists. Given the complex nature of soil food webs, common treatment-based experiments and bivariate data analyses can only capture part of that complexity. Structural equation model (SEM) is a promising multivariate technique that may help understand direct and indirect relationships in complex soil food webs. As bacterial and fungal energy channels, as well as bottom–up and top–down forces operate simultaneously, we used SEM to explore the complex interactions among multiple trophic levels in soil food webs. Further, SEM allowed us to test established theory and to refine and derive hypotheses regarding the structuring forces of soil food webs. We studied detritus-based soil food webs in a 55-year-old subtropical Illicium verum forest ecosystem by sampling different important soil food web components, including main soil microbial groups, soil nematodes, and soil microarthropods. The SEM results confirmed established theory by showing the central role of the fungal energy channel, and bottom–up effects were more important for the structure of the food web than top–down forces in the studied subtropical forest soil. In addition, we could derive some novel and refined hypotheses regarding important feeding links in soil food webs. For instance, we provide hypotheses on feeding preferences of different groups of soil biota and on the strength of bottom–up versus top–down effects in forest soil food webs. Overall, we highlight that SEM provides a framework to understand complex interactions and energy pathways in soil food webs in a multivariate context, test established theory, and propose novel experimental tests.

Aquatic macrophytes alter productivity‐richness relationships in eutrophic stream food webs

Traditional productivity‐diversity theory predicts that eutrophication will result in greater species richness due to increased resources at the bottom of the food web. However, few studies on the effects of increasing ecosystem productivity on biological communities have included responses at multiple trophic levels. We hypothesized that the effect of eutrophication on species richness would vary between different trophic levels due to shifts in community composition and trophic interactions. To investigate the mechanisms driving productivity‐richness relationships, we constructed food webs for 18 streams across a eutrophication gradient on South Island, New Zealand, and measured productivity, water quality, and habitat characteristics in each stream. A principal components analysis yielded two orthogonal axes of eutrophication: one associated with macrophytes, benthic substrate, and gross primary productivity (GPP) and the other with catchment area, temperature, and algal standing stock (chlorophyll a). Surprisingly, the majority of community response variables, especially defended primary consumer abundance and biomass, were more strongly associated with the macrophyte/habitat axis. The lack of change in abundance and declines in biomass and average mass of predatory invertebrates and fish with increasing eutrophication, despite increases in prey abundance, suggest that energy was not being passed up the food chain to higher trophic levels. The predominance of defended producers (macrophytes) and consumers (snails and microcrustacea) in eutrophic streams likely served as trophic bottlenecks; both are largely inedible by higher trophic levels. Consequently, our results suggest that managers need to focus on preserving food web linkages and energy flow, as well as biodiversity, in eutrophic systems.

Effects of alga Fucus serratus decline on benthic assemblages and trophic linkages at its retreating southern range edge

Canopy-forming seaweeds are important coastal ecosystem engineers that sustain diverse multi-trophic assemblages. Their losses, with the subsequent reduction in habitat complexity, have been documented across many parts of the world and are often attributed to climate change and other anthropogenic factors. The general aim of the present study was to understand the repercussions of the decline of the canopy-forming alga Fucus serratus L. at its retreating southern range edge in the diversity and food-web linkages of intertidal assemblages. Few studies have attempted to document changes in benthic food webs following canopy loss. We examined the differences among southern locations situated at different distances from the range margin: those at the very edge, where F. serratus experienced a dramatic decline during recent years (marginal locations), and those where F. serratus is still dominant (central locations). Comparisons were made among locations situated at the same latitude and sharing a recent history of F. serratus dominance. Trophic relationships were analyzed using natural abundances of carbon and nitrogen stable isotopes. We report clear changes in the structure of benthic assemblages and lower trophic positioning of some consumers, suggesting an overall shrinkage of the food web length at the contracting range edge of F. serratus, which will transfer to higher trophic levels. Under present and future climatic scenarios, shifts in the distribution of coastal ecosystem engineers could entail a reorganization of local natural assemblages and food webs. More attention should be given to measure how much these shifts can modify the whole coastal food webs and their functioning.

From sea ice to blubber: linking whale condition to krill abundance using historical whaling records

Krill (Euphausia superba) are fundamentally important in the Southern Ocean ecosystem, forming a critical food web link between primary producers and top predators. Krill abundance fluctuates with oceanographic conditions, most notably variation in winter sea ice, and is susceptible to environmental change. Although links between local krill availability and performance of land breeding, central place foragers are recognised, the effects of krill variability on baleen whales remain largely unclear because concurrent long-term data on whale condition and krill abundance do not exist. Here, we quantify links between whale body condition and krill abundance using a simple model that links krill abundance to sea ice extent. Body condition of humpback whales (Megaptera novaeangliae) caught in west Australian waters between 1947 and 1963 was estimated from oil yields in whaling records. Annual estimates of krill abundance in the Southern Ocean where those whales foraged (70°–130°E) were correlated significantly with contemporary annual winter sea ice extent. We hindcast sea ice extent for the whaling period from reconstructed temperature data and found that whale body condition was significantly correlated with hindcasted winter sea ice extent, supporting the hypothesis that variations in body condition were likely mediated by associated krill fluctuations. As humpback whales migrate and breed on finite energy stores accrued during summer foraging in the Antarctic, changes in sea ice and concomitant changes in krill abundance have long-term implications for their condition and reproductive success.

Movement and Habitat Use by Mottled Sculpin After Restoration of a Sand-Dominated 1st-Order Stream

Abstract Anthropogenic activities have greatly altered the natural flow regime of lotic ecosystems in many ways, including dams and culverts, which restrict sediment transport and fragment fish habitat. Sculpins, Cottus spp., are an important food-web link between macroinvertebrates and larger stream fishes and are greatly affected by culverts. Results from a previous study indicate a substantial increase in the relative abundance of mottled sculpin, Cottus bairdii, was observed upstream of a renovated road-stream crossing during the first season after construction. Redistribution of this nature from a putatively sedentary species would have required substantial movement. Our objectives were to quantify post-restoration mottled sculpin movement and habitat use in a restored stream reach. The extent of post-restoration mottled sculpin movement and habitat use were directly measured using telemetry of Passive Integrated Transponder (PIT) tag-marked fish. The maximum linear distance moved by a marked mottled...

Are Large Herbivores Vectors of Terrestrial Subsidies for Riverine Food Webs?

The tropical savannas of Africa have witnessed a dramatic reduction in native large mammalian herbivore populations. The consequences of these changes for terrestrial-aquatic food-web linkages are poorly documented. We used natural abundances of stable carbon and nitrogen isotopes (δ13C, δ15N) to determine spatial and temporal patterns in the importance of herbivore-mediated subsidies for consumers in the Mara River, Kenya. Potential primary producers (terrestrial C3 and C4 producers and periphyton) and consumers (invertebrates and fish) were collected during dry and wet seasons from different sites along the river, representing a gradient from forested highlands to natural savanna grasslands with high herbivore densities across mixed agricultural and livestock-dominated zones. Bayesian mixing models were used to estimate the relative contributions of terrestrial and algal sources of organic carbon supporting consumer trophic groups. Organic carbon sources differed for consumer groups and sites and with season. Overall, periphyton was the major energy source for most consumer groups during the dry season, but with wide 95% confidence intervals. During the wet season, the importance of terrestrial-derived carbon for consumers increased. The importance of C3 producers declined from 40 and 41% at the forested upper reaches to 20 and 8% at river reaches receiving hippo inputs during the dry and wet seasons, respectively. The reciprocal increase in the importance of C4 producers was higher than expected based on areal cover of riparian vegetation that was mainly C3. The importance of C4 producers notably increased from 18 and 10% at the forested upper reaches to 33 and 58% at river reaches receiving hippo inputs during the dry and wet seasons, respectively. This study highlights the importance of large herbivores to the functioning of riverine ecosystems and the potential implications of their loss from savanna landscapes that currently harbor remnant populations. Although the importance of C4 terrestrial carbon in most river systems has been reported to be negligible, this study shows that its importance can be mediated by large herbivores as vectors, which enhance energetic terrestrial-aquatic linkages in rivers in savanna landscapes.

Fish rely on scyphozoan hosts as a primary food source: evidence from stable isotope analysis

Predation of fish on their scyphozoan hosts has not been clearly defined using analysis of gut contents because gelatinous prey are difficult to visually detect and are dissolved by fixative solutions. Therefore, scyphomedusae have been generally considered not relevant in fish diet. To determine the contribution of their scyphozoan host tissue to the assimilated diet of age-0 Chloroscombrus chrysurus, we determined δ13C and δ15N of fish, their hosts (scyphomedusae Aurelia sp. and Drymonema larsoni) and their potential prey—small plankton ( 200 μm)—in the coastal waters of Alabama, USA. The diet of C. chrysurus was defined using the Bayesian mixing model Stable Isotope Analysis in R (SIAR). Models indicated that the scyphozoan hosts contributed on average ~90 % to fish assimilated diet. In contrast with previous dietary assessments based on analysis of gut contents, these results highlight that scyphozoans are important to the diet of fish associated with them. Because several ecologically and economically important fish species live in association with scyphomedusae, a redefinition of trophic links in marine food webs may be needed in light of the findings in this study.

How interaction strength affects the role of functional and redundant connections in food webs

Links in food webs can be classified as functional, which affect robustness, and redundant, which do not affect robustness. We explore whether this topologically based distinction may still be informative when interaction strength is considered. In 81 weighted food webs we identified functional and redundant links by exploiting the method of generalized dominators, and tested whether the two groups could be patterned according to link strength.Overall, redundant connections are weaker than functional links. However this pattern does not hold for every food web. Also, the difference in strength between functional and redundant links seems to depend on food web topology. In fact the systems in which redundant connections are significantly weaker than functional ones show greater connectance than systems in which the difference in strength between the two groups is not significant.We observed that redundant connections are fundamental in the formation of omnivory modules that have been described as being crucial for the stability/persistence of food webs. Although stability can be promoted by the specific arrangement of links magnitude within these modules, it does not necessarily imply that redundant links have to be weaker than functional connections at the whole web scale.

Feedbacks between protistan single-cell activity and bacterial physiological structure reinforce the predator/prey link in microbial foodwebs.

The trophic interactions between bacteria and their main predators, the heterotrophic nanoflagellates (HNFs), play a key role in the structuring and functioning of aquatic microbial food webs. Grazing regulation of bacterial communities, both of biomass and community structure, have been frequently reported. Additionally, bottom-up responses of the HNF at the population level (numerical responses) have also been extensively described. However, the functional response of HNF at the single-cell level has not been well explored. In this study, we concurrently measured the physiological structure of bacterial communities and HNF single-cell activities during re-growth cultures of natural aquatic communities. We found that changes in the abundance and proportion of the preferred, highly active bacterial prey, caused by the feeding activity of their predators (HNF), induced a negative feedback effect on the single-cell activity of these HNF. These shifts in the specific cellular activity of HNF occur at a much shorter time scale than population level shifts in flagellate abundance, and offer a complementary mechanism to explain not only the tight coupling between bacteria and HNF, but also the relative constancy of bacterial abundance in aquatic ecosystems.

How detectable is predation in stage-structured populations? Insights from a simulation-testing analysis.

The potential of predation to structure marine food webs is widely acknowledged. However, available tools to detect the regulation of prey population dynamics by predation are limited, partly because available population data often aggregate a population's age structure into a single biomass or abundance metric. Additionally, many food webs are relatively complex, with prey species subject to different assemblages of predators throughout their ontogeny. The goal of this study was to evaluate the extent to which stage-structured predation could be reliably detected from time series of total biomass of predators and prey. We simulated age-structured populations of four mid-trophic-level fish species with distinct life-history traits, exposed them to variable predation at different life stages and fit production models to resulting population biomass to determine how reliably the effects of predators could be detected. Predation targeting early life history and juvenile life stages generally led to larger fluctuations in annual production and was therefore more detectable. However, ecologically realistic levels of observation error and environmental stochasticity masked most predator signals. The addition of predation at a second life stage sharply decreased the ability to detect the effect of each predator. We conclude that the absence of detectable species interactions from biomass time series may be partly due to the interactive effects of environmental variability and complex food web linkages and life histories. We also note that predation signals are most robust for predator-prey systems where predators primarily act on mortality of submature life-history stages. Simulation testing can be applied widely to evaluate the statistical power of analyses to detect predation effects.

Future challenges in cephalopod research

Cephalopods (Mollusca: Cephalopoda) play an important role as keystone invertebrates in various marine ecosystems, as well as being a valuable fisheries resource. At the World Malacological Congress, held 21–28 July 2013 in Ponta Delgada, Azores, Portugal, a number of cephalopod experts convened to honour the contribution of the late Malcolm R. Clarke, FRS (1930–2013) to cephalopod research. Endorsed by the Cephalopod International Advisory Council (CIAC), the meeting discussed some of the major challenges that cephalopod research will face in the future. These challenges were identified as follows: (1) to find new ways to ascertain the trophic role and food web links of cephalopods using hard tissues, stable isotopes and novel concepts in theoretical ecology; (2) to explore new approaches to the study of cephalopod morphology; (3) to further develop cephalopod aquaculture research; (4) to find new ways to ascertain cephalopod adaptation and response to environmental change; (5) to strengthen cephalopod genetics research; and (6) to develop new approaches for cephalopod fisheries and conservation. The present paper presents brief reviews on these topics, followed by a discussion of the general challenges that cephalopod research is bound to face in the near future. By contributing to initiatives both within CIAC and independent of CIAC, the principle aim of the paper is to stimulate future cephalopod research.

Production sources and food web of a macrophyte-dominated region in Lake Taihu, based on gut contents and stable isotope analyses

We estimated the food web linkages among primary producers, invertebrates and fish in a macrophyte-dominated region of a eutrophic lake (Lake Taihu) by analyzing gut contents and C and N stable isotope ratios. Observation of the gut contents reflected a variety of feeding modes among fish species that consume a diverse assortment of prey, with limited dietary overlap. Basal food sources were distinguishable based on their δ13C isotopic signatures, and wide seasonal variations of isotope values were observed in the lake biota with a general trend towards enriched δ13C and δ15N values in summer and depleted values in winter. This pattern could be explained by a combination of environmental (e.g., irradiance and nutrient inputs) and biotic (e.g., availability of food sources and plasticity in prey item choice) features. We adopted a paired (gut contents and stable isotopes) approach to reconstruct diets that used known food items as end members in the isotopic mixing model analysis and diagrammed the food web structure of Lake Taihu describing the organic matter pathways from primary producers to predators of the upper trophic levels. Our surveys also corroborate the finding that phytoplankton were the most important primary source of organic matter for fauna within this macrophyte-dominated lake region, whereas macrophytes also made a sizeable contribution for several invertebrate and fish species, especially in winter.