Variability In Food Web Structure Research Articles

Species distribution models describe spatial variability in mesopelagic fish abundance in the Southern Ocean

IntroductionMesopelagic fishes play a central role in the transfer of energy through open-ocean food webs, particularly in the Southern Ocean where they are both important predators of zooplankton and a key prey group for many higher predators. However, they are notoriously difficult to sample, which has limited our understanding of the bio-physical predictors of their abundance and spatiotemporal variability. Species distribution models can be used to help understand species’ ecological requirements by relating records of their presence or abundance to environmental data.MethodsHere, we used data from Myctobase – a new circumpolar database of mesopelagic fishes – to model patterns in abundance of eight key myctophid species (family Myctophidae) and the genus Bathylagus in the Southern Ocean south of 45°S. We developed species-specific boosted regression tree models to obtain circumpolar predictions of abundance. Average daytime and night-time summer predictions for the period 1997 to 2011 at 0 to 200m depths were generated for each species.ResultsDepth and solar position were important predictors and species were stratified in their depth distribution. For all species, except for G. nicholsi, there was an interaction between depth of capture and solar position, reflecting diel vertical migration. Other important variables included sea surface temperature, dissolved oxygen at 200 m, chlorophyll a, and sea surface height, indicating an association with water mass properties. Circumpolar patterns of abundance varied between species with some displaying affinities for oceanic regions at Antarctic latitudes (e.g., E. antarctica and Bathylagus spp.) or sub-Antarctic latitudes (e.g., K. anderssoni and P. tenisoni); and affinities for shelf regions (e.g., P. boliniand G. nicholsi).DiscussionOur findings suggest that the abundance of mesopelagic fish is influenced by diel vertical migration and meso- and sub-mesoscale oceanographic features, with the Polar Front being a major delimiting feature. Our study showed contrasting patterns in community composition with higher species diversity north of the Polar Front that might be indicative of latitudinal variability in food web structure. Our spatial analysis is an important step toward resolving what determines important habitat for mesopelagic fishes, providing foundational information for understanding shifting food web dynamics into the future.

Microzooplankton and the plankton food web in the subtropical frontal region of the Indian Ocean sector of the Southern Ocean during austral summer 2012

The Southern Ocean is a very dynamic ecosystem comprising many fronts and interfrontal areas. Among the fronts, the Subtropical Front (STF) is a region of eddy activity exhibiting high spatial-temporal variability. The variability of food web structure in such a dynamic system and their influence on the plankton community structure is of great interest. The present study attempts to explain the variability of microzooplankton (MZP) mediation in the planktonic food web, using multi-disciplinary data collected along the zonal sections 40° S and 43° S in the STF during the austral summer 2012. Even though both zonal sections are in the STF, significant differences in the variability of their thermohaline structures were observed due to the presence of multiple mesoscale features (viz. cyclonic and anticyclonic eddies), leading to discrete environments within the STF. Our study highlights the concept of the classical food web in productive environments (cyclonic eddies), which is governed by the duration of such dynamic events. On the contrary, in the generally less productive anticyclonic eddies, the variability of trophic level interaction varies between the periphery and core regions of the eddies. The present study reveals the biological consequences of frequent mesoscale features in terms of the variability of food web structure. It also elucidates the trophic importance of MZP in the two contrasting environments within the STF.

Improving Marine Ecosystem Models with Biochemical Tracers.

Empirical data on food web dynamics and predator-prey interactions underpin ecosystem models, which are increasingly used to support strategic management of marine resources. These data have traditionally derived from stomach content analysis, but new and complementary forms of ecological data are increasingly available from biochemical tracer techniques. Extensive opportunities exist to improve the empirical robustness of ecosystem models through the incorporation of biochemical tracer data and derived indices, an area that is rapidly expanding because of advances in analytical developments and sophisticated statistical techniques. Here, we explore the trophic information required by ecosystem model frameworks (species, individual, and size based) and match them to the most commonly used biochemical tracers (bulk tissue and compound-specific stable isotopes, fatty acids, and trace elements). Key quantitative parameters derived from biochemical tracers include estimates of diet composition, niche width, and trophic position. Biochemical tracers also provide powerful insight into the spatial and temporal variability of food web structure and the characterization of dominant basal and microbial food web groups. A major challenge in incorporating biochemical tracer data into ecosystem models is scale and data type mismatches, which can be overcome with greater knowledge exchange and numerical approaches that transform, integrate, and visualize data.

Food-Web Complexity in Guaymas Basin Hydrothermal Vents and Cold Seeps.

In the Guaymas Basin, the presence of cold seeps and hydrothermal vents in close proximity, similar sedimentary settings and comparable depths offers a unique opportunity to assess and compare the functioning of these deep-sea chemosynthetic ecosystems. The food webs of five seep and four vent assemblages were studied using stable carbon and nitrogen isotope analyses. Although the two ecosystems shared similar potential basal sources, their food webs differed: seeps relied predominantly on methanotrophy and thiotrophy via the Calvin-Benson-Bassham (CBB) cycle and vents on petroleum-derived organic matter and thiotrophy via the CBB and reductive tricarboxylic acid (rTCA) cycles. In contrast to symbiotic species, the heterotrophic fauna exhibited high trophic flexibility among assemblages, suggesting weak trophic links to the metabolic diversity of chemosynthetic primary producers. At both ecosystems, food webs did not appear to be organised through predator-prey links but rather through weak trophic relationships among co-occurring species. Examples of trophic or spatial niche differentiation highlighted the importance of species-sorting processes within chemosynthetic ecosystems. Variability in food web structure, addressed through Bayesian metrics, revealed consistent trends across ecosystems. Food-web complexity significantly decreased with increasing methane concentrations, a common proxy for the intensity of seep and vent fluid fluxes. Although high fluid-fluxes have the potential to enhance primary productivity, they generate environmental constraints that may limit microbial diversity, colonisation of consumers and the structuring role of competitive interactions, leading to an overall reduction of food-web complexity and an increase in trophic redundancy. Heterogeneity provided by foundation species was identified as an additional structuring factor. According to their biological activities, foundation species may have the potential to partly release the competitive pressure within communities of low fluid-flux habitats. Finally, ecosystem functioning in vents and seeps was highly similar despite environmental differences (e.g. physico-chemistry, dominant basal sources) suggesting that ecological niches are not specifically linked to the nature of fluids. This comparison of seep and vent functioning in the Guaymas basin thus provides further supports to the hypothesis of continuity among deep-sea chemosynthetic ecosystems.

Benthic food-web structure of an Arctic fjord (Kongsfjorden, Svalbard)

Describing trophic relationships is fundamental for understanding ecosystem function and evaluating how these functions may vary under natural and human-induced changes in system drivers. The food-web structure (food-chain length, primary carbon sources, trophic positions of dominant functional groups) of an Arctic benthic community was investigated using stable isotopes of carbon and nitrogen. Suspended organic matter, benthic fauna, and benthic-feeding fishes and seabirds were collected from two locations during three seasons in and just outside of Kongsfjorden, Svalbard. Stable isotope ratios suggested relatively little variability in food-web structure over the temporal and spatial scales studied. A single food source (pelagic phytoplankton) appeared to predominate throughout the year regardless of location. Further, our results confirmed findings from other areas indicating that Arctic food chains are long, consisting of between 4 and 5 trophic levels. The prevalence of deposit-feeding taxa may buffer seasonal signals, and the highly advective nature of Kongsfjorden, and perhaps many open (non-silled) fjords, is likely responsible for similar food-web structure, despite spatial variability in benthic community composition.

Food web dynamics in an Australian Wet Tropics river

In Australia’s Wet Tropics rivers, perennial base flows punctuated by wet season floods drive instream responses across a range of spatial and temporal scales. We combined gut-content and stable-isotope analyses to produce preliminary webs depicting trophic links between fish, their main prey items and basal productivity sources. We then used these webs to test the applicability of general food web principles developed in other tropical systems. Although a range of sources appeared to underpin fish productivity, a large portion of total energy transfer occurred through a subset of trophic links. Variability in food web structure was negatively correlated with spatial scale, being seasonally stable at river reaches and variable at smaller scales. Wet Tropics rivers are similar to those in other tropical areas, but exhibit some unique characteristics. Their high degree of channel incision improves longitudinal connectivity, thereby allowing fish to move between mesohabitats and target their preferred prey items, rather than shifting their diet as resources fluctuate. However, this also inhibits lateral connectivity and limits terrestrial energy inputs from beyond the littoral zone.

Temporal and spatial variability in soil food web structure

Heterogeneity is a prominent feature of most ecosystems. As a result of environmental heterogeneity the distribution of many soil organisms shows a temporal as well as horizontal and vertical spatial patterning. In spite of this, food webs are usually portrayed as static networks with highly aggregated trophic groups over broader scales of time and space. The variability in food web structure and its consequences have seldom been examined. Using data from a Scots pine forest soil in the Netherlands, we explored (1) the temporal and spatial variability of a detrital food web and its components, (2) the effect of taxonomic resolution on the perception of variability over time and across space, and (3) the importance of organic matter quality as an explanatory factor for variability in food web composition. Compositional variability, expressed using the Bray‐Curtis similarity index, was measured over 2.5 years using a stratified litterbag design with three organic horizons per litterbag set.Variability in community composition and organic matter degradation increased over time in the litter horizon only. Seasonal variation in community composition was larger than variation between samples from the same season in different years. Horizontal spatial variability in community composition and organic matter degradation was relatively low, with no increase in variability with increasing distance between samples. Vertically, communities and organic matter degradation was more different between the non‐adjacent litter and humus horizons than between adjacent layers.These findings imply that soil food webs, at least in temperate forest plantations, are more variable than is currently appreciated in experiments and model studies, and that organic matter turnover might be an important factor explaining variability in community composition. Species composition was more variable than functional group composition, which implies that aggregated food webs will seem less sensitive to local temporal and spatial changes than they in fact are.

FOOD-WEB TOPOLOGY VARIES WITH SPATIAL SCALE IN A PATCHY ENVIRONMENT

The majority of food-web studies currently used to test ecological theory have integrated information over large spatial and temporal scales. We aimed to assess the degree to which food webs display patch-scale variation, and the consequences for emergent properties at the larger scale of the stream reach. Spatial heterogeneity in ecological conditions (habitat structure and food resources) and food-web structure were measured in three streams. All food webs were constructed using equivalent effort at a patch scale (0.06 m2) and a reach scale (30-m stream length). A mosaic of habitat structure and food resources was reflected in considerable variability in food-web structure among patches, but there was less variation within than among streams. The variability in food-web attributes among patches could not always be related to ecological conditions, but food resource availability affected connectedness of food webs and trophic structure (measured as functional feeding groups). Of particular note was the result that reach-summary food webs were consistently different from patch-specific food webs in each stream. Reach-scale food webs underestimated connectance but overestimated prey : predator ratios and the number of trophic links. Summary webs sometimes placed species together that, in fact, did not coexist in the field. Such within-site food-web heterogeneity needs to be taken into account in future multiple-site comparisons of food-web structure.

Spatial and temporal variability of food web structure during the spring bloom in the Skagerrak

During the intense spring diatom bloom, the spatial (km) and temporal (h) variability of the pelagic food web structure was investigated in the Skagerrak. Three surface water masses could be identified in the study area based on their temperature, salinity and biogeochemical characteris- tics. When the measured biological parameters were separated into water masses instead of stations, food web variability decreased by up to 41%. However, specific egg production rate by Acartia clausi and Oithona similis did not correlate with water masses and was probably influenced by other fac- tors, e.g. small-scale turbulence. Bacterial production and Calanus-grazing followed a diurnal cycle with increased activity during the night. The grazing impact by the copepod community was assessed by the egg production and the gut fluorescence method. Despite the Calanus population, the cope- pod community only grazed <3% of daily primary production. The protozooplankton, however, ingested 2 to 4 times more of primary production than the copepods. Overall, the zooplankton com- munity only grazed 17% of daily primary production, and consequently the majority of the spring bloom will leave the euphotic zone ungrazed as a potential food source for the benthos.

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

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

The roles of predation, competition, and exploitation in the trophic dynamics of a warmwater stream: a model synthesis, analysis, and application

We developed a trophic dynamic model of key populations and processes in the New River, West Virginia, to identify the mechanisms most responsible for maintaining food web structure. Key populations were represented by thirteen model components and were aquatic insects; age-1 and age-2 crayfish (three species); age-1 and age-2 hellgrammites (Corydalus cornutus larvae); non-game fishes; age-0, age-1, and adult smallmouth bass (Micropterus dolomieu); age-0, age-1, and adult rock bass (Ambloplites rupestris); and age-0, age-1 to age-3, and adult flathead catfish (Pylodictis olivaris). In this system, crayfish and hellgrammites are harvested to provide bait for the recreational fishery that extensively exploits the three predatory fish species. Predation and intraspecific regulation were represented with nonlinear algorithms, and linear terms represented fishery harvests. Interspecific competition among components occurred through predation on shared prey. Error analysis of the model suggested that predation was the most important mechanism in maintaining system structure (the disposition of biomass among system components). Further, the trophic relation between each component and its prey accounted for 34–64% of the variability in food web structure, whereas predation on each component explained 1–24% of food web structure variability. Therefore, so-called ‘bottom-up’ effects were more influential than ‘top-down’ effects. Interspecific competition and intraspecific regulation had secondary roles in maintaining New River food web structure, although intraspecific regulation was most important to aquatic insects, which were not predatory in our model. Both forms of competition are probably tempered by extensive predation and exploitation in the New River system. Exploitation was a secondary structuring agent to adult smallmouth bass, which experience a high rate of harvest in the New River.