Patterns Of Trophic Structure Research Articles

Mercury biomagnification and trophic structure patterns in neotropical coastal estuaries impacted by a Chlor-alkali plant in northeast Brazil

The present study reports the concentration of HgT and MeHg in sediments and food webs in estuarine and coastal areas of Northeast Brazil, which have been subject to different impacts, mainly by chlorine and alkali companies. The results showed that the HgT in the sediment varied from 1.7 to 1186 ng g−1 and from 3.3 to 2163 ng g−1 in the organisms. A strong positive relationship between the levels of Hg in the sediment and the organisms was detected. Methylmercury represented 42 to 99% in the muscles of fish and invertebrates. Biomagnification processes were identified for all study areas evaluated according the trophic magnification factor (TMF), which ranged from 3.1 to 12.3. We found significant negative correlations with δ13C values and mercury concentrations and positive correlations between δ15N values and mercury, typical of Hg trophodynamics. This suggests a direct impact of the level of site-specific contamination on local food web and the ecosystem, making continuous monitoring studies necessary to assess food security and the risks that the riverine community is exposed as a result of ingesting mercury contaminated fish.

Consistent predator-prey biomass scaling in complex food webs

The ratio of predator-to-prey biomass is a key element of trophic structure that is typically investigated from a food chain perspective, ignoring channels of energy transfer (e.g. omnivory) that may govern community structure. Here, we address this shortcoming by characterising the biomass structure of 141 freshwater, marine and terrestrial food webs, spanning a broad gradient in community biomass. We test whether sub-linear scaling between predator and prey biomass (a potential signal of density-dependent processes) emerges within ecosystem types and across levels of biological organisation. We find a consistent, sub-linear scaling pattern whereby predator biomass scales with the total biomass of their prey with a near ¾-power exponent within food webs - i.e. more prey biomass supports proportionally less predator biomass. Across food webs, a similar sub-linear scaling pattern emerges between total predator biomass and the combined biomass of all prey within a food web. These general patterns in trophic structure are compatible with a systematic form of density dependence that holds among complex feeding interactions across levels of organization, irrespective of ecosystem type.

Response of trophic structure and isotopic niches of the food web to flow regime in the Yellow River estuary

Variations in the flow regime lead to changes in the spatial and temporal distributions of nutrients entering the estuary area from the river, which causes corresponding changes in the trophic structure of the estuarine food web and ultimately affects the entire ecosystem. Therefore, studying the responses of estuarine food webs to changes in upstream water and sediment conditions is of great significance for protecting the biodiversity, stability, and functional integrity of estuarine ecosystems. This study focused on the world-famous sandy river, the Yellow River, and the south and north shores of the Yellow River estuary and selected sample areas with different environmental characteristics. Typical species were sampled and measured for carbon- and nitrogen-stable isotopes in the wet and dry seasons, respectively. Based on community indicators of stable isotopes, the trophic structure patterns of the aquatic ecosystem in the Yellow River estuary at different times and spaces were described. The results showed that the north shore area had greater trophic diversity and lower trophic redundancy than the south shore area. This kind of gap was more obvious in the wet season when water was abundant than in the dry season. The south shore area with lower aquatic productivity had greater reliance to input from terrestrial organic matter than the north shore area. The increase in terrestrial material input during the wet season increased the carbon-stable isotope value of the south shore food web and changed the basic food sources. The north shore food web, which did not depend on a single food source, had more abundant organic matter sources. The trophic diversity increased in the wet season, but the trophic redundancy was reduced. Our research suggests that the trophic structure patterns of estuarine ecosystems vary with the hydrological regime and environmental conditions, especially turbidity, which greatly affect the stability of estuarine ecosystems.

Latitudinal patterns in trophic structure of temperate reef‐associated fishes and predicted consequences of climate change

AbstractSome dramatic consequences of climate change are caused by shifting species interactions and associated changes to trophic structure and energy flow. In coastal ecosystems, the relative abundance of feeding guilds indicates dominant energy sources sustaining food webs. Here, we use a space‐for‐time substitution to investigate potential climate change impacts on trophic structure and energy flow in reef fish communities. We investigated latitudinal and seasonal patterns in the biomass distribution of five trophic groups across subtropical to temperate latitudes (29 to 44°S) in eastern Australia. Along western boundary currents, temperatures are increasing up to three times faster than the global average, making them ideal for studying climate change impacts. Using 10 years of Reef Life Survey data, we investigated potential determinants of fish biomass and community composition with generalized additive mixed models. Biomass decreased towards higher latitudes, from 220 g/m2 in the subtropics to 13 g/m2 in the south. Dominant trophic group also changed latitudinally, with herbivores and omnivores dominating lower latitudes (~30°S), zooplanktivores at mid‐latitudes (~35°S) and benthic invertivores at higher latitudes (~40°S). Biomass varied seasonally, with lower latitudes experiencing a 3.2‐fold increase between spring and autumn, while variation at higher latitudes was 1.9‐fold. We found strong evidence that factors linked to latitude and seasonality are important determinants in the distribution of fish trophic structure. As climate‐driven species redistributions accelerate in the 21st century, expected poleward shifts in trophic structure include overall increases in fish biomass linked to enhanced herbivory at mid‐latitudes and increased planktivory at higher latitudes.

Trophic ecology of elasmobranch and teleost fishes in a large subtropical seagrass ecosystem (Florida Big Bend) determined by stable isotope analysis

Carbon and nitrogen stable isotope analyses were used to infer relative trophic structure and examine regional variation in trophic dynamics of fishes in the Florida Big Bend, an approximately 300 km stretch of relatively pristine coastline in the eastern Gulf Of Mexico that contains over 250,000 ha of seagrass. The Florida Big Bend is home to a diverse assemblage of fauna; and the ecosystem is regionally important through its support of robust fishing (recreational and commercial) and eco-tourism industries. Stable isotope analyses suggest assemblages of fishes in the Florida Big Bend are trophically diverse, with considerable isotopic overlap across many taxa. Patterns of trophic structure corroborated the results of similar studies of these and related taxa and in other seagrass ecosystems, and there appear to be multiple channels of primary production. Large elasmobranch fishes were most enriched in δ 15N with values well above the teleost fishes sampled, while smaller and demersal elasmobranchs had δ 15N signatures comparable to several species of predatory teleosts. Results of stable isotope analyses suggested high trophic redundancy and overlap in resource use among both teleost and elasmobranch fishes. Comparisons of regional stable isotope values revealed some spatial variability and indicated the southern Big Bend is isotopically distinct, suggesting a distinct regional faunal zone in this region, potentially due to greatly reduced river influence in the southern portion of the system.

Contribution of sea ice microbial production to Antarctic benthic communities is driven by sea ice dynamics and composition of functional guilds.

Organic matter produced by the sea ice microbial community (SIMCo) is an important link between sea ice dynamics and secondary production in near-shore food webs of Antarctica. Sea ice conditions in McMurdo Sound were quantified from time series of MODIS satellite images for Sept. 1 through Feb. 28 of 2007-2015. A predictable sea ice persistence gradient along the length of the Sound and evidence for a distinct change in sea ice dynamics in 2011 were observed. We used stable isotope analysis (δ13 C and δ15 N) of SIMCo, suspended particulate organic matter (SPOM) and shallow water (10-20m) macroinvertebrates to reveal patterns in trophic structure of, and incorporation of organic matter from SIMCo into, benthic communities at eight sites distributed along the sea ice persistence gradient. Mass-balance analysis revealed distinct trophic architecture among communities and large fluxes of SIMCo into the near-shore food web, with the estimates ranging from 2 to 84% of organic matter derived from SIMCo for individual species. Analysis of patterns in density, and biomass of macroinvertebrate communities among sites allowed us to model net incorporation of organic matter from SIMCo, in terms of biomass per unit area (g/m2 ), into benthic communities. Here, organic matter derived from SIMCo supported 39 to 71 per cent of total biomass. Furthermore, for six species, we observed declines in contribution of SIMCo between years with persistent sea ice (2008-2009) and years with extensive sea ice breakout (2012-2015). Our data demonstrate the vital role of SIMCo in ecosystem function in Antarctica and strong linkages between sea ice dynamics and near-shore secondary productivity. These results have important implications for our understanding of how benthic communities will respond to changes in sea ice dynamics associated with climate change and highlight the important role of shallow water macroinvertebrate communities as sentinels of change for the Antarctic marine ecosystem.

Variable littoral‐pelagic coupling as a food‐web response to seasonal changes in pelagic primary production

Abstract Lakes are among the most seasonally forced ecosystems on Earth. Seasonal variation in temperature and light produce cyclic patterns in water column mixing, nutrient supply and phytoplankton biomass. Diet responses of consumers to these patterns have rarely been quantified. Moreover, pelagic‐littoral coupling of dietary resources by mobile consumers is commonly considered to be static over annual cycles. This study quantifies littoral‐pelagic diet responses of multiple consumers to a strong shift in pelagic phytoplankton abundance over an annual cycle (September 2014 to August 2015) in a large (area 614 km2), oligotrophic, monomictic lake (Lake Taupō, New Zealand). Intra‐annual patterns in pelagic phytoplankton (chlorophyll a) and zooplankton were determined over multiple years. Major resource and consumer δ13C and δ15N were then collected over an annual cycle. Temporal patterns in food‐web structure were examined using convex hulls as a proxy of community trophic niche size. Diet was quantified using mixing models for zooplankton, meso‐predatory zooplanktivorous common smelt (Retropinna retropinna) and benthivorous common bullies (Gobiomorphus cotidianus), as well as the top‐predator rainbow trout (Oncorhynchus mykiss). Trophic structure patterns for smelt, bullies and trout were then independently examined using compound‐specific amino acid δ15N analyses (CSIA‐AA). Lake Taupō demonstrated similar food‐web patterns to other lakes globally. Phytoplankton and zooplankton demonstrated strong seasonal oscillations of abundance driven by both bottom‐up (nutrient supply) and top‐down (stable limit cycle) drivers. The food web demonstrated the typical nested structure. It responded to seasonally low and high pelagic resource availability periods by expansion and contraction, respectively, of trophic niche space. In response to lower pelagic phytoplankton abundance during summer stratification, and phytoplankton accumulation at a deep chlorophyll maximum (DCM), zooplankton abundance reduced and their diet became dominated by phytoplankton from below the thermocline (i.e. the hypolimnion and DCM). This change may have been prompted by the combined drivers of avoidance of predation and depauperate food supply in surface waters. The diet of smelt and bullies switched from predominantly zooplankton to benthic macroinvertebrates, synchronous with the decline in pelagic zooplankton. Trout diet, inferred from comparison of isotopic signatures of tissues with different turnover rates, also increased littoral resource reliance over the stratified period. Smelt, bully and trout CSIA‐AA data confirmed estimates of trophic position and indicated a greater degree of trophic complexity in the littoral than the pelagic food chain. Food webs in large, deep lakes such as Taupō are expected to be primarily pelagic. This study demonstrates the need to re‐examine this expectation due to seasonal variations in productivity. The relatively small littoral areas in large lakes, combined with meso‐predators’ highly seasonally variable littoral resource use, may drive strong seasonal top‐down effects on littoral macroinvertebrate prey. Our study supports the notion that food‐web interactions are highly dynamic and responsive to seasonal forcing. By linking food‐web dynamics to dynamic environmental conditions, this study provides a framework for future studies research on understanding lake food‐web responses to a range of annual/seasonal and global environmental change drivers.

Human activities as a driver of spatial variation in the trophic structure of fish communities on Pacific coral reefs.

Anthropogenic activities such as land-use change, pollution and fishing impact the trophic structure of coral reef fishes, which can influence ecosystem health and function. Although these impacts may be ubiquitous, they are not consistent across the tropical Pacific Ocean. Using an extensive database of fish biomass sampled using underwater visual transects on coral reefs, we modelled the impact of human activities on food webs at Pacific-wide and regional (1,000s-10,000skm) scales. We found significantly lower biomass of sharks and carnivores, where there were higher densities of human populations (hereafter referred to as human activity); however, these patterns were not spatially consistent as there were significant differences in the trophic structures of fishes among biogeographic regions. Additionally, we found significant changes in the benthic structure of reef environments, notably a decline in coral cover where there was more human activity. Direct human impacts were the strongest in the upper part of the food web, where we found that in a majority of the Pacific, the biomass of reef sharks and carnivores were significantly and negatively associated with human activity. Finally, although human-induced stressors varied in strength and significance throughout the coral reef food web across the Pacific, socioeconomic variables explained more variation in reef fish trophic structure than habitat variables in a majority of the biogeographic regions. Notably, economic development (measured as GDP per capita) did not guarantee healthy reef ecosystems (high coral cover and greater fish biomass). Our results indicate that human activities are significantly shaping patterns of trophic structure of reef fishes in a spatially nonuniform manner across the Pacific Ocean, by altering processes that organize communities in both "top-down" (fishing of predators) and "bottom-up" (degradation of benthic communities) contexts.

Unravelling the role of allochthonous aquatic resources to food web structure in a tropical riparian forest.

The role of matter and energy flow across ecosystem boundaries for the subsidized consumer populations is well known. However, little is known on the effects of allochthonous subsidies on food web structure and trophic niche dimensions of consumers in the tropics. We excluded allochthonous aquatic insects from tropical streams using greenhouse-type exclosures to test the influence of aquatic allochthonous subsidies on the trophic structure and niche dimensions of terrestrial predators using stable isotope methods. In exclosure treatments, abundance and biomass of terrestrial predators, and biomass of phytophages decreased and increased, respectively. Vegetation-living predators were more responsive to allochthonous inputs than those living on the ground. Overall, lower availability of allochthonous inputs did not affect community-wide metrics and niche width of predators. However, the niche width of some spider families had very low overlap between treatments, and others had wider isotopic niches in the control than in the exclusion treatment. Most of the C and N in predators living in control stretches came from aquatic subsidies, and those predators living in the exclusion treatments switched their diets to terrestrial sources, showing a preference of predators for allochthonous subsidies. Our results suggest that allochthonous subsidies are also relevant to tropical fauna living upon vegetation. Moreover, allochthonous resources may amplify the niche dimension of certain predators or considerably change the trophic niche of others. Our study highlights the importance of including modern isotopic tools in elucidating the role of allochthonous resources on the patterns of trophic structure and niche dimensions of consumers from donor ecosystems.

Emergent global patterns of ecosystem structure and function from a mechanistic general ecosystem model.

Author SummaryEcosystems across the world are being rapidly degraded. This threatens their provision of natural goods and services, upon which all life depends. To be able to reduce—and one day reverse—this damage, we need to be able to predict the effects of human actions on ecosystems. Here, we present the first example of a General Ecosystem Model (GEM)—called the Madingley Model—a novel class of computational model that can be applied to any ecosystem, marine or terrestrial, and can be simulated at any spatial scale from local up to global. It covers almost all organisms in ecosystems, from the smallest to the largest, encoding the underlying biology and behaviour of individual organisms to capture the interactions between them and with the environment, to model the fate of each individual organism, and to make predictions about ecosystem structure and function. Predictions made by the Madingley Model broadly resemble what we observe in real-world ecosystems across scales from individuals through to communities, ecosystems, and the world as a whole. Our results show that ecologists can now begin modelling all nonhuman life on earth, and we suggest that this type of approach may hold promise for predicting the ecological implications of different future trajectories of human activity on our shared planet.

Stable isotope‐based community metrics as a tool to identify patterns in food web structure in east African estuaries

Summary Quantitative tools to describe biological communities are important for conservation and ecological management. The analysis of trophic structure can be used to quantitatively describe communities. Stable isotope analysis is useful to describe trophic organization, but statistical models that allow the identification of general patterns and comparisons between systems/sampling periods have only recently been developed. Here, stable isotope‐based Bayesian community‐wide metrics are used to investigate patterns in trophic structure in five estuaries that differ in size, sediment yield and catchment vegetation cover (C3/C4): the Zambezi in Mozambique, the Tana in Kenya and the Rianila, the Betsiboka and Pangalanes Canal (sampled at Ambila) in Madagascar. Primary producers, invertebrates and fish of different trophic ecologies were sampled at each estuary before and after the 2010–2011 wet season. Trophic length, estimated based on δ15N, varied between 3·6 (Ambila) and 4·7 levels (Zambezi) and did not vary seasonally for any estuary. Trophic structure differed the most at Ambila, where trophic diversity and trophic redundancy were lower than at the other estuaries. Among the four open estuaries, the Betsiboka and Tana (C4‐dominated) had lower trophic diversity than the Zambezi and Rianila (C3‐dominated), probably due to the high loads of suspended sediment, which limited the availability of aquatic sources. There was seasonality in trophic structure at Ambila and Betsiboka, as trophic diversity increased and trophic redundancy decreased from the prewet to the postwet season. For Ambila, this probably resulted from the higher variability and availability of sources after the wet season, which allowed diets to diversify. For the Betsiboka, where aquatic productivity is low, this was likely due to a greater input of terrestrial material during the wet season. The comparative analysis of community‐wide metrics was useful to detect patterns in trophic structure and identify differences/similarities in trophic organization related to environmental conditions. However, more widespread application of these approaches across different faunal communities in contrasting ecosystems is required to allow identification of robust large‐scale patterns in trophic structure. The approach used here may also find application in comparing food web organization before and after impacts or monitoring ecological recovery after rehabilitation.

Tiger salamanders in prairie potholes: A “Fish in amphibian’s garments?”

It has been hypothesized that tiger salamanders (Ambystoma tigrinum) in fishless aquatic habitats are functional equivalents of planktivorous fish. I tested this idea in a series of fishless prairie wetlands (“potholes”) in southwestern Manitoba by comparing macroinvertebrate community structure and phytoplankton standing crop across a gradient of gray tiger salamander (A. tigrinum diaboli Dunn) abundance, and in the presence and absence of tiger salamanders. Estimates of tiger salamander abundance included both adult and larval forms. Separate analyses yielded similar results. First, across a set of 45 potholes sampled over two years, I found that as tiger salamander abundance increased, aquatic insect abundance decreased and phytoplankton standing crop increased. Second, for a subset of seven potholes that had tiger salamanders one year but not the other, reverse trophic structure patterns were observed. For all analyses, abundances of mostly herbivorous and detritivorous macroinvertebrates (e.g., Gastropoda, Zooplankton) were unrelated to tiger salamander abundance suggesting that either aquatic food web relationships in prairie potholes are complex or the link between tiger salamanders and phytoplankton standing crop may partly be a function of nutrient recycling rather than only direct predator-prey interaction. These results support the idea that effects of tiger salamanders on the trophic structure of prairie potholes mimic those of planktivorous fish.

Trophic structure patterns of Brazilian reef fishes: a latitudinal comparison

AbstractAim To investigate how reef fish trophic structure responds to latitudinal changes, using a simple model: the extensive Brazilian coast.Location Six Brazilian tropical and subtropical coral and rocky coastal reefs, and the oceanic island of Atol das Rocas, between latitudes 0° and 27° S.Methods Underwater visual census data collected by the authors (five locations) or obtained from the literature (two locations) were used to estimate the relative abundance of 123 fish species belonging to 33 reef‐associated families. Cryptic species were excluded from the analysis. Fishes were grouped in eight trophic categories: roving herbivores, territorial herbivores, mobile invertebrate feeders, sessile invertebrate feeders, omnivores, planktivores, piscivores and carnivores. After a series of detailed predictions based on phylogeny, physiological constraints and anthropogenic impacts was established, the community trophic structure was analysed along a latitudinal gradient and among coastal, mid‐shore and oceanic sites.Results The trophic structure of Brazilian reef fish assemblages clearly changed with latitude. Roving herbivores such as scarids and acanthurids were proportionally more abundant at low latitudes. The browsing herbivores kyphosids followed an opposite latitudinal pattern. The parrotfish genus Sparisoma, more plastic in its feeding habits than Scarus, presented wider distribution. The relative abundance of territorial herbivores did not decrease towards higher latitudes. Mobile invertebrate feeders were the most important (in low latitudes) or the second most important trophic guild (in high latitudes) at all coastal sites. Sessile invertebrate feeders did not show any clear latitudinal trend, despite an expected increase in abundance towards low latitudes. Omnivores dominated high latitude reefs (27° S) and planktivores the oceanic island Atol das Rocas. Piscivores and carnivores were proportionally better represented in high latitudes.Main conclusions Latitudinal patterns seem to be influenced by phylogeny, physiological constraints (mainly related to temperature), and also by anthropogenic impacts. Grazing scarids and acanthurids are largely restricted to tropical reefs and show an abrupt decline beyond 23° S. This does not reflect the amount of algae present, but probably temperature‐dependent physiological constraints. Other herbivores seem to overcome this through symbiotic microbial digestive processes (kyphosids), manipulating the structure of algal turfs or increasing animal protein from within the territory (pomacentrids). Omnivores dominate the southern sites Arraial do Cabo and Arvoredo, being more adapted to environment constraints related to seasonal and/or stochastic shifts. Large carnivores (including piscivores) extend farther into high‐latitude habitats, apparently not constrained by thermal thresholds that limit the herbivores. Overfishing and/or ornamental harvesting certainly has been modifying local fish communities, but could not be detected properly at the large‐scale patterns found in this study. The data presented put in evidence for the first time how reef fish trophic structure behave in the extensive south‐western Atlantic latitudinal gradient.

Spatial and dietary overlap in the Georges Bank groundfish community

Patterns in spatial and trophic resource partitioning in the fish community of the Georges Bank region are identified, accounting for size-based changes in diets. During autumn and spring, this community is divided into geographic assemblages of species that have high spatial overlap. Similarity in spatial distribution is primarily related to similarity in depth preferences, and seasonal differences in species composition within assemblages are related to migrations. There is also important trophic structure within the Georges Bank community separating predators based upon prey size and location in the water column. Ontogenetic changes in diets are an important feature of the trophic structure in this system, particularly in the major piscivores. Seasonal changes in trophic structure reflect both predator and prey migrations. Dietary overlap among predator types is independent of either spatial overlap or depth preferences. There is spatial segregation within trophic guilds, and this spatial partitioning reduces the potential for exploitative competition within this community. Given the observed spatial patterns in trophic structure, the geographic assemblages on Georges Bank may be considered ecologically distinct functional units within this ecosystem.

A Trophic Position Model of Pelagic Food Webs: Impact on Contaminant Bioaccumulation in Lake Trout

To test how well use of discrete trophic levels represents pelagic trophic structure, we compiled dietary data from >200 lake trout and pelagic forage fish populations and calculated a continuous (fractional) measure of trophic position for each population. Although discrete trophic levels qualitatively represent broad—scale patterns in trophic structure, pelagic food webs are characterized by complexity and omnivory, thereby limiting the ability of discrete trophic levels to quantitatively represent trophic structure in terms of mass transfer and energy flow. Lake trout trophic position, which ranged from 3.0 to 4.6, explained 85% of the between—lake variability in mean PCB levels in lake trout muscle tissue, providing a significant improvement over the use of discrete trophic levels as a predictor of contaminant levels. Having demonstrated the utility of trophic position, we developed a generalized trophic position model of lake trout food webs. This approach eliminates minor trophic linkages, calculates a fractional measure of each species' trophic position, and aggregates species of similar trophic position into trophic guilds. This realized model represents trophic structure in terms of mass transfer and accounts for the complexity and omnivory that characterize aquatic food webs. In our trophic position model, smelt (a species of pelagic forage fish) were designated a trophic guild separate from other pelagic forage fish, due to their elevated trophic position. Separate consideration of smelt was supported by elevated lake trout trophic position, PCB, and Hg levels in lakes containing smelt. Consideration of omnivory caused biomagnification factors (BMFs) to be many times higher than BMFs that ignored omnivory. These omnivory—corrected BMF estimates appeared to be more consistent with values calculated using stable nitrogen isotopes (δ15N), an alternative continuous measure of trophic position. (δ15N) provided trophic position estimates that generally corresponded with our diet—derived estimates.