Trophic Organization Research Articles

Trophic mechanisms underlying bentho‐demersal community recovery in the north‐east Atlantic

Summary Bottom trawling is considered one of the greatest and most widespread causes of anthropogenic change in shelf seas, with major and prolonged impacts in areas with a long history of exploitation by fisheries such as the North Atlantic. Here, signs of recovery following the enforcement of regulations are increasingly being reported. We examined the extent to which biological diversity and functionality are restored when fishing pressure is reduced by evaluating changes in species biomass and that of the main functional groups present in the continental platform, as obtained from systematic survey (IBTS) results. Moreover, we examined how this recovery is mirrored in the trophic organization of the affected communities by assessing variations in link density and strength of the main consumer species and investigating whether variations in species richness were paralleled by changes in network properties. Finally, we investigated whether reductions in fishing pressure (fishing mortality) were correlated with the abovementioned variations in community and trophic structure of the bentho‐demersal assemblages. Our results corroborate the apparent recovery of North Atlantic fishing stocks and further substantiate the improved welfare of the bentho‐demersal assemblages of the Southern Bay of Biscay. Specifically, we found an increase in species richness and in the abundance of most functional groups, especially those more closely related to the benthos, following the reduction in fishing mortality. Increases in overall species richness were paralleled by an increase in the number of links and a reduction in mean interaction strength connecting the main consumer species with their prey items. This is in accordance with ecological theory and could explain the mechanism by which bentho‐demersal assemblages restructure their trophic network towards more stable organizations. Synthesis and applications. Detecting patterns of recovery or change to alternative stable states following stress release is essential to unravel the effects of perturbations and to design effective management strategies. Our study shows that trophic network properties provide a convincing tool to evaluate and perceive recovery patterns. The trends shown in our study appear to be related with the decline in fishing mortality resulting from the enforcement of fisheries regulations in the area. They substantiate the efficiency of these regulations as a guarantee for an ecosystem approach to fisheries management and advocate their enforcement at a wider level as a convincing measure to preserve the sustainability of marine resources and their welfare.

Implications of riparian willow invasion to instream community structure and function: a synthesis using causal criteria analysis

Invasive riparian plants are a significant threat to riverine environments and are thought to alter the structure and function of stream ecosystems. Salix spp. are a genus of highly invasive northern hemisphere trees and shrubs that have invaded substantial areas of southern hemisphere riparian corridors. We set out to review the existing peer reviewed literature surrounding the impacts of Salix spp. infestation to streams by rigorously testing a suite of cause–effect hypotheses using a causal criteria analysis. Our analysis found evidence in the literature that infestation by exotic Salix spp. can cause a decrease in incidental illumination and benthic periphyton density, increased rates of allochthonous litter leaching and decomposition and changes to secondary consumer assemblage and trophic organisation. The review also highlighted a number of aspects of Salix spp. invasion for which there are significant knowledge gaps in the literature. Our results emphasise the importance site specificity, seasonal variation, physical properties of supplanted vegetation, stream size and magnitude of infestation when predicting putative cause–effect relationships between Salix spp. invasion and stream structure and function. We show that, by possessing incongruent biological and physical characteristics to native plants, invasive terrestrial trees have the capacity to influence adjacent aquatic ecosystems.

Nitrogen loads influence trophic organization of estuarine fish assemblages

Summary Nutrient (N and P) loading may affect functioning in aquatic ecosystems by restructuring producer assemblages with flow‐on effects to consumers. Trophic niche occupancy and trophic organization of consumers are key components of ecosystem function that have been increasingly investigated using quantitative isotopic niche indices. These indices are based on the premise that the isotopic values of consumer tissues indicate their assimilated diet. Typically, isotopic niche indices are calculated using only consumer isotope data, which limit their application for spatial and temporal comparisons because consumer isotopic niches depend on isotopic variability of available autotrophs. We used measures of isotopic variability of autotrophs to standardize isotopic niche indices, which enabled us to compare trophic organization of fish assemblages in nine estuaries spanning a broad range of nutrient loading. We related standardized isotopic niche indices of fish assemblages to nitrogen and phosphorous loads and hydrological flushing of the estuaries in autumn and spring. The estuarine fish assemblages studied here showed greater trophic diversity and less redundancy given moderate to high inorganic nitrogen loading. Taxonomic richness partly influenced three isotopic niche indices measuring trophic diversity, but not measures of redundancy. Similar patterns may occur in other systems in which nitrogen loads have increased but the diversity of primary producers has not been reduced to a single dominant source. Our results demonstrate bottom‐up controls of estuarine food webs. Effects of inorganic nitrogen loading were transmitted upwards through the food web to affect the trophic organization of higher trophic levels, demonstrating the crucial role of nitrogen for estuarine trophic dynamics.

Community structure of shallow rocky shore fish in a tropical bay of the southwestern Atlantic

Abstract Southwestern Atlantic Ocean rocky shores sustain important reef fish communities. However, those communities in tropical regions are not well understood, especially in Brazil. In this present article we assess community parameters of reef fishes such as composition, trophic organization and their relationships with physical and biological factors on four tropical rocky shores in Todos os Santos Bay, southwestern Atlantic. During six months, a total of 80 visual censuses were performed, in which 3,582 fish belonging to 76 species were recorded. Territorial herbivorous fish and turf algae were dominant at all the sites. The spatial variability of fish community structure was related to the benthic cover composition and depth. The high abundance of territorial herbivores and mobile invertebrate feeders could be associated with high levels of turf cover, low wave exposure and shallow waters. Moreover, this fact could be a consequence of the low density of roving herbivores and large carnivores probably due to the pressure of intense fishing activity. Thus complementary studies are needed to evaluate the actual conservation status of these rocky shore reefs, singularly located habitats connecting inner and outer reefs in Todos os Santos Bay.

Food-web stability signals critical transitions in temperate shallow lakes.

A principal aim of ecologists is to identify critical levels of environmental change beyond which ecosystems undergo radical shifts in their functioning. Both food-web theory and alternative stable states theory provide fundamental clues to mechanisms conferring stability to natural systems. Yet, it is unclear how the concept of food-web stability is associated with the resilience of ecosystems susceptible to regime change. Here, we use a combination of food web and ecosystem modelling to show that impending catastrophic shifts in shallow lakes are preceded by a destabilizing reorganization of interaction strengths in the aquatic food web. Analysis of the intricate web of trophic interactions reveals that only few key interactions, involving zooplankton, diatoms and detritus, dictate the deterioration of food-web stability. Our study exposes a tight link between food-web dynamics and the dynamics of the whole ecosystem, implying that trophic organization may serve as an empirical indicator of ecosystem resilience.

Environmental effects and urban impacts on aquatic macroinvertebrates in a stream of central Brazilian Cerrado

The urban effects on macroinvertebrate trophic structure were evaluated in a stream located in central Brazilian Cerrado. Samplings were evaluated from May/2004 to March/2005 in the upper, middle and lower sections. Characterization of ecological conditions and habitat diversity indicated headwater as “natural” site, up and downstream the urban region were “altered” ones, and the sites located nearest to urban area were considered “impacted”. A well oxygenated and transparent water, acid pH, low electrical conductivity (<0.22 µS/cm), and low nutrients concentrations (<0.03 mg/L PO 4 −2 , <0.05 mg/L N–NH4 and <3.0 mg/L N–NO 3 −2 ) was registered for headwater area. Stretches located near the urban region had higher electrical conductivity (48–100 µS/cm), higher ammonium (7.16 mg/L), and poor oxygenated water. The trophic organization was different along the stream and related to environmental preservation degree. Shredders occurred only in the headwater (Phylloicus sp., Trichoptera). Predators were dominant in upstream sites of the urban area (>90 % of biomass). In the sites located near the urban area, an alternative predominance was observed between gathering collectors and predators, with the gathering collectors dominance in the area impacted by city sewage inflow (>90 %). There was a progressive participation of scrapers in stretches located downstream of the city. Biomass was dominated by predators (>50 % in six sites) encompassing larger organisms (>20 mm), and gathering collectors were density dominants (from 53 to 99 %). Predators were the most dominant functional feeding group (22 taxa), followed by scrapers (eight), gathering collectors (seven) and filtering collectors (five). The results state that the Sobradinho stream is under different degrees of environmental impacts with lower preservation levels of its natural conditions, even in areas close to its headwater.

Structure, functioning, and cumulative stressors of Mediterranean deep-sea ecosystems

Environmental stressors, such as climate fluctuations, and anthropogenic stressors, such as fishing, are of major concern for the management of deep-sea ecosystems. Deep-water habitats are limited by primary productivity and are mainly dependent on the vertical input of organic matter from the surface. Global change over the latest decades is imparting variations in primary productivity levels across oceans, and thus it has an impact on the amount of organic matter landing on the deep seafloor. In addition, anthropogenic impacts are now reaching the deep ocean. The Mediterranean Sea, the largest enclosed basin on the planet, is not an exception. However, ecosystem-level studies of response to varying food input and anthropogenic stressors on deep-sea ecosystems are still scant. We present here a comparative ecological network analysis of three food webs of the deep Mediterranean Sea, with contrasting trophic structure. After modelling the flows of these food webs with the Ecopath with Ecosim approach, we compared indicators of network structure and functioning. We then developed temporal dynamic simulations varying the organic matter input to evaluate its potential effect. Results show that, following the west-to-east gradient in the Mediterranean Sea of marine snow input, organic matter recycling increases, net production decreases to negative values and trophic organisation is overall reduced. The levels of food-web activity followed the gradient of organic matter availability at the seafloor, confirming that deep-water ecosystems directly depend on marine snow and are therefore influenced by variations of energy input, such as climate-driven changes. In addition, simulations of varying marine snow arrival at the seafloor, combined with the hypothesis of a possible fishery expansion on the lower continental slope in the western basin, evidence that the trawling fishery may pose an impact which could be an order of magnitude stronger than a climate-driven reduction of marine snow.

Habitat-specific food webs and trophic interactions supporting coastal-dependent fishery species: an Australian case study

Coastal ecosystems such as estuaries, tidal wetlands and shallow coastal waters are often highly productive and provide important habitats to many recreationally and commercially important fish and invertebrates that use these areas as nursery, feeding and/or reproduction grounds. The diversity of coastlines found worldwide results in differences in types of provisioning and function, and in community structure and trophic organisation. Since almost all coastal fishery species require particular components of the seascapes during specific stages of their life-cycles, it is important to understand the way fish use different habitats throughout ontogeny. Access to rich feeding environments is a key contributor to habitat value, and so knowledge on food webs and feeding relationships, and how these vary over space and time, is central to understanding the importance of the different coastal environments. However, the functional roles of the different habitats in supporting fishery species are still not well understood for most regions. In this study, we review and discuss the available literature to identify key knowledge gaps in the understanding of habitat- and context-specific food webs and trophic interactions supporting fisheries species relying on coastal ecosystems. We use Australia and Australian fisheries species as a case-study, as Australia’s extensive coastline encompasses many of the coastal ecosystems and habitats found globally. Given the ever increasing transformation of coastal landscapes by either direct human action or by sea level rise and changing climate, these knowledge gaps need to be urgently addressed for appropriate management and mitigation of various impacts.

Climate‐induced changes in river flow regimes will alter future bird distributions

Anthropogenic forcing of the climate is causing an intensification of the global water cycle, leading to an increase in the frequency and magnitude of floods and droughts. River flow shapes the ecology of riverine ecosystems and climate‐driven changes in river flows are predicted to have severe consequences for riverine species, across all levels of trophic organization. However, understanding species' responses to variation in flow is limited through a lack of quantitative modelling of hydroecological interactions. Here, we construct a Bioclimatic Envelope Model (BEM) ensemble based on a suite of plausible future flow scenarios to show how predicted alterations in flow regimes may alter the distribution of a predatory riverine species, the White‐throated Dipper (Cinclus cinclus). Models predicted a gradual diminution of dipper probability of occurrence between present day and 2098. This decline was most rapid in western areas of Great Britain and was principally driven by a projected decrease in flow magnitude and variability around low flows. Climate‐induced changes in river flow may, therefore, represent a previously unidentified mechanism by which climate change may mediate range shifts in birds and other riverine biota.

Metabolic theory predicts whole-ecosystem properties

Understanding the effects of individual organisms on material cycles and energy fluxes within ecosystems is central to predicting the impacts of human-caused changes on climate, land use, and biodiversity. Here we present a theory that integrates metabolic (organism-based bottom-up) and systems (ecosystem-based top-down) approaches to characterize how the metabolism of individuals affects the flows and stores of materials and energy in ecosystems. The theory predicts how the average residence time of carbon molecules, total system throughflow (TST), and amount of recycling vary with the body size and temperature of the organisms and with trophic organization. We evaluate the theory by comparing theoretical predictions with outputs of numerical models designed to simulate diverse ecosystem types and with empirical data for real ecosystems. Although residence times within different ecosystems vary by orders of magnitude-from weeks in warm pelagic oceans with minute phytoplankton producers to centuries in cold forests with large tree producers-as predicted, all ecosystems fall along a single line: residence time increases linearly with slope = 1.0 with the ratio of whole-ecosystem biomass to primary productivity (B/P). TST was affected predominantly by primary productivity and recycling by the transfer of energy from microbial decomposers to animal consumers. The theory provides a robust basis for estimating the flux and storage of energy, carbon, and other materials in terrestrial, marine, and freshwater ecosystems and for quantifying the roles of different kinds of organisms and environments at scales from local ecosystems to the biosphere.

Artificial reefs do increase secondary biomass production: mechanisms evidenced by stable isotopes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 509:15-26 (2014) - DOI: https://doi.org/10.3354/meps10866 Artificial reefs do increase secondary biomass production: mechanisms evidenced by stable isotopes Pierre Cresson1,2,3,*, Sandrine Ruitton1,2, Mireille Harmelin-Vivien1,2 1Aix-Marseille Université, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, 13288 Marseille, France 2Université de Toulon, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, 83957 La Garde, France 3Present address: IFREMER, Laboratoire Environnement & Ressources Provence-Azur-Corse, Zone Portuaire de Brégaillon, BP 330, 83507 La Seyne sur Mer Cedex, France *Corresponding author: pierre.cresson@ifremer.fr ABSTRACT: Artificial reefs (ARs) are used worldwide as a tool to manage and restore marine coastal ecosystems and to support small-scale fisheries, as increases in fish biomass around them commonly occur. Whether ARs actually produce biomass, or only attract fish from natural zones, is strongly debated. Using stable isotope ratios to elucidate the trophic organization of the largest Mediterranean artificial reef system, the present work demonstrates that the studied ARs effectively support biomass production, as invertebrate species directly depended on locally produced organic matter (OM). OM of pelagic origin was the main source of matter due to the predominance of filter-feeder organisms on the ARs, while benthic primary production was of secondary importance. Isotopic ratios of fishes confirmed the importance of the ARs as a food supplier. Their position in the trophic network was consistent with the hypothesis proposing the effective ability of ARs to increase fish biomass through production mechanisms. Carbon and nitrogen stable isotope ratios provided a basis for achieving an integrative view of trophic relationships and food web functioning of ARs. This work constitutes a baseline for future work on efficient management of coastal zones, including natural and artificial reefs. KEY WORDS: Attraction–Production · Coastal ecosystem · Mediterranean · Stable isotopes · Trophic functioning Full text in pdf format Supplementary material PreviousNextCite this article as: Cresson P, Ruitton S, Harmelin-Vivien M (2014) Artificial reefs do increase secondary biomass production: mechanisms evidenced by stable isotopes. Mar Ecol Prog Ser 509:15-26. https://doi.org/10.3354/meps10866 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 509. Online publication date: August 27, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Cell size as a key determinant of phytoplankton metabolism and community structure.

Phytoplankton size structure controls the trophic organization of planktonic communities and their ability to export biogenic materials toward the ocean's interior. Our understanding of the mechanisms that drive the variability in phytoplankton size structure has been shaped by the assumption that the pace of metabolism decreases allometrically with increasing cell size. However, recent field and laboratory evidence indicates that biomass-specific production and growth rates are similar in both small and large cells but peak at intermediate cell sizes. The maximum nutrient uptake rate scales isometrically with cell volume and superisometrically with the minimum nutrient quota. The unimodal size scaling of phytoplankton growth arises from ataxonomic, size-dependent trade-off processes related to nutrient requirement, acquisition, and use. The superior ability of intermediate-size cells to exploit high nutrient concentrations explains their biomass dominance during blooms. Biogeographic patterns in phytoplankton size structure and growth rate are independent of temperature and driven mainly by changes in resource supply.

Diet composition, feeding niche partitioning and trophic organisation of large pelagic predatory fishes in the eastern Arabian Sea

Information on the ecology and feeding behaviour of the large oceanic predatory fishes is crucial for the ecosystem approaches to fisheries management models. Co-existing large pelagic predators in the open oceans may avoid competition for the limited forage by resource partitioning on spatial, temporal or trophic levels. To test this, we studied the prey species composition, diet overlap, trophic level, and trophic organisation of 12 large predatory fishes co-existing in the eastern Arabian Sea. Stomach contents of 1,518 specimens caught by exploratory longline operations in the Indian Exclusive Economic Zone during the years 2006–2009 were analysed. Finfishes were dominant prey of all species except blue marlin (Makaira nigricans) and yellowfin tuna (Thunnus albacares), which fed mainly on cephalopods, and long-snouted lancetfish (Alepisaurus ferox) and pelagic stingray (Pteroplatytrygon violacea), which fed mainly on crustaceans. Common dolphinfish (Coryphaena hippurus) and yellowfin tuna fed on a wider variety of prey than the other species, while the diets of lancetfish and black marlin (Istiompax indica) were narrowest. Pelagic stingray and great barracuda (Sphyraena barracuda) fed on species occupying epipelagic waters, whereas the contribution of mesopelagic prey was higher in the diets of swordfish (Xiphias gladius) and pelagic thresher (Alopias pelagicus). Trophic levels of these fishes ranged from 4.13 to 4.37. Diet overlap index revealed that some of the large pelagic predatory fishes share common prey species. Cluster analysis of the diets revealed four distinct trophic guilds namely ‘flyingfish feeders’ (common dolphinfish and great barracuda); ‘mesopelagic predators’ (pelagic thresher and swordfish); ‘crab feeders’ (lancetfish, pelagic stingray and silky shark) and ‘squid feeders’ (yellowfin tuna, Indo-Pacific sailfish (Istiophorus platypterus), skipjack tuna (Katsuwonus pelamis), black marlin and blue marlin). Large predatory fishes of the eastern Arabian Sea target different prey types, and limit their vertical extent and time of feeding to avoid competing for prey.

Highly resolved early Eocene food webs show development of modern trophic structure after the end-Cretaceous extinction

Generalities of food web structure have been identified for extant ecosystems. However, the trophic organization of ancient ecosystems is unresolved, as prior studies of fossil webs have been limited by low-resolution, high-uncertainty data. We compiled highly resolved, well-documented feeding interaction data for 700 taxa from the 48 million-year-old latest early Eocene Messel Shale, which contains a species assemblage that developed after an interval of protracted environmental and biotal change during and following the end-Cretaceous extinction. We compared the network structure of Messel lake and forest food webs to extant webs using analyses that account for scale dependence of structure with diversity and complexity. The Messel lake web, with 94 taxa, displays unambiguous similarities in structure to extant webs. While the Messel forest web, with 630 taxa, displays differences compared to extant webs, they appear to result from high diversity and resolution of insect–plant interactions, rather than substantive differences in structure. The evidence presented here suggests that modern trophic organization developed along with the modern Messel biota during an 18 Myr interval of dramatic post-extinction change. Our study also has methodological implications, as the Messel forest web analysis highlights limitations of current food web data and models.

Perspectives on the use of lakes and ponds as model systems for macroecological research

Macroecology studies large-scale patterns aiming to identify the effects of general ecological processes. Although lakes (and ponds) are particularly suited for macroecological research due to their discrete nature and non geographically-structured variability, the development of this discipline in lentic habitats is comparatively much smaller than for terrestrial environments. This is despite the interest of limnologists for large-scale phenomena, which results in the high level of development of some disciplines such as predictive limnology. Here we discuss how current state-of-the-art in macroecology may benefit from research in lentic habitats at five topics. First, by including an island biogeography analytical framework to incorporate the effects of lake origin and history on lentic biodiversity. Second, by studying local and regional effects on the latitudinal gradients of species richness. Third, by considering lakes and ponds altogether for the study of beta diversity and metacommunity structure, which is already common ground in limnological research. Fourth, by relating species traits with ecosystem structure and functioning; here we consider in particular the potential effects of body size-determined dispersal and competitive exclusion processes on lake-wide trophic organization. And fifth, by incorporating current research in functional (i.e. trait) and phylogenetic diversity to the study of community structure. We finally conclude that lentic habitats can be particularly important for the development of the most functional aspects of macroecology, due to the relative ease of studying the different biotic and abiotic components of the system separately, compared to most terrestrial systems. This can allow teasing apart many of the confounding factors that are characteristic of macroecological research, thus helping the development of future theoretical syntheses.

Riverine subsidies for inshore filter-feeder communities: potential influences on trophic patterns among bioregions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 498:13-26 (2014) - DOI: https://doi.org/10.3354/meps10634 Riverine subsidies for inshore filter-feeder communities: potential influences on trophic patterns among bioregions Sean N. Porter1,4,*, Sven Kaehler2, George M. Branch1, Kerry J. Sink3 1Marine Research Institute, Zoology Department, University of Cape Town, Rondebosch 7701, South Africa 2Department of Botany, Rhodes University, Grahamstown 6140, South Africa 3South African National Biodiversity Institute, Claremont 7735, South Africa 4Present address: Oceanographic Research Institute, PO Box 10712, Marine Parade 4056, South Africa *Corresponding author: sporter@ori.org.za ABSTRACT: In the Natal Bioregion of eastern South Africa, biomass of marine subtidal filter feeders is particularly high and makes a central contribution to distinguishing this bioregion from adjacent ones. We analysed the trophic role of riverine suspended particulate organic matter (POM) and the extent to which subsidies from rivers may explain this high filter-feeder biomass. Using carbon, nitrogen and sulphur isotopes, and a 3-end-member Bayesian-mixing model, we determined (1) the proportions of various primary producers contributing to the inshore POM pool and (2) the relative amounts of marine seaweed, pelagic POM and riverine POM assimilated by filter-feeder communities at various distances from 4 river mouths during austral summer and winter. Of the inshore POM pool available to filter feeders, riverine POM contributed 17 to 62%, pelagic POM 18 to 77% and seaweed 6 to 53%. The contributions of riverine POM to inshore POM declined significantly with distance from river mouth, but were unaffected by season or river size. Most material assimilated by filter feeders was of marine origin, notably seaweed detritus (39-62%), but with a noteworthy uptake of riverine POM (9-33%). Only small seasonal differences (<10%) and no biologically meaningful spatial trends were detected in the proportional assimilation of the 3 food sources by filter feeders. Although important, the trophic contribution of riverine POM may be subordinate to other factors such as turbidity and productivity in explaining the high biomass of filter feeders. Collectively, however, these river-associated factors are likely to explain the contrasts in trophic organisation among marine bioregions. KEY WORDS: Rivers · Isotope · Sulphur · Seaweed · Perna perna · Pyura stolonifera Full text in pdf format PreviousNextCite this article as: Porter SN, Kaehler S, Branch GM, Sink KJ (2014) Riverine subsidies for inshore filter-feeder communities: potential influences on trophic patterns among bioregions. Mar Ecol Prog Ser 498:13-26. https://doi.org/10.3354/meps10634 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 498. Online publication date: February 17, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Contrasting perception of fish trophic level from stomach content and stable isotope analyses: A Mediterranean artificial reef experience

A large complex of artificial reefs was deployed in the Bay of Marseilles, North-Western Mediterranean, for the enhancement of commercial fisheries stocks. Carbon and nitrogen stable isotope and stomach content analyses were performed on 23 fish species collected on the artificial reefs to assess their trophic position and feeding behaviour. Results indicated that fish diets were not modified on the artificial reefs compared to natural environments, nor was the structure of their trophic network. Artificial reefs, with their complex design, provide diverse and abundant food sources for fishes. Ranges of δ13C and δ15N of artificial reef fishes were comparable to those recorded in natural Mediterranean environments, with a similar trophic organization. However, some discrepancies appeared when comparing fish trophic level based on isotopic or diet results, which calls for a careful interpretation of stable isotope values as direct indicators of trophic level.

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.

Parasites Affect Food Web Structure Primarily through Increased Diversity and Complexity

Author SummaryFood webs are networks of feeding interactions among species. Although parasites comprise a large proportion of species diversity, they have generally been underrepresented in food web data and analyses. Previous analyses of the few datasets that contain parasites have indicated that their inclusion alters network structure. However, it is unclear whether those alterations were a result of unique roles that parasites play, or resulted from the changes in diversity and complexity that would happen when any type of species is added to a food web. In this study, we analyzed many aspects of the network structure of seven highly resolved coastal estuary or marine food webs with parasites. In most cases, we found that including parasites in the analysis results in generic changes to food web structure that would be expected with increased diversity and complexity. However, in terms of specific patterns of links in the food web (“motifs”) and the breadth and contiguity of feeding niches, parasites do appear to alter structure in ways that result from unique traits—in particular, their close physical intimacy with their hosts, their complex life cycles, and their small body sizes. Thus, this study disentangles unique from generic effects of parasites on food web organization, providing better understanding of similarities and differences between parasites and free-living species in their roles as consumers and resources.

Fish assemblages on shipwrecks and natural rocky reefs strongly differ in trophic structure

In the present work fish assemblages over two metallic vessels, five and 105 years old, and two natural rocky reefs were compared. The hypothesis that shipwrecks support assemblages with trophic structure similar to that encountered on natural reefs was rejected. Artificial and natural reefs strongly differ in their trophic structure, both in their multivariate composition and in biomass of most guilds. Substrate characteristics such as rugosity and benthic cover were found to influence the trophic organisation of the communities. Moreover, slow-paced structural changes over time in both biotic and abiotic aspects of wrecks appear responsible for younger and older artificial reefs be dissimilar in respect to biomass density of most feeding guilds. However, the older artificial reef did not present any strikingly “intermediate” feature between the younger artificial reef and the natural reefs, evidencing that distinct trophic assemblages exist over wrecks. Finally, the results found indicate that the use of shipwrecks as mitigation tool for losses of natural reefs may not be fully appropriate as they greatly differ in trophic structure, and consequently in energy flow, from natural reefs. Also, setting shipwrecks near natural reefs should be avoided as they differ in resources availability for many species, which may alter the community structure of natural habitats.