Consumer Assemblages Research Articles

Zooplankton-phytoplankton biomass and diversity relationships in the Great Lakes.

Quantifying the relationship between phytoplankton and zooplankton may offer insight into zooplankton sensitivity to shifting phytoplankton assemblages and the potential impacts of producer-consumer decoupling on the rest of the food web. We analyzed 18 years (2001-2018) of paired phytoplankton and zooplankton samples collected as part of the United States Environmental Protection Agency (U.S. EPA) Great Lakes Biology Monitoring Program to examine both the long-term and seasonal relationships between zooplankton and phytoplankton across all five Laurentian Great Lakes. We also analyzed effects of phytoplankton diversity on zooplankton biomass, diversity, and predator-prey (zooplanktivore/grazer) ratios. Across the Great Lakes, there was a weak positive correlation between total algal biovolume and zooplankton biomass in both spring and summer. The relationship was weaker and not consistently positive within individual lakes. These trends were consistent over time, providing no evidence of increasing decoupling over the study period. Zooplankton biomass was weakly negatively correlated with algal diversity across lakes, whereas zooplankton diversity was unaffected. These relationships did not change when we considered only the edible phytoplankton fraction, possibly due to the high correlation between total and edible phytoplankton biovolume in most of these lakes. Lack of strong coupling between these producer and consumer assemblages may be related to lagging responses by the consumers, top-down effects from higher-level consumers, or other confounding factors. These results underscore the difficulty in predicting higher trophic level responses, including zooplankton, from changes in phytoplankton assemblages.

Resilient consumers accelerate the plant decomposition in a naturally acidified seagrass ecosystem.

Anthropogenic stressors are predicted to alter biodiversity and ecosystem functioning worldwide. However, scaling up from species to ecosystem responses poses a challenge, as species and functional groups can exhibit different capacities to adapt, acclimate, and compensate under changing environments. We used a naturally acidified seagrass ecosystem (the endemic Mediterranean Posidonia oceanica) as a model system to examine how ocean acidification (OA) modifies the community structure and functioning of plant detritivores, which play vital roles in the coastal nutrient cycling and food web dynamics. In seagrass beds associated with volcanic CO2 vents (Ischia, Italy), we quantified the effects of OA on seagrass decomposition by deploying litterbags in three distinct pH zones (i.e., ambient, low, extreme low pH), which differed in the mean and variability of seawater pH. We replicated the study in two discrete vents for 117 days (litterbags sampled on day 5, 10, 28, 55, and 117). Acidification reduced seagrass detritivore richness and diversity through the loss of less abundant, pH-sensitive species but increased the abundance of the dominant detritivore (amphipod Gammarella fucicola). Such compensatory shifts in species abundance caused more than a threefold increase in the total detritivore abundance in lower pH zones. These community changes were associated with increased consumption (52%-112%) and decay of seagrass detritus (up to 67% faster decomposition rate for the slow-decaying, refractory detrital pool) under acidification. Seagrass detritus deployed in acidified zones showed increased N content and decreased C:N ratio, indicating that altered microbial activities under OA may have affected the decay process. The findings suggest that OA could restructure consumer assemblages and modify plant decomposition in blue carbon ecosystems, which may have important implications for carbon sequestration, nutrient recycling, and trophic transfer. Our study highlights the importance of within-community response variability and compensatory processes in modulating ecosystem functions under extreme global change scenarios.

A View From Both Ends: Shifts in Herbivore Assemblages Impact Top-Down and Bottom-Up Processes on Coral Reefs

A fundamental goal in ecology is to understand the role of consumers in top-down (TD) and bottom-up (BU) processes that affect the functioning of ecosystems. Consumers ingest organic matter and excrete inorganic nutrients, and individual roles are influenced by body size and functional identity. Our study quantifies how alterations to herbivore assemblages affect both TD and BU processes on coral reefs in the South Pacific. We collected empirical data on consumption and nutrient excretion rates from 300 individual herbivorous fishes belonging to five functional groups. Individual-level traits were then scaled to a 13-year time series of fish populations from reefs that have either shifted to algal dominance or remained in the coral state. Large excavating parrotfishes and other herbivores on reefs in the coral state contributed 43% more herbivory and excreted nutrients at a higher ratio of N:P than herbivores on algal-dominated reefs; both processes may benefit coral health. Algal-dominated reefs experienced 56% higher rates of detritivory by large detritivorous fishes that remove detritus from algal surfaces, a process that may facilitate algal dominance. By scaling individual-level traits to population time series, our study provides a framework to quantify how changes to consumer assemblages impact both TD and BU processes across ecosystems undergoing change. Identifying the unique roles of consumers in processes that maintain and reinforce ecosystem states is the key to predicting the importance of shifts in diverse consumer assemblages.

Climate drives the geography of marine consumption by changing predator communities

The global distribution of primary production and consumption by humans (fisheries) is well-documented, but we have no map linking the central ecological process of consumption within food webs to temperature and other ecological drivers. Using standardized assays that span 105° of latitude on four continents, we show that rates of bait consumption by generalist predators in shallow marine ecosystems are tightly linked to both temperature and the composition of consumer assemblages. Unexpectedly, rates of consumption peaked at midlatitudes (25 to 35°) in both Northern and Southern Hemispheres across both seagrass and unvegetated sediment habitats. This pattern contrasts with terrestrial systems, where biotic interactions reportedly weaken away from the equator, but it parallels an emerging pattern of a subtropical peak in marine biodiversity. The higher consumption at midlatitudes was closely related to the type of consumers present, which explained rates of consumption better than consumer density, biomass, species diversity, or habitat. Indeed, the apparent effect of temperature on consumption was mostly driven by temperature-associated turnover in consumer community composition. Our findings reinforce the key influence of climate warming on altered species composition and highlight its implications for the functioning of Earth's ecosystems.

Seasonal variation in basal resources supporting fish biomass in longitudinal zones of the Usumacinta River Basin, southern Mexico

Aquatic food webs in tropical rivers are affected by spatial and temporal variations in basal resources and the composition of consumer assemblages. We used stable isotope analysis to estimate seasonal variation in basal resources supporting fish biomass in zones along the longitudinal gradient of the Usumacinta River Basin, the largest of Mesoamerica. A Bayesian isotope mixing model was used to estimate the proportional contributions of six basal resources: seston, filamentous algae, periphyton, aquatic macrophytes, riparian C3 plants and C4 plants. Models estimated that fish in the upper zone were largely supported by periphyton and riparian C3 plants during the dry season and by filamentous algae during the wet season. In the dry season, fishes in the middle and lower zones assimilated material derived from seston and, to a lesser extent, periphyton, whereas aquatic macrophytes were more important during the wet season. Chlorophyll-a measurements from the middle and lower zones suggested that seston contained large fractions of suspended algae. These patterns are consistent with the river wave concept, which predicts flow-associated changes in contributions of basal resources to the aquatic food web. The findings of this study reinforce the major role of flow regime in the ecological dynamics of rivers and how these dynamics vary depending on location and local conditions within fluvial networks.

Disturbance-mediated consumer assemblages determine fish community structure and moderate top-down influences through bottom-up constraints.

Disturbance is a strong structuring force that can influence the strength of species interactions at all trophic levels, but controls on the contributions to community structure of top-down and bottom-up processes across such gradients remain poorly understood. Changes in the composition of predator and consumer assemblages, and their associated traits, across gradients of environmental harshness (e.g. flooding), are likely to be a particularly important influence on the strength of top-down control and may drive bottom-up constraints. We examined how consumers with particular traits, and the predators that consumed them, varied across a gradient of stream flooding disturbance and used experiments to assess the predation impact on those contrasting consumer communities (ultimately quantifying how flood disturbance altered the strength of top-down control). Consumer community composition and mobility were strongly related to flood disturbance; the biomass and drift of protected primary consumers (i.e. those with morphological defences) decreased with increasing flood disturbance. Predatory fish species had different disturbance niches, and path analysis identified that both direct flood disturbance effects and indirect bottom-up constraints of flood disturbance on consumers influenced predatory fish composition and biomass. Fish generally fed most effectively on consumer types associated with their particular niche, but all fish were strongly size-selective when feeding on protected consumers. Although protected consumers did not grow large enough to escape predation, an in situ experiment showed protected consumers were at a reduced risk of predation as disturbance increased compared to unprotected consumers. Overall, top-down control declined with flood disturbance, but the effect depended on consumer traits. Predatory fish were only capable of exerting top-down control on protected consumers in benign habitats but impacted unprotected consumers across a larger range of the disturbance gradient. Collectively our findings suggest that a shift towards a more disturbed state will probably result in reduced predator impacts and a weakening of top-down control. Moreover, predicted increases in the frequency and intensity of climatic events causing disturbance, such as flooding, are likely to result in a community shift that disproportionately impacts protected consumers and the predators that utilize them as prey through the subsequent bottom-up constraints.

Multiple zooplankton species alter the stoichiometric interactions between producer and consumer levels

Planktonic primary consumers have been shown to strongly influence phytoplankton communities via top-down effects such as grazing and nutrient recycling. However, it remains unclear how changes in consumer richness may alter the stoichiometric constrains between producer and consumer assemblages. Here we test whether the stoichiometry of producer–consumer interactions is affected by the species richness of the consumer community (multispecies consumer assemblage vs single consumer species). Therefore, we fed a phytoplankton assemblage consisting of two flagellates and two diatom species reared under a 2 × 2 factorial combination of light and nitrogen supply to three planktonic consumer species in mono- and polycultures. As expected, phytoplankton biomass and C:nutrient ratios significantly increased with light intensity while nitrogen limitation resulted in reduced phytoplankton biomass and increasing phytoplankton C:N but lower N:P. Differences in phytoplankton stoichiometry were partly transferred to the consumer level, i.e., consumer C:N significantly increased with phytoplankton C:N. Consumer diversity significantly increased consumer biomass, resource use efficiency and nutrient uptake. In turn, consumer N:P ratios significantly decreased in consumer assemblages under high resource supply due to unequal changes in nutrient uptake. Consumer diversity further altered phytoplankton biomass, stoichiometry and species composition via increased consumption. Whether the effects of consumer diversity on phytoplankton and consumer performance were positive or negative strongly depended on the resource supply. In conclusion, the stoichiometric constraints of trophic interactions in multispecies assemblages cannot be predicted from monoculture traits alone, but consumer diversity effects are constrained by the resources supplied.

Crossing Bridges: Assembling Culture into Brands and Brands into Consumers' Global Local Cultural Lives

Fournier and Alvarez (2019—this issue) and Batra (2019—this issue), respectively, offer interpretive and psychological perspectives on how brands acquire cultural meanings. In this commentary, we discuss the opportunities for leveraging these two perspectives, and use an assemblage theory lens to uncover the dynamics of how cultural models articulated through cultural myths, metaphors, ideologies, and cultural objects circulate through the brand assemblage and through the consumer assemblage. We offer a bridge‐crossing approach to research opportunities bringing both a socio‐historical‐cultural approach and psychological approach to understand how cultural meanings are assembled into brands and how consumers assemble brands into their lives.

Avian frugivory rates at an abundant tree species are constant throughout the day and slightly influenced by weather conditions

Birds, like most animals, are expected to adjust their foraging activity in response to biotic and abiotic variations to optimize energy intake and reduce costs associated with finding and ingesting food. This may lead to temporal variation in the exploitation of food resources, which has been investigated for birds of different feeding guilds but remains poorly understood for frugivorous species. Here, we tested whether the frugivory activity of birds on Schinus terebinthifolia trees varies throughout the day during the austral autumn and winter and whether variation is related to weather conditions. For two consecutive years, we quantified frugivory events for 304 h in 19 individuals of S. terebinthifolia observed across four time periods throughout the day. We found that the number of frugivory events was similar irrespective of the time period, both for the entire assemblage (all species pooled together) and for the most common species analyzed individually. We additionally found that frugivory activity was slightly influenced by temperature and wind. The lack of any effect of daytime period on fruit consumption may relate to the high and prolonged levels of fruit availability in S. terebinthifolia and the short photoperiod of the autumn–winter, which favors frequent feeding throughout the day. Such constant removal of fruits by a diverse assemblage of generalist fruit consumers probably enhances seed dispersal and may have important implications for the population dynamics of this abundant plant. Our results also demonstrate that data on avian frugivory in temperate, open-vegetation environments can be quantified throughout the entire daytime during the cold season without temporal bias, which has important methodological implications for studies on bird–plant interactions.

Experimental N and P additions alter stream macroinvertebrate community composition via taxon‐level responses to shifts in detrital resource stoichiometry

Abstract Increases in nitrogen (N) and phosphorus (P) availability are changing animal communities, partly by altering stoichiometric imbalances between consumers and their food. Testing relationships between resource stoichiometry and consumer assemblage structure requires ecosystem‐level manipulations that have been lacking to date. We analysed patterns of macroinvertebrate community composition in five detritus‐based headwater streams subject to experimental whole‐stream N and P additions that spanned a steep gradient in dissolved N:P ratio (2:1, 8:1, 16:1, 32:1, 128:1) over 2 years, following a 1‐year pre‐treatment period. We predicted that shifts in leaf litter stoichiometry would drive overall patterns of community composition via greater responses of shredders to enrichment than other taxa, as shredders dominate primary consumer biomass and experience larger consumer–resource elemental imbalances than other functional groups in stream ecosystems. Specifically, we expected litter C:P to be a significant predictor of shredder biomass given the greater relative imbalances between shredder and litter C:P than C:N. Finally, we tested whether shredder responses to enrichment were related to other taxon‐level traits, including body size and stoichiometry, larval life span and growth rate. Whole‐community composition shifted similarly across the five streams after enrichment, largely driven by increased shredder and predator biomass. These shifts were limited to the autumn/winter seasons and related to decreased leaf litter C:P, highlighting important links between the quality of seasonal litter subsidies and community phenology. Among 10 taxa that drove structural shifts, two declined while other taxa from the same functional/taxonomic groups responded positively, suggesting that specific life‐history traits may determine sensitivity to enrichment. Increases in total shredder biomass, and in biomass of several common shredders, were associated with lower litter C:P. Body C:P did not predict shredder response to enrichment. However, weak negative relationships between shredder response and body size, and larval life span, suggest that small‐bodied and short‐lived taxa may be more responsive to shifting resource stoichiometry. Moderate anthropogenic increases in N and P availability affect resource stoichiometry and can alter animal communities, influencing additional food web and ecosystem properties. We provide support for ecological stoichiometry as a framework for predicting such outcomes based on changes in the elemental composition of resource pools. A plain language summary is available for this article.

Zooplankton and the Ocean Carbon Cycle.

Marine zooplankton comprise a phylogenetically and functionally diverse assemblage of protistan and metazoan consumers that occupy multiple trophic levels in pelagic food webs. Within this complex network, carbon flows via alternative zooplankton pathways drive temporal and spatial variability in production-grazing coupling, nutrient cycling, export, and transfer efficiency to higher trophic levels. We explore current knowledge of the processing of zooplankton food ingestion by absorption, egestion, respiration, excretion, and growth (production) processes. On a global scale, carbon fluxes are reasonably constrained by the grazing impact of microzooplankton and the respiratory requirements of mesozooplankton but are sensitive to uncertainties in trophic structure. The relative importance, combined magnitude, and efficiency of export mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations) likewise reflect regional variability in community structure. Climate change is expected to broadly alter carbon cycling by zooplankton and to have direct impacts on key species.

Communally Nesting Migratory Birds Create Ecological Hot-Spots in Tropical Australia.

Large numbers of metallic starlings (Aplonis metallica) migrate annually from New Guinea to the rainforests of tropical Australia, where they nest communally in single emergent trees (up to 1,000 birds). These aggregations create dense and species-rich faunal “hot-spots”, attracting a diverse assemblage of local consumers that utilise this seasonal resource. The starlings nested primarily in poison-dart trees (Antiaris toxicaria) near the rainforest-woodland boundary. Surveys underneath these colonies revealed that bird-derived nutrients massively increased densities of soil invertebrates and mammals (primarily wild pigs) beneath trees, year-round. Flying invertebrates, nocturnal birds, reptiles, and amphibians congregated beneath the trees when starlings were nesting (the wet-season). Diurnal birds (primarily cockatoos and bush turkeys) aggregated beneath the trees during the dry-season to utilise residual nutrients when the starlings were not nesting. The abundance of several taxa was considerably higher (to > 1000-fold) under colony trees than under nearby trees. The system strikingly resembles utilisation of bird nesting colonies by predators in other parts of the world but this spectacular system has never been described, emphasizing the continuing need for detailed natural-history studies in tropical Australia.

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.

Combined stable‐isotope and fatty‐acid analyses demonstrate that large wood increases the autochthonous trophic base of a macroinvertebrate assemblage

Summary Large wood (LW), defined as pieces of wood greater than 10 cm in diameter and 1 m long, is well known to alter river hydromorphology and the availability of potential food resources for consumers. However, there has been a lack of studies investigating whether these can cause shifts in the trophic base, which may explain alterations to the total abundance and taxonomic structure of the macroinvertebrate assemblage. We aimed to determine how the presence of LW altered the trophic base of the macroinvertebrate consumer assemblage in a lowland river, and to provide a methodological comparison of two assimilation‐based food web methods: stable‐isotope analysis (SIA) and fatty‐acid biomarker profiles (FA). To do so, we quantified the contribution of trophic resources to the diets of macroinvertebrates colonising the surface of LW, present in this study as single logs, and surrounding bed sediments with those from bed sediments of a nearby control site with minimal amounts of LW. SIA showed that the macroinvertebrate food web, even for non‐filter feeding taxa, was mostly sustained by seston exported from a lake 1 km upstream, highlighting a high degree of lake‐river coupling. The presence of wood altered the trophic base from being predominantly seston‐supported to one with increased support from epixylic autochthonous production (i.e. periphyton and bryophytes on wood). Terrestrial matter (i.e. leaves and grass) and organic sediments were a relatively unimportant fraction of the trophic base (<10%) in all locations. FA did not directly track the influence of seston, but instead differentiated between overall allochthonous (terrestrial) and autochthonous (aquatic) components of the trophic base. In particular, FA analysis demonstrated the higher nutritional value of autochthonous primary producers, and provided supporting evidence that most consumers, even seston‐feeders, were primarily supported by autochthonous resources and not by allochthonous matter. FA indicated shifts in some taxa‐specific diets not detected by stable isotopes alone. Our study demonstrated that the combined use of stable isotopes and fatty acids provides new insights into determining the trophic base of a complex food web with trophic resources of both terrestrial/aquatic and lacustrine/riverine origins. In addition, directly comparing results from both stable‐isotope and fatty‐acid analyses provided additional information on selective feeding by seston‐feeding taxa on autochthonous and allochthonous fractions of the seston. The presence of LW in the river channel decreased lake‐river coupling by providing alternative basal resources, primarily through increasing high‐quality autochthonous production on wood and by providing a superior substratum for net‐spinning caddisflies to feed on a fraction of the seston richer in essential fatty acids. River management strategies that incorporate instream LW therefore have the potential to alter energy flows and enhance ecosystem productivity by increasing the quantity and quality of available basal food resources.

Mesograzer identity, not host algae, determines consumer stable isotope ratios

ABSTRACTSpecies-specific foraging habits or feeding preferences influence the overall trophic functioning in consumer assemblages. We set out to assess the in situ trophic diversity in a mesograzer assemblage (isopods, amphipods and gastropods) associated with marine littoral macroalgae. More specifically, we set out to establish whether stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) are dependent on primary consumer identity or expressed in relation to the identity of the algal host, i.e. the habitat of the consumer. Consumer locations in bivariate isotope space revealed significant differences among grazer taxa, but no effect of the algal host. This suggests that grazer-specific foraging is more important in driving the qualitative range of feeding in the consumer assemblage than host diversity per se within a macrohabitat, such as a bay or a lagoon. The stable isotope ratios of the mesoherbivores were in line with expectations based on the known feeding ecology of the grazers. However, the trophic diversity suggested by stable isotope analysis implies that even an established concept such as the mesograzer guild may encompass simplifications in terms of functional group membership.

Integrating Ecosystem Engineering and Food Web Ecology: Testing the Effect of Biogenic Reefs on the Food Web of a Soft-Bottom Intertidal Area.

The potential of ecosystem engineers to modify the structure and dynamics of food webs has recently been hypothesised from a conceptual point of view. Empirical data on the integration of ecosystem engineers and food webs is however largely lacking. This paper investigates the hypothesised link based on a field sampling approach of intertidal biogenic aggregations created by the ecosystem engineer Lanice conchilega (Polychaeta, Terebellidae). The aggregations are known to have a considerable impact on the physical and biogeochemical characteristics of their environment and subsequently on the abundance and biomass of primary food sources and the macrofaunal (i.e. the macro-, hyper- and epibenthos) community. Therefore, we hypothesise that L. conchilega aggregations affect the structure, stability and isotopic niche of the consumer assemblage of a soft-bottom intertidal food web. Primary food sources and the bentho-pelagic consumer assemblage of a L. conchilega aggregation and a control area were sampled on two soft-bottom intertidal areas along the French coast and analysed for their stable isotopes. Despite the structural impacts of the ecosystem engineer on the associated macrofaunal community, the presence of L. conchilega aggregations only has a minor effect on the food web structure of soft-bottom intertidal areas. The isotopic niche width of the consumer communities of the L. conchilega aggregations and control areas are highly similar, implying that consumer taxa do not shift their diet when feeding in a L. conchilega aggregation. Besides, species packing and hence trophic redundancy were not affected, pointing to an unaltered stability of the food web in the presence of L. conchilega.

Resource partitioning along multiple niche axes drives functional diversity in parrotfishes on Caribbean coral reefs.

The recent loss of key consumers to exploitation and habitat degradation has significantly altered community dynamics and ecosystem function across many ecosystems worldwide. Predicting the impacts of consumer losses requires knowing the level of functional diversity that exists within a consumer assemblage. In this study, we document functional diversity among nine species of parrotfishes on Caribbean coral reefs. Parrotfishes are key herbivores that facilitate the maintenance and recovery of coral-dominated reefs by controlling algae and provisioning space for the recruitment of corals. We observed large functional differences among two genera of parrotfishes that were driven by differences in diet. Fishes in the genus Scarus targeted filamentous algal turf assemblages, crustose coralline algae, and endolithic algae and avoided macroalgae, while fishes in the genus Sparisoma preferentially targeted macroalgae. However, species with similar diets were dissimilar in other attributes, including the habitats they frequented, the types of substrate they fed from, and the spatial scale at which they foraged. These differences indicate that species that appear to be functionally redundant when looking at diet alone exhibit high levels of complementarity when we consider multiple functional traits. By identifying key functional differences among parrotfishes, we provide critical information needed to manage parrotfishes to enhance the resilience of coral-dominated reefs and reverse phase shifts on algal-dominated reefs throughout the wider Caribbean. Further, our study provides a framework for predicting the impacts of consumer losses in other species rich ecosystems.

Coastal Upwelling Drives Intertidal Assemblage Structure and Trophic Ecology.

Similar environmental driving forces can produce similarity among geographically distant ecosystems. Coastal oceanic upwelling, for example, has been associated with elevated biomass and abundance patterns of certain functional groups, e.g., corticated macroalgae. In the upwelling system of Northern Chile, we examined measures of intertidal macrobenthic composition, structure and trophic ecology across eighteen shores varying in their proximity to two coastal upwelling centres, in a hierarchical sampling design (spatial scales of >1 and >10 km). The influence of coastal upwelling on intertidal communities was confirmed by the stable isotope values (δ13C and δ15N) of consumers, including a dominant suspension feeder, grazers, and their putative resources of POM, epilithic biofilm, and macroalgae. We highlight the utility of muscle δ15N from the suspension feeding mussel, Perumytilus purpuratus, as a proxy for upwelling, supported by satellite data and previous studies. Where possible, we used corrections for broader-scale trends, spatial autocorrelation, ontogenetic dietary shifts and spatial baseline isotopic variation prior to analysis. Our results showed macroalgal assemblage composition, and benthic consumer assemblage structure, varied significantly with the intertidal influence of coastal upwelling, especially contrasting bays and coastal headlands. Coastal topography also separated differences in consumer resource use. This suggested that coastal upwelling, itself driven by coastline topography, influences intertidal communities by advecting nearshore phytoplankton populations offshore and cooling coastal water temperatures. We recommend the isotopic values of benthic organisms, specifically long-lived suspension feeders, as in situ alternatives to offshore measurements of upwelling influence.

Ecosystem-level effects of a globally spreading invertebrate invader are not moderated by a functionally similar native.

Biological invasions are a key element of human-induced global environmental change. However, lack of knowledge of the indirect consequences of invasions, combined with poor understanding of how their ecological effects depend upon competitive attributes of the receiving community, hinders our ability to manage and predict the effects of invasive species on ecosystems. We established an experiment using a combination of both additive and substitutive experimental designs to explore the effects of the globally spreading mysid shrimp Hemimysis anomala on the biological structure of outdoor pond mesocosms in the absence and presence of a functionally similar native competitor, Mysis salemaai. The naturally smaller H. anomala had considerably stronger effects on primary producers, multiple aspects of consumer assemblages and overall biological structure of the ponds in comparison with the functionally similar native. Moreover, the magnitude of these effects was generally independent of the presence of M. salemaai and even total mysid density. Hemimysis anomala reduced both the abundance and diversity of zooplankton assemblages significantly, triggering a strong trophic cascade on phytoplankton and a simultaneous increase of benthic invertebrate biomass. These findings indicate that invasion by H. anomala may exacerbate the effects of nutrient enrichment on lakes. Our results demonstrate that introduced species can, irrespective of the presence of functionally similar natives, induce complex changes to ecosystems that reach beyond direct consumptive effects. Moreover, the cascading indirect effects of invasion can exacerbate the impacts of other stressors. Disregarding the complexity of indirect effects therefore risks underestimating significantly the global ecological footprint of biological invasions.

Food web heterogeneity and succession in created saltmarshes

Summary Ecological restoration must achieve functional as well as structural recovery. Functional metrics for re‐establishment of trophic interactions can be used to complement traditional monitoring of structural attributes. In addition, topographic effects on food web structure provide added information within a restoration context; often, created sites may require spatial heterogeneity to effectively match structure and function of natural habitats. We addressed both of these issues in our study of successional development of benthic food web structure, with focus on bottom‐up‐driven changes in macroinvertebrate consumer assemblages in the saltmarshes of the Venice Lagoon, Italy. We combined quantified estimates of the changing community composition with stable isotope data (13C:12C and 15N:14N) to compare the general trophic structure between created (2–14 years) marshes and reference sites and along topographic elevation gradients within saltmarshes. Macrofaunal invertebrate consumers exhibited local, habitat‐specific trophic patterns. Stable isotope‐based trophic structure changed with increasing marsh age, in particular with regard to mid‐elevation (Salicornia) habitats. In young marshes, the mid‐elevation consumer signatures resembled those of unvegetated ponds. The mid‐elevation of older and natural marshes had a more distinct Salicornia zone food web, occasionally resembling that of the highest (Sarcocornia‐dominated) elevation. In summary, this indicates that primary producers and availability of vascular plant detritus structure consumer trophic interactions and the flow of carbon. Functionally different consumers, subsurface‐feeding detritivores (Oligochaeta) and surface grazers (Hydrobia sp.), showed distinct but converging trajectories of isotopic change over time, indicating that successional development may be asymmetric between ‘brown’ (detrital) guilds and ‘green’ (grazing) guilds in the food web. Synthesis and applications. Created marsh food webs converged into a natural state over about a decade, with successional shifts seen in both consumer community composition and stable isotope space. Strong spatial effects were noted, highlighting the utility of stable isotopes to evaluate functional equivalence in spatially heterogeneous systems. Understanding the recovery of functional properties such as food web support, and their inherent spatial variability, is key to planning and managing successful habitat restoration.