Agricultural land-use affects the nutritional quality of stream microbial communities

We investigated how the lipid composition (fatty acids and sterols) of benthic microbial mats, which represent an important basal food resource for stream food webs, differs between tropical streams located in protected pristine and agricultural Cerrado savannah areas. The total microbial biomass and lipid composition differed significantly between pristine and agricultural streams in parallel with differences in water quality and hydrodynamic characteristics. Agricultural streams exhibited lower total biomass of benthic microbial mats than pristine streams. However, the higher concentrations of essential polyunsaturated fatty acids, such as linoleic acid (LIN, 18:2ω6), α-linolenic acid (ALA, 18:3ω3), and eicosapentaenoic acid (EPA, 20:5ω3), that were observed in agricultural streams suggest enhanced lipid complexity and a higher nutritional quality of the microbial community relative to pristine streams. Meanwhile, pristine stream microbial communities had higher total concentrations of saturated fatty acids and cholesterol than those of agricultural streams, reflecting their heterotrophic microbial communities. Moreover, stream morphotype and associated differences in the hydrodynamic characteristics affected the community composition and thereby also the lipid composition of microbial mats. Land-use-induced changes in the total biomass and lipid composition of microbial communities may affect the trophic transfer of energy in stream food webs, leading to changes in the composition and productivity of primary consumers and their predators, and thereby affecting stream ecosystem functioning.     

Climate effects on fish body size–trophic position relationship depend on ecosystem type

The energetic demand of consumers increases with body size and temperature. This implies that energetic constraints may limit the trophic position of larger consumers, which is expected to be lower in tropical than in temperate regions to compensate for energy limitation. Using a global dataset of 3635 marine and freshwater ray‐finned fish species, we addressed if and how climate affects the fish body size–trophic position relationship in both freshwater and marine ecosystems, while controlling for the effects of taxonomic affiliation. We observed significant fish body size–trophic position relationships for different ecosystems. However, only in freshwater systems larger tropical fish presented a significantly lower trophic position than their temperate counterparts. Climate did not affect the fish body size–trophic position relationship in marine systems. Our results suggest that larger tropical freshwater fish may compensate for higher energetic constraints feeding at lower trophic positions, compared to their temperate counterparts of similar body size. The lower latitudinal temperature range in marine ecosystems and/or their larger ecosystem size may attenuate and/or compensate for the energy limitation of larger marine fish. Based on our results, temperature may determine macroecological patterns of aquatic food webs, but its effect is contingent on ecosystem type. We suggest that freshwater ecosystems may be more sensitive to warming‐induced alterations in food web topology and food chain length than marine ecosystems.     

Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

Chemical information transfer in freshwater plankton

The structure of aquatic ecosystems is determined by complex interactions among individual organisms at different trophic levels. Although our basic understanding of how top-down and bottom-up processes interact to determine food-web dynamics has advanced, we still lack insights into how complex interactions and feedbacks affect the dynamics and structure of food webs. It is now becoming increasingly clear that, in addition to energy transfer from one trophic level to the other, there is exchange of information between these levels facilitated by the release of infochemicals by the organisms. There is evidence from recent studies that the exchange of chemical information in freshwater ecosystems is likely to play a decisive role in shaping structure and functioning of these systems. Chemical communication among freshwater organisms mediates many aspects of both predation and interspecific competition, which play key roles in determining community structure and ecosystem functioning. For example, consumer-induced defences in phytoplankton and zooplankton include modifications in the characteristics relating to life history, behaviour, morphology and biochemistry. These inducible defences affect trophic interactions by altering predator feeding rates through changes in attack rate or handling time, or both. Also host-specific fungal parasitism in phytoplankton is probably controlled by infochemicals. The motile fungi recognise their host by host-secreted compounds. Until now models describing the functioning of ecosystems mainly considered flows of biomass and energy. Integration of new knowledge about the role of chemical communication in these models may be one of the aims of ecological informatics. In this chapter I discuss how infochemicals may affect the dynamics and structure of planktonic food webs.     

Food web structure and body size: trophic position and resource acquisition

Ulrich Brose Body size is recognized as an important determinant of trophic structure as it affects individual energetic demands, population density, and the interaction between potential prey and predators. However, its relationship with trophic position remains unclear. It has been hypothesized that a positive relationship between body size and trophic position would be associated to some particular trophic structures, which would allow larger organisms to satisfy their energetic demand and sustain viable population sizes at higher trophic positions, where fewer resources are available. To test this hypothesis, we analyzed the diet of 619 killifishes from four species (Austrolebias cheradophilus, A. luteoflammulatus, A. viarius and Cynopoecilus melanotaenia), collected in temporary ponds occurring in the grasslands of Rocha, Uruguay. Trophic position, diet richness, number of energy sources, and evenness were estimated for 20 size classes, formed by consecutive groups of 31 individuals. Gape limitation and preference for the larger available prey were evaluated as explanations for observed patterns with an individual based model (IBM). In agreement with the hypothesis, killifishes presented a strong positive relationship between trophic position and body size (R²=0.86), associated with a trophic structure that could allow larger organisms to have access to more energy from the environment. This was reflected in a positive relationship between body size and 1) prey richness, 2) number of basal energy sources (i.e. plants, detritus, phytoplankton and terrestrial prey), and 3) evenness in prey use. IBM results showed that changes in trophic structure with body size are well explained by gape limitation, but not by size preferences. Our results suggest that the fulfilment of the greater energetic demands of larger organism will depend on community diversity, which typically increases with ecosystem size, indicating a novel connection between area, diversity, body size, and food chain length.     

Warming alters the size spectrum and shifts the distribution of biomass in freshwater ecosystems

Organism size is one of the key determinants of community structure, and its relationship with abundance can describe how biomass is partitioned among the biota within an ecosystem. An outdoor freshwater mesocosm experiment was used to determine how warming of～4 °C would affect the size, biomass and taxonomic structure of planktonic communities. Warming increased the steepness of the community size spectrum by increasing the prevalence of small organisms, primarily within the phytoplankton assemblage and it also reduced the mean and maximum size of phytoplankton by approximately one order of magnitude. The observed shifts in phytoplankton size structure were reflected in changes in phytoplankton community composition, though zooplankton taxonomic composition was unaffected by warming. Furthermore, warming reduced community biomass and total phytoplankton biomass, although zooplankton biomass was unaffected. This resulted in an increase in the zooplankton to phytoplankton biomass ratio in the warmed mesocosms, which could be explained by faster turnover within the phytoplankton assemblages. Overall, warming shifted the distribution of phytoplankton size towards smaller individuals with rapid turnover and low standing biomass, resulting in a reorganization of the biomass structure of the food webs. These results indicate future environmental warming may have profound effects on the structure and functioning of aquatic communities and ecosystems.     

Soil multitrophic network complexity enhances the link between biodiversity and multifunctionality in agricultural systems

Belowground biodiversity supports multiple ecosystem functions and services that humans rely on. However, there is a dearth of studies exploring the determinants of the biodiversity–ecosystem function (BEF) relationships, particularly in intensely managed agricultural ecosystems. Here, we reported significant and positive relationships between soil biodiversity of multiple organism groups and multiple ecosystem functions in 228 agricultural fields, relating to crop yield, nutrient provisioning, element cycling, and pathogen control. The relationships were influenced by the types of organisms that soil phylotypes with larger sizes or at higher trophic levels, for example, invertebrates or protist predators, appeared to exhibit weaker or no BEF relationships when compared to those with smaller sizes or at lower trophic levels, for example, archaea, bacteria, fungi, and protist phototrophs. Particularly, we highlighted the role of soil network complexity, reflected by co-occurrence patterns among multitrophic-level organisms, in enhancing the link between soil biodiversity and ecosystem functions. Our results represent a significant advance in forecasting the impacts of belowground multitrophic organisms on ecosystem functions in agricultural systems, and suggest that soil multitrophic network complexity should be considered a key factor in enhancing ecosystem productivity and sustainability under land-use intensification.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

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Structure and functioning of two pelagic communities in the North Chilean Patagonian coastal system

The size composition of primary producers is important for how energy is channeled through a food web and on to the higher trophic levels and eventually to fisheries. To evaluate this, we studied the productive patterns for large (micro) versus small (nano) phytoplankton in two south marine Patagonian ecosystems: The Inner Sea of Chiloe—ISCh and, Moraleda Channel—MCh. We built Ecopath models (EwE), and evaluated the hypothesis that the overall primary productivity—rather than the ratio of large to small primary producers—constitutes an adequate proxy for predicting the amount of secondary and tertiary production and biomass (up to the fisheries). The EwE model included four small-scale fisheries and 36 functional groups. The functioning of both ecosystems was similar but the ecosystem parameters (biomass, energy transfer efficiencies from primary producers, secondary, and tertiary production) were twice as much in the basin with more microphytoplankton biomass. Overall, the hypothesis was rejected, albeit it was possible to highlight the importance of the quality and size spectrum of plankton on the structure of marine ecosystem, and to demonstrate the key role of the microbial loop over traditional food web in the functioning of the carbon biological pump in Patagonia ecosystems.     

Life history patterns shape energy allocation among fishes on coral reefs

Although critically important, the link between animal life histories and ecosystem energetics is seldom explored. In the pursuit of ecological simplification, ecosystem properties are typically described by models based on static counts, where organisms are aggregated into trophic- or size-based groups. Consequently, output is often based on an assumption that larger group biomass equals greater energetic contribution. Here, we modelled the individual growth of over 58,000 fishes from 74 genera within a coral reef ecosystem to investigate the role and importance of taxon-specific life histories to the division, spatial distribution and relative contribution of biomass production within 14 coral reef fish families. Rank changes among families in standing biomass to biomass production indicated that small cryptic families (e.g. Gobiidae and Blenniidae) exhibit collective growth potentials equal to or exceeding those of many other common families composed of individuals with body-sizes 1–3 orders of magnitude larger. Remaining at high risk of predation throughout their lives as a consequence of their small size, these cryptic fishes also provide a constant food resource and supply of reproductive energy to coral reefs throughout the year. Enhanced further by the strength and diversity of their trophic relationships within food webs, the highly productive nature of these small cryptic fishes suggests they make a substantial contribution to the flow of energy in coral reef ecosystems via predatory pathways. It appears that life histories leave a strong imprint on ecosystem energy fluxes and illustrate the importance of incorporating taxon-specific features when assigning values to key ecosystem processes. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Stepping in Elton's footprints: a general scaling model for body masses and trophic levels across ecosystems

Despite growing awareness of the significance of body-size and predator-prey body-mass ratios for the stability of ecological networks, our understanding of their distribution within ecosystems is incomplete. Here, we study the relationships between predator and prey size, body-mass ratios and predator trophic levels using body-mass estimates of 1313 predators (invertebrates, ectotherm and endotherm vertebrates) from 35 food-webs (marine, stream, lake and terrestrial). Across all ecosystem and predator types, except for streams (which appear to have a different size structure in their predator-prey interactions), we find that (1) geometric mean prey mass increases with predator mass with a power-law exponent greater than unity and (2) predator size increases with trophic level. Consistent with our theoretical derivations, we show that the quantitative nature of these relationships implies systematic decreases in predator-prey body-mass ratios with the trophic level of the predator. Thus, predators are, on an average, more similar in size to their prey at the top of food-webs than that closer to the base. These findings contradict the traditional Eltonian paradigm and have implications for our understanding of body-mass constraints on food-web topology, community dynamics and stability.     

The influence of biotic interactions on soil biodiversity

Belowground communities usually support a much greater diversity of organisms than do corresponding aboveground ones, and while the factors that regulate their diversity are far less well understood, a growing number of recent studies have presented data relevant to understanding how these factors operate. This review considers how biotic factors influence community diversity within major groups of soil organisms across a broad spectrum of spatial scales, and addresses the mechanisms involved. At the most local scale, soil biodiversity may potentially be affected by interactions within trophic levels or by direct trophic interactions. Within the soil, larger bodied invertebrates can also influence diversity of smaller sized organisms by promoting dispersal and through modification of the soil habitat. At larger scales, individual plant species effects, vegetation composition, plant species diversity, mixing of plant litter types, and aboveground trophic interactions, all impact on soil biodiversity. Further, at the landscape scale, soil diversity also responds to vegetation change and succession. This review also considers how a conceptual understanding of the biotic drivers of soil biodiversity may assist our knowledge of key topics in community and ecosystem ecology, such as aboveground–belowground interactions, and the relationship between biodiversity and ecosystem functioning. It is concluded that an improved understanding of what drives the diversity of life in the soil, incorporated within appropriate conceptual frameworks, should significantly aid our understanding of the structure and functioning of terrestrial communities.     

Structure of the Biotic Component in the Rybinsk Reservoir Ecosystem: Importance of Heterotrophic Bacteria (Review)

The total biomass of the biotic component of the ecosystem has been determined and the contribution of the main ecological groups—autotrophic and heterotrophic organisms from different habitats—to its formation has been estimated in a large plain meso-eutrophic reservoir (Rybinsk Reservoir, Upper Volga). Particular attention is paid to the role of heterotrophic bacteria in the structure and functioning of the biota in the reservoir. The total biomass of the biotic component of the ecosystem is 71536 t C, which is 5.2% of the total organic carbon in the reservoir. Higher aquatic plants make the largest contribution to the formation of the biomass in the reservoir. Their biomass, including epiphyton, was 6.0 and 1.9 times larger than the biomass of plankton and benthos, respectively. Heterotrophic bacteria, most of which inhabit bottom sediments, rank second in respect to their contribution to the total biomass. The comparison of the total primary production of all phototrophic organisms and the carbon demand of heterotrophic bacteria indicates the importance of allochthonous organic matter in the functioning of the reservoir ecosystem.     

Biodiversity, ecosystem functioning and food webs in fresh waters: assembling the jigsaw puzzle

1. Dramatic advances have been made recently in the study of biodiversity–ecosystem functioning (B-EF) relations and food web ecology. These fields are now starting to converge, and this fusion has the potential to improve our understanding of how environmental stressors modulate ecosystem processes and the supply of 'goods and services'.
2. Food web structure and dynamics can exert particularly strong influences on B-EF relations in fresh waters, as consumer–resource interactions (e.g. trophic cascades) are often more important than horizontal interactions within trophic levels. For instance, many freshwater food webs are size structured, with large organisms tending to occupy the higher trophic levels and often exerting powerful effects on ecosystem processes. However, because they are also vulnerable to perturbations, non-random losses of these large taxa can alter both food web structure and ecosystem functioning profoundly.
3. Recently, the focus of food web research has shifted away from exploring patterns, towards developing an understanding of processes (e.g. quantifying fluxes of individuals, biomass, energy, nutrients) and how the two interact. Many of the best-characterized food webs are from fresh waters, and these ecosystems are now being used to address some of the shortcomings of earlier B-EF studies. I have identified several key gaps in our current knowledge and highlighted potentially fruitful avenues of future B-EF and food web research.
4. A major challenge for this newly emerging research is to place it within a unified theoretical framework. The application of metabolic theory and ecological stoichiometry may help to achieve this goal by considering biological systems within the constraints imposed upon them by physical and chemical laws.     

Functional trait diversity across trophic levels determines herbivore impact on plant community biomass

Understanding the consequences of trophic interactions for ecosystem functioning is challenging, as contrasting effects of species and functional diversity can be expected across trophic levels. We experimentally manipulated functional identity and diversity of grassland insect herbivores and tested their impact on plant community biomass. Herbivore resource acquisition traits, i.e. mandible strength and the diversity of mandibular traits, had more important effects on plant biomass than body size. Higher herbivore functional diversity increased overall impact on plant biomass due to feeding niche complementarity. Higher plant functional diversity limited biomass pre‐emption by herbivores. The functional diversity within and across trophic levels therefore regulates the impact of functionally contrasting consumers on primary producers. By experimentally manipulating the functional diversity across trophic levels, our study illustrates how trait‐based approaches constitute a promising way to tackle existing links between trophic interactions and ecosystem functioning.     

Size–abundance relationships in Florida ant communities reveal how ants break the energetic equivalence rule

1. Ants are among the most abundant terrestrial organisms, yet little is known of how ant communities divide resources because it is difficult to measure the number of individuals in colonies and the density of colonies. 2. The body size–abundance relationships of the ants of five upland ecosystems in Florida were examined. The study tested whether abundance, energy use, and total biomass were distributed among species and body sizes as predicted by Damuth's energetic equivalence rule. Estimates of average worker body size, colony size, colony mass, and field metabolic rates were used to examine the relationships among body sizes, energy use, and total biomass. 3. Analyses revealed significant variation in energy use and did not support the energetic equivalence hypothesis. Specifically, the energy use and total standing biomass of species with large workers and colonies was much greater than smaller species. 4. These results suggest that larger species with larger colonies account for a disproportionate fraction of the total abundance and biomass of ants. A general model of resource allocation in colonies provides a possible explanation for why ants do not conform to the predictions of the energetic equivalence rule and for why ants are so abundant.     

Effects of land use on the structure and function of leaf‐litter microbial communities in boreal streams

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Trophic and functional cascades in tropical versus temperate aquatic microcosms

Inverse trophic cascades are a well explored and common consequence of the local depletion or extinction of top predators in natural ecosystems. Despite a large body of research, the cascading effects of predator removal on ecosystem functions are not as well understood. Developing microcosm experiments, we explored food web changes in trophic structure and ecosystem functioning following biomass removal of top predators in representative temperate and tropical rock pool communities that contained similar assemblages of zooplankton and benthic invertebrates. We observed changes in species abundances following predator removal in both temperate and tropical communities, in line with expected inverse effects of a trophic cascade, where predation release benefits the predator’s preys and competitors and impacts the preys of the latter. We also observed several changes at the community and ecosystem levels including a decrease in total abundance and mean trophic level of the community, and changes in chlorophyll-a and total dissolved particles. Our results also showed an increase in variability of both community and ecosystem processes following the removal of predators. These results illustrate how predator removal can lead to inverse trophic cascades both in structural and functioning properties, and can increase variability of ecosystem processes. Although observed patterns were consistent between tropical and temperate communities following an inverse cascade pattern, changes were more pronounced in the temperate community. Therefore, aquatic food webs may have inherent traits that condition ecosystem responses to changes in top-down trophic control and render some aquatic ecosystems especially sensitive to the removals of top predators.     

Proteomic characterization of the major arthropod associates of the carnivorous pitcher plant Sarracenia purpurea

The array of biomolecules generated by a functioning ecosystem represents both a potential resource for sustainable harvest and a potential indicator of ecosystem health and function. The cupped leaves of the carnivorous pitcher plant, Sarracenia purpurea, harbor a dynamic food web of aquatic invertebrates in a fully functional miniature ecosystem. The energetic base of this food web consists of insect prey, which is shredded by aquatic invertebrates and decomposed by microbes. Biomolecules and metabolites produced by this food web are actively exchanged with the photosynthesizing plant. In this report, we provide the first proteomic characterization of the sacrophagid fly (Fletcherimyia fletcheri), the pitcher plant mosquito (Wyeomyia smithii), and the pitcher-plant midge (Metriocnemus knabi). These three arthropods act as predators, filter feeders, and shredders at distinct trophic levels within the S. purpurea food web. More than 50 proteins from each species were identified, ten of which were predominantly or uniquely found in one species. Furthermore, 19 peptides unique to one of the three species were identified using an assembled database of 100 metazoan myosin heavy chain orthologs. These molecular signatures may be useful in species monitoring within heterogeneous ecosystem biomass and may also serve as indicators of ecosystem state.     

Linking the evolution of habitat choice to ecosystem functioning: direct and indirect effects of pond-reproducing fire salamanders on aquatic-terrestrial subsidies

Shifts in life history traits and in the behaviour of species can potentially alter ecosystem functioning. The reproduction of the central European fire salamander (Salamandra salamandra), which usually deposits its larvae in first-order streams, in small pool and pond-like habitats, is an example of a recent local adaptation in this species. Here we aimed to quantify the direct and indirect effects of the predatory larvae on the aquatic food webs in the ponds and on the flux of matter between the ponds and adjacent terrestrial habitats. Our estimates are based on biomass data of the present pond fauna as well as on the analysis of stomach content data, growth rates and population dynamics of the salamander larvae in pond habitats. By their deposition of larvae in early spring, female fire salamanders import between 0.07 and 2.86 g dry mass m^?2 larval biomass into the ponds. Due to high mortality rates in the larval phase and the relatively small size at metamorphosis of the pond-adapted salamanders compared to stream-adapted ones, the biomass export of the metamorphosed salamanders clearly falls below the initial biomass import. Catastrophic events such as high water temperatures and low oxygen levels may even occasionally result in mass mortalities of salamander larvae and thus in a net 100 % import of the salamander biomass into the pond food webs. Indirect effects further accelerate this net import of matter into the aquatic habitat, e.g. the feeding of salamanders on aquatic insect larvae with the emergence of terrestrial adults—thus preventing export—and on terrestrial organisms that fall on the water surface (supporting import). This study demonstrates that the adaptation of salamanders to pond reproduction can alter food web linkages across ecosystem boundaries by enhancing the flux of materials and energy from terrestrial (i.e. forest) to the aquatic (i.e. pond) habitat.