The functional trophic role of lotic primary consumers: generalist versus specialist strategies

1. The relative extent of generalist or specialist resource use strategies is an important question in ecology. A community dominated by specialist strategies suggests a high level of interspecific competition for resources, resulting in the evolutionary development of isolating mechanisms between species (e.g. resource specialization to avoid and/or outcompete other species). A community dominated by generalist strategies suggests less interspecific competition for resources, allowing many taxa to utilize the same resources. In stream systems, generalist food habits are a common strategy among primary consumers, but little is known about resource assimilation strategies (resources incorporated into tissue growth). Published data indicate that generalist resource assimilation strategies may prevail in lotic systems as well.
2. Functional feeding groups (FFGs) are often used to infer resource assimilation among lotic macroinvertebrates (e.g. shredder-detritivore, scraper-herbivore). While these groupings are aptly used to describe invertebrate feeding modes and community structure, the use of FFGs to describe resource assimilation among lotic consumers is not appropriate. Sufficient data now exist to seriously question how accurately FFG assignments describe the processes of energy flow and material transfer between trophic levels in stream ecosystems.
3. Because FFGs may not accurately describe functional attributes in lotic systems, an alternative approach is needed. One approach is to determine the amount of secondary production that is derived from autochthonous (e.g. periphyton and algae) and allochthonous (e.g. detritus) resources directly. A simple model of community function based on this approach is presented. The model incorporates trophic generalists into measurement of consumer–resource energetics in lotic systems.     

Longitudinal pattern of resource utilization by aquatic consumers along a disturbed subtropical urban river: Estimating the relative contribution of resources with stable isotope analysis

The utilization of food resources by aquatic consumers reflects the structure and functioning of river food webs. In lotic water systems, where food availability and predator–prey relationships vary with gradient changes in physical conditions, understanding diet assimilation by local communities is important for ecosystem conservation. In the subtropical Liuxi River, southern China, the relative contribution of basal resources to the diet assimilation of functional feeding groups (FFGs) was determined by stable carbon (¹³C) and nitrogen (¹⁵N) isotope analyses. The output of Bayesian mixing models showed that diatom‐dominated periphyton (epilithic biofilm), aquatic C₃ plants (submerged hydrophytes), and suspended particulate organic matter (SPOM) associated with terrestrial C₃ plants contributed the most to the diet assimilation of FFGs in the upper, middle, and lower reaches, respectively. The relative contribution of consumer diet assimilation was weighted by the biomass (wet weight, g/m²) of each FFG to reflect resource utilization at the assemblage level. From the upper to the lower reaches, the spatial variation in the diet assimilation of fish and invertebrate assemblages could be summarized as a longitudinal decrease in periphyton (from 57%–76% to <3%) and an increase in SPOM (from <7% to 51%–68%) with a notable midstream increase in aquatic C₃ plants (23%–48%). These results indicate that instream consumers in the Liuxi River rely more on autochthonous production (e.g., periphyton and submerged hydrophytes) than on terrestrially derived allochthonous matter (e.g., terrestrial plants). The pattern of resource utilization by consumers in the mid‐upper Liuxi River is consistent with findings from other open subtropical and neotropical rivers and provides evidence for the riverine productivity model. Our study indicates that protecting inherent producers in rivers (e.g., periphyton and submerged hydrophytes) and restoring their associated habitats (e.g., riffles with cobble substrate) are conducive to aquatic ecosystem management.     

Host specialist clownfishes are environmental niche generalists

Why generalist and specialist species coexist in nature is a question that has interested evolutionary biologists for a long time. While the coexistence of specialists and generalists exploiting resources on a single ecological dimension has been theoretically and empirically explored, biological systems with multiple resource dimensions (e.g. trophic, ecological) are less well understood. Yet, such systems may provide an alternative to the classical theory of stable evolutionary coexistence of generalist and specialist species on a single resource dimension. We explore such systems and the potential trade-offs between different resource dimensions in clownfishes. All species of this iconic clade are obligate mutualists with sea anemones yet show interspecific variation in anemone host specificity. Moreover, clownfishes developed variable environmental specialization across their distribution. In this study, we test for the existence of a relationship between host-specificity (number of anemones associated with a clownfish species) and environmental-specificity (expressed as the size of the ecological niche breadth across climatic gradients). We find a negative correlation between host range and environmental specificities in temperature, salinity and pH, probably indicating a trade-off between both types of specialization forcing species to specialize only in a single direction. Trade-offs in a multi-dimensional resource space could be a novel way of explaining the coexistence of generalist and specialists.     

Trophic Levels and Functional Feeding Groups of Macroinvertebrates in Neotropical Streams

Feeding strategies are typical traits reflecting the adaptation of species to environmental conditions. This concept is currently developed in some water quality systems (e.g. Index of Trophic completeness) and the structure of functional feeding groups (FFGs) could form part of a unified measure across communities differing in taxonomic composition. However, in South America, information about the FFG classification of invertebrates in streams is almost absent and existing studies using FFG structure follows classification from North America. But even taxonomically related species may have different diets in tropical and temperate areas and therefore, studies about FFG structure in neotropics could be biased. For this reason, we determined diet composition, trophic level and FFGs, using gut contents analysis and mouthpart observations of 49 macroinvertebrate taxa (mostly at genus level) from neotropical streams. We observed that practically all macroinvertebrates fed upon fine detritus which indicates the importance of this food resource in neotropical streams. As the assignment to a single FFG does not accurately reflect the functional profile of taxa, we transcribed the affinity of taxa to each FFG using fuzzy codes. Finally, we published the coding of diet composition and FFG of the taxa examined, which could be used in future community analyses of lotic ecosystems in the Neotropical zone.     

Intrinsic competition between larval parasitoids with different degrees of host specificity

Abstract.  1. Interspecific competition among parasitoids may play a key role in the community dynamics of tritrophic plant–herbivore–parasitoid systems and has important implications for management of herbivorous insect pests.
2. A model system was used to explore the outcome of interspecific competition between parasitoids that differ in host specificity. The system included the lepidopteran pest Heliothis virescens , the generalist parasitoid Cotesia marginiventris , and two specialist parasitoids, Microplitis croceipes and Cardiochiles nigriceps .
3. The generalist, C. marginiventris , dominated intrinsic competition when given an 8-h developmental head start over C. nigriceps or when its oviposition was simultaneous with that of M. croceipes . Microplitis croceipes and especially C. nigriceps larvae prevailed when they were allowed to oviposit prior to C. marginiventris .
4. Rates of host mortality prior to parasitoid emergence varied with parasitoid species composition and with the order of oviposition.
5. Implications for integrated pest management and the adaptive significance of competition as related to host specialisation are discussed.     

病原体感染对物种间资源竞争的影响——基于资源竞争理论

病原体感染对种间竞争的影响可能是因为改变了宿主的资源利用过程,然而竞争模型（Lotka-Volterra）由于参数化竞争系数而忽略了资源的动态变化过程,因此基于此类模型的研究无法揭示病原体对宿主资源利用的影响。基于Tilman的资源竞争理论构建了病原体感染一个物种的资源竞争模型,通过分析宿主物种资源利用效率的变化探讨了病原体对种间竞争的影响。结果表明：（1）病原体降低了宿主对资源的消耗率（消费矢量变短）,抬高了对资源的最低需求（零等倾线上移）,这意味着宿主的竞争力减弱;（2）虽然感染影响了竞争物种的密度,但不会改变共存物种的共存状态;（3）病原体可以使宿主物种的竞争对手更容易入侵,形成共存局面,极大地扩大了竞争物种共存的参数范围,本质上促进了物种多样性维持;（4）病原体的传播率和毒性也复杂地影响了竞争物种共存,传播率越大越能促进物种共存,而中等强度毒性最能促进物种共存。研究结果明确了病原体对物种资源利用模式的潜在改变,强调了病原体在物种共存和生物多样性维持中的重要性。     

Grazing and detritivory in 20 stream food webs across a broad pH gradient

Acidity is a major driving variable in the ecology of fresh waters, and we sought to quantify macroecological patterns in stream food webs across a wide pH gradient. We postulated that a few generalist herbivore-detritivores would dominate the invertebrate assemblage at low pH, with more specialists grazers at high pH. We also expected a switch towards algae in the diet of all primary consumers as the pH increased. For 20 stream food webs across the British Isles, spanning pH 5.0–8.4 (the acid sites being at least partially culturally acidified), we characterised basal resources and primary consumers, using both gut contents analysis and stable isotopes to study resource use by the latter. We found considerable species turnover across the pH gradient, with generalist herbivore-detritivores dominating the primary consumer assemblage at low pH and maintaining grazing. These were joined or replaced at higher pH by a suite of specialist grazers, while many taxa that persisted across the pH gradient broadened the range of algae consumed as acidity declined and increased their ingestion of biofilm, whose nutritional quality was higher than that of coarse detritus. There was thus an increased overall reliance on algae at higher pH, both by generalist herbivore-detritivores and due to the presence of specialist grazers, although detritus was important even in non-acidic streams. Both the ability of acid-tolerant, herbivore-detritivores to exploit both autochthonous and allochthonous food and the low nutritional value of basal resources might render chemically recovering systems resistant to invasion by the specialist grazers and help explain the sluggish ecological recovery of fresh waters whose water chemistry has ameliorated.     

Niche segregation of coexisting Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) constrains food web coupling in subarctic lakes

1. Generalist fish species are recognised as important couplers of benthic and pelagic food‐web compartments in lakes. However, interspecific niche segregation and individual specialisation may limit the potential for generalistic feeding behaviour. 2. We studied summer habitat use, stomach contents and stable isotopic compositions of the generalist feeder Arctic charr coexisting with its common resource competitor brown trout in five subarctic lakes in northern Norway to reveal population‐level and individual‐level niche plasticity. 3. Charr and trout showed partial niche segregation in all five lakes. Charr used all habitat types and a wide variety of invertebrate prey including zooplankton, whereas trout fed mainly on insects in the littoral zone. Hence, charr showed a higher potential to promote habitat and food‐web coupling compared to littoral‐dwelling trout. 4. The level of niche segregation between charr and trout and between pelagic‐caught and littoral‐caught charr depended on the prevailing patterns of interspecific and intraspecific resource competition. The two fish species had partially overlapping trophic niches in one lake where charr numerically dominated the fish community, whereas the most segregated niches occurred in lakes where trout were more abundant. 5. In general, pelagic‐caught charr had substantially narrower dietary and isotopic niches and relied less on littoral carbon sources compared to littoral‐caught conspecifics that included generalist as well as specialised benthivorous and planktivorous individuals. Despite the partially specialised planktivorous niche and thus reduced potential of pelagic‐dwelling charr to promote benthic–pelagic coupling, the isotopic compositions of both charr subpopulations suggested a significant reliance on both littoral and pelagic carbon sources in all five study lakes. 6. Our study demonstrates that both interspecific niche segregation between and individual trophic specialisation within generalist fish species can constrain food‐web coupling and alter energy mobilisation to top consumers in subarctic lakes. Nevertheless, pelagic and littoral habitats and food‐web compartments may still be highly integrated due to the potentially plastic foraging behaviour of top consumers.     

The evolution and coexistence of generalist and specialist herbivores under between-plant competition

Consumer-resource models have been used extensively to study the evolution and coexistence of generalist and specialist consumers. However, current consumer-resource models do not take into account competition between resources or only incorporate intraspecific competition phenomenologically with, for example, a logistic growth function. Here, we mechanistically incorporate competition in an existing two-resource model, by introducing nutrient-limited resource growth and setting the total amount of nutrients (free or contained in consumers and resources) to a fixed value. In addition to the three combinations of generalists and specialists found in previous models, we find four other evolutionary outcomes, depending on the strength of the consumer trade-off: coexistence of one specialist and a generalist and three types of evolutionary cycling. Furthermore, which outcomes are most likely depends strongly on the combination of intrinsic growth rate of resources and the total amount of nutrients in the system. Our results suggest that the realistic assumption of nutrient competition may shed new light on the evolution of the multitude of strategies in real systems.     

Predictable changes in trophic community structure along a spatial disturbance gradient in streams

1. We assessed insect and resource standing stocks along a spatial gradient of flood disturbance in 19 sub-alpine Swedish streams to test the prediction that change in trophic structure arises from the joint action of disturbance, which affect basal resources, and resource-control, which ties the response of the consumers to the response of the resources.
2. Trophic structure, quantified as scores of non-metric multidimensional scaling based on the biomass of insect trophic groups, changed predictably along the disturbance gradient. In early summer, predators and algae feeders decreased relative to suspension feeders with increasing disturbance; in autumn, algae feeders decreased relative to leaf feeders with increasing disturbance.
3. Across the disturbance gradient, the biomass of algae-, deposit- and leaf-feeders was principally controlled by the availability of the respective resource (algae, fine detritus and coarse detritus), while disturbance only had subsidiary effects on algae feeders in early summer.
4. Overall, patterns in trophic-group biomass along the disturbance gradient were more likely to reflect indirect effects of disturbance via impact on the resources, which reverberated to the consumers because of resource-control, rather than direct effects. In contrast with the view that stream communities are the result of stochastic colonization following disturbance events, in the study streams the trophic structure of insect assemblages is predictable and partly organized by resource-control across a broad range of disturbance conditions.     

Catchment land cover influences macroinvertebrate food‐web structure and energy flow pathways in mountain streams

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Fatty acid composition of consumers in boreal lakes – variation across species,space and time

1. Fatty acids (FAs) have been widely applied as trophic biomarkers in aquatic food web studies. However, current knowledge of inter‐ and intraspecific variation in consumer FA compositions across spatial and temporal scales is constrained to a few pelagic taxa. 2. We analysed the FAs of 22 taxa of benthic macroinvertebrates, zooplankton and fish collected from the littoral, pelagic and profundal habitats of nine boreal oligotrophic lakes over spring, summer and autumn. We quantified and compared the FA variance partitions contributed by species identity (i.e. an integrative effect of phylogenetic origin, life history and functional feeding guild of individual taxa), site and season using partial redundancy analysis both on all consumers and on benthic arthropods alone. 3. Species identity alone contributed 84.4 and 72.8% of explained FA variation of all consumers and benthic arthropods, respectively. Influences of site, season and all joint effects accounted for 0–11.3% only. Fatty acid profiles of primary consumers differentiated below class level, but those of predators were distinguishable only when they became more taxonomically distinct (i.e. among classes or higher). 4. Pelagic and profundal consumers showed stronger reliance on autochthonous resources than did their littoral counterparts as reflected by their higher ω3 to ω6 FA ratios. Polyunsaturated FAs (PUFAs) were increasingly retained with trophic levels, and saturated FAs (e.g. FA 16 : 0) gradually reduced. Ecologically, this trade‐off enhances the trophic transfer efficiency and confirms the importance of PUFA‐rich autotrophs in aquatic food webs. 5. Our findings indicate strong interspecific differences in FA requirements and assimilation among aquatic consumers from a wide range of taxonomic levels, habitats and lakes. Consumers were able to maintain homoeostasis in FA compositions across spatial and temporal changes in resource FAs, but consumer homoeostasis did not limit the effectiveness of FAs as trophic biomarkers.     

On the evolution of specialization with a mechanistic underpinning in structured metapopulations

We analyze the evolution of specialization in resource utilization in a discrete-time metapopulation model using the adaptive dynamics approach. The local dynamics in the metapopulation are based on the Beverton-Holt model with mechanistic underpinnings. The consumer faces a trade-off in the abilities to consume two resources that are spatially heterogeneously distributed to patches that are prone to local catastrophes. We explore the factors favoring the spread of generalist or specialist strategies. Increasing fecundity or decreasing catastrophe probability favors the spread of the generalist strategy and increasing environmental heterogeneity enlarges the parameter domain where the evolutionary branching is possible. When there are no catastrophes, increasing emigration diminishes the parameter domain where the evolutionary branching may occur. Otherwise, the effect of emigration on evolutionary dynamics is non-monotonous: both small and large values of emigration probability favor the spread of the specialist strategies whereas the parameter domain where evolutionary branching may occur is largest when the emigration probability has intermediate values. We compare how different forms of spatial heterogeneity and different models of local growth affect the evolutionary dynamics. We show that even small changes in the resource dynamics may have outstanding evolutionary effects to the consumers.     

Trophic disruption: a meta‐analysis of how habitat fragmentation affects resource consumption in terrestrial arthropod systems

Habitat fragmentation is a complex process that affects ecological systems in diverse ways, altering everything from population persistence to ecosystem function. Despite widespread recognition that habitat fragmentation can influence food web interactions, consensus on the factors underlying variation in the impacts of fragmentation across systems remains elusive. In this study, we conduct a systematic review and meta‐analysis to quantify the effects of habitat fragmentation and spatial habitat structure on resource consumption in terrestrial arthropod food webs. Across 419 studies, we found a negative overall effect of fragmentation on resource consumption. Variation in effect size was extensive but predictable. Specifically, resource consumption was reduced on small, isolated habitat fragments, higher at patch edges, and neutral with respect to landscape‐scale spatial variables. In general, resource consumption increased in fragmented settings for habitat generalist consumers but decreased for specialist consumers. Our study demonstrates widespread disruption of trophic interactions in fragmented habitats and describes variation among studies that is largely predictable based on the ecological traits of the interacting species. We highlight future prospects for understanding how changes in spatial habitat structure may influence trophic modules and food webs.     

Food webs in tropical Australian streams: shredders are not scarce

1. Macroinvertebrates were collected in dry and wet seasons from riffles and pools in two streams in tropical north Queensland. Total biomass, abundance and species richness were higher in riffles than in pools but did not differ between streams or seasons. 2. Gut contents of all species were identified. Cluster analysis based on gut contents identified five dietary groups: I, generalist collectors; II, generalist shredders and generalist predators; III, generalist scrapers; IV, specialist shredders; and V, specialist predators. Species were allocated to functional feeding groups (FFGs) based on these dietary groups. 3. Many species were generalist in their diets, but specialist predators and shredders were particularly prominent components of the invertebrate assemblages in terms of biomass and species richness. 4. Community composition (proportions of biomass, abundance and species richness of the different FFGs) varied between habitat types, but not between streams or seasons, although differences between riffles and pools varied with season. 5. Comparison of the fauna of 20 streams showed that our study sites were similar to, or not atypical of, low‐order streams in the Queensland wet tropics.     

Network analyses reveal the role of large snakes in connecting feeding guilds in a species‐rich Amazonian snake community

In ecological communities, interactions between consumers and resources lead to the emergence of ecological networks and a fundamental problem to solve is to understand which factors shape network structure. Empirical and theoretical studies on ecological networks suggest predator body size is a key factor structuring patterns of interaction. Because larger predators consume a wider resource range, including the prey consumed by smaller predators, we hypothesized that variation in body size favors the rise of nestedness. In contrast, if resource consumption requires specific adaptations, predators are expected to consume distinct sets of resources, thus favoring modularity. We investigate these predictions by characterizing the trophic network of a species‐rich Amazonian snake community (62 species). Our results revealed an intricate network pattern resulting from larger species feeding on higher diversity of prey and therefore promoting nestedness, whereas snakes with specific lifestyles and feeding on distinct resources, promoting modularity. Species removal simulations indicated that the nested structure is favored mainly by the presence of five species of the family Boidae, which because of their body size and generalist lifestyles connect modules in the network. Our study highlights the particular ways traits affect the structure of interactions among consumers and resources at the community level.     

Trophic structure and dynamical complexity in simple ecological models

We study the dynamical complexity of five non-linear deterministic predator–prey model systems. These simple systems were selected to represent a diversity of trophic structures and ecological interactions in the real world while still preserving reasonable tractability. We find that these systems can dramatically change attractor types, and the switching among different attractors is dependent on system parameters. While dynamical complexity depends on the nature (e.g., inter-specific competition versus predation) and degree (e.g., number of interacting components) of trophic structure present in the system, these systems all evolve principally on intrinsically noisy limit cycles. Our results support the common observation of cycling and rare observation of chaos in natural populations. Our study also allows us to speculate on the functional role of specialist versus generalist predators in food web modeling.     

Top‐down and bottom‐up diversity cascades in detrital vs. living food webs

Apex predators and plant resources are both critical for maintaining diversity in biotic communities, but the indirect (‘cascading’) effects of top‐down and bottom‐up forces on diversity at different trophic levels are not well resolved in terrestrial systems. Manipulations of predators or resources can cause direct changes of diversity at one trophic level, which in turn can affect diversity at other trophic levels. The indirect diversity effects of resource and consumer variation should be strongest in aquatic systems, moderate in terrestrial systems, and weakest in decomposer food webs. We measured effects of top predators and plant resources on the diversity of endophytic animals in an understorey shrub Piper cenocladum (Piperaceae). Predators and resource availability had significant direct and indirect effects on the diversity of the endophytic animal community, but the effects were not interactive, nor were they consistent between living vs. detrital food webs. The addition of fourth trophic level beetle predators increased diversity of consumers supported by living plant tissue, whereas balanced plant resources (light and nutrients) increased the diversity of primary through tertiary consumers in the detrital resources food web. These results support the hypotheses that top‐down and bottom‐up diversity cascades occur in terrestrial systems, and that diversity is affected by different factors in living vs. detrital food webs.     

The Effects of Spatial Scale on Trophic Interactions

Food chain models have dominated empirical studies of trophic interactions in the past decades, and have lead to important insights into the factors that control ecological communities. Despite the importance of food chain models in instigating ecological investigations, many empirical studies still show a strong deviation from the dynamics that food chain models predict. We present a theoretical framework that explains some of the discrepancies by showing that trophic interactions are likely to be strongly influenced by the spatial configuration of consumers and their resources. Differences in the spatial scale at which consumers and their resources function lead to uncoupling of the population dynamics of the interacting species, and may explain overexploitation and depletion of resource populations. We discuss how changed land use, likely the most prominent future stress on natural systems, may affect food web dynamics by interfering with the scale of interaction between consumers and their resource.     

Spatial and seasonal variability in the trophic role of aquatic insects: An assessment of functional feeding group applicability

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