Food web structure and habitat loss

In this paper we explore simple food web models to study how metacommunity structure affects species response to habitat loss. We find that patch abundances and extinction thresholds vary according to the kind of food web. Second, for intermediate species, a slight decrease in the exploration cost of the better competitor has a strong effect on the extinction threshold of the poorer competitor. When predicting extinction risk one should consider not only the amount of habitat destroyed, but also the structure of the food web in which species are embedded. Both direct and indirect interactions are critical for predicting the consequences of habitat destruction.     

Food web structure in riverine landscapes

1. Most research on freshwater (and other) food webs has focused on apparently discrete communities, in well-defined habitats at small spatial and temporal scales, whereas in reality food webs are embedded in complex landscapes, such as river corridors. Food web linkages across such landscapes may be crucial for ecological pattern and process, however. Here, we consider the importance of large scale influences upon lotic food webs across the three spatial dimensions and through time.
2. We assess the roles of biotic factors (e.g. predation, competition) and physical habitat features (e.g. geology, land-use, habitat fragmentation) in moulding food web structure at the landscape scale. As examples, external subsidies to lotic communities of nutrients, detritus and prey vary along the river corridor, and food web links are made and broken across the land–water interface with the rise and fall of the flood.
3. We identify several avenues of potentially fruitful research, particularly the need to quantify energy flow and population dynamics. Stoichiometric analysis of changes in C : N : P nutrient ratios over large spatial gradients (e.g. from river source to mouth, in forested versus agricultural catchments), offers a novel method of uniting energy flow and population dynamics to provide a more holistic view of riverine food webs from a landscape perspective. Macroecological approaches can be used to examine large-scale patterns in riverine food webs (e.g. trophic rank and species–area relationships). New multivariate statistical techniques can be used to examine community responses to environmental gradients and to assign traits to individual species (e.g. body-size, functional feeding group), to unravel the organisation and trophic structure of riverine food webs.     

Food web structure in a tropical stream ecosystem

Abstract This study investigated the structure and properties of a tropical stream food web in a small spatial scale, characterizing its planktonic, epiphytic and benthic compartments. The study was carried out in the Potreirinho Creek, a second‐order stream located in the south‐east of Brazil. Some attributes of the three subwebs and of the conglomerate food web, composed by the trophic links of the three compartments plus the fish species, were determined. Among compartments, the food webs showed considerable variation in structure. The epiphytic food web was consistently more complex than the planktonic and benthic webs. The values of number of species, number of links and maximum food chain length were significantly higher in the epiphytic compartment than in the other two. Otherwise, the connectance was significantly lower in epiphyton. The significant differences of most food web parameters were determined by the increase in the number of trophic species, represented mainly by basal and intermediate species. High species richness, detritus‐based system and high degree of omnivory characterized the stream food web studied. The aquatic macrophytes probably provide a substratum more stable and structurally complex than the sediment. We suggest that the greater species richness and trophic complexity in the epiphytic subweb might be due to the higher degree of habitat complexity supported by macrophyte substrate. Despite differences observed in the structure of the three subwebs, they are highly connected by trophic interactions, mainly by fishes. The high degree of fish omnivory associated with their movements at different spatial scales suggests that these animals have a significant role in the food web dynamic of Potreirinho Creek. This interface between macrophytes and the interconnections resultant from fish foraging, diluted the compartmentalization of the Potreirinho food web.     

Food web changes in arctic ecosystems related to climate warming

Sedimentary records from three Canadian High Arctic ponds on Ellesmere Island, spanning the last several thousand years, show major shifts in pond communities within the last ～200 years. These paleolimnological data indicate that aquatic insect (Diptera: Chironomidae) populations rapidly expanded and greatly increased in community diversity beginning in the 19th century. These invertebrate changes coincided with striking shifts in algal (diatom) populations, indicating strong food‐web effects because of climate warming and reduced ice‐cover in ponds. Predicted future warming in the Arctic may produce ecological changes that exceed the large shifts that have already occurred since the 19th century.     

Habitat subdivision causes changes in food web structure

Theory suggests that the response of communities to habitat subdivision depends on both species' characteristics and the extent to which species interact. For species with dynamics that are independent of other species, subdivision is expected to promote regional extinction as populations become small and isolated. By contrast, intermediate levels of subdivision can facilitate persistence of strongly interacting species. Consistent with this prediction, experimental subdivision lengthened persistence of some species, altering the extent of food web collapse through extinction. Extended persistence was associated with immigration rescuing a basal prey species from local extinction. As predicted by food web theory, habitat subdivision reduced population density of a top predator. Removal of this top predator from undivided microcosms increased the abundance of two other predator species, and these changes paralleled those produced by habitat subdivision. These results show that species interactions structured this community, and illustrate the need for investigations of other communities.     

Food web structure and function in two arctic streams with contrasting disturbance regimes

1. We studied the effect of substratum movement on the communities of adjacent mountain and spring tributaries of the Ivishak River in arctic Alaska (69°1′N, 147°43′W). We expected the mountain stream to have significant bed movement during summer because of storm flows and the spring stream to have negligible bed movement because of constant discharge. 2. We predicted that the mountain stream would be inhabited only by taxa able to cope with frequent bed movement. Therefore, we anticipated that the mountain stream would have lower macroinvertebrate species richness and biomass and a food web with fewer trophic levels and lower connectance than the spring stream. 3. Substrata marked in situ indicated that 57–66% of the bed moved during summer in the mountain stream and 4–20% moved in the spring stream. 4. Macroinvertebrate taxon richness was greater in the spring (25 taxa) than in the mountain stream (20 taxa). Mean macroinvertebrate biomass was also greater in the spring (4617 mg dry mass m^?2) than in the mountain stream (635 mg dry mass m^?2). Predators contributed 25% to this biomass in the spring stream, but only 7% in the mountain stream. 5. Bryophyte biomass was >1000 times greater in the spring stream (88.4 g ash‐free dry mass m^?2) than the mountain stream (0.08 g ash‐free dry mass m^?2). We attributed this to differences in substratum stability between streams. The difference in extent of bryophyte cover between streams probably explains the high macroinvertebrate biomass in the spring stream. 6. Mean food‐web connectance was similar between streams, ranging from 0.18 in the spring stream to 0.20 in the mountain stream. Mean food chain length was 3.04 in the spring stream and 1.83 in the mountain stream. Dolly Varden char (Salvelinus malma) was the top predator in the mountain stream and the American dipper (Cinclus mexicanus) was the top predator in the spring stream. The difference in mean food chain length between streams was due largely to the presence of C. mexicanus at the spring stream. 7. Structural differences between the food webs of the spring and mountain streams were relatively minor. The difference in the proportion of macroinvertebrate biomass contributing to different trophic levels was major, however, indicating significant differences in the volume of material and energy flow between food‐web nodes (i.e. food web function).     

Food web structure provides biotic resistance against plankton invasion attempts

It is generally accepted that native communities provide resistance against invaders through biotic interactions. However, much remains uncertain about the types of ecological processes and community attributes that contribute to biotic resistance. We used experimental mesocosms to examine how zooplankton community structure, invertebrate predation, and nutrient supply jointly affected the establishment of the exotic Daphnia lumholtzi. We predicted that establishment would increase with declining biomass and diversity of native zooplankton communities and that an invertebrate predator (IP) would indirectly facilitate the establishment of D. lumholtzi due to its relatively long predator-deterring spines. Furthermore, we hypothesized that elevated nutrient supply would increase algal food availability and facilitate establishment. Only when the biomass and diversity of native zooplankton were significantly reduced, was D.␣lumholtzi able to successfully invade mesocosms. Although invertebrate predation and resource supply modified attributes of native zooplankton communities, they did not influence the establishment of D. lumholtzi. Overall, our␣results are consistent with observed population dynamics in invaded reservoirs where D.␣lumholtzi tends to be present only during the late summer, coinciding with historic mid-summer declines in native zooplankton populations. Lakes and reservoirs may be more susceptible to invasion not only by D. lumholtzi, but also by other planktonic species, in the late summer when native communities exhibit characteristics associated with lower levels of biotic resistance.     

Food web structure and the strength of transient indirect effects

The relative importance of direct and indirect effects in ecological communities remains unresolved. Indirect effects may diminish as they propagate through highly reticulate food webs. We tested this hypothesis by assembling replicate food webs of different complexity in laboratory microcosms, and comparing the transmission of indirect effects through these webs. By providing the top predator ( Didinium ) with either one ( Paramecium ) or two ( Paramecium and Colpidium ) species of protists as prey, we created linear or reticulate food webs where we could examine the transient response of predators to an indirect effect. Addition of Chlamydomonas , a small alga consumed by Paramecium , but not by Colpidium , perturbed the system and generated an indirect effect on Didinium . We expected the proportional response of Didinium to Chlamydomonas addition would be smaller in the reticulate web containing alternative, unperturbed prey ( Colpidium ). We measured predator response as predator yield, the maximum number of predators produced prior to overexploitation of prey and subsequent predator decline. The ratio of yield in perturbed bottles to yield in unperturbed bottles measures the proportional response of Didinium to Chlamydomonas addition. We expected this ratio to be smaller with Colpidium present. Contrary to expectations, alternative prey enhanced rather than diminished predator response to the perturbation. This resulted from competition between the prey species, a factor ignored in some simple verbal arguments. Food web complexity may have unanticipated consequences for the strength of indirect effects.     

食物网结构与功能: 理论进展与展望

食物网刻画了物种间通过捕食而形成的复杂网络关系。阐明食物网结构与功能之间的关系, 既是生态学的基本理论问题, 也是预测全球变化背景下生态系统响应的重要依据。早期关于食物网结构与功能的研究往往是分离的, 或是基于食物链等的简单网络模型, 而近期研究基于复杂食物网模型取得了重要理论进展。本文综述了食物网研究的理论方法和近期进展, 特别介绍了复杂食物网中的结构、多样性和功能的度量指标、结构-多样性-功能之间的关系以及全球变化对食物网结构与功能的影响。本文最后对未来的一些研究方向进行了展望, 包括与功能性状和化学计量学的整合、食物网与其他网络类型的整合以及拓展食物网研究的空间和时间尺度。     

Food web changes associated with drought and invasive species in a tropical semiarid reservoir

Hydrobiologia - Fish and invertebrates are introduced in freshwaters around the world for commercial purposes, despite widely known impacts on food webs and biological invasions. As a proxy for...     

Surviving winter: Food,but not habitat structure,prevents crashes in cyclic vole populations

Vole population cycles are a major force driving boreal ecosystem dynamics in northwestern Eurasia. However, our understanding of the impact of winter on these cycles is increasingly uncertain, especially because climate change is affecting snow predictability, quality, and abundance. We examined the role of winter weather and snow conditions, the lack of suitable habitat structure during freeze‐thaw periods, and the lack of sufficient food as potential causes for winter population crashes. We live‐trapped bank voles Myodes glareolus on 26 plots (0.36 ha each) at two different elevations (representing different winter conditions) in southeast Norway in the winters 2013/2014 and 2014/2015. We carried out two manipulations: supplementing six plots with food to eliminate food limitation and six plots with straw to improve habitat structure and limit the effect of icing in the subnivean space. In the first winter, all bank voles survived well on all plots, whereas in the second winter voles on almost all plots went extinct except for those receiving supplemental food. Survival was highest on the feeding treatment in both winters, whereas improving habitat structure had no effect. We conclude that food limitation was a key factor in causing winter population crashes.     

Artificial selection on allometry: change in elevation but not slope

To what extent within-species (static) allometries constitute a constraint on evolution is the subject of a long-standing debate in evolutionary biology. A prerequisite for the constraint hypothesis is that static allometries are hard to change. Several studies have attempted to test this hypothesis with artificial-selection experiments, but their results remain inconclusive due to various methodological issues. Here, we present results from an experiment in which we selected independently on the slope and the elevation of the allometric relationship between caudal-fin size and body size in male guppies (Poecilia reticulata). After three episodes of selection, the allometric elevation (i.e. intercept at constant slope) had diverged markedly between the lines selected to increase or decrease it, and showed a realized heritability of 50%. In contrast, the allometric slope remained unaffected by selection. These results suggest that the allometric elevation is more evolvable than the allometric slope, this latter representing a potential constraint on adaptive trait evolution. To our knowledge, this study is the first artificial-selection experiment that directly tests the evolvability of static allometric slopes.     

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.     

Food web structure of aphids and their parasitoids in Belgian fruit agroecosystems

Community structures of aphids and their parasitoids were studied in fruit crop habitats of eastern Belgium in 2014 and 2015. Quantitative food webs of these insects were constructed separately for each year, and divided into subwebs on three host‐plant categories, fruit crop plants, non‐crop woody and shrub plants and non‐crop herbaceous plants. The webs were analyzed using the standard food web statistics designed for binary data. During the whole study period, 78 plant species were recorded as host plants of 71 aphid species, from which 48 parasitoid species emerged. The community structure, aphid / parasitoid species‐richness ratio and trophic link number varied between the two years, whereas the realized connectance between parasitoids and aphids was relatively constant. A new plant–aphid–parasitoid association for Europe was recorded. Dominant parasitoid species in the study sites were Ephedrus persicae, Binodoxys angelicae and Praon volucre: the first species was frequently observed on non‐crop trees and shrubs, but the other two on non‐crop herbaceous plants. The potential influence, through indirect interactions, of parasitoids on aphid communities was assessed with quantitative parasitoid‐overlap diagrams. Symmetrical links were uncommon, and abundant aphid species seemed to have large indirect effects on less abundant species. These results show that trophic indirect interactions through parasitoids may govern aphid populations in fruit crop habitats with various non‐crop plants, implying the importance for landscape management and biological control of aphid pests in fruit agroecosystems.     

Nest microhabitats and tree size mediate shifts in ant community structure across elevation in tropical rainforest canopies

Declines or mid-elevation peaks in invertebrate diversity with elevation are often attributed to climate and geometric constraints. However, vegetation structure may also drive diversity patterns, especially for tree-dwelling species, via its effects on microhabitat use and competitive interactions. Here we investigate these effects on the diversity and community structure of tree-nesting ants over elevation. We exhaustively sampled ant nests in 1254 trees within continuous plots of primary rainforest at low (200 m a.s.l.), mid (900 m a.s.l.) and high (1800 m a.s.l.) elevation in Papua New Guinea. Ant diversity, nest abundance and tree occupancy peaked at mid-elevation. Although host tree diversity also peaked at mid-elevation, there was low specialisation of ant species to tree species at all elevations. Mid-elevation trees hosted more species, more nests and a greater diversity of nest types than trees of a similar size at low or high elevation. Tree size and nest microhabitat use were the strongest predictors of species composition, explaining twice as much of the variability in the communities than elevation. At mid to high elevation there were proportionally fewer large nests than in the lowlands, with an increase in smaller nests in live hollow twigs and epiphytes. There was high species turnover between elevations, and between trees within elevations. Species co-occurrence patterns within trees differed with tree size, and with elevation. In large trees species tended to co-occur at random at low and high elevation, but co-occurred more often than expected by chance at mid elevation, indicating an elevational shift in competitive interactions. We conclude that the more extreme diurnal temperatures at higher elevations, combined with increased epiphyte availability, drive ants to nest in more insulated microhabitats. This results in smaller colony sizes and a decrease in interspecific competition, thereby boosting species co-existence at mid elevation.     

Food web modelling on the structure and functioning of a Mediterranean lentic system

A number of modelling results suggested thermocline shifts as a consequence of global climate change in stratifying lakes. Abundance and composition of the phytoplankton assemblage is strongly affected by the stratification patterns, and therefore, change in the thermocline position might have a substantial effect on this community or even on the whole lake ecosystem. In this study, thermocline depths in large mesocosms installed in Lake Stechlin (Germany) were deepened by 2 meters and phytoplankton changes were analysed by comparing changes to untreated mesocosms. Higher amounts of SRP were registered in the hypolimnion of treatment mesocosms than in the controls, and there were no differences in the epilimnion. Small but significant changes were observed on the phytoplankton community composition related to the effect of deepening the thermocline; however, it was weaker than the yearly successional changes. The most remarkable differences were caused by Planktothrix rubescens and by chlorophytes. P. rubescens became strongly dominant at the end of the experiment in the mesocosms, and in the open lake as well. The results of the experiment cannot clearly support the proliferation of cyanobacteria in general; however, the deepened thermocline can modify the behaviour of some species, as was observed in case of P. rubescens.     

Food web structure in Mediterranean streams: exploring stabilizing forces in these ecosystems

We constructed the food webs of six Mediterranean streams in order to determine ecological generalities derived from analysis of their structure and to explore stabilizing forces within these ecosystems. Fish, macroinvertebrates, primary producers and detritus are the components of the studied food webs. Analysis focused on a suite of food web properties that describe species’ trophic habits, linkage complexity and food chains. A great structural similarity was found in analyzed food webs; we therefore suggest average values for the structural properties of Mediterranean stream food webs. Percentage of omnivorous species was positively correlated with connectance, and there was a predominance of intermediate trophic level species that had established simple links with detritus. In short, our results suggest that omnivory and the weak interactions of detritivores have a stabilizing role in these food webs.     

Food web structure and interaction strength pave the way for vulnerability to extinction

This paper focuses on how food web structure and interactions among species affects the vulnerability, due to environmental variability, to extinction of species at different positions in model food webs. Vulnerability is here not measured by a traditional extinction threshold but is instead inspired by the IUCN criteria for endangered species: an observed rapid decline in population abundance. Using model webs influenced by stochasticity with zero autocorrelation, we investigate the ecological determinants of species vulnerability, i.e. the trophic interactions between species and food web structure and how these interact with the risk of sudden drops in abundance of species. We find that (i) producers fulfil the criterion of vulnerable species more frequently than other species, (ii) food web structure is related to vulnerability, and (iii) the vulnerability of species is greater when involved in a strong trophic interaction than when not. We note that our result on the relationship between extinction risk and trophic position of species contradict previous suggestions and argue that the main reason for the discrepancy probably is due to the fact that we study the vulnerability to environmental stochasticity and not extinction risk due to overexploitation, habitat destruction or interactions with introduced species. Thus, we suggest that the vulnerability of species to environmental stochasticity may be differently related to trophic position than the vulnerability of species to other factors. Earlier research on species extinctions has looked for intrinsic traits of species that correlate with increased vulnerability to extinction. However, to fully understand the extinction process we must also consider that species interactions may affect vulnerability and that not all extinctions are the result of long, gradual reductions in species abundances. Under environmental stochasticity (which importance frequently is assumed to increase as a result of climate change) and direct and indirect interactions with other species some extinctions may occur rapidly and apparently unexpectedly. To identify the first declines of population abundances that may escalate and lead to extinctions as early as possible, we need to recognize which species are at greatest risk of entering such dangerous routes and under what circumstances. This new perspective may contribute to our understanding of the processes leading to extinction of populations and eventually species. This is especially urgent in the light of the current biodiversity crisis where a large fraction of the world's biodiversity is threatened.     

Antibiotic disturbance affects aquatic microbial community composition and food web interactions but not community resilience

Notwithstanding the fundamental role that environmental microbes play for ecosystem functioning, data on how microbes react to disturbances are still scarce, and most factors that confer stability to microbial communities are unknown. In this context, antibiotic discharge into the environment is considered a worldwide threat for ecosystems with potential risks to human health. We therefore tested resilience of microbial communities challenged by the presence of an antibiotic. In a continuous culture experiment, we compared the abundance, composition and diversity of microbial communities undisturbed or disturbed by the constant addiction of tetracycline in low (10 µg/L) or intermediate (100 µg/L) concentration (press disturbance). Further, the bacterial communities in the three treatments had to face the sudden pulse disturbance of adding an allochthonous bacterium (Escherichia coli). Tetracycline, even at low concentrations, affected microbial communities by changing their phylogenetic composition and causing cell aggregation. This, however, did not coincide with a reduced microbial diversity, but was mainly caused by a shift in dominance of specific bacterial families. Moreover, the less disturbed community (10 µg/L tetracycline) was sometimes more similar to the control and sometimes more similar to heavily disturbed community (100 µg/L tetracycline). All in all, we could not see a pattern where the communities disturbed with antibiotics were less resilient to a second disturbance introducing E. coli, but they seemed to be able to buffer the input of the allochthonous strain in a similar manner as the control.