Variable dependence on detrital and grazing food webs by generalist predators: aerial insects and web spiders

Recent studies have shown that organisms from the detritus food web subsidize generalist predators in aboveground food webs, but its significance in space and time is largely unknown. Here we report seasonal dynamics of aerial insects from grazing and detritus food webs in both forest and grassland habitats, and show how these patterns influence the dependence of web spiders on the detritus food web. Detrital insects were more abundant in spring, decreased in summer, and then increased slightly in autumn. This pattern was most conspicuous in Nematocera. Due to different seasonal activity patterns of grazing and detrital insects, the proportion of detrital insects was greater in spring and autumn. Detrital insects were relatively more abundant in the forest than in the grassland. Prey captured by web spiders generally reflected seasonal and spatial patterns of aerial insect abundance. In particular, Leucauge spiders reversed their dependence on the two food webs seasonally. Body size of spiders was negatively correlated with the proportion of detrital prey, suggesting that the detrital subsidy is essential for relatively small predators. This size effect probably resulted from interaction of the following two factors: 1) the maximum body size of prey that can be caught increased with spider body size, 2) larger body size classes of aerial insects included a higher proportion of insects from the grazing food web.     

A test for top-down cascade in a detritus-based food web by litter-dwelling web spiders

A top-down cascade in detritus-based food webs has the potential to alter ecosystem functioning but its prevalence in terrestrial systems remains unclear. We manipulated the density of the dominant web spider Neriene brongersmai for 1.5 years to see whether the cascading effect would propagate down to the litter decomposition rate in the forest-floor of a Japanese cedar forest. Decreasing spider abundance increased collembolan density, but it did not change the biomass of microorganisms or the litter decomposition rate. We propose possible explanations for the absence of a top-down cascade and discuss the difficulty in generalizing the presence and absence of a cascading effect in soil ecosystems.     

The characteristics of species in an evolutionary food web model

We explore the consequences of modifying the way in which species are defined in an evolutionary food web model. In the original version of the model, the species were defined in terms of a fixed number of features, chosen from a large number of possibilities. These features represented phenotypic and behavioural characteristics of the species. Speciation consisted in occasionally replacing one of the features by another. Here we modify this scheme by firstly allowing for a richer structure and secondly by testing whether we are able to eliminate the need for an explicit choice of features altogether. In the first case we allow for changing the number of features which define a species, as well as their nature, and find that in the resulting webs the higher trophic levels typically contain species with the greatest number of features. In the second case, by a simplification of the mechanisms for inter and intra-species competition, we construct a model without any explicit features and find that we are still able to grow model food webs. We assess the quality of the food webs produced and discuss the consequences of our findings for the future modelling of food webs.     

Terrestrial food web complexity in Amazonian forests decays with habitat loss

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Intraguild interactions between spiders and ants and top-down control in a grassland food web

In most terrestrial ecosystems ants (Formicidae) as eusocial insects and spiders (Araneida) as solitary trappers and hunters are key predators. To study the role of predation by these generalist predators in a dry grassland, we manipulated densities of ants and spiders (natural and low density) in a two-factorial field experiment using fenced plots. The experiment revealed strong intraguild interactions between ants and spiders. Higher densities of ants negatively affected the abundance and biomass of web-building spiders. The density of Linyphiidae was threefold higher in plots without ant colonies. The abundance of Formica cunicularia workers was significantly higher in spider-removal plots. Also, population size of springtails (Collembola) was negatively affected by the presence of wandering spiders. Ants reduced the density of Lepidoptera larvae. In contrast, the abundance of coccids (Ortheziidae) was positively correlated with densities of ants. To gain a better understanding of the position of spiders, ants and other dominant invertebrate groups in the studied food web and important trophic links, we used a stable isotope analysis (¹⁵N and ¹³C). Adult wandering spiders were more enriched in ¹⁵N relative to ¹⁴N than juveniles, indicating a shift to predatory prey groups. Juvenile wandering and web-building spiders showed δ¹⁵N ratios just one trophic level above those of Collembola, and they had similar δ¹³C values, indicating that Collembola are an important prey group for ground living spiders. The effects of spiders demonstrated in the field experiment support this result. We conclude that the food resource of spiders in our study system is largely based on the detrital food web and that their effects on herbivores are weak. The effects of ants are not clear-cut and include predation as well as mutualism with herbivores. Within this diverse predator guild, intraguild interactions are important structuring forces.     

Tracing the evolutionary origin of a visual signal: the coincidence of wrap attack and web decorating behaviours in orb web spiders (Araneidae)

The silk decorations that adorn the webs of many orb-web spiders are thought to have a signal function, but the evolution of the decorating behaviour remains unresolved. The decoration signal is maintained apparently because it improves foraging efficiency, through either increased encounter rates with prey or reduced damage to the web. Recent investigations suggest that the decorations may originate in a regulation of the activity of the aciniform silk glands, which produce silk for both decorating the web and wrapping prey. This view predicts a link between decorating behaviour and a preference for restraining prey by wrapping with silk, which is evident among species of Argiope spiders. Here I compare the frequency of the wrap attack behaviour in four species of orb-web spiders that occupy the same habitat, but differ in their silk decorating behaviour: two species, Plebs bradleyi and Gea theridioides, build silk decorations, while the other two, Araneus hamiltoni and Backobourkia brounii do not. Spiders were presented with prey items that varied in the ease with which they could be captured, with houseflies being more easily subdued than house crickets. As predicted, the silk decorating species used wrap attacks significantly more often than non-decorating spiders, irrespective of the prey species. These data support the view that both behaviours are evolutionary linked. I propose that silk decorating originated from the evolution of wrap attacking, and that silken web decorations have later evolved into a signal and are now maintained for that function.     

Microzooplankton feeding behaviour: grazing on the microbial and the classical food web of African soda lakes

We investigated the feeding behaviour of the dominant microzooplankton of saline lakes in the East African Rift Valley. A set of grazing experiments revealed high ingestion rates of the two euryhaline rotifers Brachionus dimidiatus and Brachionus plicatilis and of the large-sized omnivorous ciliates Frontonia sp. and Condylostoma magnum reflecting the unique nature of tropical saline systems. The size spectrum of ingested particles was broad and even included filamentous cyanobacteria such as the commonly dominating Arthrospira fusiformis. Feeding selectivity on cyanobacteria, however, was rather low showing higher values for cryptomonads and small ciliates. Bacterial biomass was favoured by the presence of grazers, as small bacterivorous predators were reduced at an average of 13.9%, showing the cascading effect of large zooplankton on the food web structure. Overall, based on this first-time study of the microzooplankton feeding behaviour in East African soda lakes, a strong structuring effect of rotifers and large ciliates on microbial plankton communities is assumed, especially in times of high consumer biomass.     

Species richness and food web structure of soil decomposer community as affected by the size of habitat fragment and habitat corridors

While most ecologists agree that the effects of fragmentation on diversity of organisms are predominantly negative and that the scale of fragmentation defines their severity, the role of habitat corridors in mitigating those effects still remains controversial. This ambiguousness rests largely on various difficulties in experimentation, a problem partially solved in the present paper by the use of easily manipulated soil communities. In this 2.5‐year‐long field experiment, we investigated the responses of soil decomposer organisms (from microbes to mesofaunal predators) to habitat fragment size, in the presence or absence of habitat corridors connecting the fragments. The habitat fragments and corridors, composed of forest humus soil, were embedded in mineral soil representing an uninhabitable (or nonpreferred) matrix for the decomposer organisms. The results demonstrate that soil decomposer organisms do respond to changes in their habitat size: the species richness of microarthropods (mites and collembolans) increased as the size of the fragments increased. Especially collembolan species and predatory mites proved to be sensitive to the restricted habitat size, which is suggested to be a consequence of the large proportion of rare species and small and fluctuating population sizes in these groups. Contrary to our expectations, the presence of corridors had no positive effects on species richness or abundance of any of the studied faunas, possibly because of the low quality of the corridors. On the other hand, the biomass of soil fungi increased in the presence of corridors, which apparently provided a preferred pathway for vegetative dispersal of the fungi. Our results indicate that despite their characteristic underground environment, the response of soil decomposer organisms – in particular that of microarthropods – to habitat size is not unlike to that of the larger organisms in aboveground habitats.     

Potential importance of protozoan grazing on the accumulation of polychlorinated biphenyls (PCBs) in the pelagic food web

Experiments were conducted to study the distribution of three selectedpolychlorinated biphenyl (PCB) congeners within the microbial food web attwo different nutrient levels; control and nutrient enriched. The objectivewas to quantify the uptake of PCBs through grazing by protozoa. The¹⁴C-PCBs tested were 4-chlorobiphenyl (IUPAC # 3),2,2,5,5-tetrachlorobiphenyl (IUPAC # 52), and2,2,4,4,5,5-hexachlorobiphenyl (IUPAC # 153). EachPCB was incubated in triplicate seawater samples at 20 idref;Cover one week. Daily, samples were separated into four fractions; <0.2µm (dissolved), 0.2-2 µm (bacteria), 2-10 µm(flagellate), and > 10 µm (microplankton; phytoplankton andprotozoa) by selective filtration. Of the PCB fraction that initiallyadsorbed to particles, 60–100% was associated to the bacterialfraction and 0–5% to the microplankton fraction. The totaluptake was highest in the nutrient enriched samples, but when normalized tothe carbon biomass the concentration was lower or equal to the control inall particle fractions. The recovery of the PCBs in the particulatefractions depended on the degree of chlorination, as the highest values wereobserved for the 2,2,4,4,5,5-hexachlorobiphenyl and thelowest for the 4-chlorobiphenyl. The concentrations in the bacterial andflagellate fractions decreased over the first 48–96 hours whilst theconcentration increased in the highest trophic level (>10 µmfraction). Approximately 75% of the increase in concentration of the2,2,4,4,5,5-hexachlorobiphenyl in the > 10 µmfraction was estimated to be the result of bacterivory. Our results indicatethe microbial food web can contribute to a rapid uptake of higherchlorinated PCBs, particularly in oligotrophic ecosystems where thebacterial biomass dominates.     

Nutrient dynamics and pelagic food web interactions in oligotrophic and eutrophic environments: an overview

The concept of limiting nutrients is a cornerstone of theories concerning the control of production, structure and dynamics of freshwater and marine plankton. The current dogma is that nitrogen is limiting in most marine environments while freshwater ecosystems are mostly phosphorus-limited, although evidence of phytoplankton limitation by either N or P has been found in both environments.However, the same considerations apply to the availability of phosphorus in freshwater as to nitrogen in oceans. In resource-limited environments the plankton dynamics depend mostly on the internal mechanisms which act to recycle the limiting nutrient many times over within the surface waters. As the overall productivity increases, this dependence on nutrient regeneration decreases.The relationship between the stock of limiting nutrient, rates of supply and plankton dynamics must therefore be seen in the light of the processes operating within the entire food chain over quite different time scales. There is strong evidence that process-rates are mostly size-dependent and that food web interactions at the microbial level (picophytoplankton, bacteria, microheterotrophs) strongly effect the production of carbon and the regeneration of nutrients in the pelagic zone.     

A model for the evolutionary maintenance of monogyny in spiders

Sexual selection theory predicts that males should attempt to mate with several females, unless the benefits of male promiscuity are trumped by alternative benefits associated with male monogamy (monogyny). Here we use a game theory model to address the adaptive value of a monogynous strategy, which has the sole benefit of enhancing a male's paternity share in the context of competition with other males. We consider two ways in which monogynists might enhance their paternity: by outcompeting rival ejaculates in sperm competition, and by reducing the probability that a female remates with rival males. The model is based on the biology of some particularly well-studied spider species, in which males are morphologically restricted to mate with either one or at most two females in their lifetime. Our results suggest that, regardless of the mechanism of paternity enhancement involved, a male-biased sex ratio is generally required for the evolution and maintenance of monogyny. Moreover, we show that there is a large region of parameter space where monogyny and bigyny can coexist as alternative mating strategies under negative frequency dependent selection. There is also a narrow range of conditions where either monogyny or bigyny can be evolutionarily stable. Our results are in qualitative agreement with empirical findings in spiders.     

The food web in the pelagic environment

Summary 1. The study of grazing along ecological successions helps in understanding how the mechanisms of transfer of energy evolve and provides a measure of its effectiveness in the different situations. Phytoplankton populations show regular changes along succession. Average size of cells and relative abundance of mobile organisms increase, and productivity or rate of multiplication slows down; there are also changes in the chemical composition, exemplified in the plant pigments by an absolute and relative decrease of chlorophyll a.2. Along a succession, animals are offered different kinds of food, and the resulting selection produces a shift in the composition of zooplankton populations. The speed at which phytoplankton succession proceeds is a very important factor, and grazing may be effective in the regulation of such speed. In a general way, along usual successions, food in the form of small particles, richly suspended in a more or less turbulent environment, is replaced by scarcer food concentrated in bigger units and dispersed in a more organized (stratified) environment.3. Indiscriminate filter feeders are at an advantage in the first stages of succession, but it can be shown that, given the usual properties of food organisms in later stages, it pays to adopt a more selective and hunting behaviour and to concentrate more and more on bigger prey. The effectiveness of such adaptation depends on the distribution of food in size classes and also on its mobility or other clues that prey organisms can offer, and, in general, on the predictability of their distribution. Divergence between microphagous passive filterers and macrophagous hunters must be rapid. Distribution of populations of copepods along time and in relation with phytoplankton distribution sustain such views.4. In general, in later stages of succession, total transfer of energy may be lower, buts its efficiency seems to be regularly improved. Similar considerations could be extended to the discussion of energy transfer between other, superior, trophic levels.

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Das Nahrungsnetz in der pelagischen Umwelt

Kurzfassung Vergleichende Studien über die Organisation des Nahrungsnetzes während verschiedener Stadien von Planktonsukzessionen können Aufschlüsse liefern über Gesetzmäßigkeiten der Energietransformation. Von dieser Annahme ausgehend, werden zunächst verschiedene Stadien von Phytoplanktonsukzessionen beschrieben und dann an Hand von Beispielen aus der Literatur und theoretischen Erwägungen allgemeinere Gesetzmäßigkeiten abgeleitet. Phytoplanktonpopulationen verändern sich im Verlaufe von Sukzessionen in ganz bestimmter Weise: Die Durchschnittgröße der Zellen und die relative Häufigkeit von Organismen mit Eigenbewegung nehmen zu, während Produktivität oder Zellteilungsrate abnehmen; ferner kommt es zu Veränderungen in der chemischen Zusammensetzung, etwa zu einer Abnahme der Menge an Chlorophyll. Im Verlauf der Sukzessionen verändert sich das Nahrungsangebot für die Zooplankter; dieser Umstand führt zu Verschiebungen in der Komposition der Zooplanktonpopulationen. Die Geschwindigkeit, mit welcher Phytoplanktonsukzessionen fortschreiten, ist ein sehr wichtiger Faktor; grazing ist hier möglicherweise als Geschwindigkeitsregulativ wirksam. Ganz allgemein wird im Verlauf gewöhnlicher Sukzessionen Nahrung, welche in Form von kleinen dispers suspendierten Partikeln in einem mehr oder minder turbulenten Wasserkörper vorliegt, ersetzt durch geringere Nahrungsmengen, welche in größeren Partikeln konzentriert sind und in einer stärker organisierten (stratifizierten) Umwelt vorkommen. In den ersten Sukzessionsstadien sind unspezifische Filtrierer im Vorteil; in späteren Stadien werden jedoch räuberische Formen und solche, die sich auf immer größere Nahrungsorganismen konzentrieren, bevorzugt. In den späteren Sukzessiontadien ist die Gesamtmenge der transferierten Energie möglicherweise geringer, aber der Nutzeffekt scheint größer zu sein.

     

A theoretical approach to structuring mechanisms in the pelagic food web

In the literature there is a commonly used idealized concept of the foodweb structure in the pelagic photic zone food web, based to a large extenton size dependent relationships. An outline is here given of how theelementary size-related physical laws of diffusion and sinking, combinedwith the assumption of predators being size selective in their choice ofprey, give a theoretical foundation for this type of structure. It is shownhow such a theoretical fundament makes it possible to relate a broad specterof phenomena within one generic and consistent framework. Phenomena such asHutchinson‘s and Goldman‘s paradoxes, the influence of nutrients and watercolumn stability on the balance between microbial and classical food webs,bacterial carbon consumption, new production and export of DOC and POC tothe aphotic zone, eutrophication and diversity, can all be approached fromthis perspective. By including host-specific viruses, this approach gives ahierarchical structure to the control of diversity with nutrient contentcontrolling the maximum size of the photic zone community, size selectivityof predators regulating how the nutrient is distributed between size-groupsof osmotrophic and phagotrophic organisms, and viral host specificityregulating how the nutrients within a size group is distributed between hostgroups. I also briefly discuss how some biological strategies may besuccessful by not conforming to the normal rules of such a framework.Analyzing the behavior of these idealized systems is thus claimed tofacilitate our understanding of the behavior of complex natural food webs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lipid composition of zooplankton in relation to the sub-arctic food web

Summary Seasonal changes in the lipid class composition and fatty acid and fatty alcohol composition of neutral lipids were determined for Calanus finmarchicus, Metridia longa and Sagitta sp. in Balsfjord, northern Norway. Similar analyses were obtained for C. hyperboreus and Parathemisto abyssorum in an adjacent fjord, Ullsfjord, in spring. C. finmarchicus, C. hyperboreus, M. longa, and Parathemisto abyssorum all contained large amounts of wax esters whereas Sagitta sp. contained small amounts of triacylglycerols and traces of wax esters. the levels of wax ester in C. finmarchicus and M. longa were highest in late autumn (respectively 88% and 84% of total lipid) and lowest in early spring (respectively 85% and 27% of total lipid). The accumulation of these neutral lipids in spring and summer is related to the feeding activity during the primary production period, while their decline in late winter is associated with the mobilisation of metabolic energy for production of gonads. The major fatty alcohols in the wax esters of C. finmarchicus and C. hyperboreus and Parathemisto abyssorum were 20:1 and 22:1 while those in the wax esters of M. longa were 14:0 and 16:0. The traces of wax esters in Saqitta were rich in 20:1 and 22:1 fatty alcohols. These analyses are consistent with C. finmarchicus and C. hyperboreus being strictly herbivorous, M. longa being more carnivorous and both Sagitta sp. and Parathemisto being highly carnivorous, probably ingesting substantial amounts of calanoid copepods.     

Changes in the pelagic microbial food web due to artificial eutrophication

The effect of nutrient enrichment on the structure and carbon flow in the pelagic microbial food web was studied in mesocosm experiments using seawater from the northern Baltic Sea. The experiments included food webs of at least four trophic levels; (1) phytoplankton–bacteria, (2) flagellates, (3) ciliates and (4) mesozooplankton. In the enriched treatments high autotrophic growth rates were observed, followed by increased heterotrophic production. The largest growth increase was due to heterotrophic bacteria, indicating that the heterotrophic microbial food web was promoted. This was further supported by increased growth of heterotrophic flagellates and ciliates in the high nutrient treatments. The phytoplankton peak in the middle of the experiments was mainly due to an autotrophic nanoflagellate, Pyramimonas sp. At the end of the experiment, the proportion of heterotrophic organisms was higher in the nutrient enriched than in the nutrient-poor treatment, indicating increased predation control of primary producers. The proportion of potentially mixotrophic plankton, prymnesiophyceans, chrysophyceans and dinophyceans, were significantly higher in the nutrient-poor treatment. Furthermore, the results indicated that the food web efficiency, defined as mesozooplankton production per basal production (primary production + bacterial production − sedimentation), decreased with increasing nutrient status, possibly due to increasing loss processes in the food web. This could be explained by promotion of the heterotrophic microbial food web, causing more trophic levels and respiration steps in the food web.     

Subsidy from the detrital food web, but not microhabitat complexity, affects the role of generalist predators in an aboveground herbivore food web

The activity and density of generalist predators, such as carabid beetles, rove beetles and spiders, may increase in response to: (1) increased availability of prey from the belowground subsystem and/or (2) enhanced complexity of aboveground vegetation. Organic farming practices support decomposer populations and enhance habitat complexity due to an increased weed density. A response by generalist predators to such below‐ or aboveground changes could affect predation rates on herbivores in the aboveground food web. We tested this hypothesis in a replicated field experiment conducted in a winter wheat field, where increased predator activity could lead to improved control of herbivorous pests. In a crossed design, we increased and lowered densities of decomposer prey, and manipulated vegetation complexity using artificial plants in order to examine the effect of structural complexity in isolation from effects of plant‐attracted additional prey. Isotomid Collembola exhibited lowest activity‐densities (AD) in plots treated with soil insecticide and had gradually increasing AD in untreated plots and plots receiving detrital subsidies. Carabid beetles and cursorial spiders did not respond to increased availability of isotomid prey, and they unexpectedly displayed higher AD in the structurally less‐complex plots. Aphid density mirrored the positive response of isotomids to detrital subsidies, suggesting that aphids benefited from reduced predation due to predators switching to abundant prey in the decomposer subsystem. The absence of a numerical response by surface‐active predators apparently strengthened this indirect effect of isotomids on aphids. Our results suggest that indirect predator‐mediated prey‐prey interactions can reduce beneficial effects of detrital subsidies on pest suppression. We further demonstrated that generalist predators may not per se benefit from structural complexity. Both results document the challenges associated with management practices that support generalist predators, as these measures may not necessarily improve herbivore suppression.     

Selective alteration of soil food web components by invasive giant goldenrod Solidago gigantea in two distinct habitat types

Apart from relatively well‐studied aboveground effects, invasive plant species will also impact the soil food web. So far, most research has been focusing on primary decomposers, while studies on effects at higher trophic levels are relatively scarce. Giant goldenrod Solidago gigantea, native to North America, is a widespread and common invasive species in most European countries. We investigated its impact on plant communities and on multiple trophic levels of the soil food web in two contrasting habitats: riparian zones and semi‐natural grasslands. In 30 pairs of invaded and uninvaded plots, floristic composition, pH, fungal biomass and the densities of 11 nematode taxa were determined by using a quantitative PCR‐based method. In the two habitats, the invader outcompeted both rare and dominant plant species. Belowground, S. gigantea invasion reduced pH, increased overall fungal biomass as well as the density of a single lineage of fungivorous nematodes, the family Aphelenchoididae. The densities of two other, phylogenetically distinct lineages of fungivorous nematodes, Aphelenchidae and Diphtherophoridae, were unaffected by the local increase in fungal biomass. Apparently this plant species induces a local asymmetric boost of the fungal community, and only Aphelenchoididae were able to benefit from this invader‐induced change. The alternative explanation – the results are explained by a subtle, S. gigantea‐induced 0.1–0.2 units decrease of pH – seems unlikely, as pH optima for nematode taxa are relatively broad. Thus, apart from readily observable aboveground effects, the invasive plant species S. gigantea affects fungal biomass as well as a specific part of the fungivorous nematode community in a soil type‐independent manner.     

Responses to food web manipulation in a shallow waterfowl lake

The effects of fish stock reduction have been studies in 3 Dutch lakes (Lake Zwemlust, Lake Bleiswijkse Zoom and Lake Noorddiep) and 1 Danish lake (Lake Væng) during 4–5 years. A general response id described. The fish stock reduction led in general to a low fish stock, low chlorophyii-a, high transparency and high abunuance of macrophytes. Large Daphnia became abundant, but their density decreased, due to food limitation and predation by fish. The total nitrogen concentration became low due to N-uptake by macrophytes and enhanced denitrification. In Lake Bleiswijkse Zoom the water transparency deteriorated and the clear water state was not stable. The fish stock increased and the production of young fish in summer was high. lear water occurred only in spring. Large daphnids were absent in summer and the macrophytes decreased.In Lake Zwemlust, Lake Væng and Lake Noorddiep the water remained clear during the first five years. In summer of the sixth year (1992) transparency decreased in Lake Zwemlust (with high P-concentration of 1.0 mg P l^-1). Also in Lake Væng (with a low nutrient concentration of 0.15 mg P.^-1) a short term turbid stage (1.5 month) occurred in summer 1992 after a sudden collapse of the macrophytes. Deterioration of the water quality seems to start in summer and seems related to a collapse in macrophytes. At a low planktivorous fishstock (e.g. Lake Væng)thhe duration of the turbid state is shorter. than in presence of a high planktivorous fish biomass (e.g. Lake Zwemlust, and later years of Lake Bleiswijkse Zoom).