Detrital Food Webs Research Articles

Two microcrustaceans affect microbial and macroinvertebrate‐driven litter breakdown

Summary Leaf litter degradation in fresh waters is a fundamental ecosystem process performed by a wide array of decomposers. The meiofauna is an important component of aquatic heterotrophic assemblages, which can provide a trophic link between plant detritus and associated microbial and macroinvertebrate communities, but their contribution to leaf breakdown remains poorly understood. We hypothesised that, through their feeding activity, microcrustaceans influence the structure of fungal assemblages and consequently microbially mediated litter breakdown. Litter‐associated microcrustaceans were predicted to change the pathways of energy transfer in the food web according to the positive (e.g. complementarity) or negative (e.g. predation) interactions with macroinvertebrate detritivore taxa. We evaluated experimentally in the laboratory, over 6 and 13 days, the potential contribution of two freshwater microcrustaceans (a cladoceran and a copepod) to litter breakdown in the presence of microfungi (aquatic hyphomycetes), with and without macroinvertebrate detritivores (a trichopteran and a gammarid amphipod). The presence of microcrustaceans enhanced leaf mass loss by 62 and 22% in treatments with fungi or trichopteran alone, respectively, while no significant effect was observed for treatments with the amphipod. Microcrustaceans strongly increased the production of fine particulate organic matter, particularly in treatments with fungi alone (+637%). The leaf consumption rate by the amphipod significantly decreased (−61%) at 13 days in the presence of microcrustaceans, likely due to predation on cladocerans. Our study supports the potential role of microcrustaceans in the detrital food web of streams and rivers. Interestingly, microcrustaceans may interact with microbial and macroinvertebrate decomposers in either positive or negative ways. Therefore, microcrustaceans add complexity to detrital food webs by increasing vertical diversity and modulating biotic interactions with important consequences for carbon and energy transfers in stream ecosystems.

Short‐term incorporation of freshly fixed plant carbon into the soil animal food web: field study in a spruce forest

AbstractWe analyzed the dynamics of the short‐term incorporation of recently fixed carbon into the below‐ground food web in a boreal forest. Five young spruce trees (Picea abies) were pulse‐labeled with13CO2and the isotopic label was traced in soil invertebrates during 5 weeks. The freshly fixed plant carbon quickly entered both litter‐located and soil‐located compartments of the detrital food web. Among invertebrates inhabiting the mineral soil layers, a trophic link to the root‐derived C was most pronounced in species with higherδ15N values, suggesting this energy source to be more important in deeper mineral soil horizons. The label appeared faster in saprophagous animals than in predators (the median time lag after labeling was 6 and 12 days, respectively), but the difference was not significant. The label was recovered in 15 of 38 species of saprophagous animals and in 20 of 63 species of predators. Among saprophages, the frequency and intensity of the label was relatively high in endogeic collembolans and in bibionid larvae, but earthworms and enchytraeids were not labeled. Several groups of predators, lithobiid centipedes in particular, quickly acquired the root‐derived carbon, possibly indicating the feeding on live roots or mycorrhizal mycelium. In total, only 35 % of species or genera examined acquired the label. This suggests that majority of invertebrate taxa in the decomposer food web are unlikely to depend heavily on freshly fixed plant carbon provided by roots and root‐associated microorganisms.

Enzymatic biomarkers can portray nanoCuO-induced oxidative and neuronal stress in freshwater shredders

Commercial applications of nanometal oxides have increased concern about their release into natural waters and consequent risks to aquatic biota and the processes they drive. In forest streams, the invertebrate shredder Allogamus ligonifer plays a key role in detritus food webs by transferring carbon and energy from plant litter to higher trophic levels. We assessed the response profiles of oxidative and neuronal stress enzymatic biomarkers in A. ligonifer after 96h exposure to nanoCuO at concentration ranges <LC30. To better understand the contribution of ionic form in nanoCuO-induced stress, Cu2+ released from nanoCuO was quantified and the enzymatic responses to Cu2+ exposure at similar effective concentrations were compared. The highest activities of superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione reductase (GR) were observed at concentrations <LC5. The enzymatic activities decreased at effective concentrations between LC10 and LC30. GR activity remained higher than in control at all concentrations. The activity of glutathione S-transferase (GST) increased whereas that of catalase (CAT) decreased at concentrations between LC10 and LC30. The response patterns suggested that antioxidant enzymes could prevent oxidative stress at low concentrations (<LC10) of nanoCuO, thereby contributing to the survival of A. ligonifer. At concentrations between LC10 and LC30, effects of nanoparticulate or released ionic copper on enzyme activities were concentration-dependent, and led to oxidative stress and even to animal death. The activity of acetylcholinesterase (AChE) was strongly inhibited even at concentrations <LC10, suggesting neuronal stress in A. ligonifer.

Predicted rainfall changes disrupt trophic interactions in a tropical aquatic ecosystem.

Changes in the distribution of rainfall and the occurrence of extreme rain events will alter the size and persistence of aquatic ecosystems. Such alterations may affect the structure of local aquatic communities in terms of species composition, and by altering species interactions. In many aquatic ecosystems, leaf litter sustains detrital food webs and could regulate the responses of communities to changes in rainfall. Few empirical studies have focused on how rainfall changes will affect aquatic communities and none have evaluated if basal resource diversity can increase resistance to such rainfall effects. In this study, we used water-holding terrestrial bromeliads, a tropical aquatic ecosystem, to test how predicted rainfall changes and litter diversity may affect community composition and trophic interactions. We used structural equation modeling to investigate the combined effects of rainfall changes and litter diversity on trophic interactions. We demonstrated that changes in rainfall disrupted trophic relationships, even though there were only minor direct effects on species abundance, richness, and community composition. Litter diversity was not able to reduce the impact of changes in rainfall on trophic interactions. We suggest that changes in rainfall can alter the way in which species interact with each other, decreasing the linkages among trophic groups. Such reductions in biotic interactions under climate change will have critical consequences for the functioning of tropical aquatic ecosystems.

Biomass or growth? How to measure soil food webs to understand structure and function

Food web links reflect the feeding rate of organisms and should thus capture the biomass production rate of the consumer. This information is rarely available for detrital food webs and has instead been inferred from biomass estimates to construct quantitative detrital food webs. Published method comparisons between biomass, growth and mineralisation show that microbial biomass is a poor predictor for process rates. This calls into question the current practise of parameterising soil detrital food webs. Emerging methods to estimate microbial growth rates and growth efficiencies are promising new avenues. If quantitative assessments of detrital food webs are revised by incorporating information that reflects the feeding rates of organisms, they could represent a powerful conceptual tool to investigate fundamental ecological theory, including how stability links to complexity and how function depends on the structure of whole food webs.

Consumption of aquatic subsidies by soil invertebrates in coastal ecosystems

Routes of aquatic allochthonous inputs (aquatic subsidies) to detrital food webs are studied, as is the effect of aquatic subsidies on the functional and taxonomic structure of soil invertebrate communities in coastal ecosystems. The study took place in the coastal zone of an oxbow lake of the Pra River in the Oka Reserve. The results indicate a strong dependence of soil animals in the coastal habitats on aquatic subsidies. Isotopic analysis shows that aquatic resources enter soil food webs not only via predators feeding on flying insects or aquatic prey, but also via saprophages decomposing organic debris of aquatic origin. The contribution of aquatic subsidies to the energy balance of soil invertebrates decreases rapidly with increasing distance from the lake. The fraction of aquatic carbon in tissues of collembolans and saprophages is negligible already a few meters from the water edge. The dependence of predatory invertebrates on aquatic resources can be traced at somewhat greater distance (tens of meters).

Aquatic hyphomycetes, benthic macroinvertebrates and leaf litter decomposition in streams naturally differing in riparian vegetation

Small forest streams and their riparian vegetation are closely linked ecosystems. Stream consumers obtain most of their energy from leaf litter provided by the terrestrial vegetation. Thus, understanding the relationship between riparian vegetation, aquatic communities and litter decomposition may help explaining the variability in aquatic communities and processes among non-impacted streams, and anticipate their responses to anthropogenic-induced changes in the riparian vegetation. We surveyed 10 small non-impacted forest streams in central Portugal for riparian vegetation (species richness), benthic litter (species richness and biomass), aquatic hyphomycete conidia in transport (species richness and conidia concentration) and macroinvertebrates associated with benthic litter (taxon richness, density and biomass), during the litter fall peak. We found significant correlations between (a) aquatic communities and riparian vegetation species richness, (b) aquatic communities and benthic litter species richness and biomass and (c) within aquatic communities. Oak litter decomposition rates (from a previous experiment on the same streams) were also correlated with riparian tree species richness. This survey showed that spatial variability in riparian vegetation, benthic litter, aquatic communities and litter decomposition can be high even within a relatively small area, and allowed the identification of complex interactions between these components of the aquatic detrital food web. The positive correlation between aquatic hyphomycete species richness, macroinvertebrate taxon richness, litter decomposition and riparian tree species richness suggests that anthropogenic-induced decreases in riparian species richness may affect aquatic communities and processes. Surveys over streams naturally differing in environmental conditions may allow forecasting the response of aquatic communities and processes to anthropogenic activities.

Functional redundancy dampens the trophic cascade effect of a web-building spider in a tropical forest floor

The trophic cascade effect of predators on ecosystem functioning is generally believed to be less frequent and weaker in detritus-based than primary producer-based food webs, in part because of functional redundancy among soil fauna. Despite this view, no empirical studies have explicitly examined roles of different soil fauna within trophic levels in mediating cascading effects of predators in detritus food webs. Here we manipulated the density of a dominant funnel-web building spider Macrothele yunnanica in permanent plots (1 m2) for one year. Three spider treatments were applied: 0 spiders, 6 spiders (natural density) and 10 spiders (high density). We found that although changes in spider densities caused large shifts in litter-dwelling Collembola community composition on average, modifying spider densities did not generate a trophic cascade effect and alter litter decomposition in litter bags with coarse mesh (2 mm). Our data supports the hypothesis that functional redundancy among Collembola species may weaken the strength of spider-initiated cascading effects. Consequently, changes in Collembola diversity occupying the same trophic level may not significantly alter ecosystem function in tropical forest-floor ecosystems.

Linking the green and brown worlds through nonconsumptive predator effects

Predators can indirectly affect lower trophic levels by either consuming their prey (consumptive effect, CE) or by changing the physiology or behavior of their prey (nonconsumptive effect, NCE). Cascading effects of predators on primary producers are common, and can be propagated by CEs, NCEs, or a combination of both mechanisms.Predator impacts in detrital food webs (the ‘brown world’) have received considerably less attention than their effects on systems with primary producers at the base (the ‘green world’), and only recently have we begun to appreciate the importance of above‐ground predators indirectly impacting below‐ground processes.Numerous studies reveal the total impact (CEs and NCEs) of predators in brown food webs, but our understanding of the role of isolated NCEs is limited. Many habitats and major taxa have not been studied, and patterns are difficult to distinguish due to frequent reporting of mixed effects.Predators play an important role as connectors between brown and green worlds when they feed from both food webs (multichannel feeding). We are only beginning to understand how NCEs influence detrital food webs, and it is unknown whether multichannel fear is an essential component of predator–prey ecology that regulates ecosystem function.SynthesisPredators have been shown to impact ecosystems through both consumptive and nonconsumptive effects on their prey Historically, herbivory‐based ‘green’ systems have been the venue for documenting these predator effects, while detritus‐based ‘brown’ systems received considerably less attention. However, similar mechanisms exist in green and brown worlds, suggesting strong parallels. We review and synthesize predator effects in detrital systems, highlighting important shortcomings in current understanding. Furthermore, we build upon the idea of multichannel feeding (i.e. consumption of prey from both green and brown food webs) to propose the existence of ‘multichannel fear’. We provide a framework for documenting multichannel fear to facilitate continued exploration of how predators link seemingly disparate systems.

Invasive plants have different effects on trophic structure ofgreen and brown food webs in terrestrial ecosystems: a meta-analysis.

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta-analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.

Epidermal shedding in Ascophyllum nodosum (Phaeophyceae): seasonality, productivity and relationship to harvesting

:Epidermal shedding, a process that effectively controls epiphyte accumulation on certain long-lived macroalgae, was quantified weekly for over 1 year in a population of Ascophyllum nodosum from Nova Scotia. About 25% of the frond epidermis is shed per week and for at least 9 months of the year, this shedding occurred in population-level cycles of skin shedding followed by reaccumulation of epiphytes. Using a geometric model based on cross-sectional area of branches assigned to one of four size categories, we determined that about 1% of frond biomass is shed during each monthly cycle. Taking into account the differences in frond architecture at 10 sites from around Nova Scotia, an estimated 10% of vegetative frond biomass enters coastal waters each year solely as a result of epidermal shedding. This natural contribution to coastal detritus amounts to about 2 t ha−1 of Ascophyllum beds in Lobster Bay, the primary harvest region for Ascophyllum in Nova Scotia. This is a significant and previously unconsidered addition to the environmental impact of the annual harvest of over 20 t ha−1 that is reported by industry from this area. These data form part of a wider assessment of the impact of the commercial harvest of Ascophyllum on biomass removal from coastal detrital food webs.

A Conceptual Model of the Aquatic Food Web of the Upper San Francisco Estuary

Aquatic trophic interactions in the upper San Francisco Estuary are synthesized here as a conceptual food web model, using over 35 years of scientific research, and highlighting key uncertainties for restoration. The food web was created as part of the Delta Regional Ecosystem Restoration Implementation Program to evaluate the benefits of restoration actions. Historic changes to the hydrology and geomorphology of the region have decreased ecosystem resiliency. More recently, pressures from water export, alien species introductions, and nutrient loading have disrupted the food web and increased the vulnerability of pelagic and juvenile fishes. One of the key features of the contemporary food web is a decoupling of pelagic and the detrital pathways. Low production and high mortality of phytoplankton since the 1980s have led to declines of pelagic organisms, including zooplankton, mysids, and planktivorous fish. In contrast, detrital pathways support abundant epibenthic invertebrates, such as amphipods and crayfish, which have become a dominant food source for adult demersal and piscivorous fish. Fishes that are obligate to the pelagic web will likely continue to decline, although fishes able to use the detrital pathway may be more robust. Fishes with pelagic larvae may be vulnerable to recruitment failures if they are unable to obtain planktonic food during the critical period of their ontological development. Options for increasing pelagic production at large scales are limited, but may include management of clams, nutrient ratios, and off-channel habitat subsidies. Restorations at small to intermediate scales may produce pelagic food, but volumetric constraints will limit the extent of subsidies. Creating spatial opportunities where pelagic and detrital food webs can re-integrate may offer some opportunities for local recruitment, and species able to use localized detritally-based webs will benefit strongly from such activities.

From water to land: How an invasive clam may function as a resource pulse to terrestrial invertebrates

Resource pulses are episodes of low frequency, large magnitude and short duration that result in increased resource availability in space and time, with consequences for food web dynamics. Studies assessing the importance of resource pulses by invasive alien species in the interface between terrestrial and aquatic ecosystems are rare, especially those in the direction from water to land. This study assessed the importance of massive die-offs of the Asian clam Corbicula fluminea (Müller, 1774) as a resource pulse to the terrestrial invertebrate community after an extreme climatic event using a manipulative experiment. We used 5 levels of C. fluminea density (0, 100, 500, 1000 and 2000ind·m−2), with terrestrial invertebrates being censused 7, 30 and 90days after C. fluminea addition. We also assessed the possible effect of plots position, where plots that delimited the experiment were assigned as edge plots and the remaining as core plots. Clear differences were detected in abundance, biomass, richness and diversity of terrestrial invertebrates depending on the C. fluminea density, time and position. Interestingly, the highest abundance of adult Diptera was observed 7days after C. fluminea addition, whereas that of the other terrestrial invertebrates was on day 30, both with C. fluminea densities higher than 500ind·m−2 located on the edge of the experimental design. This study highlights the importance of major resource pulses after massive die-offs of invasive bivalves, contributing with remarkable amounts of carrion for adjacent terrestrial systems. Part of this carrion can be consumed directly by a great number of invertebrate species while the remainder can enter the detrital food web. Given the high density and biomass attained by several invasive bivalves worldwide and the predicted increase in the number, intensity and magnitude of extreme climatic events, the ecological importance of this phenomenon should be further investigated.

Revisiting the hypothesis that fungal‐to‐bacterial dominance characterizes turnover of soil organic matter and nutrients

Resolving fungal and bacterial groups within the microbial decomposer community is thought to capture disparate life strategies for soil microbial decomposers, associating bacteria with an r‐selected strategy for carbon (C) and nutrient use, and fungi with a K‐selected strategy. Additionally, food‐web model‐based work has established a widely held belief that the bacterial decomposer pathway in soil supports high turnover rates of easily available substrates, while the slower fungal‐dominated decomposition pathway supports the decomposition of more complex organic material, thus characterizing the biogeochemistry of the ecosystem. We used a field experiment, the Detritus Input and Removal Treatments, or DIRT, experiment (Harvard Forest Long‐Term Ecological Research Site, USA) where litter and root inputs (control, no litter, double litter, or no tree roots) have been experimentally manipulated during 23 years, generating differences in soil C quality. We hypothesized (1) that δ13C enrichment would decrease with higher soil C quality and that a higher C quality would favor bacterial decomposers, (2) that the C mineralized in fungal‐dominated treatments would be of lower quality and also depleted in δ13C relative to bacterial‐dominated high‐quality soil C treatments, and (3) that higher C mineralization along with higher gross N mineralization rates would occur in bacterial‐dominated treatments compared with more fungal‐dominated treatments. The DIRT treatments resulted in a gradient of soil C quality, as shown by up to 4.5‐fold differences between the respiration per soil C between treatments. High‐quality C benefited fungal dominance, in direct contrast with our hypothesis. Further, there was no difference between the δ13CO2 produced by a fungal compared with a bacterial‐dominated decomposer community. There were differences in C and N mineralization between DIRT treatments, but these were not related to the relative dominance of fungal and bacterial decomposers. Thus we find no support for the hypothesized differences between detrital food webs dominated by bacteria compared to those dominated by fungi. Rather, an association between high‐quality soil C and fungi emerges from our results. Consequently there is a need to revise our basic understanding for microbial communities and the processes they regulate in soil.

Ecological Dynamics of Two Distinct Viruses Infecting Marine Eukaryotic Decomposer Thraustochytrids (Labyrinthulomycetes, Stramenopiles).

Thraustochytrids are cosmopolitan osmotrophic or heterotrophic microorganisms that are considered as important decomposers in coastal ecosystems. However, because of a lack of estimation method for each genus or systematic group of them, relatively little is known about their ecology in situ. Previously, we reported two distinct types of virus infecting thraustochytrids (AuRNAV: reported as SssRNAV, and SmDNAV) suggesting they have wide distributions in the host-virus systems of coastal environments. Here we conducted a field survey from 2004 through 2005 to show the fluctuation pattern of thraustochytrids and their viruses in Hiroshima Bay, Japan. During the field survey, we monitored the dynamics of the two types of thraustochytrid-infecting virus: small viruses causing lysis of Aurantiochytrium sp. NIBH N1-27 (identified as AuRNAV) and the large viruses of Sicyoidochytrium minutum NBRC 102975 (similar to SmDNAV in physiology and morphology). Fluctuation patterns of the two distinct types of virus were different from each other. This may reflect the difference in the preference of organic substrates; i.e., it may be likely the host of AuRNAV (Aurantiochytrium sp.) increases utilizing algal dead bodies or feeble cells as the virus shows a large increase in abundance following raphidophyte blooms; whereas, the trophic nutrient supply for S. minutum may primarily depend on other constantly-supplied organic compounds because it did not show any significant change in abundance throughout the survey. Further study concerning the population composition of thraustochytrids and their viruses may demonstrate the microbial ecology (especially concerning the detrital food web) of marine environments.

Do alterations in mesofauna community affect earthworms?

Interactions between the saprotrophic animal groups that strongly control soil microbial activities and the functioning of detrital food webs, such as earthworms and mesofauna, are not well understood. Earthworm trophic and engineering activities strongly affect mesofauna abundance and diversity through various direct and indirect pathways. In contrast, mesofauna effects on earthworm populations are less evident; however, their importance may be high, considering the keystone significance of earthworms for the functioning of the soil system. We studied effects of a diverse mesofauna community of a deciduous forest on two earthworm species representing epigeic (Lumbricus rubellus) and endogeic (Aporrectodea caliginosa) ecological groups. In microcosms, the density of total mesofauna or its separate groups (enchytraeids, collembolans, gamasid mites) was manipulated (increased) and responses of earthworms and soil systems were recorded. A rise in mesofauna density resulted in a decrease of biomass and an increased mortality in L. rubellus, presumably due to competition with mesofauna for litter resources. In contrast, similar mesofauna manipulations promoted reproduction of A. caliginosa, suggesting a facilitated exploitation of litter resources due to increased mesofauna activities. Changes of microcosm respiration rates, litter organic matter content and microbial activities across the manipulation treatments indicate that mesofauna modify responses of soil systems in the presence of earthworms. However, similar mesofauna manipulations could induce different responses in soil systems with either epigeic or endogeic lumbricids, which suggests that earthworm/mesofauna interactions are species-specific. Thus, mesofauna impacts should be treated as a factor affecting the engineering activities of epigeic and endogeic earthworms in the soil.

Stable isotope composition (δ(13)C and δ(15)N values) of slime molds: placing bacterivorous soil protozoans in the food web context.

Data on the bulk stable isotope composition of soil bacteria and bacterivorous soil animals are required to estimate the nutrient and energy fluxes via bacterial channels within detrital food webs. We measured the isotopic composition of slime molds (Myxogastria, Amoebozoa), a group of soil protozoans forming macroscopic spore-bearing fruiting bodies. An analysis of largely bacterivorous slime molds can provide information on the bulk stable isotope composition of soil bacteria. Fruiting bodies of slime molds were collected in a monsoon tropical forest of Cat Tien National Park, Vietnam, and analyzed by continuous-flow isotope ratio mass spectrometry. Prior to stable isotope analysis, carbonates were removed from a subset of samples by acidification. To estimate the trophic position of slime molds, their δ(13) C and δ(15) N values were compared with those of plant debris, soil, microbial destructors (litter-decomposing, humus-decomposing, and ectomycorrhizal fungi) and members of higher trophic levels (oribatid mites, termites, predatory macroinvertebrates). Eight species of slime molds represented by at least three independent samples were 3-6‰ enriched in (13) C and (15) N relative to plant litter. A small but significant difference in the δ(13) C and δ(15) N values suggests that different species of myxomycetes can differ in feeding behavior. The slime molds were enriched in (15) N compared with litter-decomposing fungi, and depleted in (15) N compared with mycorrhizal or humus-decomposing fungi. Slime mold sporocarps and plasmodia largely overlapped with oribatid mites in the isotopic bi-plot, but were depleted in (15) N compared with predatory invertebrates and humiphagous termites. A comparison with reference groups of soil organisms suggests strong trophic links of slime molds to saprotrophic microorganisms which decompose plant litter, but not to humus-decomposing microorganisms or to mycorrhizal fungi. Under the assumption that slime molds are primarily feeding on bacteria, the isotopic similarity of slime molds and mycophagous soil animals indicates that saprotrophic soil bacteria and fungi are similar in bulk isotopic composition.

The DIRT Experiment

Resolving carbon use by fungi and bacteria is thought to capture disparate life strategies for soil microbial decomposers, associating bacteria with a fast but leaky use of carbon (C) and nutrients, and fungi with a slow but efficient use. We tested these ideas using the Detritus Input and Removal Treatments (DIRT) experiment, where litter and root inputs had been experimentally manipulated during 23 years, generating differences in soil C quality. We find no support for the presumed differences between fungal- and bacterial-dominated detrital food webs in these soils, calling for a revision of our basic understanding of microbial communities and the processes they regulate. The different litter manipulation treatments of the DIRT experiment. Photo by Mel Knorr. These photographs illustrate the article ”Revisiting the hypothesis that fungal-to-bacterial dominance characterizes turnover of soil organic matter and nutrients” by Johannes Rousk and Serita D. Frey, tentatively scheduled to appear in Ecological Monographs 85(3), August 2015. http://dx.doi.org/10.1890/14-1796.1

The sign of cascading predator effects varies with prey traits in a detrital system.

Theory and experiments show that the nature of 'green' trophic cascades, between predators, herbivores and plants, varies with several key species traits: predator hunting mode and predator and prey habitat domains. Meanwhile, 'brown' cascades between predators, environment-modifying detritivores and plants have been largely overlooked and the roles of species traits, particularly prey traits, in determining the nature of these cascades remains unclear. We hypothesize that, in predator-detritivore-plant interaction chains, the burrowing ability of plant-facilitating detritivores determines their response to predators and thus the sign of indirect effect transmitted. In the dung-decomposer food web of an alpine meadow, we predicted that in the presence of above-ground predacious beetles: (i) non-burrowing detritivores will suffer mortality due to predation and transmit negative indirect effects to plants, whereas (ii) burrowing detritivores will escape predation by retreating deeper into the soil, transmitting positive indirect effects to plants. In support of predictions, experiments showed that a single species of predacious beetle (i) reduced the density of the non-burrowing species and indirectly reduced dung loss rate, soil nutrient concentrations and plant biomass, but (ii) drove the burrowing species deeper, indirectly improved soil conditions and increased plant biomass. These results show that the burrowing ability of a detritivore can determine whether it transmits a negative indirect effect mediated by a reduction in its density, or a positive indirect effect mediated by its behavioural response to predation risk. We call for further tests of our detritivore-trait hypothesis in different regions and ecosystems to further develop a general trait-based framework for trophic cascades in detrital food webs. We further advance the general hypothesis that the locomotion traits of prey species (e.g. burrowing/flying ability) may help explain their behavioural response to predation risk and the nature of indirect effect they transmit from predators to plants.

CAN Canopy Addition of Nitrogen Better Illustrate the Effect of Atmospheric Nitrogen Deposition on Forest Ecosystem?

Increasing atmospheric nitrogen (N) deposition could profoundly impact community structure and ecosystem functions in forests. However, conventional experiments with understory addition of N (UAN) largely neglect canopy-associated biota and processes and therefore may not realistically simulate atmospheric N deposition to generate reliable impacts on forest ecosystems. Here we, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. UAN and reviewed the merits and pitfalls of the two approaches. The following hypotheses will be tested: i) UAN overestimates the N addition effects on understory and soil processes but underestimates those on canopy-associated biota and processes, ii) with low-level N addition, CAN favors canopy tree species and canopy-dwelling biota and promotes the detritus food web, and iii) with high-level N addition, CAN suppresses canopy tree species and other biota and favors rhizosphere food web. As a long-term comprehensive program, this experiment will provide opportunities for multidisciplinary collaborations, including biogeochemistry, microbiology, zoology, and plant science to examine forest ecosystem responses to atmospheric N deposition.