Feeding of biofilm-dwelling nematodes examined using HPLC-analysis of gut pigment contents

The natural feeding behaviour of the nematodes Chromadorina bioculata (Schultze in Carus 1857) and Chromadorina viridis (Linstow 1876) was studied in situ, within epilithic biofilms of the Garonne River (France). Based on their feeding-type characteristics and population dynamics, it was hypothesised that these species feed selectively on microphytobenthos (MPB) within the biofilm, and that among MPB groups, diatoms are preferred. High-performance liquid chromatography (HPLC) was used for separation, identification and quantification of pigments both in nematode guts and in the biofilm. This is the first time that nematode gut pigment contents were examined under natural conditions. Diatoms dominated the MPB which also comprised cyanobacteria and green microalgae. The comparison between chlorophyll a content in nematode guts versus in the biofilm showed that C. bioculata and C. viridis fed opportunistically (non-selectively) on MPB within the biofilm. Only diatom biomarker pigments were found in nematode guts suggesting that they could preferentially fed on diatoms among MPB groups. However, the non-detection of biomarker pigments for other microphyte groups could be also linked to HPLC detection limits. It was estimated that Chromadorina nematodes daily ingested on average 0.03–0.67% of the MPB standing stock. This grazing covered only a small part of their energetic requirements, suggesting that besides MPB they probably also fed on other biofilm food sources. Some considerations on the applicability of the HPLC gut pigment analysis technique for the examination of nematode feeding are also presented.     

Bacterial Community Succession in Natural River Biofilm Assemblages

Temporal bacterial community changes in river biofilms were studied using 16S rRNA gene-based polymerase chain reaction–denaturing gradient gel electrophoresis (DGGE) followed by sequence analysis. Naturally occurring biofilms were sampled in 2001 during an undisturbed 7-month low-water period in the River Garonne (SW France). During the sampling period epilithic biomass exhibited a particular pattern: two 3-month periods of accumulation that resulted in two peaks in summer and fall, each at about 25 g ash-free dry mass per square meter. Bacterial community DGGE profiles differed between the summer and fall biomass peaks and shared only 30% common operational taxonomic units (OTUs), suggesting the influence of seasonal factors on these communities. During the second biomass accrual phase, bacterial richness and the appearance of new OTUs fitted a conceptual model of bacterial biofilm succession. During succession, five OTUs (corresponding to Dechloromonas sp., Nitrospira sp., and three different Spirosoma spp.) exhibited particular patterns and were present only during clearly defined successional stages, suggesting differences in life-history strategies for epilithic bacteria. Co-inertia analysis of DGGE banding patterns and physical–chemical data showed a significant relationship between community structure and environmental conditions suggesting that bacterial communities were mainly influenced by seasonal changes (temperature, light) and hydrodynamic stability. Within the periods of stability, analysis of environmental variables and community patterns showed the dominant influence of time and maturation on bacterial community structure. Thus, succession in these naturally occurring epilithic biofilm assemblages appears to occur through a combination of allogenic (seasonal) and autogenic changes.     

Understorey benthic microalgae and their consumers depend on habitat complexity and light in a microtidal coastal ecosystem

Presence of habitat-forming macroalgae is supposed to mitigate effects of altered resources on benthic microalgae and their consumers. In a field experiment in a microtidal area of the Western Baltic Sea, we tested the interactive effects of nutrient enrichment, artificial shading, and habitat complexity on microalgal biomass and diversity as well as invertebrate abundance and richness. Habitat complexity comprised three levels, the presence of macroalgal canopy of Fucus vesiculosus, the presence of macroalgal propagules, and the absence of both (=control). Microalgal biomass (and richness) was significantly reduced by canopy presence (−88%, compared to control) and shading (−42%), with the highest biomass in the absence of both canopy and macroalgal propagules at ambient light. Within the microalgal assemblage, higher biomass was related to lower evenness (higher dominance). Density of two main invertebrate groups (snails and amphipods) strongly increased with canopy presence (on average from 53 to 154 individuals m⁻² stone area for snails, and from 234 to 1203 individuals m⁻² for amphipods) and so did invertebrate richness (from 4.3 to 10.3). Additionally, snail density doubled with increasing light availability. The snail responses to light and canopy were independent, the former relating to higher availability of microalgal prey, the latter to more structure. Microalgal taxon richness and biomass decreased with increasing invertebrate richness and with density of snails and amphipods. Our experiment thus showed that the presence of habitat-forming macroalgae alters biomass and diversity across trophic levels in benthic coastal communities as well as their response to resource manipulations.     

Biodiversity acts as insurance of productivity of bacterial communities under abiotic perturbations

Anthropogenic disturbances are detrimental to the functioning and stability of natural ecosystems. Critical ecosystem processes driven by microbial communities are subjected to these disturbances. Here, we examine the stabilizing role of bacterial diversity on community biomass in the presence of abiotic perturbations such as addition of heavy metals, NaCl and warming. Bacterial communities with a diversity gradient of 1–12 species were subjected to the different treatments, and community biomass (OD₆₀₀) was measured after 24 h. We found that initial species richness and phylogenetic structure impact the biomass of communities. Under abiotic perturbations, the presence of tolerant species in community largely contributed in community biomass production. Bacterial diversity stabilized the biomass across the treatments, and differential response of bacterial species to different perturbations was the key reason behind these effects. The results suggest that biodiversity is crucial for maintaining the stability of ecosystem functioning and acts as ecological insurance under abiotic perturbations. Biodiversity in natural ecosystems may also uphold the ecosystem functioning under anthropogenic disturbance.     

Experimental evidence that the ecosystem effects of aquatic herbivory by moose and beaver may be contingent on water body type

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Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

An insight into the feeding ecology of deep-sea canyon nematodes — Results from field observations and the first in-situC feeding experiment in the Nazaré Canyon

Submarine canyon systems provide a heterogeneous habitat for deep-sea benthos in terms of topography, hydrography, and the quality and quantity of organic matter present. Enhanced meiofauna densities as found in organically enriched canyon sediments suggest that nematodes, as the dominant metazoan meiobenthic taxon, may play an important role in the benthic food web of these sediments. Very little is known about the natural diets and trophic biology of deep-sea nematodes, but enrichment experiments can shed light on nematode feeding selectivity and trophic position. An in-situ pulse-chase experiment (Feedex) was performed in the Nazaré Canyon on the Portuguese margin in summer 2007 to study nematode feeding behaviour. ¹³C-labelled diatoms and bacteria were added to sediment cores which were then sampled over a 14-day period. There was differential uptake by the nematode community of the food sources provided, indicating selective feeding processes. ¹³C isotope results revealed that selective feeding was less pronounced at the surface, compared to the sediment subsurface. This was supported by a higher trophic diversity in surface sediments (Θ⁻¹ = 3.50 ± 0.2) compared to the subsurface (2.78 ± 0.6), implying that more food items may be used by the nematode community at the sediment surface. Predatory and scavenging nematodes contributed relatively more to biomass than other feeding types and can be seen as key contributors to the nematode food web at the canyon site. Non-selective deposit feeding nematodes were the dominant trophic group in terms of abundance and contributed substantially to total nematode biomass. The high levels of ‘fresh’ (bioavailable) organic matter input and moderate hydrodynamic disturbance of the canyon environment lead to a more complex trophic structure in canyon nematode communities than that found on the open continental slope, and favours predator/scavengers and non-selective deposit feeders.     

Monitoring Soil Ecosystem Recovery Following Bioremediation of a Terrestrial Crude Oil Spill With and Without a Fertilizer Amendment

The effect of fertilizer as an amendment in the bioremediation of a terrestrial crude oil spill has been investigated in terms of the subsequent recovery of the soil ecosystem following bioremediation. Two different spills in the same area with different initial hydrocarbon concentrations (33,500 mg kg^-1 and 4,800 mg kg^-1) were compared. At the higher initial hydrocarbon concentration fertilizer addition increased the rate of bioremediation (first-order rate constant of 0.0033 days-1 with fertilizer amendment vs. 0.0020 days-1 without) and resulted in more rapid recovery of soil bacteria (numbers, community structure, diversity) and nematodes (trophic diversity and community structure). The effect of the fertilizer amendment was more significant at the higher initial concentration of crude oil hydrocarbons, presumably due to greater depletion of soil nutrient pools in the absence of the amendment. A second objective of this work was to identify sensitive and cost-effective ecological indicators useful for monitoring the recovery of soil ecosystems impacted by crude oil. Ecological indicators used included: microbial numbers, community structure, and activity as revealed by biomarker analysis (phospholipid fatty acids); nitrogen availability; nematode numbers and community structure (trophic groups and colonizer-persister classes); and ultimately, plant cover and diversity. All ecological indicators investigated were sensitive to disturbances in the soil food web in a hydrocarbon-impacted site. However, nematode community structure analysis offered the greatest sensitivity coupled with low cost and readily available sources for the analysis.     

Nematode biomass changes along an elevational gradient are trophic group dependent but independent of body size

Aboveground, large and higher trophic-level organisms often respond more strongly to environmental changes than small and lower trophic-level organisms. However, whether this trophic or size-dependent sensitivity also applies to the most abundant animals, microscopic soil-borne nematodes, remains largely unknown. Here, we sampled an altitudinal transect across the Tibetan Plateau and applied a community-weighted mean (CWM) approach to test how differences in climatic and edaphic properties affect nematode CWM biomass at the level of community, trophic group and taxon mean biomass within trophic groups. We found that climatic and edaphic properties, particularly soil water-related properties, positively affected nematode CWM biomass, with no overall impact of altitude on nematode CWM biomass. Higher trophic-level omnivorous and predatory nematodes responded more strongly to climatic and edaphic properties, particularly to temperature, soil pH, and soil water content than lower trophic-level bacterivorous and fungivorous nematodes. However, these differences were likely not (only) driven by size, as we did not observe significant interactions between climatic and edaphic properties and mean biomasses within trophic groups. Together, our research implies a stronger, size-independent trophic sensitivity of higher trophic-level nematodes compared with lower trophic-level ones. Therefore, our findings provide new insights into the mechanisms underlying nematode body size structure in alpine grasslands and highlight that traits independent of size need to be found to explain increased sensitivity of higher trophic-level nematodes to climatic and edaphic properties, which might affect soil functioning.     

乌拉特荒漠草原红砂(Reaumuria soongorica)灌丛林地地面节肢动物群落季节分布特征

以乌拉特荒漠草原红砂(Reaumuria soongorica)灌丛林地作为研究样地,分别于2018年春季、夏季和秋季,利用国际通用的陷阱诱捕法,调查了地面节肢动物群落季节动态分布特征及其与环境因子间的关系。结果显示:(1)3个季节内共捕获地面节肢动物560只,隶属于10目23科。其中,春季优势类群为拟步甲科,夏季为拟步甲科和蚁科,秋季为拟步甲科,其个体数分别占总个体数的51.54%、59.11%和64.88%;常见类群春季和夏季均有8类,秋季有10类,其个体数分别占总个体数的43.08%,37.33%和32.68%;稀有类群春季有7类,夏季有6类,秋季有3类,其个体数分别占总个体数的5.38%、3.56%和2.44%。(2)3个季节内生物量占比超过10%的地面节肢动物类群仅有拟步甲科,春季、夏季和秋季其分别占群落总生物量的88.59%、72.56%和80.90%;1%—10%的地面节肢动物类群生物量,春季、夏季和秋季其分别占群落总生物量的9.76%、25.51%和17.78%;小于1%的地面节肢动物类群生物量,春季、夏季和秋季其分别占群落总生物量的1.62%、1.94%和1.32%。(3)随季节变化,地面节肢动物的密度、类群数、Shannon-Wiener指数和Simpson指数均存在显著差异性(P0.05),表现为夏季显著高于春季或秋季。(4)RDA和Spearman相关分析结果表明,降雨量、气温、土壤含水量、土壤pH和电导率及土壤粒径组成是地面节肢动物季节分布的重要影响因素。研究表明,随着季节更替乌拉特荒漠草原红砂灌丛林地地面节肢动物群落组成季节分布差异性较大,夏季地面节肢动物多样性较高,而且不同类型环境因子对于地面节肢动物的个体数分布影响不同。     

Abiotic mosaics affect seasonal variation of plant resources and influence the performance and mortality of a leaf-miner in Gambel’s oak (Quercus gambelii, Nutt.)

Abstract Conceptual models of terrestrial trophic dynamics have emphasized the potential influence of various abiotic factors, though empirical studies have found generalities to be scarce. Progress may result through an increased use of experimental gradients in tandem with existing, natural gradients of abiotic factors that are more difficult to manipulate. Along an elevation/climatic gradient, a fertilization experiment was conducted to examine the impact of environmentally induced variation in foliar nitrogen on tri-trophic interactions for Phyllonorycter sp., a leaf-miner (Lepidoptera: Gracillariidae), in Rocky Mountain white oak, Quercus gambelii. Microclimate determined the relative effect of host-plant and natural-enemy effects for Phyllonorycter sp. Cooler, more humid microclimates resulted in a higher biomass payoff (total biomass/density). Further, increased seasonal variation in foliar nitrogen content at warmer, drier sites significantly increased parasitism rates. Fertilization treatments increased foliar nitrogen content and resulted in increased early-instar mortality. Fertilization treatment also acted in a non-linear fashion with microclimate to influence spatial patterns in parasitism rates. Overall, microclimate was found to affect characteristics of each trophic level, including host-plant nitrogen dynamics, Phyllonorycter sp. performance, and parasitism rates. Additionally, nutrient availability altered patterns of parasitoid-related mortality in Phyllonorycter sp. within microclimates. These results suggest that local topographic variation in combinations of abiotic factors, or abiotic mosaics, has important effects for spatial patterns of tri-trophic interactions.     

Upward cascading effects of nutrients: shifts in a benthic microalgal community and a negative herbivore response

We evaluated the effects of nutrient addition on interactions between the benthic microalgal community and a dominant herbivorous gastropod, Cerithidea californica (California horn snail), on tidal flats in Mugu Lagoon, southern California, USA. We crossed snail and nutrient (N and P) addition treatments in enclosures on two tidal flats varying from 71 to 92% sand content in a temporally replicated experiment (summer 2000, fall 2000, spring 2001). Diatom biomass increased slightly (~30%) in response to nutrient treatments but was not affected by snails. Blooms of cyanobacteria (up to 200%) and purple sulfur bacteria (up to 400%) occurred in response to nutrient enrichment, particularly in the sandier site, but only cyanobacterial biomass decreased in response to snail grazing. Snail mortality was 2–5 times higher in response to nutrient addition, especially in the sandier site, corresponding to a relative increase in cyanobacterial biomass. Nutrient-related snail mortality occurred only in the spring and summer, when the snails were most actively feeding on the microalgal community. Inactive snails in the fall showed no response to nutrient-induced cyanobacterial growths. This study demonstrated strongly negative upward cascading effects of nutrient enrichment through the food chain. The strength of this upward cascade was closely linked to sediment type and microalgal community composition.     

Relation between nematode communities and trophic state in southern Swedish lakes

The aim of this study was to examine whether littoral nematode community patterns are shaped by lake trophic state. It was hypothesized that trophic level is associated negatively with the proportion of omnivores and positively with the percentages of bacterial feeders, but not at all with the diversity, abundance, and biomass of freshwater nematodes. Sediment samples were taken at littoral sites of eight southern Swedish lakes of different trophy in spring and autumn 2007. Trophic level was found to strongly influence species richness, as oligotrophic and mesotrophic lakes supported the greatest species numbers, whereas nematode abundance, biomass, and Shannon index were unaffected. Furthermore, our results indicated effects on the nematode community’s trophic structure, with a larger proportion of predatory nematodes in oligotrophic and mesotrophic lakes but no differences in the other feeding types (bacteria, algae and suction feeders, omnivorous species). Multivariate analysis indicated a shift in species compositions along the threshold from mesotrophic to eutrophic conditions, with the presence of Tobrilus gracilis, Monhystera paludicola, Brevitobrilus stefanskii, and Ethmolaimus pratensis related to the latter. Nematode communities in oligotrophic and mesotrophic lakes were characterized by a similar species composition, with pronounced occurrences of Eumonhystera longicaudatula, Semitobrilus cf. pellucidus, Prodesmodora circulata, and Rhabdolaimus terrestris. Overall, the results suggested that lake trophic state is a major factor structuring littoral nematode communities, although intra-lake variations might be of importance as well.     

Biofilm formation at warming temperature: acceleration of microbial colonization and microbial interactive effects

River biofilms that grow on wet benthic surface are mainly composed of bacteria, algae, cyanobacteria and protozoa embedded in a polysaccharide matrix. The effects of increased river water temperature on biofilm formation were investigated. A laboratory experiment was designed employing two temperatures (11.1-13.2°C, night-day; 14.7-16.0°C, night-day) and two nutrient levels (0.054 mg P l(-1), 0.75 mg N l(-1); 0.54 mg P l(-1), 7.5 mg N l(-1)). Biofilm formation at the higher temperature was faster, while the biomass of the mature biofilm was mainly determined by nutrient availability. The specific response of the three microbial groups that colonized the substrata (algae, bacteria and ciliates) was modulated by interactions between them. The greater bacterial growth rate and earlier bacterial colonization at the higher temperature and higher nutrient status was not translated into the accrual of higher bacterial biomass. This may result from ciliates grazing on the bacteria, as shown by an earlier increase in peritrichia at higher temperatures, and especially at high nutrient conditions. Temperature and ciliate grazing might determine the growth of a distinctive bacterial community under warming conditions. Warmer conditions also produced a thicker biofilm, while functional responses were much less evident (increases in the heterotrophic utilization of polysaccharides and peptides, but no increase in primary production and respiration). Increasing the temperature of river water might lead to faster biofilm recolonization after disturbances, with a distinct biofilm community structure that might affect the trophic web. Warming effects would be expected to be more relevant under eutrophic conditions.     

The sea-ice algal community of seasonal pack ice in the southwestern Kara Sea in late winter

The composition and abundance of ice algae have been studied in a 64-cm core of seasonal pack ice taken in the southwestern Kara Sea in February 1996. The cell numbers, biomass and taxonomic diversity of the ice microalgal assemblages were rather low, and there was a predominance of diatoms. The comparison of its composition with that of the pelagic microalgal community demonstrated a great difference between them. The ecological function of the seasonal ice habitat as a depository for seeding material of the early-spring pelagic diatom is suggested. Accepted: 7 August 2000     

根际效应对大豆田土壤线虫群落组成及多样性的影响

根际作为重要的环境界面是植物与环境之间物质能量交换的场所,关于根际效应的研究已成为土壤生态学的新兴热点领域,然而有关大豆根际效应对土壤动物多样性影响的研究报道并不多见。在三江平原选择连续耕作15a的大豆田,对大豆根际区与非根际区土壤线虫群落结构组成进行了对比分析。结果表明:大豆根际区土壤线虫总数、辛普森多样性指数(Dom)显著高于非根际区,根际区的物种数(S)、物种丰富度指数(SR)显著低于非根际区。说明大豆根际效应增加土壤线虫的丰度,但降低了线虫群落结构的复杂性。大豆根际区植物寄生线虫(PP)、食真菌线虫(FF)和食细菌线虫(BF)数量显著高于非根际区,而PP类群的比例在根际区却显著低于非根际区。这一研究结果表明食微线虫(FF和BF)类群在大豆根际区的比例增加更显著。食真菌与食细菌线虫数量比值(F/B)指示大豆根际区细菌生物量相对高于真菌生物量。研究结果丰富了农田土壤线虫多样性的研究内容,并为我国东北大豆田线虫病害的防治及定制相应的农业管理措施提供参考。     

Intertidal meiofauna of a high-latitude glacial Arctic fjord (Kongsfjorden, Svalbard) with emphasis on the structure of free-living nematode communities

Meiofauna communities of four intertidal sites, two sheltered and two more exposed, in Kongsfjorden (Svalbard) were investigated in summer 2001 at two different tidal levels (i.e. the low-water line and close below the driftline, referred to as mid-water (MW) level). A total of seven meiofaunal higher taxa were recorded with nematodes, oligochaetes and turbellarians being numerically dominant. Mean total meiofaunal densities ranged between 50 ind. 10 cm⁻² and 903 ind. 10 cm⁻². Our data showed a clear decrease in total meiofaunal densities with increasing coarseness of the sediment. Total meiofaunal biomass varied from 0.2 g dwt m⁻² to 2 g dwt m⁻² and, in general, was high even at low meiofaunal densities, i.e. larger interstitial spaces in coarser sediments supported larger meiofauna, especially turbellarians. The results on the vertical distribution of meiofauna contrasted sharply with typical meiobenthic depth profiles on other beaches, probably in response to ice-scouring and concomitant salinity fluctuations. Oligochaetes were the most abundant taxon, with a peak density of 641 ind. 10 cm⁻² at Breoyane Island. They were mainly comprised of juvenile Enchytraeidae, which prohibited identification to species/genus level. Nematode densities ranged between 4 ind. 10 cm⁻² and 327 ind. 10 cm⁻². Nematodes were identified up to genus level and assigned to trophic guilds. In total, 28 nematode genera were identified. Oncholaimus and Theristus were the most abundant genera. The composition of the nematode community and a dominance of predators and deposit feeders were in agreement with results from other arctic and temperate beaches. Nematode genus diversity was higher at the more sheltered beaches than at the more exposed ones. Low-water level stations also tended to harbour a more diverse nematode communities than stations at the MW level. Differences in nematode community structure between low- and MW stations of single beaches were more pronounced than community differences between different beaches and were mainly related to resources quality and availability.     

Seasonal and spatial functional shifts in phytoplankton communities of five tropical reservoirs

Trait-based approaches have become increasingly important and valuable in understanding phytoplankton community assembly and composition. These approaches allow for comparisons between water bodies with different species composition. We hypothesize that similar changes in environmental conditions lead to similar responses with regard to functional traits of phytoplankton communities, regardless of trophic state or species composition. We studied the phytoplankton (species composition, community trait mean and diversity) of five reservoirs in Brazil along a trophic gradient from ultra-oligotrophic to meso-eutrophic. Samples at two seasons (summer/rainy and winter/dry) with a horizontal and vertical resolution were taken. Using multivariate analysis, the five reservoirs separated, despite some overlap, according to their environmental variables (mainly total phosphorus, conductivity, pH, chlorophyll a). However, between the seasonal periods, the reservoirs shifted in a similar direction in the multi-dimensional space. The seasonal response of the overall phytoplankton community trait mean differed between the ultra-oligotrophic and the other reservoirs, with three reservoirs exhibiting a very similar community trait mean despite considerable differences in species composition. Within-season differences between different water layers were low. The functional diversity was also unrelated to the trophic state of the reservoirs. Thus, seasonal environmental changes had strong influence on the functional characteristics of the phytoplankton community in reservoirs with distinct trophic condition and species composition. These results demonstrate that an ataxonomic trait-based approach is a relevant tool for comparative studies in phytoplankton ecology.     

Plant diversity and identity effects on predatory nematodes and their prey

There is considerable evidence that both plant diversity and plant identity can influence the level of predation and predator abundance aboveground. However, how the level of predation in the soil and the abundance of predatory soil fauna are related to plant diversity and identity remains largely unknown. In a biodiversity field experiment, we examined the effects of plant diversity and identity on the infectivity of entomopathogenic nematodes (EPNs, Heterorhabditis and Steinernema spp.), which prey on soil arthropods, and abundance of carnivorous non‐EPNs, which are predators of other nematode groups. To obtain a comprehensive view of the potential prey/food availability, we also quantified the abundance of soil insects and nonpredatory nematodes and the root biomass in the experimental plots. We used structural equation modeling (SEM) to investigate possible pathways by which plant diversity and identity may affect EPN infectivity and the abundance of carnivorous non‐EPNs. Heterorhabditis spp. infectivity and the abundance of carnivorous non‐EPNs were not directly related to plant diversity or the proportion of legumes, grasses and forbs in the plant community. However, Steinernema spp. infectivity was higher in monocultures of Festuca rubra and Trifolium pratense than in monocultures of the other six plant species. SEM revealed that legumes positively affected Steinernema infectivity, whereas plant diversity indirectly affected the infectivity of Heterorhabditis EPNs via effects on the abundance of soil insects. The abundance of prey (soil insects and root‐feeding, bacterivorous, and fungivorous nematodes) increased with higher plant diversity. The abundance of prey nematodes was also positively affected by legumes. These plant community effects could not be explained by changes in root biomass. Our results show that plant diversity and identity effects on belowground biota (particularly soil nematode community) can differ between organisms that belong to the same feeding guild and that generalizations about plant diversity effects on soil organisms should be made with great caution.     

Long‐term cattle grazing shifts the ecological state of forest soils

Cattle grazing profoundly affects abiotic and biotic characteristics of ecosystems. While most research has been performed on grasslands, the effect of large managed ungulates on forest ecosystems has largely been neglected. Compared to a baseline seminatural state, we investigated how long‐term cattle grazing of birch forest patches affected the abiotic state and the ecological community (microbes and invertebrates) of the soil subsystem. Grazing strongly modified the soil abiotic environment by increasing phosphorus content, pH, and bulk density, while reducing the C:N ratio. The reduced C:N ratio was strongly associated with a lower microbial biomass, mainly caused by a reduction of fungal biomass. This was linked to a decrease in fungivorous nematode abundance and the nematode channel index, indicating a relative uplift in the importance of the bacterial energy‐channel in the nematode assemblages. Cattle grazing highly modified invertebrate community composition producing distinct assemblages from the seminatural situation. Richness and abundance of microarthropods was consistently reduced by grazing (excepting collembolan richness) and grazing‐associated changes in soil pH, Olsen P, and reduced soil pore volume (bulk density) limiting niche space and refuge from physical disturbance. Anecic earthworm species predominated in grazed patches, but were absent from ungrazed forest, and may benefit from manure inputs, while their deep vertical burrowing behavior protects them from physical disturbance. Perturbation of birch forest habitat by long‐term ungulate grazing profoundly modified soil biodiversity, either directly through increased physical disturbance and manure input or indirectly by modifying soil abiotic conditions. Comparative analyses revealed the ecosystem engineering potential of large ungulate grazers in forest systems through major shifts in the composition and structure of microbial and invertebrate assemblages, including the potential for reduced energy flow through the fungal decomposition pathway. The precise consequences for species trophic interactions and biodiversity–ecosystem function relationships remain to be established, however.