Nematode consumption by mite communities varies in different forest microhabitats as indicated by molecular gut content analysis

Soil animals live in complex and heterogeneous habitats including litter of various types but also microhabitats such as mosses, fungal mats and grass patches. Soil food webs have been separated into a slow fungal and a fast bacterial energy channel. Bacterial-feeding nematodes are an important component of the bacterial energy channel by consuming bacteria and forming prey for higher trophic levels such as soil microarthropods. Investigating the role of nematodes as prey for higher trophic level consumers has been hampered by methodological problems related to their small body size and lack in skeletal structures which can be traced in the gut of consumers. Recent studies using molecular gut content analyses suggest that nematodes form major prey of soil microarthropods including those previously assumed to live as detritivores. Using molecular markers we traced nematode prey in fourteen abundant soil microarthropod taxa of Mesostigmata and Oribatida (both Acari) from three different microhabitats (litter, grass and moss). Consumption of nematodes varied between mite species indicating that trophic niche variation contributes to the high diversity of microarthropods in deciduous forests. Further, consumption of nematodes by Mesostigmata (but not Oribatida) differed between microhabitats indicating that trophic niches vary with habitat characteristics. Overall, the results suggest that free-living bacterial-feeding nematodes form important prey for soil microarthropods including those previously assumed to live as detritivores.     

Intra- and interspecific variation in trophic ecology of ‘predatory’ ants in the subfamily Ponerinae

1. The characterisation of energy flow through communities is a primary goal of ecology. Furthermore, predator–prey interactions can influence both species abundance and community composition. The ant subfamily Ponerinae includes many predatory species that range from generalist insectivores to highly specialised hunters that target a single prey type. Given their high diversity and ubiquity in tropical ecosystems, measuring intra- and interspecific variation in their trophic ecology is essential for understanding the role of ants as predators of insect communities. 2. The stable isotopic composition of nitrogen of 22 species from the ant subfamily Ponerinae was measured, relative to plants and other predatory and herbivorous insects at two Atlantic Forest sites in Argentina. The study tested the general assumption that ponerine ants are all predatory, and examined intra- and interspecific variation in trophic ecology relative to habitat, body size and cytochrome c oxidase subunit 1 sequences (DNA barcoding). 3. Stable isotope analysis revealed that most ponerines occupy high trophic levels (primary and secondary predators), but some species overlapped with known insect herbivores. Species residing at low trophic levels were primarily arboreal and may rely heavily on nectar or other plant-based resources in their diet. In addition, larger species tend to occupy lower trophic positions than smaller species. 4. Although some of the species were divided into two or more genetic clusters by DNA barcoding analysis, these clusters did not correspond to intraspecific variation in trophic position; therefore, colony dietary flexibility most probably explains species that inhabit more than one trophic level.     

Body size,trophic level,and the use of fish as transmission routes by parasites

Within food webs, trophically transmitted helminth parasites use predator–prey links for their own transfer from intermediate prey hosts, in which they occur as larval or juvenile stages, to predatory definitive hosts, in which they reach maturity. In large taxa that can be used as intermediate and/or definitive hosts, such as fish, a host species’ position within a trophic network should determine whether its parasite fauna consists mostly of adult or larval helminths, since vulnerability to predation determines an animal’s role in predator–prey links. Using a large database on the helminth parasites of 303 fish species, we tested whether the proportion of parasite species in a host that occur as larval or juvenile stages is best explained by their trophic level or by their body size. Independent of fish phylogeny or habitat, only fish body length emerged as a significant predictor of the proportion of parasites in a host that occur as larval stages from our multivariate analyses. On average, the proportion of larval helminth taxa in fish shorter than 20 cm was twice as high as that for fish over 100 cm in length. This is consistent with the prediction that small fishes, being more vulnerable to predation, make better hosts for larval parasites. However, trophic level and body length are strongly correlated among fish species, and they may have separate though confounded effects on the parasite fauna exploiting a given species. Helminths show varying levels of host specificity toward their intermediate host when the latter is the downstream host involved in trophic transmission toward an upstream definitive host. Given this broad physiological compatibility of many helminths with fish hosts, our results indicate that fish body length, as a proxy for vulnerability to predators, is a better predictor of their use by helminth larvae than their trophic level based on diet content.     

Experimental assessment of trophic ecology in a generalist spider predator: Implications for biocontrol in Uruguayan crops

Conservative biological control promotes the use of native natural enemies to limit the size and growth of pest populations. Although spiders constitute one of the most important groups of native predators in several crops, their trophic ecology remains largely unknown, especially for several generalist taxa. In laboratory, we assessed the predatory behaviour of a wandering spider (the wolf spider Lycosa thorelli (Keyserling, 1877) against several arthropods varying in size and trophic positions, all found in South American soybean and rice crops. As prey we used the bug Piezodorus guildinii (Westwood, 1837) as well as larvae and adults of the moth Spodoptera frugiperda (Smith, 1797), both being considered important pests in Uruguayan crops. We also used several non-pest arthropods as prey, sarcophagid flies, carabid beetles and wolf spiders. All prey were attacked in more or less high, although not statistically differing, proportions. However, carabids were not consumed, and bugs were consumed in significantly lower proportions than flies. A negative correlation was found between prey size and acceptance rate. Immobilization times were longer against larvae when compared to moths and flies, while predatory sequences were longer for bugs when compared to flies, moths and spiders. In addition, we found a positive effect of prey size on predatory sequence length and complexity. Our results confirm the ability of spiders to attack and feed upon prey with different morphologies, included well-defended arthropods, and their potential use as natural enemies of several pests in South American crops.     

The response of a three trophic level soil food web to the identity and diversity of plant species and functional groups

Despite considerable recent interest in how biodiversity may influence ecosystem properties, the issue of how plant diversity and composition may affect multiple trophic levels in soil food webs remains essentially unexplored. We conducted a glasshouse experiment in which three plant species of each of three functional groups (grasses, N‐fixing legumes and forbs) were grown in monoculture and in mixtures of three species (with the three species being in the same or different functional groups) and all nine species. Plant species identity had important effects on the biomasses or population densities of belowground primary consumers (microbial biomass, herbivorous nematodes) and two groups of secondary consumers (microbe‐feeding nematodes and enchytraeids); the third consumer trophic level (predatory nematodes) was marginally not significantly affected at P=0.05. Plant species also influenced the relative importance of the bacterial‐based and fungal‐based energy channels for both the primary and secondary consumer trophic levels. Within‐group diversity of only the soil microflora and herbivorous nematodes (both representing the basal consumer trophic level) were affected by plant species identity. However, community composition within all trophic groupings considered (herbivorous nematodes, microbes, microbe‐feeding nematodes, predatory nematodes) was strongly influenced by what plant species were present. Despite the strong responses of the soil biota to plant species identity, there were few effects of plant species or functional group richness on any of the belowground response variables measured. Further, net primary productivity (NPP) was unaffected by plant diversity. Since some belowground response variables were correlated with NPP across treatments, it is suggested that belowground responses to plant diversity might become more apparent in situations when NPP itself responds to plant diversity. Our results point to plant species identity as having important multitrophic effects on soil food webs, both at the whole trophic group and within‐group levels of resolution, and suggest that differences in plant traits across species may be important in driving the decomposer subsystem.     

Influence of prey body characteristics and performance on predator selection

At the time of settlement to the reef environment, coral reef fishes differ in a number of characteristics that may influence their survival during a predatory encounter. This study investigated the selective nature of predation by both a multi-species predator pool, and a single common predator (Pseudochromis fuscus), on the reef fish, Pomacentrus amboinensis. The study focused on the early post-settlement period of P. amboinensis, when mortality, and hence selection, is known to be highest. Correlations between nine different measures of body condition/performance were examined at the time of settlement, in order to elucidate the relationships between different traits. Single-predator (P. fuscus) choice trials were conducted in 57.4-l aquaria with respect to three different prey characteristics [standard length (SL), body weight and burst swimming speed], whilst multi-species trials were conducted on open patch reefs, manipulating prey body weight only. Relationships between the nine measures of condition/performance were generally poor, with the strongest correlations occurring between the morphological measures and within the performance measures. During aquaria trials, P. fuscus was found to be selective with respect to prey SL only, with larger individuals being selected significantly more often. Multi-species predator communities, however, were selective with respect to prey body weight, with heavier individuals being selected significantly more often than their lighter counterparts. Our results suggest that under controlled conditions, body length may be the most important prey characteristic influencing prey survival during predatory encounters with P. fuscus. In such cases, larger prey size may actually be a distinct disadvantage to survival. However, these relationships appear to be more complex under natural conditions, where the expression of prey characteristics, the selectivity fields of a number of different predators, their relative abundance, and the action of external environmental characteristics, may all influence which individuals survive.     

Free‐living nematodes as prey for higher trophic levels of forest soil food webs

Nematodes are the most abundant invertebrates in soils and are key prey in soil food webs. Uncovering their contribution to predator nutrition is essential for understanding the structure of soil food webs and the way energy channels through soil systems. Molecular gut content analysis of consumers of nematodes, such as soil microarthropods, using specific DNA markers is a novel approach for studying predator–prey interactions in soil. We designed new specific primer pairs (partial 18S rDNA) for individual soil‐living bacterial‐feeding nematode taxa (Acrobeloides buetschlii, Panagrellus redivivus, Plectus velox and Plectus minimus). Primer specificity was tested against more than 100 non‐target soil organisms. Further, we determined how long nematode DNA can be traced in the gut of predators. Potential predators were identified in laboratory experiments including nine soil mite (Oribatida, Gamasina and Uropodina) and ten springtail species (Collembola). Finally, the approach was tested under field conditions by analyzing five mite and three collembola species for feeding on the three target nematode species. The results proved the three primer sets to specifically amplify DNA of the respective nematode taxa. Detection time of nematode DNA in predators varied with time of prey exposure. Further, consumption of nematodes in the laboratory varied with microarthropod species. Our field study is the first definitive proof that free‐living nematodes are important prey for a wide range of soil microarthropods including those commonly regarded as detritivores. Overall, the results highlight the eminent role of nematodes as prey in soil food webs and for channelling bacterial carbon to higher trophic levels.     

Variations of Predatory Tactics of a Water Bug during Development

At all developmental stages, Diplonychus indicus Venk. & Rao (Heteroptera: Belostomatidae) bugs make predatory attempts and catch prey both from ambush and while foraging actively. The distribution of predatory movements between these two foraging modes does not vary significantly during development, although a bug generally performs more predatory attempts from ambush than during active search. Both modes were subdivided and six different predatory movement types were recorded. These different predatory types are not all performed at the same frequency at all developmental stages. The proportion of predatory attempts including a preliminary lunge decreases with age whereas the proportion of simple predatory attempts (strike) increases with age. There is a significant effect of developmental stage and of predatory mode on capture success. General capture success rate increases with age and success rates vary with predatory type. Capture success of predatory attempts made while swimming is always lower than capture success of simple predatory attempts (strike). These rates do not vary during development. On the contrary, success rates for the predatory types dive and lunge increase with developmental stage. For some predatory categories, capture success varies with predator-prey distance. The data then suggest that the bugs are undershooting.     

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.     

Intraguild Interactions Between the Predatory Mites Neoseiulus californicus and Phytoseiulus persimilis

Species at the same trophic level may interact through competition for food, but can also interact through intraguild predation. Intraguild predation is widespread at the second and third trophic level and the effects may cascade down to the plant level. The effects of intraguild predation can be modified by antipredator behaviour in the intraguild prey. We studied intraguild predation and antipredator behaviour in two species of predatory mite, Neoseiulus californicus and Phytoseiulus persimilis, which are both used for control of the two-spotted spider mite in greenhouse and outdoor crops. Using a Y-tube olfactometer, we assessed in particular whether each of the two predators avoids odours emanating from prey patches occupied by the heterospecific predator. Furthermore, we measured the occurrence and rate of intraguild predation of different developmental stages of P. persimilis and N. californicus on bean leaves in absence or in presence of the shared prey. Neither of the two predator species avoided prey patches with the heterospecific competitor, both when inexperienced with the other predator and when experienced with prey patches occupied by the heterospecific predator. Intraguild experiments showed that N. californicus is a potential intraguild predator of P. persimilis. However, P. persimilis did not suffer much from intraguild predation as long as the shared prey was present. This is probably because N. californicus prefers to feed on two-spotted spider mites rather than on its intraguild prey.     

Chemosensory responses of Aporcelaimellus nivalis (Nematoda: Dorylaimida) towards kairomones emitted by prey nematodes belonging to different trophic groups

Abstract

Observations were made on the attraction of Aporcelaimellus nivalis towards kairomones/attractants emitted by prey nematodes belonging to different trophic categories, viz., saprophagous, epidermal, migratory semi-endodermal, predatory nematodes, virus vectors and cortical feeders. Aporcelaimellus nivalis responded positively and significantly to prey kairomones, but showed variation in their individual behaviour. Predators are most attracted towards epidermal feeders and least attracted to virus vectors. The differential responses of A. nivalis towards different prey were attributed to the inert behaviour of predators, their preference for a particular species of prey, chemical composition, concentration, quality, quantity of prey attractant, formation of minimum perceptible attraction gradient of prey and minimum response threshold of predators. Various factors such as prey density, period of prey incubation, starvation of predators, temperature, agar concentration, agar thickness and distance of predators from the source of attraction (prey) govern chemosensory responses of predator. Aporcelaimellus nivalis maximum response was towards Hirschmanniella oryzae, when tested as 10 day starved predators in agar plates containing 2 mm thick layer of 1% water-agar with 200 prey individuals previously incubated for 16 h at 30°C. Prey kairomones were most attractive when A. nivalis were tested from a distance of 2 and 3 cm.     

The signature of phylogenetic constraints on food-web structure

Understanding the processes underlying food-web structure and organization remains one of the major tasks of ecology. While first attempts were mostly based on niche theory, with body size of species imposing a hierarchical structure for consumer species, it has been recently suggested that phylogenetic constraints may be more fundamental to understand who eats whom in natural communities. Models of food-web structure built on basic evolutionary assumptions are able to adequately reproduce the topology of real food-webs. Here, we analyze different implications of phylogenetic constraints on trophic structure, and present preliminary results. Our exploration of the relationship between trophic and taxonomic similarity in food-webs shows that phylogeny and trophic structure are closely linked. Interestingly, the relationship is stronger for trophic similarity between prey (similarity measured by shared predators species, or predatory similarity) than between consumer species (similarity measured by shared prey species, or dietary similarity). When relating body mass of prey and predators, slopes of major axis regressions within taxonomic groups differ markedly from the global pattern; similar differences between taxonomic levels appear when exploring the relationship between body mass of predators and the range in body mass of their prey, and vice versa. These results are important to understand how evolutionary processes shaping body sizes can affect food-web structure.     

Disentangling mite predator‐prey relationships by multiplex PCR

Gut content analysis using molecular techniques can help elucidate predator‐prey relationships in situations in which other methodologies are not feasible, such as in the case of trophic interactions between minute species such as mites. We designed species‐specific primers for a mite community occurring in Spanish citrus orchards comprising two herbivores, the Tetranychidae Tetranychus urticae and Panonychus citri, and six predatory mites belonging to the Phytoseiidae family; these predatory mites are considered to be these herbivores’ main biological control agents. These primers were successfully multiplexed in a single PCR to test the range of predators feeding on each of the two prey species. We estimated prey DNA detectability success over time (DS₅₀), which depended on the predator‐prey combination and ranged from 0.2 to 18 h. These values were further used to weight prey detection in field samples to disentangle the predatory role played by the most abundant predators (i.e. Euseius stipulatus and Phytoseiulus persimilis). The corrected predation value for E. stipulatus was significantly higher than for P. persimilis. However, because this 1.5‐fold difference was less than that observed regarding their sevenfold difference in abundance, we conclude that P. persimilis is the most effective predator in the system; it preyed on tetranychids almost five times more frequently than E. stipulatus did. The present results demonstrate that molecular tools are appropriate to unravel predator‐prey interactions in tiny species such as mites, which include important agricultural pests and their predators.     

Evaluating the predatory potential of carnivorous nematodes against Rotylenchulus reniformis and Meloidogyne incognita

Predatory behavior of a nematode is usually determined through gut content observation or prey delimitation counts. In this experiment, Mononchus and Neoactinolaimus predation of Rotylenchulus reniformis or Meloidogyne incognita was determined using a PCR-based nematode gut content analysis. Soil samples naturally infested with Mononchus were placed in tubes and potential prey nematodes R. reniformis, M. incognita, or a mixture of both were introduced. The gut contents of Mononchus were assayed for the DNA from R. reniformis or M. incognita using PCR specific primers. A higher % of Mononchus tested positive for DNA of R. reniformis than for M. incognita when the prey were added alone. However, when provided with both prey species, Mononchus was tested positive for DNA of M. incognita more frequently than for R. reniformis. Percent Mononchus testing positive for DNA of R. reniformis correlated positively with the abundance of R. reniformis, but this relationship was not observed between Mononchus and M. incognita. Neoactinolaimus was added to aqueous solution containing a mixture of free-living nematodes and R. reniformis. More Neoactinolaimus tested positive for DNA of R. reniformis than other predatory or omnivorous nematodes in the same samples. Based on regression analysis, the presence of fungivorous and other predatory nematodes in the soil could distract Neoactinolaimus from predation on R. reniformis. Our results suggested that Prismatolaimus, Mesodiplogasteroides and Eudorylaimus could also prey on R. reniformis. Although less than 40% of the predatory or omnivorous nematodes tested preyed on R. reniformis, this level of predation could contribute to reducing the population densities of plant-parasitic nematodes in the soil.     

Diet composition,feeding niche partitioning and trophic organisation of large pelagic predatory fishes in the eastern Arabian Sea

Information on the ecology and feeding behaviour of the large oceanic predatory fishes is crucial for the ecosystem approaches to fisheries management models. Co-existing large pelagic predators in the open oceans may avoid competition for the limited forage by resource partitioning on spatial, temporal or trophic levels. To test this, we studied the prey species composition, diet overlap, trophic level, and trophic organisation of 12 large predatory fishes co-existing in the eastern Arabian Sea. Stomach contents of 1,518 specimens caught by exploratory longline operations in the Indian Exclusive Economic Zone during the years 2006–2009 were analysed. Finfishes were dominant prey of all species except blue marlin (Makaira nigricans) and yellowfin tuna (Thunnus albacares), which fed mainly on cephalopods, and long-snouted lancetfish (Alepisaurus ferox) and pelagic stingray (Pteroplatytrygon violacea), which fed mainly on crustaceans. Common dolphinfish (Coryphaena hippurus) and yellowfin tuna fed on a wider variety of prey than the other species, while the diets of lancetfish and black marlin (Istiompax indica) were narrowest. Pelagic stingray and great barracuda (Sphyraena barracuda) fed on species occupying epipelagic waters, whereas the contribution of mesopelagic prey was higher in the diets of swordfish (Xiphias gladius) and pelagic thresher (Alopias pelagicus). Trophic levels of these fishes ranged from 4.13 to 4.37. Diet overlap index revealed that some of the large pelagic predatory fishes share common prey species. Cluster analysis of the diets revealed four distinct trophic guilds namely ‘flyingfish feeders’ (common dolphinfish and great barracuda); ‘mesopelagic predators’ (pelagic thresher and swordfish); ‘crab feeders’ (lancetfish, pelagic stingray and silky shark) and ‘squid feeders’ (yellowfin tuna, Indo-Pacific sailfish (Istiophorus platypterus), skipjack tuna (Katsuwonus pelamis), black marlin and blue marlin). Large predatory fishes of the eastern Arabian Sea target different prey types, and limit their vertical extent and time of feeding to avoid competing for prey.     

Functional response of Euseius concordis to densities of different developmental stages of the cassava green mite

Both prey density and developmental stage of pests and natural enemies are known to influence the effectiveness of biological control. However, little is known about the interaction between prey density and population structure on predation and fecundity of generalist predatory mites. Here, we evaluated the functional response (number of prey eaten by predator in relation to prey density) of adult females and nymphs of the generalist predatory mite Euseius concordis to densities of different developmental stages of the cassava green mite Mononychellus tanajoa, as well as the fecundity of adult females of the predator. We further assessed the instantaneous rate of increase, based on fecundity and mortality, of E. concordis fed on eggs, immatures and adults of M. tanajoa. Overall, nymphs and adults of E. concordis feeding on eggs, immatures and females of M. tanajoa had a type III functional response curve suggesting that the predator increased prey consumption rate as prey density increased. Both nymphs and adult females of the predator consumed more eggs than immatures of M. tanajoa from the density of 20 items per leaf disc onwards, revealing an interaction between prey density and developmental stage in the predatory activity of E. concordis. In addition, population growth rate was higher when the predator fed on eggs and immatures in comparison with females. Altogether our results suggest that E. concordis may be a good candidate for the biological control of M. tanajoa populations. However, the efficiency of E. concordis as a biological control agent of M. tanajoa is contingent on prey density and population structure.     

Impact of differential predation potential on eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis>) faunal community structure

The trophic structure of fauna within eelgrass beds (Zostera marina) was assessed at two sites in Little Egg Harbor, New Jersey, USA during the summer of 1999. Although the sites were similar with respect to both Z. marina shoot density and plant biomass, they differed significantly in the relative distribution of large predatory fish (e.g., Cynoscion regalis, Paralichthys dentatus, Morone saxatilis). Site One, Marsh Elder, was characterized by a significantly greater catch per unit effort for large predators than Site Two, Shelter Island. Gut content analysis provided direct evidence of trophic linking and significant declines between these fish and four of the five most abundant organisms collected in throw traps used to analyze the density of large benthic prey/small predators. The densities of grass shrimp (Palaemonetes spp. Hippolyte zostericola), blue crab (Callinectes sapidus), and small predatory fish (e.g., Syngnathus fuscus, Opsanus spp., Tautoga onitus) were significantly reduced at Marsh Elder, potentially as a direct impact of large predatory fish. In turn, the differences in the density of small predators observed between sites produced either a significant positive or negative effect on the distribution of small benthic prey (e.g., polychaetes, amphipods), resulting in a two-step trophic cascade within the system. Additionally, an analysis of similarities defined each site independently for both large prey/small predators and small benthic prey community structure. Although the mechanism which produced the differences in the distribution of large predatory fish remains unknown, their impact on faunal community structure mediated not only the distribution of their potential prey, but also subsequent lower trophic levels.     

The influence of feeding on predatory tactics in a water bug

Abstract. The tropical belostomatid Diplonychus indicus Venk. & Rao (Heter-optera: Belostomatidae) forages both actively and from ambush, changing frequently from one predatory mode to the other. Analyses presented here of the influence of feeding and of developmental stage on several locomotory and predatory parameters show that the fact that a bug was eating did not prevent it from reacting to and from seizing prey items, but it did influence the mode of predatory attempts. Feeding bugs were less active and they foraged actively less frequently, but feeding did not affect the frequency of predatory attempts from ambush. No important variations during development were found and first stadium larvae behaved similarly to adults. Capture success rates were higher at all developmental stages during a meal than when a bug was not eating and rejection rates were lower during a meal. These results suggest that, when eating, Diplonychus indicus bugs aim more successfully at prey items they are more likely to capture and to eat.     

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.     

Prey state shapes the effects of temporal variation in predation risk

The ecological impacts of predation risk are influenced by how prey allocate foraging effort across periods of safety and danger. Foraging decisions depend on current danger, but also on the larger temporal, spatial or energetic context in which prey manage their risks of predation and starvation. Using a rocky intertidal food chain, we examined the responses of starved and fed prey (Nucella lapillus dogwhelks) to different temporal patterns of risk from predatory crabs (Carcinus maenas). Prey foraging activity declined during periods of danger, but as dangerous periods became longer, prey state altered the magnitude of risk effects on prey foraging and growth, with likely consequences for community structure (trait-mediated indirect effects on basal resources, Mytilus edulis mussels), prey fitness and trophic energy transfer. Because risk is inherently variable over time and space, our results suggest that non-consumptive predator effects may be most pronounced in productive systems where prey can build energy reserves during periods of safety and then burn these reserves as ‘trophic heat’ during extended periods of danger. Understanding the interaction between behavioural (energy gain) and physiological (energy use) responses to risk may illuminate the context dependency of trait-mediated trophic cascades and help explain variation in food chain length.