Methods to identify the prey of invertebrate predators in terrestrial field studies

Quantitative description of six fish species' gut contents and prey abundances in the Baltic Sea (1968-1978)

The dataset presents a compilation of stomach contents from six demersal fish species from two functional groups inhabiting the Baltic Sea. It includes detailed information on prey identities, body masses, and biomasses recovered from both the fish's digestive systems and their surrounding environment. Environmental parameters, such as salinity and temperature levels, have been integrated to enrich this dataset. The juxtaposition of information on prey found in stomachs and in the environment provides an opportunity to quantify trophic interactions across different environmental contexts and investigate how fish foraging behaviour adapts to changes in their environment, such as an increase in temperature. The compilation of body mass and taxonomic information for all species allows approaching these new questions using either a taxonomic (based on species identity) or functional trait (based on body mass) approach.

DNA High-Throughput Sequencing for Arthropod Gut Content Analysis to Evaluate Effectiveness and Safety of Biological Control Agents

The search for effective biological control agents without harmful non-target effects has been constrained by the use of impractical (field direct observation) or imprecise (cage experiments) methods. While advances in the DNA sequencing methods, more specifically the development of high-throughput sequencing (HTS), have been quickly incorporated in biodiversity surveys, they have been slow to be adopted to determine arthropod prey range, predation rate and food web structure, and critical information to evaluate the effectiveness and safety of a biological control agent candidate. The lack of knowledge on how HTS methods could be applied by ecological entomologists constitutes part of the problem, although the lack of expertise and the high cost of the analysis also are important limiting factors. In this review, we describe how the latest HTS methods of metabarcoding and Lazaro, a method to identify prey by mapping unassembled shotgun reads, can serve biological control research, showing both their power and limitations. We explain how they work to determine prey range and also how their data can be used to estimate predation rates and subsequently be translated into food webs of natural enemy and prey populations helping to elucidate their role in the community. We present a brief history of prey detection through molecular gut content analysis and also the attempts to develop a more precise formula to estimate predation rates, a problem that still remains. We focused on arthropods in agricultural ecosystems, but most of what is covered here can be applied to natural systems and non-arthropod biological control candidates as well.

Effects of retained dead wood on predation pressure on herbivores in young pine forests

Retention of logging residue as dead wood could be a method to simultaneously increase biodiversity and predation rates of pest insects, in managed forests. Managed forests are generally low in diversity, and dead wood has been demonstrated to increase species diversity. Moreover, managed forests are predicted to suffer from higher frequency of insect outbreaks in the future, particularly in the northern hemisphere. In this study, we explore the effect of dead wood removal and addition in managed pine forest stands in Sweden on arthropod diversity and abundance and predation rates. We performed a controlled field experiment, focusing on logging residue type of dead wood. We used pitfall traps and sticky traps to measure arthropod diversity and abundance and plasticine larvae to assess predation rates. We specifically targeted generalist arthropods (i.e. non-wood living species), and predation rate on tree-dwelling larvae (corresponding to defoliating outbreak pests). We found no effect of dead wood addition on arthropod abundance or diversity, neither did we find an effect on predation rate. Despite the lack of effects in our study, we argue that dead wood can be an important component for both biodiversity of generalist arthropod and for pest control, but the effect may depend on both the specific arthropod group targeted and the specific life stage of the pest insect as well as on inherent components of the dead wood, such as age.

Diet variation in a critically endangered marine predator revealed with stable isotope analysis

Understanding the foraging ecology of animals gives insights into their trophic relationships and habitat use. We used stable isotope analysis to understand the foraging ecology of a critically endangered marine predator, the Māui dolphin. We analysed carbon and nitrogen isotope ratios of skin samples (n = 101) collected from 1993 to 2021 to investigate temporal changes in diet and niche space. Genetic monitoring associated each sample with a DNA profile which allowed us to assess individual and population level changes in diet. Potential prey and trophic level indicator samples were also collected (n = 166; 15 species) and incorporated in Bayesian mixing models to estimate importance of prey types to Māui dolphin diet. We found isotopic niche space had decreased over time, particularly since the 2008 implementation of a Marine Mammal Sanctuary. We observed a decreasing trend in ∂¹³C and ∂¹⁵N values, but this was not linear and several fluctuations in isotope values occurred over time. The largest variation in isotope values occurred during an El Niño event, suggesting that prey is influenced by climate-driven oceanographic variables. Mixing models indicated relative importance of prey remained constant since 2008. The isotopic variability observed here is not consistent with individual specialization, rather it occurs at the population level.

Effect of Conservation Agriculture on Aphid Biocontrol by Generalist (Carabid Beetle) and Specialist (Parasitoids Wasp) Natural Enemy Communities in Winter Wheat

Adoption of practices that reduce the risk of pest outbreaks is one of the pillars of agroecology and is largely based on biological control. Multiple infield and landscape parameters affect biocontrol, but the effects of conservation soil management on biological control have been poorly investigated over crop season. By comparing winter wheat fields within the same landscape but with different soil management, the direct and indirect effects of soil management (conservation and conventional systems) on natural enemies’ communities and their biological control on aphids was studied from the tillering stage to the harvest. In addition to aphid infestation, two families of the main natural enemies’ guilds were monitored, as well as their associated services: aphid parasitoid, a specialist and flying natural enemy, with parasitism service, and carabid beetles, a generalist and ground-dwelling predator, with aphidophagy service. Soil conservation system hosted more abundant and diverse carabid beetles’ assemblages, and received higher aphidophagy service in June than conventional system. However, neither parasitoid abundance, nor parasitism rates, were affected by soil management. Aphid infestation and its associated damage did not depend on soil management either. Our results suggest that ground-dwelling natural enemies are more impacted by soil management than foliage-dwelling natural enemies, which is partly reflected in aphid biocontrol. In agricultural systems with reduced soil perturbation, direct mortality on ground-dwelling communities due to tillage may be lower than in a conventional system, but habitat heterogeneity is also greater, increasing the number of ecological niches for natural enemies. Both factors are supposed to favor an early presence of natural enemies and a tendency toward a precocious aphidophagy service is indeed observed in conservation system.

Artefactual depiction of predator-prey trophic linkages in global soils

Soil invertebrates contribute to multiple ecosystem services, including pest control, nutrient cycling, and soil structural regulation, yet trophic interactions that determine their diversity and activity in soils remain critically understudied. Here, we systematically review literature (1966-2020) on feeding habits of soil arthropods and macrofauna and summarize empirically studied predator-prey linkages across ecosystem types, geographies and taxa. Out of 522 unique predators and 372 prey organisms (constituting 1947 predator-prey linkages), the vast majority (> 75%) are only covered in a single study. We report a mean of just 3.0 ± 4.7 documented linkages per organism, with pronounced taxonomic biases. In general, model organisms and crop pests (generally Insecta) are well-studied, while important soil-dwelling predators, fungivores and detritivores (e.g., Collembola, Chilopoda and Malacostraca) remain largely ignored. We argue that broader food-web based research approaches, considering multiple linkages per organism and targeting neglected taxa, are needed to inform science-driven management of soil communities and associated ecosystem services.

Disturbance-Induced Trophic Niche Shifts In Ground Beetles (Coleoptera: Carabidae) In Restored Grasslands

Ecosystem restoration is a critical component of land management, countering the loss of native biodiversity. Restoration efforts are enhanced by reintroducing naturally occurring ecosystem processes, including disturbances that may impact species characteristics such as niche position or niche size. In grasslands, grazing and fire affect plant diversity and habitat complexity, which potentially influence insect dietary behaviors and thus their contributions to functions like seed and arthropod predation. Using carbon and nitrogen stable isotopes, we characterized variation in the dietary niche of six ground beetle species (Coleoptera: Carabidae) in response to grazing by reintroduced bison and prescribed fire disturbances in twenty tallgrass prairies. Management disturbances did not affect activity density for most beetle species and mean trophic position was mostly unaffected. However, five of six species exhibited increased trophic niche area and breadth with disturbances, indicating a switch to a more generalist diet that incorporated a wider range of food items. The combination of bison and fire impacts may increase vegetation patchiness and heterogeneity, driving these diet changes. Morphological traits and microhabitat preferences might mediate response to disturbances and the resulting heterogeneity. Combining prescribed fire and grazing, which increases plant diversity and vegetation structural diversity, may help beetle communities establish over time and support the ecological functions to which these insects contribute.

Prey naïveté rather than enemy release dominates the relation of an invasive spider toward a native predator

Ecosystems may suffer from the impact of invasive species. Thus, understanding the mechanisms contributing to successful invasions is fundamental for limiting the effects of invasive species. Most intuitive, the enemy release hypothesis predicts that invasive species might be more successful in the exotic range than resident sympatric species owing to the absence of coevolution with native enemies. Here, we test the enemy release hypothesis for the invasion of Europe by the North American spider Mermessus trilobatus. We compare the susceptibility of invasive Mermessus trilobatus and a native species with similar life history to a shared predator with which both species commonly co-occur in Europe. Contrary to our expectations, invasive Mermessus trilobatus were consumed three times more frequently by native predators than their native counterparts. Our study shows that invasive Mermessus trilobatus is more sensitive to a dominant native predator than local sympatric species. This suggests that the relation between the invasive spider and its native predator is dominated by prey naïveté rather than enemy release. Further studies investigating evolutionary and ecological processes behind the invasion success of Mermessus trilobatus, including testing natural parasites and rapid reproduction, are needed to explain its invasion success in Europe.

Deciphering the diet of a wandering spider (Phoneutria boliviensis; Araneae: Ctenidae) by DNA metabarcoding of gut contents

Arachnids are the most abundant land predators. Despite the importance of their functional roles as predators and the necessity to understand their diet for conservation, the trophic ecology of many arachnid species has not been sufficiently studied. In the case of the wandering spider, Phoneutria boliviensis F. O. Pickard-Cambridge, 1897, only field and laboratory observational studies on their diet exist. By using a DNA metabarcoding approach, we compared the prey found in the gut content of males and females from three distant Colombian populations of P. boliviensis. By DNA metabarcoding of the cytochrome c oxidase subunit I (COI), we detected and identified 234 prey items (individual captured by the spider) belonging to 96 operational taxonomic units (OTUs), as prey for this wandering predator. Our results broaden the known diet of P. boliviensis with at least 75 prey taxa not previously registered in fieldwork or laboratory experimental trials. These results suggest that P. boliviensis feeds predominantly on invertebrates (Diptera, Lepidoptera, Coleoptera, and Orthoptera) and opportunistically on small squamates. Intersex and interpopulation differences were also observed. Assuming that prey preference does not vary between populations, these differences are likely associated with a higher local prey availability. Finally, we suggest that DNA metabarcoding can be used for evaluating subtle differences in the diet of distinct populations of P. boliviensis, particularly when predation records in the field cannot be established or quantified using direct observation.

Identification of novel bacterial biomarkers to detect bird scavenging by invasive rats

Rapid advances in genomic tools for use in ecological contexts and non-model systems allow unprecedented insight into interactions that occur beyond direct observation. We developed an approach that couples microbial forensics with molecular dietary analysis to identify species interactions and scavenging by invasive rats on native and introduced birds in Hawaii. First, we characterized bacterial signatures of bird carcass decay by conducting 16S rRNA high-throughput sequencing on chicken (Gallus gallus domesticus) tissues collected over an 11-day decomposition study in natural Hawaiian habitats. Second, we determined if field-collected invasive black rats (Rattus rattus; n = 51, stomach and fecal samples) had consumed birds using molecular diet analysis with two independent PCR assays (mitochondrial Cytochrome Oxidase I and Cytochrome b genes) and Sanger sequencing. Third, we characterized the gut microbiome of the same rats using 16S rRNA high-throughput sequencing and identified 15 bacterial taxa that were (a) detected only in rats that consumed birds (n = 20/51) and (b) were indicative of decaying tissue in the chicken decomposition experiment. We found that 18% of rats (n = 9/51) likely consumed birds as carrion by the presence of bacterial biomarkers of decayed tissue in their gut microbiome. One species of native bird (Myadestes obscurus) and three introduced bird species (Lophura leucomelanos, Meleagris gallopavo, Zosterops japonicus) were detected in the rats' diets, with individuals from these species (except L. nycthemera) likely consumed through scavenging. Bacterial biomarkers of bird carcass decay can persist through rat digestion and may serve as biomarkers of scavenging. Our approach can be used to reveal trophic interactions that are challenging to measure through direct observation.

Can the carbon and nitrogen isotope values of offspring be used as a proxy for their mother's diet? Using foetal physiology to interpret bulk tissue and amino acid δ15N values

The measurement of bulk tissue nitrogen (δ¹⁵N) and carbon isotope values (δ¹³C) chronologically along biologically inert tissues sampled from offspring can provide a longitudinal record of their mothers' foraging habits. This study tested the important assumption that mother-offspring stable isotope values are positively and linearly correlated. In addition, any change in the mother-offspring bulk tissues and individual amino acids that occurred during gestation was investigated. Whiskers sampled from southern elephant seal pups (Mirounga leonina) and temporally overlapping whiskers from their mothers were analyzed. This included n = 1895 chronologically subsampled whisker segments for bulk tissue δ¹⁵N and δ¹³C in total and n = 20 whisker segments for amino acid δ¹⁵N values, sampled from recently weaned pups (n = 17), juvenile southern elephant seals (SES) < 2 years old (n = 23) and adult female SES (n = 17), which included nine mother-offspring pairs. In contrast to previous studies, the mother-offspring pairs were not in isotopic equilibrium or linearly correlated during gestation: the Δ¹⁵N and Δ¹³C mother-offspring offsets increased by 0.8 and 1.2‰, respectively, during gestation. The foetal bulk δ¹⁵N values were 1.7 ± 0.5‰ (0.9-2.7‰) higher than mothers' δ¹⁵N values before birth, while the foetal δ¹³C increased by ~1.7‰ during gestation and were 1.0 ± 0.5‰ (0.0-1.9‰) higher than their mothers' δ¹³C at the end of pregnancy. The mother-offspring serine and glycine Δ¹⁵N differed by ~4.3‰, while the foetal alanine δ¹⁵N values were 1.4‰ lower than that of their mothers during the third trimester of pregnancy. The observed mother-offspring δ¹⁵N differences are likely explained by shuttling of glutamate-glutamine and glycine-serine amongst skeletal muscle, liver, placenta and foetal tissue. Foetal development relies primarily on remobilized endogenous maternal proteinaceous sources. Researchers should consider foetal physiology when using offspring bulk tissue isotope values as biomarkers for the mother's isotopic composition as part of monitoring programmes.

Insights into aphid prey consumption by ladybirds: Optimising field sampling methods and primer design for high throughput sequencing

Elucidating the diets of insect predators is important in basic and applied ecology, such as for improving the effectiveness of conservation biological control measures to promote natural enemies of crop pests. Here, we investigated the aphid diet of two common aphid predators in Central European agroecosystems, the native Coccinella septempunctata (Linnaeus) and the invasive Harmonia axyridis (Pallas; Coleoptera: Coccinellidae) by means of high throughput sequencing (HTS). For acquiring insights into diets of mobile flying insects at landscape scale minimizing trapping bias is important, which imposes methodological challenges for HTS. We therefore assessed the suitability of three field sampling methods (sticky traps, pan traps and hand-collection) as well as new aphid primers for identifying aphid prey consumption by coccinellids through HTS. The new aphid primers facilitate identification to species level in 75% of the European aphid genera investigated. Aphid primer specificity was high in silico and in vitro but low in environmental samples with the methods used, although this could be improved in future studies. For insect trapping we conclude that sticky traps are a suitable method in terms of minimizing sampling bias, contamination risk and trapping success, but compromise on DNA-recovery rate. The aphid diets of both field-captured ladybird species were dominated by Microlophium carnosum, the common nettle aphid. Another common prey was Sitobion avenae (cereal aphid), which got more often detected in C. septempunctata compared to H. axyridis. Around one third of the recovered aphid taxa were common crop pests. We conclude that sampling methodologies need constant revision but that our improved aphid primers offer currently one of the best solutions for broad screenings of coccinellid predation on aphids.

Beyond polyphagy and opportunism: natural prey of hunting spiders in the canopy of apple trees

Spiders (Araneae) form abundant and diverse assemblages in agroecosystems such as fruit orchards, and thus might have an important role as natural enemies of orchard pests. Although spiders are polyphagous and opportunistic predators in general, limited information exists on their natural prey at both species and community levels. Thus, the aim of this study was to assess the natural prey (realized trophic niche) of arboreal hunting spiders, their role in trophic webs and their biological control potential with direct observation of predation events in apple orchards. Hunting spiders with prey in their chelicerae were collected in the canopy of apple trees in organic apple orchards in Hungary during the growing seasons between 2013 and 2019 and both spiders and their prey were identified and measured. Among others, the composition of the actual (captured by spiders) and the potential (available in the canopy) prey was compared, trophic niche and food web metrics were calculated, and some morphological, dimensional data of the spider-prey pairs were analyzed. Species-specific differences in prey composition or pest control ability were also discussed. By analyzing a total of 878 prey items captured by spiders, we concluded that arboreal hunting spiders forage selectively and consume a large number of apple pests; however, spiders’ beneficial effects are greatly reduced by their high levels of intraguild predation and by a propensity to switch from pests to alternative prey. In this study, arboreal hunting spiders showed negative selectivity for pests, no selectivity for natural enemies and positive selectivity for neutral species. In the trophic web, the dominant hunting spider taxa/groups (Carrhotus xanthogramma, Philodromus cespitum, Clubiona spp., Ebrechtella tricuspidata, Xysticus spp. and ‘Other salticids’) exhibit different levels of predation on different prey groups and the trophic web’s structure changes depending on the time of year. Hunting spiders show a high functional redundancy in their predation, but contrary to their polyphagous nature, the examined spider taxa showed differences in their natural diet, exhibited a certain degree of prey specialization and selected prey by size and taxonomic identity. Guilds (such as stalkers, ambushers and foliage runners) did not consistently predict either prey composition or predation selectivity of arboreal hunting spider species. From the economic standpoint, Ph. cespitum and Clubiona spp. were found to be the most effective natural enemies of apple pests, especially of aphids. Finally, the trophic niche width of C. xanthogramma and Ph. cespitum increased during ontogeny, resulting in a shift in their predation. These results demonstrate how specific generalist predators can differ from each other in aspects of their predation ecology even within a relatively narrow taxonomic group.

Feeding studies take guts - critical review and recommendations of methods for stomach contents analysis in fish

Studies on the feeding ecology of fish are essential for exploring and contrasting trophic interactions and population and community dynamics within and among aquatic ecosystems. In this respect, many different methods have been adopted for the analysis of fish stomach contents. No consensus has, however, been reached for a standardised methodology despite that for several decades there has been an ongoing debate about which methodical approaches that should be preferred. Here, we critically review and scrutinise methods, addressing their strengths and weaknesses and emphasising inherent problems and possible pitfalls in their use. Although our critical assessment reveals that no completely ideal approach exists, appropriate and reliable procedures can be adopted through careful considerations and implementation. In particular, we advocate that different objectives require different methodical approaches and the choice of method should therefore be closely linked to the research questions that are addressed. For a standardisation of methods, we recommend a combination of the relative-fullness and presence-absence methods as the optimal approach for the commonly applied feeding studies addressing relative dietary composition in terms of prey diversity and abundance. Additionally, we recommend the gravimetric method for objectives related to the quantification of food consumption rates and the numerical method for prey selection studies. DNA-based dietary analysis provides a new and promising complementary approach to visual examination of stomach contents, although some technical challenges still exist. The suggested method standardisation facilitates comparisons across species, ecosystems and time and will enhance the applicability and benefits of fish feeding studies in trophic ecology research.

Selective insecticides secure natural enemies action in cotton pest management

Cotton hosts a variety of arthropod pests requiring intensive control mostly with insecticides, which in turn may impact beneficial insects and the environment. Therefore, insect control in cotton fields preconizes the use of selective insecticides that offer pest control but conserve natural enemies. In this work, we measured the impact of recommended insecticides on the abundance of predatory insects and predation upon sentinel preys in the field. Further, the survival of four key selected predatory insects of cotton ecosystem, representing chewing and sucking feeding habits and different pest species attacked [Chrysoperla externa Hagen, Eriopis connexa (Germar), Podisus nigrispinus (Dallas) and Orius insidiosus (Say)], were assessed when exposed to the dried residues of the tested insecticides. Mortality of sentinel prey caused by natural enemies was higher in areas treated with selective insecticides relative to the non-selective ones, and most of time similar to the untreated areas. Furthermore, areas treated with non-selective insecticides experienced prolonged impact between sprays depending on the insecticide applied. Seasonal abundance of predatory insects was 2× greater in fields under selective and untreated fields compared to those under non-selective recommendation. Survival of predators exposed to the dried residues of the selective insecticides pymetrozine, chlorantraniliprole, pyriproxyfen, and cyantraniliprole were greater than when exposed to the non-selective lambda-cyhalothrin, malathion, dimethoate, and thiamethoxam. Among the non-selective insecticides, malathion and dimethoate exhibited shorter residual time compared to the thiamethoxam and lambda-cyhalothrin + thiamethoxam. Therefore, the recommendation of selective insecticides provides benefits for cotton pest management by maintaining the action of the natural enemies present in the field.

Combining stable isotope analysis with DNA metabarcoding improves inferences of trophic ecology

Knowing what animals eat is fundamental to our ability to understand and manage biodiversity and ecosystems, but researchers often must rely on indirect methods to infer trophic position and food intake. Using an approach that combines evidence from stable isotope analysis and DNA metabarcoding, we assessed the diet and trophic position of Anthene usamba butterflies, for which there are no known direct observations of larval feeding. An earlier study that analyzed adults rather than caterpillars of A. usamba inferred that this butterfly was aphytophagous, but we found that the larval guts of A. usamba and two known herbivorous lycaenid species contain chloroplast 16S sequences. Moreover, chloroplast barcoding revealed high sequence similarity between chloroplasts found in A. usamba guts and the chloroplasts of the Vachellia drepanolobium trees on which the caterpillars live. Stable isotope analysis provided further evidence that A. usamba caterpillars feed on V. drepanolobium, and the possibilities of strict herbivory versus limited omnivory in this species are discussed. These results highlight the importance of combining multiple approaches and considering ontogeny when using stable isotopes to infer trophic ecology where direct observations are difficult or impossible.

Diet assessment of two land planarian species using high-throughput sequencing data

Geoplanidae (Platyhelminthes: Tricladida) feed on soil invertebrates. Observations of their predatory behavior in nature are scarce, and most of the information has been obtained from food preference experiments. Although these experiments are based on a wide variety of prey, this catalog is often far from being representative of the fauna present in the natural habitat of planarians. As some geoplanid species have recently become invasive, obtaining accurate knowledge about their feeding habits is crucial for the development of plans to control and prevent their expansion. Using high throughput sequencing data, we perform a metagenomic analysis to identify the in situ diet of two endemic and codistributed species of geoplanids from the Brazilian Atlantic Forest: Imbira marcusi and Cephaloflexa bergi. We have tested four different methods of taxonomic assignment and find that phylogenetic-based assignment methods outperform those based on similarity. The results show that the diet of I. marcusi is restricted to earthworms, whereas C. bergi preys on spiders, harvestmen, woodlice, grasshoppers, Hymenoptera, Lepidoptera and possibly other geoplanids. Furthermore, both species change their feeding habits among the different sample locations. In conclusion, the integration of metagenomics with phylogenetics should be considered when establishing studies on the feeding habits of invertebrates.

Predation pressure in maize across Europe and in Argentina: an intercontinental comparison

Humankind draws important benefits from large-scale ecological processes termed ecosystem services, yet the status of several of them is declining. Reliable monitoring methods are essential for tracking the status of ecosystem services. Predation is the mainstay of natural pest control, a key ecosystem service. We used green plasticine caterpillars to monitor predation pressure, and to obtain baseline data on predator activity in transgenic Bt versus non-Bt maize fields in Old and New World countries. Predation pressure was measured at ground and canopy levels using an identical, small-plot experimental design in four European countries (Denmark, Slovakia, Romania and Italy) and Argentina. Total predation rate in maize was 11.7%d^-1 (min. 7.2%d^-1 in Argentina, max. 29.0%d^-1 in Romania). Artificial caterpillars were attacked both by invertebrates (mostly chewing insects with 42.0% of the attack marks, and ants with 7.1%, but also predatory and parasitoid wasps, spiders and slugs), and vertebrates (small mammals 25.5%, and birds 20.2%). Total predation at ground level (15.7%d^-1 ) was significantly higher than in maize canopies (6.0%d^-1 ) in all countries, except Argentina. We found no significant differences between predator pressure in Bt versus non-Bt maize plots. The artificial caterpillar method provided comparable, quantitative data on predation intensity, and proved to be suitable for monitoring natural pest control. This method usefully expands the existing toolkit by directly measuring ecological function rather than structure.

Molecular analysis indicates high levels of carabid weed seed consumption in cereal fields across Central Europe

Carabid beetles are abundant in temperate agroecosystems and can play a pivotal role as biocontrol agents. While there is good knowledge regarding their effects on invertebrate pests in some systems, comparably little is known on the rate of seed feeding under field conditions. Molecular approaches are ideally suited for investigating carabid feeding interactions; to date, however, they have only been applied to animal prey. We sampled adult carabid beetles in organic cereal fields in three regions along a Central European transect. Regurgitates from populations of the three most common species, Poecilus cupreus, Pseudoophonus rufipes and Pterostichus melanarius, were screened for plant DNA, cereal aphids, collembolans and earthworms. The frequency of carabid individuals positive for plant DNA was high (> 70%) and independent of carabid species, sex, region and the time point of sampling. Detections for non-pest and pest prey were comparably lower, with 21.6% for collembolans, 18.1% for earthworms and 4.2% for aphids, respectively. Despite the prolonged detection period of plant DNA in carabid guts, as compared to animal prey, these first results suggest that weed seeds form an important part of the adult carabid diet. It would also lend support to the hypothesis that seed-feeding carabids are biocontrol agents of weeds, with effects of regulation on the weed seedbank that depend on behavioural and contextual factors including carabid species preferences for weed seed species, their life stage and tillage practices.

Organic farming at local and landscape scales fosters biological pest control in vineyards

While organic farming practices, which are often promoted as models of ecological intensification, generally enhance biodiversity, their effects on the delivery of ecosystem services, such as biological pest control, are still unknown. Here, using a multi-scale hierarchical design in southwestern France, we examined the effects of organic farming and seminatural habitats at the local and landscape scales on biological control services of three pests, including weeds and insects, in 42 vineyards. Organic farming at the local and landscape scales was beneficial to the mean and temporal stability of biological control services, while the proportion of seminatural habitats in the landscape reduced the level of biological pest control potential. The effects of organic farming and seminatural habitats across spatial scales varied with the type of prey considered and with time. Egg moth removal rates were higher in fields under organic management compared to conventional management while weed seed removal rates increased with the proportion of organic farming in the landscape. Larval removal rates as well as seed removal rates were always more stable within time in organic fields than in conventional fields. Moreover, independently of farming system type, local variables describing the agricultural management intensity, such as pesticide use or crop productivity, were also found to be important variables explaining levels of biological control services. Pesticide use tended to reduce biological control potential, while crop productivity was associated with contrasting biological control responses depending on the pest type. Our study demonstrates the need to target multiple spatial scales and to consider farming practices, as well as the proportion of seminatural habitats, to design functional landscapes that optimize biological pest control services.

Predation-mediated ecosystem services and disservices in agricultural landscapes

Ecological intensification may reduce environmental externalities of agriculture by harnessing biodiversity to benefit regulating ecosystem services. However, to propose management options for the production of such services, there is a need to understand the spatiotemporal dynamics of net effects between ecosystem services and disservices provided by wild organisms across taxonomic groups in relation to habitat and landscape management. We studied the contribution of predatory vertebrates and invertebrates (including both carnivores and seed herbivores) to regulating ecosystem services and disservices in 16 cereal fields in response to a local habitat contrast and a landscape complexity gradient. From May to November 2016, we provided weed (predation reflects an ecosystem service) and crop (predation reflects a disservice) seeds, as well as pest (predation reflects an ecosystem service) and beneficial (predation reflects a disservice) invertebrate prey to predators. Seed predation was dominated by vertebrates, while vertebrates and invertebrates contributed equally to predation of animal prey. Before harvest, predation steadily increased from very low levels in May to high levels in July independent of the resource type. After harvest, ecosystem services declined more rapidly than disservices. The presence of adjacent seminatural grasslands promoted crop seed predation, but reduced pest prey predation. Predation on beneficial prey decreased with increasing proportions of seminatural grassland in the landscape. Predatory vertebrates and invertebrates provide important ecosystem services due to the consumption of pests. However, beneficial invertebrates and crop seeds were often consumed to a similar or even higher extent than harmful invertebrates or weed seeds. Our results therefore raise concerns that management options aimed at enhancing service providers may simultaneously increase levels of disservices. By considering positive and negative effects simultaneously, this study addresses an important knowledge gap and highlights the importance of interactions between local management, landscape composition, and service and disservice provision across taxa and over time. Considering trade-offs between ecosystem services and disservices when evaluating the net effects of biodiversity conservation measures on ecosystem service provision is crucial. Future agri-environment schemes that offer payments for seminatural habitats may need to provide higher compensation for farmers in cases where net effects are likely to be negative.

Rodents, not birds, dominate predation-related ecosystem services and disservices in vertebrate communities of agricultural landscapes

To understand the relationship between conservation measures and agricultural yields, we need to know the contributions of organisms to both ecosystem services and disservices. We studied the activity and contribution of birds and mammals to intermediate ecosystem services (predation of weed seeds or invertebrate pests) and disservices (predation of crop seeds or beneficial invertebrates) in southern Sweden between June and November 2016. We measured seed and invertebrate predation rates using trays placed in front of 32 wildlife cameras in 16 cereal fields with a local habitat contrast (8 fields adjacent to another crop field and 8 fields adjacent to a semi-natural grassland) and along a landscape heterogeneity gradient (amount of semi-natural grassland). Both activity and predation were dominated by small mammals (mainly rodents), yet only a few species contributed to predation services and disservices according to camera records. Small mammal activity and predation varied considerably over time. Small mammal activity was significantly higher at trays with crop seeds or beneficial invertebrate prey compared to trays with pest prey, and crop seed predation by small mammals was significantly higher than weed seed predation. In contrast, bird activity and predation did not differ significantly between resource types, but varied over time depending on the habitat contrast. Predation of animal prey by birds was highest after cereal harvest, independent of habitat contrast. Our study highlights that birds and in particular rodents provide important intermediate ecosystem services, but also disservices, which fluctuate strongly in intensity over time.

Unveiling community patterns and trophic niches of tropical and temperate ants using an integrative framework of field data, stable isotopes and fatty acids

Background

The use and partitioning of trophic resources is a central aspect of community function. On the ground of tropical forests, dozens of ant species may be found together and ecological mechanisms should act to allow such coexistence. One hypothesis states that niche specialization is higher in the tropics, compared to temperate regions. However, trophic niches of most species are virtually unknown. Several techniques might be combined to study trophic niche, such as field observations, fatty acid analysis (FAA) and stable isotope analysis (SIA). In this work, we combine these three techniques to unveil partitioning of trophic resources in a tropical and a temperate community. We describe patterns of resource use, compare them between communities, and test correlation and complementarity of methods to unveil both community patterns and species' niches.

Methods

Resource use was assessed with seven kinds of bait representing natural resources available to ants. Neutral lipid fatty acid (NLFA) profiles, and δ¹⁵N and δ¹³C isotope signatures of the species were also obtained. Community patterns and comparisons were analyzed with clustering, correlations, multivariate analyses and interaction networks.

Results

Resource use structure was similar in both communities. Niche breadths (H') and network metrics (Q and H₂') indicated similar levels of generalization between communities. A few species presented more specialized niches, such as Wasmannia auropunctata and Lasius fuliginosus. Stable isotope signatures and NLFA profiles also indicated high generalization, although the latter differed between communities, with temperate species having higher amounts of fat and proportions of C18:1n9. Bait use and NLFA profile similarities were correlated, as well as species' specialization indices (d') for the two methods. Similarities in δ¹⁵N and bait use, and in δ¹³C and NLFA profiles, were also correlated.

Discussion

Our results agree with the recent view that specialization levels do not change with latitude or species richness. Partition of trophic resources alone does not explain species coexistence in these communities, and might act together with behavioral and environmental mechanisms. Temperate species presented NLFA patterns distinct from tropical ones, which may be related to environmental factors. All methods corresponded in their characterization of species' niches to some extent, and were robust enough to detect differences even in highly generalized communities. However, their combination provides a more comprehensive picture of resource use, and it is particularly important to understand individual niches of species. FAA was applied here for the first time in ant ecology, and proved to be a valuable tool due to its combination of specificity and temporal representativeness. We propose that a framework combining field observations with chemical analysis is valuable to understand resource use in ant communities.

Cover Crop Species and Management Influence Predatory Arthropods and Predation in an Organically Managed, Reduced-Tillage Cropping System

Agricultural practices affect arthropod communities and, therefore, have the potential to influence the activities of arthropods. We evaluated the effect of cover crop species and termination timing on the activity of ground-dwelling predatory arthropods in a corn-soybean-wheat rotation in transition to organic production in Pennsylvania, United States. We compared two cover crop treatments: 1) hairy vetch (Vicia villosa Roth) planted together with triticale (×Triticosecale Wittmack) after wheat harvest, and 2) cereal rye (Secale cereale Linnaeus) planted after corn harvest. We terminated the cover crops in the spring with a roller-crimper on three dates (early, middle, and late) based on cover crop phenology and standard practices for cash crop planting in our area. We characterized the ground-dwelling arthropod community using pitfall traps and assessed relative predation using sentinel assays with live greater waxworm larvae (Galleria mellonella Fabricius). The activity density of predatory arthropods was significantly higher in the hairy vetch and triticale treatments than in cereal rye treatments. Hairy vetch and triticale favored the predator groups Araneae, Opiliones, Staphylinidae, and Carabidae. Specific taxa were associated with cover crop condition (e.g., live or dead) and termination dates. Certain variables were positively or negatively associated with the relative predation on sentinel prey, depending on cover crop treatment and stage, including the presence of predatory arthropods and various habitat measurements. Our results suggest that management of a cover crop by roller-crimper at specific times in the growing season affects predator activity density and community composition. Terminating cover crops with a roller-crimper can conserve generalist predators.

Combined effects of agrochemicals and ecosystem services on crop yield across Europe

Simultaneously enhancing ecosystem services provided by biodiversity below and above ground is recommended to reduce dependence on chemical pesticides and mineral fertilisers in agriculture. However, consequences for crop yield have been poorly evaluated. Above ground, increased landscape complexity is assumed to enhance biological pest control, whereas below ground, soil organic carbon is a proxy for several yield-supporting services. In a field experiment replicated in 114 fields across Europe, we found that fertilisation had the strongest positive effect on yield, but hindered simultaneous harnessing of below- and above-ground ecosystem services. We furthermore show that enhancing natural enemies and pest control through increasing landscape complexity can prove disappointing in fields with low soil services or in intensively cropped regions. Thus, understanding ecological interdependences between land use, ecosystem services and yield is necessary to promote more environmentally friendly farming by identifying situations where ecosystem services are maximised and agrochemical inputs can be reduced.

Patterns and dynamics of neutral lipid fatty acids in ants - implications for ecological studies

BACKGROUND

Trophic interactions are a fundamental aspect of ecosystem functioning, but often difficult to observe directly. Several indirect techniques, such as fatty acid analysis, were developed to assess these interactions. Fatty acid profiles may indicate dietary differences, while individual fatty acids can be used as biomarkers. Ants are among the most important terrestrial animal groups, but little is known about their lipid metabolism, and no study so far used fatty acids to study their trophic ecology. We set up a feeding experiment with high- and low-fat food to elucidate patterns and dynamics of neutral lipid fatty acids (NLFAs) assimilation in ants. We asked whether dietary fatty acids are assimilated through direct trophic transfer, how diet influences NLFA total amounts and patterns over time, and whether these assimilation processes are similar across species and life stages.

RESULTS

Ants fed with high-fat food quickly accumulated specific dietary fatty acids (C18:2n6, C18:3n3 and C18:3n6), compared to ants fed with low-fat food. Dietary fat content did not affect total body fat of workers or amounts of fatty acids extensively biosynthesized by animals (C16:0, C18:0, C18:1n9). Larval development had a strong effect on the composition and amounts of C16:0, C18:0 and C18:1n9. NLFA compositions reflected dietary differences, which became more pronounced over time. Assimilation of specific dietary NLFAs was similar regardless of species or life stage, but these factors affected dynamics of other NLFAs, composition and total fat.

CONCLUSIONS

We showed that ants accumulated certain dietary fatty acids via direct trophic transfer. Fat content of the diet had no effect on lipids stored by ants, which were able to synthesize high amounts of NLFAs from a sugar-based diet. Nevertheless, dietary NLFAs had a strong effect on metabolic dynamics and profiles. Fatty acids are a useful tool to study trophic biology of ants, and could be applied in an ecological context, although factors that affect NLFA patterns should be taken into account. Further studies should address which NLFAs can be used as biomarkers in natural ant communities, and how factors other than diet affect fatty acid dynamics and composition of species with distinct life histories.

Methods to identify the prey of invertebrate predators in terrestrial field studies

Predation is an interaction during which an organism kills and feeds on another organism. Past and current interest in studying predation in terrestrial habitats has yielded a number of methods to assess invertebrate predation events in terrestrial ecosystems. We provide a decision tree to select appropriate methods for individual studies. For each method, we then present a short introduction, key examples for applications, advantages and disadvantages, and an outlook to future refinements. Video and, to a lesser extent, live observations are recommended in studies that address behavioral aspects of predator–prey interactions or focus on per capita predation rates. Cage studies are only appropriate for small predator species, but often suffer from a bias via cage effects. The use of prey baits or analyses of prey remains are cheaper than other methods and have the potential to provide per capita predation estimates. These advantages often come at the cost of low taxonomic specificity. Molecular methods provide reliable estimates at a fine level of taxonomic resolution and are free of observer bias for predator species of any size. However, the current PCR‐based methods lack the ability to estimate predation rates for individual predators and are more expensive than other methods. Molecular and stable isotope analyses are best suited to address systems that include a range of predator and prey species. Our review of methods strongly suggests that while in many cases individual methods are sufficient to study specific questions, combinations of methods hold a high potential to provide more holistic insights into predation events. This review presents an overview of methods to researchers that are new to the field or to particular aspects of predation ecology and provides recommendations toward the subset of suitable methods to identify the prey of invertebrate predators in terrestrial field research.