Selectivity underlies the dissociation between seasonal prey availability and prey consumption in a generalist predator

Two wild carnivores selectively forage for prey but not amino acids

In nutritional ecology the intake target is the diet that maximises consumer fitness. A key hypothesis of nutritional ecology is that natural selection has acted upon the behavioural and physiological traits of consumers to result in them Selectively Consuming prey to match the Intake Target (SCIT). SCIT has been documented in some herbivores and omnivores, which experience strong heterogeneity in the nutritional quality of available foods. Although carnivores experience a prey community with a much more homogeneous nutrient composition, SCIT by carnivores has nevertheless been deemed highly likely by some researchers. Here we test for SCIT for micronutrients (amino acids) in two freshwater carnivores: the river blackfish and the two-spined blackfish. Although both blackfishes exhibited non-random consumption of prey from the environment, this resulted in non-random consumption of amino acids in only one species, the river blackfish. Non-random consumption of amino acids by river blackfish was not SCIT, but instead an artefact of habitat-specific foraging. We present hypotheses to explain why wild populations of freshwater carnivores may not exhibit SCIT for amino acids. Our work highlights the need for careful, critical tests of the hypotheses and assumptions of nutritional ecology and its application to wild populations.

Poison frog dietary preference depends on prey type and alkaloid load

The ability to acquire chemical defenses through the diet has evolved across several major taxa. Chemically defended organisms may need to balance chemical defense acquisition and nutritional quality of prey items. However, these dietary preferences and potential trade-offs are rarely considered in the framework of diet-derived defenses. Poison frogs (Family Dendrobatidae) acquire defensive alkaloids from their arthropod diet of ants and mites, although their dietary preferences have never been investigated. We conducted prey preference assays with the Dyeing Poison frog (Dendrobates tinctorius) to test the hypothesis that alkaloid load and prey traits influence frog dietary preferences. We tested size preferences (big versus small) within each of four prey groups (ants, beetles, flies, and fly larvae) and found that frogs preferred interacting with smaller prey items of the fly and beetle groups. Frog taxonomic prey preferences were also tested as we experimentally increased their chemical defense load by feeding frogs decahydroquinoline, an alkaloid compound similar to those naturally found in their diet. Contrary to our expectations, overall preferences did not change during alkaloid consumption, as frogs across groups preferred fly larvae over other prey. Finally, we assessed the protein and lipid content of prey items and found that small ants have the highest lipid content while large fly larvae have the highest protein content. Our results suggest that consideration of toxicity and prey nutritional value are important factors in understanding the evolution of acquired chemical defenses and niche partitioning.

Assassin snails (Anentome helena) as a biological model for exploring the effects of individual specialisation within generalist predators

Quantifying feeding behaviour of generalist predators at the population and individual levels is crucial for understanding the structure and functioning of food webs. Individual predator/consumer feeding niches can be significantly narrower than that of the population across animal taxa. In such species, the population of a generalist predator becomes essentially an ensemble of specialist individuals and this often highly affects the dynamics of the prey-predator interactions. Currently, few experimental systems exist that are both easily technically manipulated in a lab and are reliable to accurately assess effects of individual specialisation within generalist predators. Here we argue that a freshwater predaceous snail, Anentome helena (also known as an ‘assassin snail’), is a convenient and reliable experimental system to study feeding of a generalist predator on multiple food types which exhibits well-pronounced specialisation of foraging individuals. Using A. helena we experimentally test: (i) how relative prey abundances in the environment affect the feeding patterns, (ii) whether the feeding patterns are consistent over the duration of the experimental period, and (iii) compare the feeding niche breadth of individuals to that of the laboratory population. By offering four different prey snail species, at a range of relative abundances, we show that there are consistent patterns in feeding. Importantly, the consumption of each prey was independent of the relative abundance at which they were present. Individual predators showed selectivity to a particular prey, i.e. the population of assassin snails seems to be formed of individuals that specialise on different prey. Our findings would contribute to the recent revision and the ongoing debate on the classification of predator species into generalists and specialists.

Rich and abundant spider communities result from enhanced web capture breadth and reduced overlap in urban greenspaces

Urbanization is a key contributor to biodiversity loss, but evidence is mounting that cities can support rich arthropod communities, including rare and threatened species. Furthermore, greenspace is growing within hundreds of "shrinking cities" that have lost population resulting in a need to demolish an overabundance of infrastructure creating vacant land. Efforts are underway to transform vacant lots, often viewed as blighted areas, into habitats that promote biodiversity and generate ecosystem services, such as urban agroecosystems. To understand how reconfiguring these greenspaces might influence species conservation, elucidation of the factors that drive the distribution of an urban species pool is needed. In particular, the importance of species interactions in structuring urban communities is poorly understood. We tested hypotheses that (1) greater breadth of prey captured by web-building spiders and reduced overlap of prey capture among individuals facilitates the conservation of genera richness and abundance and (2) heterogeneity within a greenspace patch facilitates enhanced dietary niche breadth and greater resource partitioning. In 2013 and 2014, the abundance, breadth and degree of overlap in prey capture of sheet web spiders (Linyphiidae) was measured using web mimic traps at 160 microsites (0.25 m² ) situated in four urban vacant lots and four urban farms in the city of Cleveland, Ohio, USA. Within a subset of 40 microsites, we used vacuum sampling and hand collection to measure the abundance and genera richness of Linyphiidae. Spider richness and abundance were significantly reduced within urban farms relative to vacant lots. The distribution of spiders and prey was explained by habitat structure, with microsites dominated by tall grasses and flowering plants, with a high bloom abundance and richness, supporting greater prey capture and a higher genera richness and abundance of spiders. In 2014, web capture overlap was significantly greater within microsites dominated by bare ground. These findings illustrate that urban greenspace conservation efforts that focus on reducing bare ground and incorporating a diversity of grasses and flowering plant species can promote linyphiid spiders, potentially by relaxing exploitative competition for shared prey.

Predator preferences shape the diets of arthropodivorous bats more than quantitative local prey abundance

Although most predators are generalists, the majority of studies on the association between prey availability and prey consumption have focused on specialist predators. To investigate the role of highly generalist predators in a complex food web, we measured the relationships between prey consumption and prey availability in two common arthropodivorous bats. Specifically, we used high-throughput amplicon sequencing coupled with a known mock community to characterize seasonal changes in little brown and big brown bat diets. We then linked spatiotemporal variation in prey consumption with quantitative prey availability estimated from intensive prey community sampling. We found that although quantitative prey availability fluctuated substantially over space and time, the most commonly consumed prey items were consistently detected in bat diets independently of their respective abundance. Positive relationships between prey abundance and probability of consumption were found only among prey groups that were less frequently detected in bat diets. While the probability of prey consumption was largely unrelated to abundance, the community structure of prey detected in bat diets was influenced by the local or regional abundance of prey. Observed patterns suggest that while little brown and big brown bats maintain preferences for particular prey independently of quantitative prey availability, total dietary composition may reflect some degree of opportunistic foraging. Overall, our findings suggest that generalist predators can display strong prey preferences that persist despite quantitative changes in prey availability.

Energy balance and metabolic changes in an overwintering wolf spider, Schizocosa stridulans

Winter provides many challenges for terrestrial arthropods, including low temperatures and decreased food availability. Most arthropods are dormant in the winter and resume activity when conditions are favorable, but a select few species remain active during winter. Winter activity is thought to provide a head start on spring growth and reproduction, but few studies have explicitly tested this idea or investigated tradeoffs associated with winter activity. Here, we detail biochemical changes in overwintering winter-active wolf spiders, Schizocosa stridulans, to test the hypothesis that winter activity promotes growth and energy balance. We also quantified levels of putative cryoprotectants throughout winter to test the prediction that winter activity is incompatible with biochemical adaptations for coping with extreme cold. Body mass of juveniles increased 3.5-fold across winter, providing empirical evidence that winter activity promotes growth and therefore advancement of spring reproduction. While spiders maintained protein content throughout most of the winter, lipid content decreased steadily, suggesting either a lack of available prey to maintain lipids, or more likely, an allometric shift in body composition as spiders grew larger. Carbohydrate content showed no clear seasonal trend but also tended to be higher at the beginning of the winter. Finally, we tested the hypothesis that winter activity is incompatible with cryoprotectant accumulation. However, we observed accumulation of glycerol, myo-inositol, and several other cryoprotectants, although levels were lower than those typically observed in overwintering arthropods. Together, our results indicate that winter-active wolf spiders grow during the winter, and while cryoprotectant accumulation was observed in the winter, the modest levels relative to other species could make them susceptible to extreme winter events.

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.

High-throughput sequencing for community analysis: the promise of DNA barcoding to uncover diversity, relatedness, abundances and interactions in spider communities

Large-scale studies on community ecology are highly desirable but often difficult to accomplish due to the considerable investment of time, labor and, money required to characterize richness, abundance, relatedness, and interactions. Nonetheless, such large-scale perspectives are necessary for understanding the composition, dynamics, and resilience of biological communities. Small invertebrates play a central role in ecosystems, occupying critical positions in the food web and performing a broad variety of ecological functions. However, it has been particularly difficult to adequately characterize communities of these animals because of their exceptionally high diversity and abundance. Spiders in particular fulfill key roles as both predator and prey in terrestrial food webs and are hence an important focus of ecological studies. In recent years, large-scale community analyses have benefitted tremendously from advances in DNA barcoding technology. High-throughput sequencing (HTS), particularly DNA metabarcoding, enables community-wide analyses of diversity and interactions at unprecedented scales and at a fraction of the cost that was previously possible. Here, we review the current state of the application of these technologies to the analysis of spider communities. We discuss amplicon-based DNA barcoding and metabarcoding for the analysis of community diversity and molecular gut content analysis for assessing predator-prey relationships. We also highlight applications of the third generation sequencing technology for long read and portable DNA barcoding. We then address the development of theoretical frameworks for community-level studies, and finally highlight critical gaps and future directions for DNA analysis of spider communities.

Consuming alternative prey does not influence the DNA detectability half-life of pest prey in spider gut contents

BACKGROUND

Key natural enemy-pest interactions can be mapped in agricultural food webs by analysing predator gut content for the presence of a focal pest species. For this, PCR-based approaches are the most widely used methods providing the incidence of consumption of a focal pest in field sampled predators. To interpret such data the rate of prey DNA decay in the predators' gut, described by DNA detectability half-life (t 1/2), is needed. DNA decay may depend on the presence of alternative prey in the gut of generalist predators, but this effect has not been investigated in one of the major predatory arthropod groups, spiders.

METHODS

In a laboratory feeding experiment, we determined t 1/2 of the key cereal pest virus vector leafhopper Psammotettix alienus in the digestive tracts of its natural enemy, the spider Tibellus oblongus. We followed the fate of prey DNA in spiders which received only the focal prey as food, or as an alternative prey treatment they also received a meal of fruit flies after leafhopper consumption. After these feeding treatments, spiders were starved for variable time intervals prior to testing for leafhopper DNA in order to establish t 1/2.

RESULTS

We created a PCR protocol that detects P. alienus DNA in its spider predator. The protocol was further calibrated to the digestion speed of the spider by establishing DNA decay rate. Detectability limit was reached at 14 days, where c. 10% of the animals tested positive. The calculated t 1/2 = 5 days value of P. alienus DNA did not differ statistically between the treatment groups which received only the leafhopper prey or which also received fruit fly. The PCR protocol was validated in a field with known P. alienus infestation. In this applicability trial, we showed that 12.5% of field collected spiders were positive for the leafhopper DNA. We conclude that in our model system the presence of alternative prey did not influence the t 1/2 estimate of a pest species, which makes laboratory protocols more straightforward for the calibration of future field data.

Does prey encounter and nutrient content affect prey selection in wolf spiders inhabiting Bt cotton fields?

Wolf spiders are abundant and voracious predators at the soil-plant interface in cotton crops. Among other prey, they attack late-instar larvae of the cotton bollworm Helicoverpa spp., an economically important pest. Consequently, wolf spiders in transgenic Bt cotton could provide significant biological control of Bt-resistant Helicoverpa larvae that descend to the soil to pupate. The predator-prey interactions between wolf spiders and Helicoverpa could, however, be constrained by the presence of alternative prey and intraguild predators. This study used laboratory enclosures to analyse the effect of alternative prey on predatory selection of the wolf spider Tasmanicosa leuckartii Thorell. The prey included another wolf spider Hogna crispipes Koch (potential intraguild predator), the ground cricket Teleogryllus commodus Walker (minor pest), and Helicoverpa armigera larvae (major pest). We tested if encounter rates, prey vulnerability, and prey nutritional content influenced the likelihood that a prey was attacked. In three-way food webs, Tasmanicosa encountered and attacked Teleogryllus and Helicoverpa in similar frequencies. However, in the presence of a competing intraguild predator and potential prey (Hogna) in a four-way food web, Tasmanicosa did not always attack Teleogryllus at first encounter, but still attacked Helicoverpa at each encounter. Helicoverpa (protein-poor) and Hogna (protein-rich) were consumed by Tasmanicosa in similar proportions, suggesting that Tasmanicosa might benefit from nutrient balance as an outcome of diverse prey in this food web. As Teleogryllus (protein rich) escapes quicker than Helicoverpa and Hogna, Hogna may be an easier protein-rich option than Teleogryllus. Field surveys showed that while Teleogryllus was the most common prey, wolf spiders feed on diverse insect taxa, as well as other spiders. That Tasmanicosa readily attacked Helicoverpa larvae in the presence of alternative prey is an encouraging result that supports the potential of Tasmanicosa predation to assist in the control of Bt-resistant Helicoverpa larvae and thereby inhibit the proliferation and spread of resistance.

Seasonally varying marine influences on the coastal ecosystem detected through molecular gut analysis

Terrestrial predators on marine shores benefit from the inflow of organisms and matter from the marine ecosystem, often causing very high predator densities and indirectly affecting the abundance of other prey species on shores. This indirect effect may be particularly strong if predators shift diets between seasons. We therefore quantified the seasonal variation in diet of two wolf spider species that dominate the shoreline predator community, using molecular gut content analyses with general primers to detect the full prey range. Across the season, spider diets changed, with predominantly terrestrial prey from May until July and predominantly marine prey (mainly chironomids) from August until October. This pattern coincided with a change in the spider age and size structure, and prey abundance data and resource selection analyses suggest that the higher consumption of chironomids during autumn is due to an ontogenetic diet shift rather than to variation in prey abundance. The analyses suggested that small dipterans with a weak flight capacity, such as Chironomidae, Sphaeroceridae, Scatopsidae and Ephydridae, were overrepresented in the gut of small juvenile spiders during autumn, whereas larger, more robust prey, such as Lepidoptera, Anthomyidae and Dolichopodidae, were overrepresented in the diet of adult spiders during spring. The effect of the inflow may be that the survival and growth of juvenile spiders is higher in areas with high chironomid abundances, leading to higher densities of adult spiders and higher predation rates on the terrestrial prey next spring.