Predator diversity effects cascade across an ecosystem boundary

Food webs in different ecosystems are often connected through spatial resource subsidies. As a result, biodiversity effects in one ecosystem may cascade to adjacent ecosystems. I tested the hypothesis that aquatic predator diversity effects cascade to terrestrial food webs by altering a prey subsidy (biomass and trophic structure of emerging aquatic insects) entering terrestrial food webs, in turn altering the distribution of a terrestrial consumer (spider) that feeds on emerging aquatic insects. Fish presence, but not diversity, altered the trophic structure of emerging aquatic insects by strongly reducing the biomass of emerging predators (dragonflies) relative to non‐feeding taxa (chironomid midges). Fish diversity reduced emerging insect biomass through enhanced effects on the most common prey taxa: predatory dragonflies Pantala flavescens and non‐feeding chironomids. Terrestrial spiders (Tetragnathidae) primarily captured emerging chironomids, which were reduced in the high richness (3 spp.) treatment relative to the 1 and 2 species treatments. As a result, terrestrial spider abundance was lower above pools with high fish richness (3 species) than pools with 1 and 2 species. Synergistic predation effects were mostly limited to the high richness treatment, in which fish occupied each level of vertical microhabitat in the water‐column (benthic, middle, surface). This study demonstrates that predator diversity effects are not limited to the habitat of the predator, but can propagate to adjacent ecosystems, and demonstrates the utility of using simple predator functional traits (foraging domain) to more accurately predict the direction of predator diversity effects.     

Predator foraging strategy mediates the effects of predators on local and emigrating prey

Predation is a dominant structuring force in ecosystems, but its effects are almost always measured in the ecosystem of the predator. However, the effects of predators can potentially extend across ecosystem boundaries during ontogenetic niche shifts in prey. We compared the effects of fish predation on benthic versus emerging aquatic insects, and hypothesized that the relative effects of fish on these two stages of prey are mediated by fish foraging strategy (benthic versus water‐column feeders). Benthic‐feeding smallmouth buffalo reduced benthic insect biomass in the freshwater ecosystem by 89%, and reduced insect emergence to the terrestrial ecosystem by 65%. In contrast, water‐column feeding sunfish had no effect on benthic biomass in the freshwater ecosystem, but reduced emergence to the terrestrial ecosystem by 44% relative to the fishless control. When smallmouth and sunfish were combined in a substitutive design that kept total fish density the same as the single species treatments, their effects on benthic insects (50% reduction) were weaker than expected based on predictions from the single species treatments. In contrast, their combined effects on emergence (46% reduction) were additive. Tetragnathid spider densities increased during peak emergence, but did not respond to changes in emergence among treatments. These results demonstrate that the effects of fish on prey flux to the terrestrial ecosystem are not the same as their effects on benthic prey biomass in the aquatic ecosystem, and that this difference is likely mediated by foraging strategy.     

Evidence of exploitative competition among young stages of the wolf spider Schizocosa ocreata

Summary Previous research by many investigators has demonstrated food limitation in both web-building and wandering spiders. Field experiments have tested for exploitative competition for prey in web-building, but not wandering species. As a first step to examining the question of whether spiders without webs exhibit exploitative competition, we manipulated densities of young stages of a common wolf spider, Schizocos ocreata, and measured (1) spider growth rate and (2) numbers of Collembola, a potential prey organism. Replicate populations of recently hatched S. ocreata were established in 1-m² fenced plots at four levels: 0×, 0.25×, 1× and 4× natural density. Increasing spider density had a negative effect on spider growth rate, defined as increase in weight or cephalothorax width. Early in the experiment spider density had a weak negative effect on Collembola numbers [p(F)=0.08]. Taken together, this probable response by Collembola and the clear effect of spider density on growth rate constitute the first experimental evidence of intraspecific exploitative competition for prey in a species of wandering spider. We discuss (1) the strength of this evidence given the constraints of the experiment's design, and (2) the implications of the strong convergence in spider densities that had occurred after 2.5 months.     

Differences in venom composition between orb-weaving and wandering Hawaiian Tetragnatha (Araneae)

Spider venoms are complex mixtures of toxins that are used primarily for immobilizing prey. There is evidence of chemical variation in spider venoms among close relatives, yet few studies have analysed their evolution within an ecological and phylogenetic framework. On the Hawaiian archipelago, Tetragnatha, a cosmopolitan orb-weaving genus, has undergone a radiation in which a monophyletic lineage has abandoned web-building and become obligately wandering foragers. This study compares venom composition and details of feeding behaviour between orb-weaving and wandering Hawaiian Tetragnatha. Protein gel electrophoresis patterns indicated that relative to orb-weavers, wandering species had a reduced concentration of low molecular weight (<14kDa) components. Both orb-weaving and wandering Tetragnatha captured flying prey (adult lepidopterans, dipterans), but wandering spiders also captured non-flying prey (insect larvae, spiders). There were no distinct differences between orb-weavers and wanderers in prey capture and immobilization sequences, or in the paralytic effects of bites on prey. However, prey bitten by wanderers took longer to be permanently immobilized than prey bitten by orb-weavers. Contrary to predictions, there was no indication that web-loss in this group was associated with an increase in venom potency.     

The importance of sward architectural complexity in structuring predatory and phytophagous invertebrate assemblages

Abstract.  1. Although the importance of plant community assemblages in structuring invertebrate assemblages is well known, the role that architectural complexity plays is less well understood. In particular, direct empirical data for a range of invertebrate taxa showing how functional groups respond to plant architecture is largely absent from the literature.
2. The significance of sward architectural complexity in determining the species richness of predatory and phytophagous functional groups of spiders, beetles, and true bugs, sampled from 135 field margin plots over 2 years was tested. The present study compares the relative importance of sward architectural complexity to that of plant community assemblage.
3. Sward architectural complexity was found to be a determinant of species richness for all phytophagous and predatory functional groups. When individual species responses were investigated, 62.5% of the spider and beetle species, and 50.0% of the true bugs responded to sward architectural complexity.
4. Interactions between sward architectural complexity and plant community assemblage indicate that the number of invertebrate species supported by the plant community alone could be increased by modification of sward architecture. Management practices could therefore play a key role in diversifying the architectural structure of existing floral assemblages for the benefit of invertebrate assemblages.
5. The contrasting effects of sward architecture on invertebrate functional groups characterised by either direct (phytophagous species) or indirect (predatory species) dependence on plant communities is discussed. It is suggested that for phytophagous taxa, plant community assemblage alone is likely to be insufficient to ensure successful species colonisation or persistence without appropriate development of sward architecture.     

Effects of Spider Chemotactile Cues on Arthropod Behavior

Predation risk can strongly affect the behavior of prey species. However, empirical evidence for changes in behavior driven by spider cues is restricted to relatively few taxa. Here, we conducted a series of behavioral experiments to test for changes in activity among a wide range of terrestrial arthropods. We confronted 13 insect and eight spider species with chemotactile cues of three spider species. We applied two different experimental setups: In the ‘no-choice experiment’ prey individuals were either put on control filter papers or on filter papers previously occupied by a spider. In the ‘choice experiment’, the prey individuals were able to choose between filter paper halves with and without spider cues. In both setups, the response to spider cues depended significantly on prey species, with some species increasing and others decreasing their activity. Surprisingly few prey species responded to the spider cues at all. Our results indicate that predator recognition upon contact with cue bearing filter papers is strongly prey-specific and that behavioral effects driven by spider chemotactile cues are an exception rather than the rule among terrestrial arthropods.     

Prey preference,intraguild predation and population dynamics of an arthropod food web on plants

The theory of intraguild predation (IGP) largely studies effects on equilibrium densities of predators and prey, while experiments mostly concern transient dynamics. We studied the effects of an intraguild (IG) predator, the bug Orius laevigatus, on the population dynamics of IG-prey, the predatory mite Phytoseiulus persimilis, and a shared prey, the phytophagous two-spotted spider mite Tetranychus urticae, as well as on the performance of cucumber plants in a greenhouse. The interaction of the predatory mite and the spider mite is highly unstable, and ends either by herbivores overexploiting the plant or predators exterminating the herbivores. We studied the effect of IGP on the transient dynamics of this system, and compared the dynamics with that predicted by a simple population-dynamical model with IGP added. Behavioural studies showed that the predatory bug and the predatory mite were both attracted to plants infested by spider mites and that the two predators did not avoid plants occupied by the other predator. Observations on foraging behaviour of the predatory bug showed that it attacks and kills large numbers of predatory mites and spider mites. The model predicts strong effects of predation and prey preference by the predatory bugs on the dynamics of predatory mites and spider mites. However, experiments in which the predatory bug was added to populations of predatory mites and spider mites had little or no effect on numbers of both mite species, and cucumber plant and fruit weight.     

Importance of habitat structure to the arthropod food-web in Douglas-fir canopies

This study tested the hypothesis that habitat structure dictates the distribution and community composition of arboreal arthropods. A diverse arthropod assemblage of Douglas-fir canopies, which included Araneae, Psocoptera, Collembola and Homoptera, was chosen as a model system. Habitat structural diversity, defined as needle density and branching complexity of Douglas-fir branches, was manipulated in a four-month experiment by needle removal, thinning and tying of branches. Abundance of canopy spiders declined significantly following needle density reduction and branch thinning, branch tying significantly increased spider abundance. Distinct habitat utilization patterns were found among individual spider guilds. Orb weavers (Araneidae) dominated spider assemblages in structurally simple habitats, whereas tied branches were colonized primarily by sheet-web weavers (Linyphiidae) and nocturnal hunting spiders (Anyphaenidae and Clubionidae). Spider species richness and average body size of several spider species increased in structurally more complex habitats. Arboreal spiders appeared to be limited by strong bottom-up effects in the form of habitat quality and, to a lesser degree, prey availability. Habitat manipulations did not affect densities or biomass of flying arthropod colonists in the branch vicinity. Needle removal and branch thinning led to a significant decline in the abundance of Psocoptera and Collembola. Tying of branches resulted in an eight-fold increase in Collembola numbers, organisms most sensitive to habitat alterations. Canopy habitat structure modified vertical dispersal of Collembola from forest litter, which may have significant implications for arboreal consumers. Our results lend strong support to the importance of habitat structural diversity in explaining general patterns of arthropod abundance and diversity on plants.     

Phytophagous insect assemblages and the regional species pool: patterns and asymmetries

We describe three models predicting relationships between: (a) the taxonomic composition of the regional species pool of phytophagous insects and the composition of the phytophagous insect fauna on a host taxon; and (b) the faunal composition of two host taxa. The predictions of these models were compared with empirical data representing the regional pool of phytophages in Central Europe and the faunas of two plant taxa: the cabbage plants (Brassicaceae) and the thistles (Asteraceae: Cardueae). Three important findings emerge at a general level. (1) Different taxonomic levels of insects (orders, families, genera) of the regional pool and on the investigated host taxa are well correlated in terms of species richness, but there is no consistent trend in the variance explained by this correlation across taxonomic levels. (2) The model considering evolutionary interactions and speciation processes is consistent with patterns found in the empirical data. (3) Asymmetries in sampled species numbers of insect families on both host taxa may be accounted for by reference to the biology of these insects. We conclude that the faunas of single host taxa can provide the basis for extrapolating to the regional pool, at least at high taxonomic levels.     

Direct and indirect effects of predatory young‐of‐year fishes in a dryland river food web

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稀土叶面肥对葡萄园节肢动物群落和食饵功能团的影响

为阐明稀土叶面肥对江淮丘陵和黄河故道葡萄园节肢动物群落、中性昆虫亚群落和食饵功能团组成的影响,通过系统调查和数学分析得出,稀土元素镧、钕和醋(CK2)对肥东葡萄园节肢动物总群落、植食性昆虫亚群落和捕食性天敌亚群落的物种数、个体数和物种丰富度的影响均不显著。稀土叶面肥对萧县总群落的物种数影响显著,镧对总群落物种丰富度影响显著,其余影响不显著。镧和钕元素对两地葡萄园中性昆虫亚群落的物种数和物种丰富度影响极显著,对个体数影响不显著,CK2对中性昆虫亚群落的物种数、个体数和物种丰富度影响均不显著;肥东县葡萄园镧肥区、钕肥区和CK2与CK1之间食饵功能团物种数的t值为3.4384、2.3911和2.0528,镧肥区和钕肥区与CK2之间食饵功能团物种数的t值为1.6397和0.6357;萧县葡萄园镧肥区、钕肥区和CK2与CK1之间食饵功能团的物种数t值为2.2909、2.3223和0.3674,镧肥和钕肥区与CK2之间的t值为2.7533和2.7744,表明镧肥和钕肥对两地食饵功能团的物种数影响均显著。肥东县镧肥区、钕肥区和CK2与CK1之间食饵功能团的个体数t值为1.3047、1.0338和0.2300,镧肥区和钕肥区与CK2之间的t值为1.6014和1.1835;萧县镧肥区、钕肥区和CK2与CK1之间食饵功能团的个体数t值为1.0431、1.0245和0.7369,镧肥区和钕肥区与CK2之间的t值为0.9495和0.9490;两地处理与与CK1和CK2之间食饵功能团的个体数t值均小于t0.05,表明稀土叶面肥对两地食饵功能团个体数影响不显著。稀土叶面肥对肥东葡萄园食饵功能团物种丰富度影响很小,萧县镧肥和钕肥与CK2之间物种丰富度的t值为2.1709和2.0226,差异显著。综上所述,稀土叶面肥对葡萄园中性昆虫亚群落和食饵功能团的物种数影响显著。     

Complementary effects of resident natural enemies on the suppression of the introduced moth Epiphyas postvittana

Generalist predators may disrupt or complement biological control by parasitoids. Past studies have examined how predators and parasitoids interact to affect aphid suppression, but more information is needed from other host taxa. Here, we explore the interactive effects of a spider (Cheiracanthium mildei) and a generalist parasitoid (Meteorus ictericus) on the light brown apple moth (Epiphyas postvittana), a recent introduction to North America. The spider negatively affected adult parasitoids in a field experiment, and reduced numbers of parasitized larvae in the laboratory. Nonetheless, the combined effects of parasitoids and spiders on larval mortality of the moth were additive. Percent parasitism was not affected by the presence of the spider in field or laboratory experiments, and results were similar when single or multiple larval instars of the moth were included. The spider’s lack of prey preference for unparasitized or parasitized larvae likely precluded any disruptive effects on parasitism. Results suggest that resident generalist parasitoids and predators can work in conjunction to hinder the invasion success of a novel herbivore prey species.     

Biodiversity vs. biocontrol: positive and negative effects of alternative prey on control of slugs by carabid beetles

Environment-friendly farming techniques seek to increase invertebrate biodiversity in part with the intention of encouraging greater numbers of predators that will help to control crop pests. However, in theory, this effect may be negated if the availability of a greater abundance and diversity of alternative prey diverts predators away from feeding on pests. The hypothesis that access to alternative prey can lead to reduced pest suppression under semi-field conditions was tested. Alternative prey type and diversity were manipulated in 70 mesocosms over 7+ weeks in the presence of the carabid Pterostichus melanarius (Illiger), a known predator of slugs, and reproducing populations of the slug Deroceras reticulatum (Müller). Significantly fewer slugs survived where no alternative prey were provided. Maximum slug numbers and biomass were found in treatments containing either carabids plus a high diversity of alternative prey (many species of earthworm and three of Diptera larvae) or a single additional prey (blowfly larvae, Calliphora vomitoria Linnaeus). In these treatments slug numbers and biomass were as high as in plots lacking predators. The effects of alternative prey were taxon-specific. Alternative prey strongly affected carabid fitness in terms of biomass and egg load. The fittest predators (those with access to high alternative prey diversity or C. vomitoria larvae) reduced slug numbers the least. The mean individual slug weights were greater in treatments with alternative prey than where no alternative prey were provided to the carabids. These results suggest that pests may survive and reproduce more rapidly in patches where predators have access to alternative prey.     

A cross‐system meta‐analysis reveals coupled predation effects on prey biomass and diversity

Predator diversity and abundance are under strong human pressure in all types of ecosystems. Whereas predator potentially control standing biomass and species interactions in food webs, their effects on prey biomass and especially prey biodiversity have not yet been systematically quantified. Here, we test the effects of predation in a cross‐system meta‐analysis of prey diversity and biomass responses to local manipulation of predator presence. We found 291 predator removal experiments from 87 studies assessing both diversity and biomass responses. Across ecosystem types, predator presence significantly decreased both biomass and diversity of prey across ecosystems. Predation effects were highly similar between ecosystem types, whereas previous studies had shown that herbivory or decomposition effects differed fundamentally between terrestrial and aquatic systems based on different stoichiometry of plant material. Such stoichiometric differences between systems are unlikely for carnivorous predators, where effect sizes on species richness strongly correlated to effect sizes on biomass. However, the negative predation effect on prey biomass was ameliorated significantly with increasing prey richness and increasing species richness of the manipulated predator assemblage. Moreover, with increasing richness of the predator assemblage present, the overall negative effects of predation on prey richness switched to positive effects. Our meta‐analysis revealed strong general relationships between predator diversity, prey diversity and the interaction strength between trophic levels in terms of biomass. This study indicates that anthropogenic changes in predator abundance and diversity will potentially have strong effects on trophic interactions across ecosystems. Synthesis The past centuries we have experienced a dramatic loss of top–predator abundance and diversity in most types of ecosystems. To understand the direct consequences of predator loss on a global scale, we quantitatively summarized experiments testing predation effects on prey communities in a cross‐system meta‐analysis. Across ecosystem types, predator presence significantly decreased both biomass and diversity of prey, and predation effects were highly similar. However, with increasing predator richness, the overall negative effects of predation on prey richness switched to positive ones. Anthropogenic changes in predator communities will potentially have strong effects on prey diversity, biomass, and trophic interactions across ecosystems.     

Spatial and temporal distribution patterns of the macrozoobenthos assemblage in the salt marshes of Tejo estuary (Portugal)

The present study focuses on the spatial and temporal distribution of the macroinvertebrate community of the salt marsh areas of the Tejo estuary, based on surveys conducted from autumn 1998 to summer 2000. Samples were collected quarterly in five different intertidal areas along an elevation gradient in: mudflats, creek mouths, creeks, pioneer salt marsh areas and middle marsh areas. A total of 36 benthic invertebrate taxa were identified. Insect larvae were the most well represented group, with 10 taxa identified. Oligochaetes and ostracods were the most numerically abundant taxa, whereas bivalves dominated in biomass. Benthic macroinvertebrate assemblages were dominated, both in number and biomass, by deposit feeders. Three distinct macroinvertebrate assemblages were distinguished along the elevation gradient, based on species presence, density and biomass: the unvegetated muddy areas with a macrobenthic assemblage composed mostly by infauna; the salt marsh pioneer areas of Spartina maritima in which several epibenthic taxa were found, as well as endobenthic taxa characteristic of muddy sediment; and the creek margins, with epifauna taxa such as insect larvae and crustaceans and a low abundance of benthic infauna. Total biomass in the unvegetated and Spartina areas was higher during spring and summer mainly due to the increase in biomass of Scrobicularia plana and Hydrobia ulvae. No decreases in the salt marsh macroinvertebrate biomass values were observed during the highest densities of their potential nektonic predators (summer). This fact might indicate that macroinvertebrates are not a limiting resource for the nektonic species, and that the natural biomass increment of these invertebrate species could be masking the predation/disturbance caused by the nektonic species.     

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.     

Spatial distribution of predators and prey affect biological control of twospotted spider mites by Phytoseiulus persimilis in greenhouses

We evaluated the effects of predator release pattern and prey distribution on rate of suppression of the twospotted spider mite, Tetranychus urticae Koch (Acari, Tetranychidae) and visual damage to the ornamental plant, Impatiens wallerana Hook.f., in a greenhouse. Sixteen impatiens plants were arranged in a square and infested with the same total number of spider mites distributed either evenly (equal numbers on all plants) or clumped (divided equally among the 4 central plants), simulating a “hot spot.” The predatory mite, Phytoseiulus persimilis Athias-Henriot, was released at a 1:4 predator:prey ratio based on total spider mites in the experimental unit, but the pattern of release was either even or clumped, which simulated broadcast or point-release strategies, respectively. Nine days after predator release, spider mite populations were reduced in all treatments, but only in the clumped pest-clumped predator treatment were spider mites undetectable. Poorest pest suppression occurred in the clumped spider mite-even predator treatment. Eighteen days after predator release, spider mites were eliminated in all treatments, but a reduction in average plant damage occurred only in treatments in which the predator release pattern matched the spider mite distribution (i.e., even-even or clumped-clumped) with the greatest reduction in the even-even treatment. Results suggest that there is an advantage to releasing predators in “hot spots” provided that the recommended predator:prey ratio is maintained within infested patches. If more uniform predator releases are planned, overall predator numbers need to be kept sufficiently high so that the predator:prey ratio of 1:4 shown to prevent damage on impatiens is achieved in higher-density spider mite patches.     

Diet composition of two larval headwater stream salamanders and spatial distribution of prey

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Contributions of Robert (Bob) J. O’Neil to the field of predaceous Heteroptera

Predaceous Heteroptera are important components of natural and managed habitats. The extent of predatory habits among taxa ranges from those that are totally (or nearly totally) predaceous to those that are nearly totally phytophagous, but occasionally feed on arthropod prey. Because of that variability in predatory habit, the contributions of heteropteran taxa to management of pest species will vary greatly. The late Robert (Bob) J. O’Neil (1955–2008) added significantly to understanding the roles of and benefits from facultatively predaceous Heteroptera. His focus on behavioral and physiological attributes that permitted facultatively predaceous species to persist in agricultural habitats demonstrated that facultative predators could help prevent pest outbreaks, but were less effective at reducing numbers of species already considered above economic thresholds.     

Tree Species Richness Strengthens Relationships between Ants and the Functional Composition of Spider Assemblages in a Highly Diverse Forest

In species‐rich ecosystems, such as subtropical and tropical forests, higher trophic level interactions are key mediators of ecosystem functioning. Plant species loss may alter these interactions, but the effects of plant diversity might be modified by intraguild interactions, particularly among predators. We analyzed the relationships between spiders and ants, two dominant predatory arthropod taxa, on tree saplings across a gradient from medium to high woody plant species richness in a subtropical forest in Southeast China. Neither ant nor spider total biomass was significantly related to plant species richness. By contrast, the biomass distribution of web‐building and hunting spiders changed and spider family richness increased in the presence of ants, resulting in more web builder‐dominated assemblages. However, these relationships depended on the plant communities, and were stronger in plots with higher plant species richness. Our results indicate that in addition to potential effects of ants on hunting spiders in particular, ants could indirectly influence intraguild interactions within spider assemblages. The observed shifts in the spider assemblages with increasing ant presence and plant species richness may have functional consequences, as web‐building and hunting spiders have distinct prey spectra. The relationships among ants, spiders, and plant species richness might contribute to explaining the non‐significant relationship between the overall effects of predators and plant diversity previously observed in the same forest plots. Our findings thus give insight into the complexity of biotic interactions in such species‐rich ecosystems.