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Intraguild predation (IGP)–predation between generalist predators (IGPredator and IGPrey) that potentially compete for a shared prey resource–is a common interaction module in terrestrial food webs. Understanding temporal variation in webs with widespread IGP is relevant to testing food web theory. We investigated temporal constancy in the structure of such a system: the spider-focused food web of the forest floor. Multiplex PCR was used to detect prey DNA in 3,300 adult spiders collected from the floor of a deciduous forest during spring, summer, and fall over four years. Because only spiders were defined as consumers, the web was tripartite, with 11 consumer nodes (spider families) and 22 resource nodes: 11 non-spider arthropod taxa (order- or family-level) and the 11 spider families. Most (99%) spider-spider predation was on spider IGPrey, and ~90% of these interactions were restricted to spider families within the same broadly defined foraging mode (cursorial or web-spinning spiders). Bootstrapped-derived confidence intervals (BCI's) for two indices of web structure, restricted connectance and interaction evenness, overlapped broadly across years and seasons. A third index, % IGPrey (% IGPrey among all prey of spiders), was similar across years (~50%) but varied seasonally, with a summer rate (65%) ~1.8x higher than spring and fall. This seasonal pattern was consistent across years. Our results suggest that extensive spider predation on spider IGPrey that exhibits consistent seasonal variation in frequency, and that occurs primarily within two broadly defined spider-spider interaction pathways, must be incorporated into models of the dynamics of forest-floor food webs. [ABSTRACT FROM AUTHOR]

Shifts in densities of apex predators may indirectly affect fundamental ecosystem processes, such as decomposition, by altering patterns of cascading effects propagating through lower trophic levels. These top–down effects may interact with anthropogenic impacts, such as climate change, in largely unknown ways.We investigated how changes in densities of large predatory arthropods in forest leaf‐litter communities altered lower trophic levels and litter decomposition. We conducted our experiment in soil communities that had experienced different levels of long‐term average precipitation. We hypothesized that altering abundances of apex predators would have stronger effects on soil communities inhabiting dry forests, due to lower secondary productivity and greater resource overexploitation by lower trophic levels compared to wet forests.We experimentally manipulated abundances of the largest arthropod predators (apex predators) in field mesocosms replicated in the leaf‐litter community of Iberian beech forests that differed in long‐term mean annual precipitation by 25% (three dry forests with MAP < 1,250 mm and four wet forests with MAP > 1,400 mm). After one year, we assessed abundances of soil fauna in lower trophic levels and indirect impacts on leaf‐litter decomposition using litter of understorey hazel, Corylus avellana.Reducing densities of large predators had a consistently negative effect on final abundances of the different trophic groups and several taxa within each group. Moreover, large predatory arthropods strongly impacted litter decomposition, and their effect interacted with the long‐term annual rainfall experienced by the soil community. In the dry forests, a 50% reduction in the densities of apex predators was associated with a 50% reduction in decomposition. In wet forests, the same reduction in densities of apex soil predators did not alter the rate of litter decomposition.Our results suggest that predators may facilitate lower trophic levels by indirectly reducing competition and resource overexploitation, cascading effects that may be more pronounced in drier forests where conditions have selected for greater competitive ability and more rapid resource utilization. These findings thus provide insights into the functioning of soil invertebrate communities and their role in decomposition, as well as potential consequences of soil community responses to climate change. [ABSTRACT FROM AUTHOR]

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta-analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure. [ABSTRACT FROM AUTHOR]

Exploitation of forests for biofuel results in variable amounts of fine woody debris (FWD) being left on the forest floor. Such practices have the potential to affect the diversity and taxonomic composition of litter-dwelling organisms. The ability to predict the consequences of harvesting biofuel will be strengthened by knowledge of how variation in FWD affects litter organisms in non-harvested forests. A field experiment with three treatments [Control (no manipulation), FWD Removal, and FWD Addition] was carried out in replicated open plots in an unexploited forest in order to determine how the amount of fallen FWD affects the abundance, diversity and community composition of spiders, selected because of their importance as predators in the leaf-litter food web. Manipulation of FWD started in August, and by the following July total spider density in the FWD Addition treatment was 1.69 that of FWD Removal plots. Spider density in FWD Removal plots was 30% lower than the Control treatment, with no statistically significant difference between Control and FWD Addition. Manipulating FWD had no statistically significant effects on standard indices of spider diversity. However, multivariate community-level analyses revealed statistically significant differences in spider community structure between the FWD Removal and FWD Addition treatments. Two dominant genera of web builders contributed the most to this effect of manipulating FWD. Our results, and the findings of studies of woody debris in younger forests, suggest that the relatively small effects of FWD in our field experiment may reflect the age of the forest, with effects on the fauna likely being larger in forests younger than the one we studied. [ABSTRACT FROM AUTHOR]

The activity and density of generalist predators, such as carabid beetles, rove beetles and spiders, may increase in response to: (1) increased availability of prey from the belowground subsystem and/or (2) enhanced complexity of aboveground vegetation. Organic farming practices support decomposer populations and enhance habitat complexity due to an increased weed density. A response by generalist predators to such below- or aboveground changes could affect predation rates on herbivores in the aboveground food web. We tested this hypothesis in a replicated field experiment conducted in a winter wheat field, where increased predator activity could lead to improved control of herbivorous pests. In a crossed design, we increased and lowered densities of decomposer prey, and manipulated vegetation complexity using artificial plants in order to examine the effect of structural complexity in isolation from effects of plant-attracted additional prey. Isotomid Collembola exhibited lowest activity-densities (AD) in plots treated with soil insecticide and had gradually increasing AD in untreated plots and plots receiving detrital subsidies. Carabid beetles and cursorial spiders did not respond to increased availability of isotomid prey, and they unexpectedly displayed higher AD in the structurally less-complex plots. Aphid density mirrored the positive response of isotomids to detrital subsidies, suggesting that aphids benefited from reduced predation due to predators switching to abundant prey in the decomposer subsystem. The absence of a numerical response by surface-active predators apparently strengthened this indirect effect of isotomids on aphids. Our results suggest that indirect predator-mediated prey-prey interactions can reduce beneficial effects of detrital subsidies on pest suppression. We further demonstrated that generalist predators may not per se benefit from structural complexity. Both results document the challenges associated with management practices that support generalist predators, as these measures may not necessarily improve herbivore suppression. [ABSTRACT FROM AUTHOR]

Interactions among predators that prey on each other and are potential competitors for shared prey (intraguild [IG] predators) are widespread in terrestrial ecosystems and have the potential to strongly influence the dynamics of terrestrial food webs. Ants and spiders are abundant and ubiquitous terrestrial IG predators, yet the strength and consequences of interactions between them are largely unknown. In the leaf-litter food web of a deciduous forest in Kentucky (USA), we tested the direct and indirect effects of ants on spiders and a category of shared prey (Collembola) by experimentally subsidizing ants in open plots in two field experiments. In the first experiment, ant activity was increased, and the density of ants in the litter was doubled, by placing carbohydrate and protein baits in the center of each plot. Gnaphosa spiders were almost twice as abundant and Schizocosa spiders were half as abundant in baited plots relative to controls. There were more tomocerid Collembola in baited plots, suggesting possible indirect effects on Collembola caused by ant-spider interactions. The second experiment, in which screening of two mesh sizes selectively excluded large and small worker ants from a sugar bait, revealed that the large ants, primarily Camponotus, could alone induce similar effects on spiders. Gnaphosa biomass density was almost twice as high in the plots where large ants were more active, whereas Schizocosa biomass density was reduced by half in these plots. Although tomocerid densities did not differ between treatments, tomocerid numbers were negatively correlated with the activity of Formica, another large ant species. Path analysis failed to support the hypothesis that the ant Camponotus indirectly affected tomocerid Collembola through effects on densities of spiders. However, path analysis also revealed other indirect effects of Camponotus affecting tomocerids. These results illustrate the complexity of interactions between and within two major IG predator groups with disparate predatory behaviors, complexities that will have consequences for functioning of the forest-floor food web. [ABSTRACT FROM AUTHOR]

The article focuses on a study of stable isotopes that assert the prey consumption patterns of generalist predators. Stable isotopes showed which generalist predators will consume pest species in the presence of greater densities of alternative prey. The biological control efficiency of generalist predators can be decreased if the accrued availability of alternative prey causes every predator to decrease their consumption of crop pests. Thus, farming methods that increases the density of microbiodetritivores in the detrital food web can produce large densities of generalist predators that feed on pests. The net biocontrol impact of increased predator densities can be anticipated depending on the extent to which individual predators may shift to detrital prey and feed less on crop pests.

Cannibalism among generalist predators has implications for the dynamics of terrestrial food webs. Spiders are common, ubiquitous arthropod generalist predators in most natural and managed terrestrial ecosystems. Thus, the relationship of spider cannibalism to food limitation, competition, and population regulation has direct bearing on basic ecological theory and applications such as biological control. This review first briefly treats the different types of spider cannibalism and then focuses in more depth on evidence relating cannibalism to population dynamics and food web interactions to address the following questions: Is cannibalism in spiders a foraging strategy that helps to overcome the effects of a limited supply of calories and/or nutrients? Does cannibalism in spiders reduce competition for prey? Is cannibalism a significant density-dependent factor in spider population dynamics? Does cannibalism dampen spider-initiated trophic cascades? [ABSTRACT FROM AUTHOR]

1. The high number of potential predatory interactions among the many arthropod generalist predators in terrestrial food webs makes exhaustive testing of interaction strengths by field experiments unfeasible. Thus, correlative patterns and laboratory observations of behaviour often form the basis of inferences about the strength of interaction pathways involving generalist predators (intraguild predation). 2. Previous research has revealed a negative correlation between survival of juvenile wolf spiders of the genus Schizocosa (Lycosidae) and densities of another abundant spider family, the Gnaphosidae. 3. Feeding trials in laboratory microcosms with a leaf-litter substrate revealed that gnaphosids prey on juvenile Schizocosa in a structurally complex habitat. 4. Gnaphosid densities were manipulated in two different field experiments, each conducted in a different year, in order to test directly the hypothesis that intraguild predation by gnaphosids limits densities of juvenile Schizocosa. 5. Reducing numbers of gnaphosids, and doubling their numbers to two times the mean natural density, had no impact on the survival of juvenile Schizocosa in either field experiment. This finding suggests that correlative patterns in nature and feeding trials in the laboratory may at times provide deceptively simple and potentially misleading generalisations about the strengths of interaction pathways in complex networks of generalist predators. [ABSTRACT FROM AUTHOR]

In a long-term field experiment, densities of wandering spiders (i.e. species that do not build webs to capture prey) were reduced in order to determine whether or not a major group of microbi-detritivores, the Collembola (springtails), would increase in response to lowered spider predation. Thirty 4-m2 fenced plots (15 spider-removal, 15 control plots) and 15 unfenced, undisturbed reference areas were established in a deciduous forest. Spiders were taken from the removal-treatment after being collected by periodic trapping with pitfall traps that contained no preservative, and by searching the litter surface. Judging from a comparison of the numbers captured during periodic censuses in spider-removal and control plots (one census period was two consecutive days of pitfall trapping), wandering spiders were rapidly reduced by ca. 50% and were maintained at levels lower than control plots for 1.3 years. The reduction in numbers was greatest for wolf spiders (Lycosidae; &ap;90%). Of six families of Collembola abundant in the leaf litter during the experiment, one family, the Tomoceridae, exhibited a significant release from spider predation. Densities of tomocerids, which are the largest and most active springtails on the research site, gradually increased until they were consistently 2× higher in the spider-removal treatment than in either fenced control plots or unfenced, open reference areas. [Copyright &y& Elsevier]

Deals with a study which evaluated the overall magnitude of trophic cascades in terrestrial arthropod-dominated food webs and conditions affecting their occurrence and strength. Background of the theory of trophic cascades; Data and methodology used; Results and discussion.

Examines the bottom-up limitation of predaceous arthropods in a detritus-based terrestrial food web. Effects of an increase in the resource base of a food web; Food-web structure and dynamics; Detrital food web; Detritivores; Fungivores; Terrestrial arthropod community.