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Aim To investigate the overall effect of spiders on pest suppression and crop performance, and to explore the extent to which the biocontrol efficacy of spiders depends on the characteristics of spiders, pests, agroecosystems, climate and geography. Location Global. Time period 1970–2017. Major taxa studied Spiders. Methods We performed a meta‐analysis of 58 published studies where we investigated (a) the overall effect of spiders on pest density and crop performance; (b) the extent to which the biocontrol efficacy of spiders depends on the taxonomy of pests (aphids, leafhoppers, beetles, and lepidopteran larvae), the hunting strategy of spiders (hunters, web‐weavers), crop type (vine, cabbage, wheat, rice), climate, and geography. Results Spiders suppressed agricultural pest insects in 79% of cases. The mean effect size of increased spider density on pest suppression was large (Hedge's d = 0.89; 95% confidence interval (CI95 )= 0.66–1.12). Spider pest suppression efficacy slightly increased also with taxonomic diversity (d = 0.33; CI95 = 0.05–0.61). The effects of spiders cascaded down and improved crop performance (d = 2.3, CI95 = 0.70–3.84). The effects of spiders seemed to escalate rather than attenuate down through the agricultural food‐chains (regression slopes > 1). The biocontrol efficacy of spiders was highest in rice followed by grape, cabbage and wheat. The pest suppression efficacy of spiders and the positive effect of spiders on crop yield slightly increased towards the tropics and with mean annual temperature. Spiders suppressed the four pest groups with similar efficacy. Main conclusions The meta‐analysis provides strong evidence that spiders are effective in natural pest control and improve crop performance. However, the efficacy of spiders differed among crops. Our study substantiates the few earlier findings that predation pressure and the intensity of trophic cascades in terrestrial ecosystems intensify towards the tropics.

Traditional woodland management created a mosaic of differently aged patches providing favorable conditions for a variety of arthropods. After abandonment of historical ownership patterns and traditional management and the deliberate transformation to high forest after World War II, large forest areas became darker and more homogeneous. This had significant negative consequences for biodiversity. An important question is whether even small-scale habitat structures maintained by different levels of canopy openness in abandoned coppiced forest may constitute conditions suitable for forest as well as open habitat specialists. We investigated the effect of canopy openness in former traditionally coppiced woodlands on the species richness, functional diversity, activity density, conservation value, and degree of rareness of epigeic spiders. In each of the eight studied locations, 60-m-long transect was established consisting of five pitfall traps placed at regular 15 m intervals along the gradient. Spiders were collected from May to July 2012. We recorded 90 spider species, including high proportions of xeric specialists (40%) and red-listed threatened species (26%). The peaks of conservation indicators, as well as spider community abundance, were shifted toward more open canopies. On the other hand, functional diversity peaked at more closed canopies followed by a rapid decrease with increasing canopy openness. Species richness was highest in the middle of the canopy openness gradient, suggesting an ecotone effect. Ordinations revealed that species of conservation concern tended to be associated with sparse and partly opened canopy. The results show that the various components of biodiversity peaked at different levels of canopy openness. Therefore, the restoration and suitable forest management of such conditions will retain important diversification of habitats in formerly coppiced oak forest stands. We indicate that permanent presence of small-scale improvements could be suitable conservation tools to prevent the general decline of woodland biodiversity in the intensified landscape of Central Europe.

The morphological, physiological, and behavioural traits of organisms are often used as surrogates for actual ecological functions. However, differences in these traits do not necessarily lead to functional differences and/or can be context-dependent. Therefore, it is necessary to explicitly test whether the surrogates have general ecological relevance. To investigate the relationship between the hunting strategies of predators (i.e., how, where, and when they hunt) and their function, we used euryphagous spiders as a model group. We used published data on the diet composition of 76 spider species based on natural prey and laboratory prey acceptance experiments. We computed differences in the position and width of trophic niches among pairs of sympatrically occurring species. Pairs were made at different classification levels, ranked according to the dissimilarity in their hunting strategies: congeners, confamiliars (as phylogenetic proxies for similarity in hunting strategy), species from the same main class of hunting strategy, from the same supra-class, and from different supra-classes. As for niche position computed from the natural prey analyses, species from the same class differed less than species from different classes. A similar pattern was obtained from the laboratory studies, but the congeners differed less than the species from the same classes. Niche widths were most similar among congeners and dissimilar among species from different supra-classes. Functional differences among euryphagous spiders increased continuously with increasing difference in their hunting strategy. The relative frequency of hunting strategies within spider assemblages can, therefore, influence the food webs through hunting strategy-specific predator-prey interactions.

Summary1. The role of generalist predators in pest control has been neglected because generalists are not able to track pest populations. Generalist predators are suggested to be important in spring before specialist predators become active. 2. Here, we show that some generalist predators are important even during winter, when the majority of arthropod pests and their enemies are dormant. We quantified the role of winteractive generalist predators on the suppression of pear psylla during winter using a discrete nonlinear model of an intraguild predation system. 3. To parameterize our model, we conducted a series of experiments on (i) functional responses, (ii) prey preferences and (iii) ontogenetic development and made observations on the population densities of spiders and potential prey. We ran the model for different winter scenarios, that is, for very cold and very warm winters. 4. Synthesis and applications. We found that winter-active predators considerably suppressed the pear psylla population. Predators exerted a stronger effect in a warmer winter than in a colder one. Orchard growers thus should avoid use of non-selective pesticides during this period and instead aim to support the community of generalist predators. Our results suggest more attention should be given to encouraging generalist predator populations in other systems, even in periods when crops are not producing.

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