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The niche variation hypothesis states that the population niche width expands with increasing interindividual differences in prey utilization (i.e., individual dietary specialization). The main ecological drivers forming this relationship include a) ecological opportunity, b) food limitation and exploitative competition, and c) intraguild interference. Only a limited number of empirical studies have tested the impact of these drivers on the niche variation–width relationship and focused only on vertebrates. Using molecular gut content analysis, we investigated how prey diversity (proxy for ecological opportunity), prey abundance (proxy for exploitative competition / food-limitation), and activity density of guild members (proxy for intraguild interference) affect the short-term individual dietary specialization and consequently the population niche width in local communities of 13 species of predatory beetles and spiders. The study took place in 10 spring barley fields in Sweden in 2011. We found that the niche variation and consequently the average population niche width of the species in the predator community decreased with prey abundance but increased with activity density of guild members. The results indicate that intraguild interference and exploitative competition / food limitation increased dietary variation. The increased diet variation led to the observed population diet expansion. Our results support the niche variation hypothesis and, in contrast to the traditional view, show that negative intraguild interactions may act as a diversifying force. [ABSTRACT FROM AUTHOR]

The suppression of agricultural pests by natural enemies, including generalist arthropod predators, is an economically important regulating ecosystem service. Besides pests, generalist predators may also consume non-pest extraguild and intraguild prey, which can affect their impact on pest populations. This may either reduce the impact of generalist predators on pest populations, because they are diverted from pest predation, or increase it, as it helps them survive periods of low pest availability. However, the availability of pest prey and alternative, non-pest prey can vary over the crop growing season and between farming systems, potentially affecting predator-prey interactions and the levels of biological control. We have limited information about how farming systems and environmental variation over the crop growing season influence predator diets. This limits our ability to predict the importance of generalist predators as natural enemies of agricultural pests. Here we utilize molecular gut content analyses to assess detection frequencies of extra- and intraguild prey DNA in generalist predator communities in replicated organically and conventionally managed cereal fields at two key periods of the cropping season for aphid biological control. This is done in order to understand how farming system, crop season, prey availability and predator community composition determine the composition of predator diets. Aphid pests and decomposers (springtails) were equally important prey for generalist predators early in the growing season. Later in the season, the importance of aphid prey increased with increasing aphid densities while springtail predation rates were positively correlated to abundance of this prey at both early and late crop growth stages. Intraguild predation was unidirectional: carabids fed on spiders, whereas spiders rarely fed on carabids. Carabids had higher detection frequencies for the two most common spider families in organically compared to conventionally managed fields. Our study documents that predation by generalist predator communities on aphid pests increases with pest numbers independently of their generally widespread consumption of alternative, non-pest prey. Therefore, conservation strategies in agricultural fields could promote biological control services by promoting high levels of alternative non-pest prey for generalist predator communities. [ABSTRACT FROM AUTHOR]

The arthropod predator community of an organically cultivated 0.3 ha field was sampled by pitfall trapping during 1 year and analysed for small-scale spatial distribution patterns and their seasonal variation. Due to the small size of the study site, most of the investigated species were able to colonise the whole field. Ordination analyses revealed significant differences in community composition (i) between field centre and margin in summer and (ii) between field parts adjacent to meadows or conventionally cultivated arable land in all seasons. Results changed considerably if only adult carabids or spiders were used instead of the whole predator community.During summer the following significant differences in community parameters were found: (i) total catches in the centre were nearly twice as high as near the margin, (ii) predator richness did not differ between centre and margin while significantly more species were found close to meadows and (iii) evenness and heterogeneity were significantly higher at the margin versus the centre.The arthropod predator community even of a small field site showed distinct spatial distribution patterns with seasonal changes. Selecting appropriate taxonomic groups is crucial for the outcome of such studies and should be tailored to the aim of the investigation. [Copyright &y& Elsevier]

Abstract: Molecular gut content analysis provides a highly specific and sensitive tool to examine the diet of soil invertebrates. Here, we present new polymerase chain reaction (PCR) assays for the detection of twelve prey taxa common in Central European forest soils. The assays target five species of collembolans as well as dipterans, gamasid and oribatid mites, lithobiid centipedes, spiders, staphylinid beetles and woodlice at the group level, amplifying 123–299 bp long DNA fragments. Cross-reactivity tests against 119 soil invertebrate taxa confirm their specificity. These new PCR assays were found to be highly sensitive, revealing the consumption of five different prey taxa in field-collected centipedes. Thus they provide a ready-to-use approach for unravelling trophic interactions among soil arthropods. [Copyright &y& Elsevier]