Field experiments with manipulations of natural enemies of plant-feeding insects may show how a diverse enemy group ensures an important ecosystem function such as naturally occurring biological pest control. We studied cereal aphid populations in winter wheat under experimentally reduced densities of: (i) ground-dwelling generalist predators (mostly spiders, carabid and staphylinid beetles); (ii) flying predators (coccinellid beetles, syrphid flies, gall midges, etc.) and parasitoids (aphidiid wasps), and a combination of (i) and (ii), compared with open controls. Aphid populations were 18% higher at reduced densities of ground-dwelling predators, 70% higher when flying predators and parasitoids were removed, and 172% higher on the removal of both enemy groups. Parasitoid wasps probably had the strongest effect, as flying predators occurred only in negligible densities. The great importance of parasitism is a new finding for aphid control in cereal fields. In conclusion, a more detailed knowledge of the mechanisms of natural pest control would help to develop environmentally sound crop management with reduced pesticide applications.

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Summary 1.Most studies in applied ecology use measures of α -diversity measures, i.e. the mean diversity on a site, to compare biodiversity effects of different management schemes. The total or α -diversity within a region, however, need not be correlated with the mean α -diversity within any site of the region. Thus, analyses of α -diversity alone may misrepresent the contributions of other diversity components ( β ) to total diversity ( γ ). 2. We apply a biodiversity-partitioning approach to species richness from a comparison between paired organic and conventional wheat fields in 21 sites from three regions in Germany, where we recorded plants, bees, carabids, staphylinids and spiders in the centre and edge of the fields. 3. Relative values of α -and β -diversity depended on taxon. Both between-site and between-region β -diversity were very high (in total 60 -85%). α -Diversity and between-site β -diversity was larger on the edge than in the centre of fields for all taxa. 4. α -Diversity, between-site β -diversity of plants and bees and between-region β -diversity of bees were higher in organic than in conventional fields, providing local as well as larger-scale species richness benefits. α -Diversity did not differ between management types for the epigaeic arthropods. Lower between-site β -diversity was found for spiders in organic fields than in conventional fields, resulting in higher total species richness in conventionally managed wheat. 5. Similarity in composition of landscapes surrounding the study fields was correlated with similarity in species composition for epigaeic arthropods in conventional fields. For this group of organisms the variability of landscapes in the sample contributed to increasing β -diversity. 6. Synthesis and applications. β -Diversity accounts for the major part of species richness in agro-ecosystems. Implementing an agri-environment scheme such as organic agriculture may result in either an additional increase of total diversity, as could be shown for plants and bees, or in a decrease in total diversity as was the case for the spiders. Therefore, β -diversity needs to be included in the evaluation of different management schemes for conservation. For plant and bees it is recommended to implement agri-environment schemes in contrasting landscapes and in different regions to maximize total species richness benefits.

We studied the response of carabid species richness and density to landscape simplification (measured as percentage cover of non-crop habitat surrounding each study site) in 36 wheat fields using pitfall traps. Carabids were divided to trophic groups following the literature. The number of species from different trophic groups declined with increasing landscape simplification in the order: carnivores > phytophages > omnivores. Density compensation of both carnivores and phytophages suggests that species decline is caused by the loss of specific resources rather than by an overall reduction in food availability. Increasing evenness indicates that a greater share of phytophagous species contributes to density compensation at poorer sites. A comparison with data from complementing studies shows that marked differences in species numbers (carnivores > omnivores > phytophages) are due to a different sensitivity of trophic groups to agricultural management. Since our findings seem to be partly due to increasing sensitivity to landscape changes with trophic rank, and partly to decreasing sensitivity of depauperate communities to local environmental stress, species loss can best be explained by the co-action of factors at local and regional scales. Species richness decline might significantly alter the role of carabids as biocontrol agents.

Aim A better understanding of the processes driving local species richness and of the scales at which they operate is crucial for conserving biodiversity in cultivated landscapes. Local species richness may be controlled by ecological processes acting at larger spatial scales. Very little is known about the effect of landscape variables on soil biota. The aim of our study was to partly fill this gap by relating the local variation of surface-dwelling macroarthropod species richness to factors operating at the habitat scale (i.e. land use and habitat characteristics) and the landscape scale (i.e. composition of the surrounding matrix).Location An agricultural landscape with a low-input farming system in Central Hesse, Germany.Methods We focused on five taxa significantly differing in mobility and ecological requirements: ants, ground beetles, rove beetles, woodlice, and millipedes. Animals were caught with pitfall traps in fields of different land use (arable land, grassland, fallow land) and different habitat conditions (insolation, soil humidity). Composition of the surrounding landscape was analysed within a radius of 250 m around the fields. ResultsFactors from both scales together explained a large amount of the local variation in species richness, but the explanatory strength of the factors differed significantly among taxa. Land use particularly affected ground beetles and woodlice, whereas ants and rove beetles were more strongly affected by habitat characteristics, namely by insolation and soil characteristics. Local species richness of diplopods depended almost entirely on the surrounding landscape. In general, the composition of the neighbouring landscape had a lower impact on the species richness of most soil macroarthropod taxa than did land use and habitat characteristics. Main conclusionsWe conclude that agri-environment schemes for the conservation of biodiversity in cultivated landscapes have to secure management for both habitat quality and heterogeneous landscape mosaics.