Bees and wasps in agricultural landscapes: effects of dispersal corridors and land-use intensity at multiple spatial scales

Abstract

Agricultural landscapes are characterized by close interactions between annually ploughed crop fields and relatively little disturbed noncrop areas. Many species have adapted to these mosaic landscapes and species-rich agroecosystems developed as a result of moderate anthropogenic changes. During the last decades, agricultural intensification has caused a rapid decline of biodiversity in agroecosystems. Intensive farming practices with high agrochemical inputs displaced low-intensity farming, crop fields expanded at the cost of noncrop habitats, and hedgerows and fallow strips were removed in the course of field enlargement. These substantial changes in land-use intensity threat biodiversity and may disrupt trophic interactions and ecosystem services.In this study, we investigated how organic farming, landscape complexity and dispersal corridors may contribute to the diversity of bees and wasps in agroecosystems. Further, we examined how interactions with natural enemies were influenced. Our investigations were conducted in agricultural landscapes in the vicinity of the city of Göttingen, the Soester Börde and the Lahn-Dill-Bergland. Landscapes were selected to encompass a gradient from homogeneous crop-dominated landscapes to heterogeneous landscapes dominated by a diversity of noncrop habitats. Bees, wasps and their natural enemies were recorded in altogether 74 organic and conventional wheat fields, 74 fallow strips, 32 grass strip corridors, 24 hedges and 12 forest edges.Diversity of flower-visiting bees in wheat fields was greatly enhanced by organic farming due to a higher availability of flowering noncrop plants in organic compared to conventional fields. Differences in bee diversity between organic and conventional fields increased with the proportion of crop fields in the surrounding landscape (1 km radius), thereby indicating that processes at the landscape level have the potential to modify the effectiveness of agri-environment schemes. Organic farming in homogeneous landscapes with few remaining flower-rich habitats may reach highest relative effectiveness.Bee communities in fallow strips were influenced by organic crop fields at a local and a landscape scale. At the local scale, species richness of bees and abundance of solitary and bumble bees were higher in fallow strips adjacent to organic than to conventional fields. At the landscape scale, species richness and abundances of bees were enhanced by a high proportion of organic crop fields in the surrounding landscape. An increase of the proportion of organic crop fields from now 4.4 % on average in Germany to 20 % as aimed by the government may enhance species richness of bees in fallow strips by 54 %, density of solitary bees by 66 % and bumble bee density by 156 %.Nest colonization of cavity-nesting bees and wasps was enhanced by heterogeneous landscapes, organic farming and fallow strips. Wasps benefited more from fallow strips than from organic compared to conventional farming. Wasps in fallow strips were not influenced by the farming system of the adjacent field. This suggests that wasps preferred dispersing along fallow strips and that wasps nesting in fallow strips did not substantially utilize resources provided by the adjacent field. The positive effect of fallow strips at the local scale was consistent with a positive effect of high edge densities providing dispersal structures at a landscape scale. In contrast, bee colonization was enhanced by organic farming in both field centres and adjacent fallow strips, which did not significantly differ in bee abundance. This suggests that bees nesting in noncrop habitats benefited from resources in neighbouring organic wheat fields and did not depend on noncrop habitats for foraging or dispersal. A positive effect of high proportions of noncrop habitats in the surrounding landscape underlined that nest colonization of bees was influenced by source habitats.Grass strip corridors connecting forest edges (source habitats) and standardized nesting sites in cropland landscapes enhanced the abundance of solitary wasps in nest patches by 400 % compared to isolated nest patches. Natural enemies largely reflected the patterns found for their hosts, and mortality due to natural enemies did not depend on the presence of a corridor. In agricultural landscapes, where nesting sites are limited and food availability changes frequently, rapid colonization of nest patches may be linked to high population viability.