Die Bedeutung der Landschaftsstruktur für die Bienendiversität und Bestäubung auf unterschiedlichen räumlichen Skalen

Abstract

Pollination is one of the most important processes in terrestrial ecosystems. This free nature service depends on a homogeneous distribution and sufficient density of flower-visiting insects, mainly wild bees and managed honey bees. Habitat destruction and habitat fragmentation and intensive land-use led to species loss on multiple trophic levels.The aim of this work was to answer the question whether pollination is still sufficiently ensured by wild bees and honey bees in intensively managed agricultural landscapes. For this, it was necessary to quantify local honey bee abundance and wild bee species composition and density in relation to landscape structure at different spatial scales. Thus it was tested, how solitary wild bees, social bumble bees and honey bees interact with the surrounding landscape (chapter 4). Furthermore, the influence of possible pollination limitation due to low densities of flower-visiting bees on yield of oilseed rape was studied in dependence of the surrounding landscape (chapter 5). Finally, the effect of different pollination vectors on oilseed rape was measured to quantify the relative importance of bee pollination and wind pollination for fruit set. The spatial distribution and density of managed honey bees was measured using Geographical Information Systems, for the district of Göttingen (Germany). In Germany, in a period of twelve years (1989 2000), the total number of managed honey bees decreased by 26% representing a loss of about 260.000 honey bee locations. Bee diseases and the high percentage of old beekeepers led to this situation. In conclusion, the current situation of beekeeping in the district of Göttingen has reached a critical level. The conservation of wild pollinator and the support of beekeeping must be a central topic to ensure pollination services for agricultural crops. If not, a full-coverage pollination by honey bees cannot be guaranteed.An aim of this study was the analysis of the effects of landscape context and the influence of the percentage of semi-natural habitats on species composition of trap-nesting wild bee communities a gradient of structurally poor to structurally rich landscapes. An increasing proportion of the semi-natural habitats in the surrounding landscape had no positive effect on species diversity at any spatial scale. In contrast, an increasing proportion of rapecrop fields influenced the number of brood cells per nest and the total nest number of Osmia rufa (Hymenoptera: Apidae) at small spatial scales. In conclusion, all studied agricultural landscapes only supported relatively low numbers of solitary bees and wasps.Bee visitation is necessary for pollination. To analyse the value of the degree of flower-visiting bees we analysed, if an increasing diversity and density of flower-visiting bees may have positive effects on the yield of oilseed rape (Brassica napus L. ssp. oleifera (Metzg.), Brassicaceae). Flower visitor observations indicated significantly increased seed set per flower with increasing honey bee densities, whereas the abundance of flower-visiting wild bees had no measurable effect on yields of oilseed rape. Bee visitation had no effect on real yield (g/m²), probably as a result of the high compensatory-growth ability of oilseed rape and variation in cultivation methods or local soil conditions between the landscapes. In contrast, rape yield was positively related to a higher percentage of rape crop fields in the surrounding landscape, possibly due to higher pollen densities in the air and enhanced wind pollination.Summarizing this study, species richness and abundance of solitary wild bees depends on landscape context and habitat composition. Species diversity and abundance of flower-visiting bees increase with higher percentage of semi-natural habitats in the surrounding landscape. Generally, habitat fragmentation and destruction of ecologically valuable habitats can cause decreasing bee diversity (solitary wild bees, social bumble bees and honey bees). Pollination as a free nature service depends on a high pollinator density and may result in a higher fruit set and yield thus confirming the economical importance of pollination. In conclusion, a further loss of pollinators must be avoided to protect most wild plant species and to ensure high yields of insect-pollinated cultivated plants.