Landscape context of bee, wasp and parasitoid diversity: grass-strip corridors, fallows and food webs

The world population is growing, resulting in an increasing demand for food and energy plants. During the last decades the modification of landscapes into agricultural production units and the intensification of agricultural practises have led to a decline in biodiversity. Biodiversity is crucial for the stability of ecosystem services driven by biotic interactions, such as pollination and biological control. Pollination is important for reproduction of wild plants and it is estimated that 75 % of the world crops need animal pollination for obtaining optimal yields. In the northern hemisphere bees are the most important pollinators in the agricultural landscape. But there is evidence that we are facing a pollinator crisis since managed honey bees as well as wild bee populations are declining, especially in intensively managed areas. Agricultural intensification affects also predatory wasps, which are contributing to biological control of pest species. Parasitoids play an important role in regulating host species. Biodiversity loss of functionally important groups such as pollinators, predators and parasitoids can destabilize trophic interactions leading to cascading extinctions. The aim of this study was to quantify the impact of different landscape elements and habitat types, which are common in the agricultural landscape, on the communities of bees, wasps and their parasitoids with changing importance of habitat type along a gradient of landscape complexity. The study was carried out in the vicinity of the city of Gottingen, Lower Saxony, Germany. In the first part of this thesis I tested the hypothesis that grassy field margin strips can function as corridors, facilitating the movement of hymenopterans and antagonists in the agricultural landscape. Additionally possible barrier effects of dense hedges, which were hypothesized to inhibit the movement of wild bees, were tested. Species-rich calcareous grasslands were taken as source habitats for bees, wasps and their parasitoids and connected and unconnected grass strips in varying distances were chosen to account for possible corridor effects on abundance and species richness, whereas dense hedges at the edge of the grasslands were expected to be barriers for foraging bees. Foraging bees were monitored using pan traps, while above-nesting bees, wasps and their parasitoids were experimentally analysed using standardized nesting resources (trap nests). The results showed that grassy strips can function as alternative nesting and foraging habitats when semi–natural habitats are nearby. I found strong isolation effects for nearly all observed groups. Abundance and species richness of foraging solitary bees, above ground-nesting bees, wasps and parasitoids declined with increasing distance to the calcareous grassland. Bumblebees were an exception to this, showing no decline in abundance or species richness. The hypotheses that grass strip corridors mitigate the tested 100m isolation effects on any of the observed groups and that hedges function as barriers on the foraging bees had to be rejected. In the second part of this thesis the relative importance of different habitat types, landscape attributes (percent fallows and semi-natural habitats) and landscape complexity on species richness and abundance of pollinators was assessed over a time span of three years. Five habitat types that are common in the agricultural landscapes, namely (i) wheat field, ii) grassy field margin strip, iii) meadow, iv) fallow and v) forest edge were chosen in 12 non-overlapping landscape sectors with a radius of 1000m around each habitat type. These landscapes covered a gradient in landscape complexity, which was measured as amount of arable land ranging from structurally complex (14% of arable land) to structurally poor (89% of arable land). Pollinator populations in the habitat types were monitored, using pan traps for foraging bees in three years and standardized trap nests for above-nesting bees in two years. Abundance and species richness of the observed ground-nesting bee groups were primarily affected by landscape attributes, rather than habitat types, while above nesting ground-nesting bees were mainly influenced by habitat type. In the three study years the amount of fallows declined more than 50% in the landscapes. Abundance and species richness of solitary ground–nesting bees declined over the years as well, while the abundance and species richness of above ground-nesting bees remained stable. As hypothesized, abundance of bumblebees was positively influenced by the amount of fallow in the landscape. Species richness of bumblebees and abundance and species richness of solitary ground-nesting bees, as well as abundance of above-ground nesting bees showed a positive response to the amount of semi–natural habitats in the landscape. Abundance and species richness of above ground-nesting bees was positively influenced by higher quality habitats, hosting higher numbers of plant species, than intensively managed habitats (wheat field and field margin strip). The third study was based on the previous, using the same design to assess the effects of habitat types and landscape complexity on the trophic interactions and food web structure of bees, wasps and their natural enemies in the trap nests in two years. Abundance and species richness of the observed groups were lowest in intensively managed habitat types (wheat field and field margin strip). Parasitism rates were higher in high quality habitats (forest edge) in both years, though species composition varied. Shannon diversity and linkage density of interactions in the food webs were negatively affected by low habitat quality. Interestingly, the connectance of species interactions in food webs increased with decreasing landscape complexity, which may result in enhanced consumer effects in simple landscapes. In conclusion, bees, wasps and their parasitoids depend strongly on high quality habitats, such as extensively managed grasslands and forest edges as source habitats in the landscape, but have the ability to use alternative foraging and nesting sites of lower quality, if these are supported by a certain amount of semi-natural habitats in the surrounding. Intensively used habitats, such as arable fields and field margin strips, and decreasing landscape complexity affect abundance and diversity of pollinators, predators and parasitoids negatively and alter their interactions. The results of the studies show that conservation of semi-natural habitats is essential to maintain viable populations of bees, wasps and parasitoids in the agricultural landscape and ensure ecosystem services provided by them.