Pollinating insects like bees and hoverflies play an important role in natural ecosystems and in agriculture. They pollinate the majority of wild plant and crop species worldwide while visiting flowers for their own food supply. Like many other insect groups wild (= unmanaged) pollinators are in decline. A lot of evidence for this decline comes from Europe, where intensified agriculture was identified as the main driver. One of the major issues is the lack of floral resources due to a loss in flower-rich habitats. Measures for enhancing pollinators therefore typically aim for increasing the availability of floral resources in agricultural landscapes. This can for example be achieved by means of flower strips, which are prominently promoted in form of EU agri-environmental schemes. But also semi-natural habitats like hedges or herbaceous structures along slopes or ditches can be valuable flower providers and can be established as enhancement measures in agro-ecosystems. However, flower strips, hedges and herbaceous semi-natural habitats may vary in their effect on wild pollinators due to differences in floral resources and other habitat characteristics, e.g. the provision of nesting sites or a beneficial microclimate. Furthermore, their impact may depend on the dominant agricultural land use in their surroundings and the resources that are available there. Intensive apple orchards for example seem to be of limited value for wild pollinators because apart from the apple bloom they provide only few floral resources. In combination with the fact that apples require insect pollination this makes intensive apple orchards relevant research objects for finding suitable pollinator enhancement measures. In this theses, I aimed at understanding how wild pollinators react on different enhancement measures in intensive apple orchards and whether the measures complement each other in a beneficial way. The overall goal was to find out how wild pollinators can efficiently be enhanced in landscapes with high proportions of intensive apple orchards. The main focus is on pollinator conservation, but I also address the question whether enhancement measures help increasing apple pollination.
In Chapter I I compare the effectiveness of perennial flower strips, hedges and improved hedges (complemented with a sown herb layer). I found that wild bee abundance and species richness were highest in flower strips followed by improved hedges. Also hoverflies were most abundant in flower strips, but not more species rich than at control sites. Flower abundance was the main driver for wild bee diversity, whereas hoverflies were largely unaffected by floral resources. Only the wild bee but not the hoverfly community composition differed between the control orchard edge and the enhancement measures. The enhancement measures had neither an effect on the pollinator diversity within the orchards nor on apple flower visitation. I conclude that perennial flower strips are the most effective measure to enhance wild pollinators in intensive apple orchards though hedges should not be ignored as they attracted a different bee community than the flower strips. However, the increased diversity of wild pollinators did not translate in increased apple-flower visits, thus had no effect on pollination.
Chapter II sheds light on the temporal interplay of the enhancement measures. Diverse wild bee communities require a continuous flower supply over the entire vegetation season that sustains long-lived social species as well as solitary species, which are active only temporarily. I hypothesised that hedges and flower strips complement each other in providing flowers at different periods of the year and that the improved hedges combined both flowering periods. Indeed, hedges and flower strips complemented each other. The hedges flowered from spring to early summer and attracted most wild bees in this period. In early summer the flower strips started to provide a continuous high floral supply until late summer. In contrast to the flower strips, the hedges showed fluctuating floral resources. This difference in the continuity of flower provision was a main reason for the generally higher wild bee diversity in flower strips as found in Chapter I. The improved hedges, which would have in theory been superior to both hedges and flower strips, performed less well than expected mostly due to varying establishment success of the sown herb layers. The flower strips started flowering much earlier in the second year after establishment and showed a different flower composition, which both translated in differences in the wild bee community across the years. In the third year wild bee diversity however decreased in comparison to the first two years in the flower strips. I conclude that woody vegetation and flower strips should be managed together to enhance bee pollinator communities in agricultural landscapes. To stabilise the fluctuating short-term flower supply in hedges the diversity of bee-attractive shrubs with complementary phenology should be promoted. The establishment of herb layers along hedges may be useful, but only if enough space and light is available. Flower strips should be perennial because they attract different bee communities at different ages and start flowering much earlier from the second year on. Maintenance measures should ensure long term flower richness in the flower strips.
Chapter III focuses on other semi-natural habitat types than hedges, namely small-scale patches with spontaneous herbaceous vegetation. I compared explicitly flower-rich semi-natural habitat patches along slopes, overgrown fences or ditches with the flower strips. As before, the flower strips were superior in attracting a high pollinator diversity due to a higher floral richness, whereas in the semi-natural habitat patches the pollinator diversity varied strongly due to differences in the flower supply. However, the bee species composition differed between the flower strips and the semi-natural habitat patches. The patches attracted bee species with different pollen specialization than the flower strips. I conclude that herbaceous semi-natural habitat patches are not as attractive for pollinators as flower strips, but nevertheless play an important role for pollinator diversity by providing different flower species at different times. Semi-natural habitat patches typically do not compete with agricultural land use, can often be improved or enlarged at low costs and thus have a high potential to promote pollinator conservation in intensive agricultural landscapes.
Chapter IV investigates the plant-pollinator-networks in the orchards and whether these are affected by the presence of hedges or flower strips. Of special interest in this context are those pollinator species that visit and potentially pollinate apple flowers. Therefore, only apple-pollinators were considered in the network analysis. The networks were regarded from two perspectives: from the orchard plant perspective and from the pollinator perspective. For this, two indices were calculated: plant generality, which indicates the number of visiting pollinator species per plant species, and apple-pollinator generality, i.e. the number of plant species visited per pollinator species. Plant generality in orchards was not influenced by the presence of enhancement measures. So plants within the orchards were not visited by a higher diversity of species when flower strips or hedges were present. This applied also to the apple flowers. In contrast, apple-pollinator generality was higher in the orchards with enhancement measures, which indicates a larger food supply for apple pollinators before and after the apple flower. This was especially true for flower strips. Thus, planting hedges and flower strips does not increase the diversity of wild pollinators visiting apple flowers, but particularly flower strips are beneficial for apple pollinators after the mass-flowering event and help stabilizing apple pollinator populations.
In summary, the enhancement measures have the potential to increase pollinator diversity in intensive apple orchards, but the enhancement measures differ in their effect on wild pollinators. The perennial flower strips turned out highly efficient in enhancing wild bees, but the hedges and semi-natural habitat patches also played a non-neglectible role. Their habitat quality could be increased by improvements and maintenance measures. Hoverflies in contrast to wild bees could only be enhanced in terms of abundance, but not in species richness, which either calls for further research on how to enhance this species group in intensive agrarian landscapes or suggests the conclusion that hoverflies are not a meaningful target species group in the studied landscape. Though the enhancement measures were relatively successful in terms of pollinator conservation, they did not increase the visitation rate of apple flowers. Yet, they may indirectly stabilize the apple-pollinator populations by providing them with floral resources before and after the apple bloom.