Local and landscape threats to bee and wasp populations in agricultural landscapes

German Research Association (DFG) Research Training Group 1644 "Scaling Problems in Statistics", grant no. 152112243

The transformation and intensified use of the agricultural landscape in central Germany in the past decades has led to the fragmentation and loss of semi-natural habitats, threatening biodiversity including insects. Bees and wasps are highly diverse groups of insects providing ecosystem services such as pollination and pest control. Different species experience the landscape at different scales, have different requirements regarding food and nesting habitats, and show different responses to the exposure to stressors such as insecticides. Calcareous grasslands are biodiversity hotspots in the agricultural landscape around Göttingen, but their numbers and sizes have vastly decreased in the past. Their importance as habitat for bees and wasps, also compared to effects of the surrounding landscape, and how they contribute to the export of pollinators and their services to the surrounding landscape have been studied and are reported in this thesis. The thesis consists of four chapters: 1) A framework chapter giving an overview of the entirety of the thesis and providing context for the following chapters 2) Floral resource diversification promotes solitary bee reproduction and may offset insecticide effects – evidence from a semi-field experiment 3) Calcareous grassland fragments as sources of bee pollinators for the surrounding agricultural landscape 4) Trophic level and specialization moderate effects of habitat loss and landscape diversity on cavity-nesting bees, wasps and their parasitoids In the second chapter, the importance of the availability and diversity of food resources and of the exposure to insecticides for orchard bee populations were studied in a highly replicated semi-field experiment. Bees were kept in mesocosms with flowering oilseed rape, which was either treated with a neonicotinoid insecticide or untreated, and flower strips differing regarding plant identity and diversity. We found that the availability of flower strips per se, flower strip diversity, and the availability of specific flowering plant species all positively affected brood cell production of bees. Exposure to insecticide-treated oilseed-rape reduced bee larval to adult development only in mesocosms with oilseed-rape monocultures, which suggests complementary flower resources offsetting these insecticide effects. Our findings emphasize the importance of alternative and diverse food resources for bee populations in the agricultural landscapes, which can be provided, for example, by plants in flower strips, hedgerows, or field margins. In the third chapter, the focus is on the export of pollinators and pollination services from calcareous grasslands into the surrounding landscape. Calcareous grasslands are biodiversity hotspots in the agricultural landscape, and provide food and nesting resources for pollinators, which also forage in the close surroundings, thereby pollinating flowering crop and wild plants. This spillover was studied for solitary bees, social bees and hoverflies by establishing potted flowering plants at different distances (0-400m) from the focal grasslands and observing pollinator visits, as well as measuring pollination success by counting seeds developed by the exposed plants. The number of visits decreased with distance only for solitary bees, suggesting the importance of calcareous grasslands as their main habitat. The size of the grasslands was correlated with solitary bee numbers, with larger grasslands supporting more than twice as many bees individuals. Seed production of the focal plant did not decrease with distance, suggesting that other pollinators, such as bumble bees and hoverflies, which seemed to be less dependent on the grasslands, or to have greater foraging ranges, were compensating for the loss of solitary bees. These results show the significance of calcareous grasslands as major bee habitats, and calls for their conservation and restoration to support bee populations, which pollinate plants in the surrounding landscape. The fourth and final chapter is about the effects of habitat loss and landscape diversity on cavity-nesting insects of different trophic levels and grades of food and habitat specialization. Using trap nests on calcareous grasslands differing regarding size and diversity of the surrounding landscape, the following patterns were revealed: Species from higher trophic levels were positively affected by semi-natural habitat at larger spatial scales, compared to those from lower trophic levels, but only, when they were generalists. Specialist species responded to habitat loss at the same scales as their hosts, suggesting bottom-up effects as the driving factor. While bees, being habitat specialists, were mainly driven by local habitat loss, wasps as habitat generalists were mostly affected by landscape diversity. These findings highlight the need to consider different special scales contingent on trophic level and specialization of target species for conservation, maintaining or restoring both local habitats and high landscape diversity. In summary, this thesis shows the importance of a unique habitat type (calcareous grasslands) especially for bees, and how their pollination services spillover into the surrounding landscape, the different effects of local and landscape factors on species with different trophic levels, and the scales, at which they experience the landscape. Furthermore, the significance of the availability and diversity of food resources in supporting bee populations and potentially offsetting negative effects of other stressors was shown. The findings can help to evaluate the relative importance of different stressors affecting bees in the agricultural landscape and they underline the need to conserve and restore semi-natural habitats in the landscape, and to foster diverse landscapes, to support populations of various beneficial insects.

2023-12-08