Theresia Krausl

Most flowering plants depend on insect pollinators for their reproduction, yet these pollinators have been facing considerable losses in abundance and diversity in recent decades. The ongoing pollinator declines, caused by stressors including land-use and climate change, have raised concerns for species-rich plant communities such as those found in semi-natural grasslands. These communities contain a high diversity of herbaceous flowering plants – forbs – which are associated with important ecosystem functions including resource provisioning to pollinator populations in the form of nectar and pollen. Recent studies have indicated that pollinator declines could already be altering the species composition in semi-natural plant communities, favouring wind-pollinated plants like grasses at the cost of insect-pollinated forbs. Thus far, these studies have been mainly correlative, and empirical evidence for these dynamics remains limited. In addition, there is still an incomplete understanding as to how landscape-scale processes and climate change shape plant-pollinator interactions, and how these effects cascade into the composition of the plant community and the functioning of ecosystems. With this thesis, I aim to contribute to a deeper understanding of plant-pollinator interactions under environmental change by applying knowledge synthesis as well as observational and experimental methods at both landscape and local scales. This thesis provides empirical evidence that changes in pollinator availability rapidly shape plant communities to the extent that changes in trait distribution become apparent. These changes have the potential to create reinforcing feedback loops further threatening pollinator populations. Furthermore, I show that the surrounding landscape can affect plant species differently depending on their pollinator dependence, and that relationships between pollinator availability and plant reproductive success are not always linear. Finally, the results of this work indicate that pollinator availability has a more consistent impact on plant reproduction than water and nutrient availability, although I also found combined effects of these factors. This thesis highlights the importance of stabilising pollinator communities to protect the species diversity and ecosystem functioning of semi-natural grasslands. Taken together, the findings further our understanding of how land-use and climate change impact plant-pollinator interactions and associated ecosystem functions and can inform conservation strategies and policy development.