Pollination is one of the most essential ecosystems services, it is crucial to the reproduction of most flowering plants, and plays an important role in crop production. This fundamental ecosystem service is threatened by the ongoing global change, which includes climatic change, changes in land use, and the introduction and spread of alien species. Alien plants may have a negative impact on native plants when they compete for pollination services. Effects of competi- tion for pollinators may have an impact on the quantity as well as the quality of pollination. The quantity of pollination adresses the number of visits a plant receives, while the quality of pollina- tion describes the number of transferred pollen grains. The complex interactions of these factors determines the outcome of seed set for a plant. Ultimately, a negtive affection of these compo- nents may lead to a reduction of plant fitness. The fact that alien plants often have large showy inflorescences as they were commonly introduced as ornamentals makes them potentially supe- rior competitors. Alien plants have been shown to affect the quantity as well as the quality of co- flowering native species. Furthermore, it has to be taken into consideration that species do not interact in a vacuum, plants and pollinators form a mutualistic interaction network. While there is evidence that alien plants are well-integrated into interaction networks, there is still a demand to know if this integration translates into effects on the quality of pollination and reproductive success of a community of co-flowering plants.

Aim of this thesis was to scrutinize the impacts of the alien invasive plant Heracleum mantegaz- zianum on native plant-pollinator systems. Due to its tall growth and large compound inflores- cences H. mantegazzianum definitively has the potential to affect the pollination of native plants. In order to find out about the pollinators of the invader a field study was conducted, which included observation of flowers and measurements of pollinator efficiency. The same measure- ments were made for the native closely related Heracleum sphondylium in order to detect an overlap of pollinator faunas. An experimental garden setup was used in order to determine the effect of the invader on insect visitation and set seed of an array of co-flowering plants. Addi- tionally, I used a network approach to explore the impact of H. mantegazzianum on two- and one-mode network parameters. Furthermore, the transport of alien pollen was quantified and network parameters of networks based on visitation, interaction and pollen transfer were com- pared.

Results show that the large inflorescences of the invader were highly attractive to a broad range of insects, yet there were just a few insects truly important pollinators, most of all the honeybee Apis mellifera. There was low potential for competition between the two Heracleum-species, yet the invader might have a potential to influence native plant-pollinator interactions due to its association with the super-generalist A. mellifera. However, results of the experimental garden revealed that visitation rates of co-flowering plants were (although statistically non-significant) generally enhanced, and the effect on seed set was neutral. H. mantegazzianum was found not only to be well integrated, but dominant in invaded visitation networks. Nevertheless, in com- bination with the results for seed set it was concluded that the impact of the invader was not detrimental to the invaded plant-pollinator system. Additionally, I could show that, while flower visitors of H. mantegazzianum were less constant than on other plants and there were plant-to- plant interactions for all co-flowering plants, still little alien pollen was transferred. Neverthe- less, there was a potentially competitive effect of conspecific pollen loss depending on the plant species. Network analysis showed that the alien plant was only dominant in terms of visitation but not in terms of interspecific pollen potentially transferred. This highlights that both, the quantity and the quality component, need to be assessed to create an adequate estimate on the impact of an invader on plant-pollinator systems.