Thesis #999

Angiosperms represent the most diverse and dominant group of plant organisms. With more than 250’000 species worldwide they outnumber by far other plant groups like mosses, ferns or gymnosperms. Flowering plants conquered the world’s terrestrial habitats within a short period of 70 million years. Interestingly, insects co-radiated during the same period. Does the novelty of plant-pollinator interactions explain their success story? Doubtlessly, the establishment of animal-mediated pollination represented a dramatic key innovation in the realm of plant organisms. Since plants are sessile, genetic exchange was restricted predominantly to the very proximate neighborhood in the pre-pollinator era. Thus, gametic exchange mediated by animal vectors must have revolutionized sexual reproduction of plants. For the first time pollen was transported rather target-oriented. The evolution of plant-pollinator interactions was only possible, since one group of these organisms is sessile whereas the other is mobile. All this certainly set the stage for a rapid co-evolution of one of the most common mutualistic relationships with tremendous ecological consequences. Obviously, since its appearance plant-pollinator interactions promoted and maintained biodiversity in both the plants’ and animals’ evolutionary history.
Today plant-pollinator interactions represent one of the most important ecological interactions in terrestrial ecosystems. Virtually every life-form on land depends directly or indirectly on the primary production of angiosperms, whereas the majority of these flowering plants in turn depends intimately on myriads of animal pollinators belonging to diverse taxonomic groups. However, we are still far from being able to quantify the ecological risks (e.g. for human welfare) in response to a potential global pollination crisis, i.e. the consequences of the rapid loss of pollinator species. It is high time to realize that we are on the way to experience a dramatic loss in biodiversity with unpredictable consequences and that loosing pollinator species might even accelerate this threat.
Therefore, we have to quantitatively explore numerous particular plant-pollinator relationships before we are able to deduce some general conclusions. Accordingly, we need to measure carefully each component of pollinator-mediated fitness, and thus clarify whether and how plant species are threatened by disappearing pollinator species. Furthermore, measured pollinator-mediated fitness components (abundance, visitation frequency and pollination efficiency) contributing to plant reproduction will illuminate which mechanisms promote pollinator-mediated selection, and thus how flower traits evolve in response to pollinator-mediated selection.
Thus, in the present thesis we consider evolutionary, ecological and conservational aspects of plant-pollinator interactions using two closely related carnation species, Dianthus carthusianorum and D. sylvestris (Caryophyllaceae), and their pollinator species. The two species represent ideal model systems for studying plant-pollinator interactions. Both species are rather specialized in pollination and restricted to fairly narrow ecological conditions, i.e. dry and nutrient poor habitats. Such habitats are more and more threatened due to human activities. Additionally, these ecological conditions are also associated with rather threatened pollinator species. Thus, these circumstances provide ideal premises to investigate plant-pollinator interactions in the context of the main contemporary challenges in conservation biology. As mentioned above the two carnation species are closely related. Despite their obvious similarity they possess some very distinctive features, e.g. they differ in flower depth, inflorescence architecture and color. These differences are likely, at least in part, the result of differential pollinator-mediated selection regimes. Since the two carnation species occasionally occur in sympatry, where some rare hybrids are found, we are in the fortunate position to investigate pollinator-mediated selection, the mechanisms upholding species barriers as well as the processes that potentially lead to the completion of speciation. The recorded rare hybrids indicate that the two carnation species are interfertile. However, the rareness of hybrids indicates also mechanisms impeding that the two species interbreed randomly. Observations suggest that the pollinators might play a crucial role in upholding the species isolation in sympatry. In the following four studies we investigated aspects of pollinator-mediated selection, extinction risk with respect to the plant-pollinator relationship, and isolation mechanism between the two closely related carnation species.