Interactions between plants, microbes and insects

Plants interact with an astonishing diversity of insects and microorganisms. Both above- and belowground, plants are attacked by herbivores and pathogens, and interact with mutualists such as pollinating insects and beneficial microorganisms. Insects and microorganisms interacting with plants may also affect one another when sharing the same host, leading to direct and indirect interactions between plants, microorganisms and insects. These interactions may have important ecological and evolutionary consequences for all involved species, and interaction outcomes might be dependent on the timing of the interaction as well as the abiotic and biotic context. Thus, in order to predict the outcome of plant–microbe–insect interactions, we need insights into how interactions vary over time and space and how these are influenced by biotic context, from the perspective of all involved species. In this thesis, my overarching aim was to investigate when and where species interact, and to examine the influence of relative timing and biotic context on the outcomes of plant–microbe–insect interactions from a multi-species perspective. I focussed on two study systems, the pedunculate oak Quercus robur and the wood anemone Anemone nemorosa, and the insects and microorganisms associated with these plants. First, I looked into the drivers behind the spatiotemporal distribution of several fungal pathogens on plants. Specifically, I investigated whether life history traits of pathogens were linked to their metapopulation dynamics. Second, I examined the relative importance of direct and insect-mediated effects of these pathogens on plant performance. Third, I conducted multifactorial growth chamber experiments to investigate performance impacts of interactions from the perspectives of all involved species (plant, insect and pathogen). Lastly, I investigated the influence of time of attacker arrival, early arriving attackers and soil microbial communities on interaction outcomes. I found that life history traits of pathogens were related to some aspects of metapopulation dynamics. These pathogens had direct, negative impacts on plant growth and fitness, while I did not find evidence for insect-mediated effects. For the multifactorial experiments, I observed some impacts of multiple attackers (pathogens and insects) on plant growth, though plants were mostly tolerant to attack. Attackers that shared a host could affect each other’s performances, with effects mostly being asymmetric, though the interaction outcomes were dependent on the time of arrival, early arriving attackers and soil microbial communities. In conclusion, my findings show that i) life history traits may influence where pathogen species occur in space and time, that ii) direct effects of pathogens on plant fitness can dominate insect-mediated effects, and that iii) the outcome of species interactions is often asymmetric and dependent on relative timing as well as biotic context. By identifying some of the drivers behind, and consequences of, plant–microbe–insect interactions, this thesis contributes to the development of a predictive framework for species interactions