Impacts of insect herbivory and nitrogen eutrophication on grassland ecosystems

Nitrogen (N) eutrophication causes severe species richness declines in grassland ecosystems worldwide. Species interactions, for example between plants and herbivores, are likely to mediate the grassland’s response to increased N availability. However, attempts to investigate how herbivores might modify ecosystem-level N impacts have so far focused near-exclusively on the aboveground subsystem. Such approaches overlook the importance of the great number of belowground interactions that occur among plants, herbivores and other organisms. A great challenge in global change ecology is to understand how the above- and belowground subsystems interact to determine the ultimate, ecosystem-level impact of N eutrophication and other global change drivers. In this thesis, I present controlled experiments that were carried out to explore the role of above- and belowground insect herbivory in mediating interactions among plant species, and how the interaction between above- and belowground herbivory determines the response of the grassland to N eutrophication. In a field experiment, I found that above- and belowground herbivory determined the impact of N eutrophication on the grassland ecosystem, in terms of plant community composition, ecosystem functioning, and the soil food web. N and herbivory synergistically conferred a competitive advantage to forbs over grasses, as the former strongly benefited from the presence of both herbivory types at elevated N. Nutrient turnover displayed inverted responses to above- and belowground herbivory at contrasting N levels. Additionally, the shifts in the plant community extended to shifts in the functional composition of the soil nematode community. In a parallel plant competition experiment, I found that belowground herbivory mitigated an increased competitive asymmetry caused by aboveground herbivory. I conclude that insect herbivores can be important mediators of the impact of N eutrophication on grasslands, and that their mediating role will depend on whether both above- and ...