| Plants interact with a diversity of microorganisms including enemies and mutualists. Plant pathogens and mutualistic fungi are two classes of microorganisms that directly impact the plant and may in turn alter each other's success. While their roles have often been considered independently, few researchers have considered their concurrent role. Dynamics of these two groups of widespread microbes may modify plant nutrient allocation in response to abiotic environmental changes. Furthermore, early models suggest that mutualists and pathogens may profoundly impact not only their shared host plant, but each other. In one of the first thorough explorations of three-species interactions, I use both experimental and theoretical approaches to investigate the interaction between plants, their pathogenic enemies and fungal mutualists in the context of changing abiotic conditions.;In two separate greenhouse experiments I show that mutualistic strategy is important for determining the direction of change by which mutualists alter pathogen dynamics. In additional work, I also confrim the reverse can also be true in that pathogen infection influences mutualists. Both mathematical theory and an experiment indicate that a pathogen can alter host-mutualist dynamics and consequently alter long-term co-existence of a host and a mutualist. Finally, my thesis shows that changing abiotic environmental conditions can modify the relationships between hosts, mutualists and pathogens. Using greenhouse and field experiments, I demonstrate that increases in atmospheric CO 2, temperature and precipitation all modify mutualist-host-pathogen relationships. Overall, my thesis demonstrates that mutualists and pathogens can have important impacts on not only the host but also on the success of each other. Such dynamics can be further modified by changes in the abiotic environment. Precipitation, temperature, and atmospheric CO2 are all expected to continue to change for the foreseeable future. Thus, in order to make accurate projections about ecosystem, community or population dynamics, changes in microorganisms and their interactions must be included in those projections. |
