Hammarlund, Sarah2022-04-132022-04-132022-02https://hdl.handle.net/11299/226932University of Minnesota Ph.D. dissertation. 2022. Major: Ecology, Evolution and Behavior. Advisor: William Harcombe. 1 computer file (PDF); 160 pages.Mutualisms—reciprocally positive interspecies interactions—are critical for the survival and reproduction of many organisms, from bacteria and fungi to plants and animals. Despite the ubiquity of mutualism, there are many questions that remain underexplored. For example, it is unclear why mutualisms sometimes break down when environmental conditions change. Another open area of research is the ecology and evolution of so- called multiple mutualist communities that contain many interacting species. My research aims to understand how environmental factors alter interactions and stability within mutualisms, to explore how competitors within multiple mutualist communities coexist despite competition, and to identify the selective forces that influence whether mutualists interact with one or multiple partners. I use an experimental system of two species of bacteria that engage in mutualism via cross-feeding—the consumption of nutrients secreted by another species—as well as mathematical and computational modelling. My findings contribute to our understanding of mutualism broadly, and may be useful for efforts to manipulate microbial communities and to protect mutualistic species that are vulnerable to extinction.encontext-dependencecross-feedingecological stabilitygeneralismmultiple mutualist communitiesmutualismThesis or Dissertation