Date of Award
Doctor of Philosophy (PhD)
Ecology & Evolutionary Biology
The processes of community assembly shape all groups of coexisting organisms across all environments where life is found. Even for environments that are currently sterile, when they are colonized by life, community assembly processes will occur. The processes governing formation and diversification of microbial communities are vital to an understanding of how ecosystems develop in the human body, in emerging landscapes, and everywhere else. In this dissertation, I use community assembly theory to understand how microbial communities are shaped by geography, and also by the resources available to them. Using a microcosm-based nutrient addition experiment, I show that microbial communities in the plant-free, oligotrophic debris on top of the Middle Fork Toklat Glacier (Denali National Park and Preserve, Alaska) are strongly structured by the lack of available phosphorus, a result which differs significantly from ecological dogma that suggests early-successional primary producers are limited by nitrogen instead. I expand on my findings of P-limitation with an in situ nutrient addition experiment in soils exposed by the retreating Puca glacier (Cordillera Vilcanota, Peru), which showed that phosphorus limits primary producers (both microbes and plants), shaping the structure and function of microbial communities. The biogeochemical properties affecting microbial communities often have a geospatial component, and I found that atop the Toklat Glacier, geographic space strongly shapes the interaction between biogeochemistry and microbial communities. But surprisingly, microbial community compositional data revealed that the top of the glacier is likely a hidden chronosequence, disguised as supraglacial debris. Chronosequences are natural experiments where space and time are conveniently conflated, and to better understand these time-series from a microbiological perspective, I developed a mathematical model that estimates the degree to which microbial communities form nepotistically. Using this model with time-series data from human microbiome studies, I found that in skin, feces and tongue, microbes are more likely to join those communities if a closely-related species is already present. Together, the chapters of this dissertation show how the environment, space, and time shape microbial communities on glaciers, near glaciers, on people, and in people.
Darcy, John Lawrence, "Biogeographic and Biogeochemical Drivers of Microbial Community Assembly" (2017). Ecology & Evolutionary Biology Graduate Theses & Dissertations. 101.