Environmental context and host plant extinction as drivers of microbial symbiont population prevalence and metacommunity diversity
Donald, Marion L
Miller, Thomas EX
Doctor of Philosophy
Microbiomes, the communities of microorganisms living in and on a host, are often comprised heritable symbionts and symbionts that are dispersed across host species. I studied these two types of microbial partners to understand the drivers underlying microbial prevalence and diversity within host populations and communities. My first chapter used concepts and tools developed in the field of population demography to address the puzzling observation that heritable symbionts are often observed at intermediate prevalence within host populations. I coupled experimental populations of a heritable fungal endophyte and its grass host with demographic models to infer long-term symbiont prevalence within host populations and assess the effects of the symbiont on host vital rates. I found that intermediate symbiont prevalence can arise from multiple mechanisms, which stem from the outcomes of context dependent symbiont effects on host vital rates. Zooming out from a core microbial partner to consider a community of host-associated microorganisms, I addressed how the local extinction of a host species and urbanization affect microbial diversity across multiple host species. For these questions, I used tools and concepts developed in the field of community ecology and addressed these questions using nectar microbial metacommunities as the study system. My second chapter addressed how the extinction of a plant host affects nectar microbial metacommunity diversity. I found that host plant extinction reduced local species richness, species turnover, and overall richness of the nectar microbial metacommunity through the coextinction of host specific bacterial taxa, rather than a reduction in dispersal. My final chapter determined how urbanization affects nectar microbial metacommunity diversity. I found that while microbial communities carried by nectar-feeding birds -- microbial dispersal agents in this system -- had higher diversity in suburban compared to rural areas, nectar filtered the microbial communities, which resulted in similar composition and diversity across suburban and rural sites. However, bacterial taxa sorted across host types and additional host types at suburban sites drove the increase in bacterial beta diversity. Collectively, this body of work enhances our understanding of the drivers underpinning microbial prevalence within host populations and diversity within host communities.