Top-Down Processes Drive Taxonomic and Phylogenetic Diversity in a Model Aquatic Microbial Community

While the relative importance of bottom-up and top-down processes in structuring patterns of aquatic taxonomic diversity has been widely explored for many taxa, the response of aquatic microbial communities to bottom-up: top-down processes is not well understood. Further, studies of bottom-up: top-down processes in aquatic systems often focus on taxonomic diversity, leaving the response of other components of diversity (e.g., phylogenetic diversity) less clear. We used the model aquatic food web found in leaves of the pitcher plant Sarracenia purpurea to explore the influence of bottom-up: top-down processes on patterns of bacterial taxonomic and phylogenetic α- and β-diversity. To explore the status of Wyeomyia smithii mosquito larvae as a prospective keystone predator of bacteria in this system, we developed two experiments where we manipulated resource level and the abundance of the top predator, W. smithii, in the presence or absence of intermediate consumers. Bacterial communities were assessed by high-throughput sequencing of bacterial 16S rRNA genes and were interrogated using multiple components of taxonomic and phylogenetic α- and β-diversity. Taxonomic α-diversity decreased in the presence of mosquito larvae predators but was unaffected by predator number or resource level. Phylogenetic α-diversity responses to bottom-up and top-down processes depended on the metric. Taxonomic and phylogenetic β-diversity increased largely as a result of mosquito larvae predation; however, there was some influence of resource level, as well as differences in dispersion among levels of predation. We show that W. smithii mosquito larvae exerted strong top-down control over taxonomic α- and β-diversity in S. purpurea bacterial communities but did not act as a keystone predator. Predation by these larvae altered phylogenetic diversity through the removal of less abundant phylogenetic lineages, shaping both α- and β-diversity. These experiments clarify the role of W. smithii in the S. purpurea model food web, provide novel insights into the response of bacterial communities to both bottom-up and top-down processes, and highlight the importance of using multiple components of diversity in bottom-up: top-down studies.