Experimental studies of the causes and consequences of biodiversity over ecological and evolutionary timescales

This dissertation presents four microbial microcosm-based experimental studies addressing questions related to the causes and consequences of biodiversity. All four studies adopted an approach that integrates ecology and evolutionary biology. Two studies explored the utility of knowledge on species phylogenetic relationships for understanding community assembly (chapter 1) and invasibility (chapter 3). The other two studies investigated the impacts of important ecological factors, including competition (chapter 2) and temporal niches (chapter 4), on adaptive radiation, using the rapidly diversifying bacterium Pseudomonas fluorescens SBW25 as the model organism. The first study, described in Chapter 1, examined how phylogenetic relatedness between competing species affected the strength of priority effects and ecosystem functioning during community assembly. Strong priority effects emerged only when competing bacterial species were phylogenetically most closely related, resulting in multiple community states associated with different assembly histories. In addition, the phylogenetic diversity of bacterial communities effectively predicted bacterial production and decomposition. The second study, described in Chapter 2, explored the role of competition in the adaptive radiation of P. fluorescens. The adaptive radiation was generally suppressed by competition, but its effect was strongly modulated by the phylogenetic relatedness between the diversifying and competing species and their immigration history. The inhibitive effect of competition on adaptive radiation was strongest when phylogenetic relatedness was high and when competitors were introduced earlier. The third study, described in Chapter 3, evaluated the relative importance of phylogenetic relatedness between resident and invading species and phylogenetic diversity of resident communities for invasibility. Laboratory bacterial communities containing a constant number of resident species with varying phylogenetic diversity and relatedness to invaders were challenged by nonresident bacterial species. Whereas invader abundance decreased as phylogenetic relatedness increased as predicted by Darwin's naturalization hypothesis, it was unaffected by phylogenetic diversity. The final study, described in Chapter 4, presented the first experimental demonstration of the maintenance of biodiversity that emerged from adaptive radiation in the presence of temporal niches. Only when provided with temporal niche opportunities were multiple derived phenotypes of P. fluorescens able to coexist as a result of negative frequency-dependent selection. When temporal niche was absent, the specialized phenotypes either did not emerge or were predominated by one superior phenotype.Ph.D