The causes and consequences of biodiversity in multitrophic communities

The work described in this dissertation is linked by the common theme of biodiversity and its relationship to microbially-mediated functions in ecosystems. In the first chapter, I present results from a study where I evaluated the consequences of diversity by manipulating species richness in model aquatic communities. I showed that bacterial abundance remained constant with increasing eukaryotic species richness at low productivity, but significantly declined at high productivity. Furthermore, eukaryotic species richness together with productivity influenced the composition of the bacterial community, and food web diversity and productivity interact to influence bacterial community composition and function. In more diverse food webs, bacterial activity (decomposition) increased despite lower population abundance. In chapters two and three, I present results from one experiment in which I measured responses of microbial diversity and multiple trophic levels to an environmental perturbation in naturally occurring forest soil food webs from two geographically different locations. In the second chapter, I showed that diversity of the bacterial and fungal communities (measured by colony and ectomycorrhizal morphotype respectively) responded differently to nitrogen addition depending on geographic context. The composition of the bacterial community differed with nitrogen addition and geographic site, while the composition of the fungal community did not. In chapter three, I evaluated the relative importance of trophic control in the soil micro-food webs from the same two geographic sites (Florida and New Jersey). I found that the FL site supported greater biomass of bacteria and fungi than NJ, and the NJ site supported greater density of measured soil animal groups (collembola, oribatid mites and predatory mites) than FL. I found evidence for top down control by soil animals on microbial biomass, and at the same time, I also found evidence for bottom up control on microbial biomass through limitation of NO3 and PO4. This dissertation demonstrates that microbially mediated-ecosystem functions depend upon trophic interactions with producers, consumers and predators in food webs. Furthermore, it demonstrates that the response of these communities is context dependent. Biotic and abiotic factors play a critical role in shaping a community's diversity, composition and functioning.Ph.D.Includes bibliographical references