Herbivores and soil microorganisms: Their effect on perennial and woody plant species in a grassland ecosystem

A fundamental goal in ecology is to understand what limits the distribution and abundance of species. However the complexity of biological and abiotic interactions makes it challenging to determine which links or variables are the mast significant. Abiotic factors, such as soil nutrient availability, often exist as a gradient, upon which biotic factors like herbivory, mutualism, and competition are layered. Through their effects on the population dynamics of individual species, these interacting factors ultimately determine the organization of communities and the functioning of ecosystems. Therefore, more insight is needed to understand the biotic interactions which shape plant species distributions if we are to make accurate predictions about how communities and ecosystems will function in the future. In this dissertation I detail my exploration of plant-herbivore and plant-nitrogen-soil microbial interactions at the Cedar Creek Long Term Ecological Research Site in Minnesota. This ecosystem is characterized by sandy, nitrogen poor soils. The vegetation there is a mix of prairie, savanna, deciduous and boreal species, though my research was focused on prairie and savanna species. I explore the impact of different types of herbivores, both mammalian and insect, on the reproductive success (Ch. 2) and survivorship (Ch. 3) of different grassland and savanna plants. I consider the relative importance of environmental variation versus a bias generated by scientific measurement to the study of plant-insect herbivore interactions (Ch. 4 & Ch. 5). I also examine the response of soil microbial communities in to added nitrogen in both (1) a long running experiment where, shifts in plant communities due to excess nitrogen have been well-studies, and (2) a shorter term study where the plant species are the same in the different nitrogen treatments (Ch. 6). Finally, I finish with a study about the limited effects of above and belowground vegetation on the rates of carbon and nitrogen accumulation in the vertical soil profile (Ch. 7).