Understanding the role of biological diversity to ecosystem function in ecological communities is particularly important now in the face of unprecedented, human-driven global change. Anthropogenic impacts such as climate change, nitrogen deposition, and land-use changes that modify natural disturbance regimes and promote the introduction of invasive species are all altering environmental conditions in such a way as to impact both diversity and ecosystem functioning. Diversity can be described in several ways and until recently, most research has focused on the relationship between the number and diversity of species and ecosystem function. Recently however, there has been a growing focus on the importance of genetic diversity within a dominant species and its impact on community and ecosystem processes. In this dissertation, I examine the relationship between community and population diversity, evaluate how diversity at both levels is maintained under varying environmental conditions, and test the relative effects of population and community level diversity on ecosystem function. First, I utilized a long-term experimental burn manipulation to examine the impact of fire frequency on genetic diversity of the dominant species and assessed the relationship between genetic diversity of the dominant species, species diversity, and productivity across the entire fire gradient. Genetic diversity of the dominant species was highest in a four year burn cycle and lowest in an annual and two year burn cycle. However, there was not a significant relationship between genetic and species diversity or between genetic diversity of the dominant species and productivity across the disturbance Gradient. In the second and third study, I explored in further detail the relationship between genetic diversity of the dominant species, species diversity, and evaluate the direct and indirect effects of both levels of diversity on productivity and invasion resistance. Contrary to expectations, genetic diversity of the dominant species did not have a direct effect on invasion resistance or productivity. Rather the effects of diversity at both the community and population level are primarily indirect via traits. Genetic differences between naturally co-occurring genotypes of the perennial dominant grass species were more subtle, and thus also highlighted the necessity of conducting studies in intact communities with naturally co-occurring individuals. Finally, to tease apart the possible mechanisms for how diversity within the population is maintained and subsequently impacts community and ecosystem processes, I conducted a pairwise competition experiment with the four most common genotypes of the dominant species and manipulated light, water, and nitrogen resources. Phenotypic differences between genotypes resulted in different competitive successes depending on resource conditions. Some genotypes were able to take advantage of increased water or nitrogen resources under low and high light conditions, providing evidence for niche complementarity. There was evidence that some genotypes were more antagonistic under low versus high resource conditions while other genotypes were more synergistic. This differential success in resource acquisition, biomass accumulation, and subsequent competitive ability translated to variation in vegetative reproductive success, which has implications for the population dynamics of this perennial, primarily asexually reproductive, dominant C4 grass. Together the results from these four studies suggest that the roles of population and community level diversity are complex in intact, natural tallgrass prairie where diversity effects on ecosystem function may be primarily indirect via traits.