Disentangling competitive processes to better understand their drivers and consequences in a native grassland community

Abstract

Competition is often highlighted as a major force influencing community structure. However, there are multiple facets of competition that may have independent and differential impacts, making the understanding of what drives competition and how it structures communities difficult. In this thesis, I disentangle two facets of competition, competitive intensity and size-asymmetry, to better understand their drivers and relative importance for community assembly. First, I test the assumption that competition for soil nutrients is size-symmetric. I find that the modification of soil nutrient abundance and distribution can alter the degree of competitive size-asymmetry independent of competitive intensity, suggesting that competitive size-asymmetries may be more common than originally thought. Second, I indirectly test the importance of competitive size-asymmetries for species diversity by examining how the incorporation of community size-structure increases the explanatory power of the productivity-diversity relationship. I find that variation in size structure is common, even in a typically non-light limited system, and its incorporation can increase the explained variance of the productivity-diversity relationship by up to 30%. Next, I directly test the relative importance of competitive intensity and size-asymmetry for species turnover. I find that competitive size-asymmetries are associated with increased species loss and decreased species gain, while competitive intensity has no impact. Finally, I test an alternative mechanism that may promote increased beta-diversity in a landscape: whether the removal of vegetation, which is often associated with changes to competitive processes, results in distinct seedbank communities. I find that vegetation removal leads to distinct seedbank communities by decreasing seed rain and increasing germination rates. Overall, the results of this thesis promote new understanding of how competition structures communities, the mechanisms of competitive size-asymmetry, and the mechanisms that support beta-diversity.