Historical Contingencies in the Ecology and Evolution of Species Diversity

Ecologists have long-sought to explain the high diversity of species in biological communities, given that classic theory predicts that diversity is limited by available niche space. In recent years, ecologists have looked towards â historical contingenciesâ , the persistent effects of past ecological and evolutionary processes, as possible mechanisms that maintain diverse communities, either by relaxing the constraints of niche availability or by adding temporal dimensions to speciesâ niches. In this thesis, I use field and greenhouse experiments to explore three ways in which historical contingencies manifest in annual plant communities. First, my work on maternal effects shows that abiotic (ch. 2) and biotic (ch.3) conditions in the maternal generation have diverse effects on offspring phenotypes across an assemblage of species. Because species differences in environmental responses can facilitate coexistence, these studies suggest that maternal effects could act as a form of niche differentiation, and motivate future research to clarify their influences on coexistence outcomes. Second, I performed, to my knowledge, the first experimental decoupling of dispersal limitation and environmental sorting in a natural landscape by manipulating entire seed pools of annual plants (ch. 4). In doing so, I was able to identify the pervasive and scale-specific influences of dispersal limitation that constrain species distributions in plant communities. Lastly, I used competitive trials to identify macroevolutionary divergence in competitive interactions among species (ch. 5), and found evidence that divergence is contingent on historical competitive interactions in ways that are consistent with character displacement. In sum, my dissertation work has expanded our understanding of (i) the number of potential niche dimensions that might allow species to differentiate, (ii) how this differentiation can arise over evolutionary time, and (iii) the interplay of current and historical conditions in the maintenance of species diversity, and the timescales over which they play out.Ph.D.2019-02-09 00:00:0