Species Diversity and Assembly History in Ecological Communities: Microcosm Experiments and Computer Simulations

Despite its academic and applied importance, it has proven difficult to understand patterns of species diversity. This is in large part because multiple processes operating at various scales interact to influence diversity patterns emerging at different scales. Here I examine how the history of community assembly may interact with other ecological variables to influence species diversity. I consider four variables: the level of productivity, the size of ecosystem, the rate of dispersal, and the size of species pool. These variables have received considerable attention as major determinants of species diversity. However, their joint effects with assembly history remain largely unexplored. In a laboratory experiment with bacteria, algae, protists, and rotifers, I show that assembly history can interact with productivity to create a remarkable variety of productivity-diversity patterns. In another experiment, I show that community assembly can interact with ecosystem size to affect diversity and that, through this interaction, assembly history can dictate when a significant size-diversity relationship is observed. Using computer simulations of community assembly, I also show that internal and external dispersal, though generally studied separately, reciprocally provide the context in which the other influences diversity at multiple spatial scales. I further show that assembly history and the size of the species pool can jointly affect diversity patterns and that their interaction has implications for determining the relative importance of local and regional processes governing diversity. One common theme that emerges from these studies is the presence of significant historical effects in the absence of alternative stable states. Overall, these studies suggest that it is promising, though rarely attempted, to incorporate the dynamics of community assembly into a conceptual framework for species diversity.