Most marine organisms, insects, many lower invertebrates and plants have life histories that involve an obligate dispersive stage. Because of this dispersive stage, recruitment of individuals to the adult population can be highly variable in space and time. I investigated how variation in recruitment of the acorn barnacles Balanus glandula and Chthamalus fissus/dalli affected the adult population along the California coast. I found that differences in large-scale climatic variation between years had dramatic effects on recruitment. These differences in recruitment resulted in differences in the adult population sizes between years. In the second chapter of my dissertation, I investigated how variability in reproductive output and variation in the intensity of a disturbance combined with recruitment variability to influence population dynamics using a mathematical model. The results indicated that although the changes in reproductive output influenced recruitment dynamics over time, the intensity of disturbance had a much stronger effect on recruitment and subsequent population dynamics.

After establishing that recruitment variation influenced the adult barnacle population, I investigated the effects of this variation on community dynamics. In the third chapter of my dissertation, I used a mathematical model to simulate spatial and temporal variability in recruitment and assessed the resultant effects on community structure. The results of the model indicated that spatial variability in recruitment explained 25 times more of the variance in community structure than temporal variability in recruitment. Further, there was a non-linear relationship between spatial variability in recruitment and community structure. The final chapter of my dissertation was an empirical investigation of the role of recruitment variability in structuring communities. These experiments showed that community composition differed as a function of recruit density and the strength of the relationship between recruitment intensity and community structure increased over time as the relative importance of the different species interactions to community structure changed over time.

To summarize, I found that at the population level, processes such as climatic variation and disturbance interact with variation in recruitment to influence barnacle population dynamics. If recruitment variation influences population dynamics, I reasoned, and subsequently found, that there should be effects at the community level.

More information is available at www.stanford.edu/~sforde