The interaction of dispersal and density dependence: Resource variation, competition, and gregarious settlement

In chapter 1, microcosms of bacteria and protozoa were used to investigate the interaction of resource variation and dispersal. Consistent with source-sink models, and contrary to balanced dispersal models, there was a net flux of individuals from high to low resource patches. Unlike the simplest source-sink models, intermediate rates of dispersal led to highest abundances in low resource patches. There was strong density dependence in local population dynamics and differences in average protozoan size between high and low resource patches. Parameterization and analysis of a two-patch model showed that high migration from high to low resource patches could have depressed population density in low resource patches, creating pseudosinks.; Gregarious settlement is common among marine organisms with larval dispersal. Since it increases aggregation, it should arise only when competitive costs are offset by benefits of aggregation, e.g., facilitation and habitat quality information. In chapters 2 and 3, I assess the costs and benefits of gregarious settlement in Petrolisthes cinctipes (Porcellanidae: Anomura), a filter-feeding crab of the northeast Pacific. Gregarious settlement is experimentally demonstrated to be a saturating function of adult density. In field and laboratory experiments, P. cinctipes growth rate declined with increasing density and this decline was more severe for smaller individuals. Smaller individuals were also more sensitive to changes in food level. P. cinctipes fed less frequently at high density and this effect was more pronounced for smaller individuals. Previous studies demonstrated that adults protect settlers from predation. In this study, the intertidal stichaeid, Anoplarchus purpurescens, had a Type III functional response to settler density without adults, but a Type I response with adults. A model evaluating the costs and benefits of gregarious settlement revealed the critical life-history features that resulted in the empirical settlement distribution. First, there was a tradeoff between competition and predation risk with increasing adult density. Second, individual fitness depended on settler density, which increased predation rate or competition. Third, positive correlation between habitat quality and adult density offset increasing competition with increasing adult density. When this correlation was high enough, settlement increased then saturated with adult density, in accord with empirical data.