ABSTRACT: The relative importance of recruitment and post-recruitment processes in determining adult density among marine species varies considerably between species and populations. This study aimed to investigate the relative effects of variation inrecruitment and survival rates on the density dynamics of 3 spatially separated natural populations of the barnacle Semibalanus balanoides (Isle of Man, SW Ireland and west Sweden). We analysed 2 different data-based stochastic matrix models ofopen populations with space-limited recruitment. Assumption testing supported the application of matrix-model theory for studying and comparing population variables of open populations. Recruitment was found to be partly dependent on free space, andmortality was size-specific and independent of other vital rates. Model simulations showed that the relative importance of recruitment and survival in shaping adult density depends on the existing variation of these vital rates at each location. In theIsle of Man the variation in density was primarily caused by variability in recruitment, while in SW Ireland the variability in survival induced the most variation. In west Sweden variation was high due to variability in both recruitment and survival. Alife-cycle elasticity analysis across locations indicated that the population dynamics were generally more sensitive to changes in survival than to changes in recruitment. Also, locations with high variability in vital rates seemed better able to buffersudden changes in vital rates. In addition, differences in survival and growth produced population structures that varied between populations. In the Isle of Man and west Sweden the populations consisted of mainly Size Category 1 individuals, while thepopulation in SW Ireland had a linear relationship between size category and the proportion of individuals.