An experimental manipulation of barnacles successfully recruiting to two seashores tested the consequences of these variations to predation by whelks and the eventual population structure of the barnacles. Very young barnacle spat were removed from some areas with and without predatory whelks. The whelks moved away from areas without the juvenile barnacles, but stayed and ate barnacles present on the treatment with barnacle recruitment. Predation resulted in almost complete elimination of the cohort of barnacles. In contrast, barnacles survived and grew to reproductive sizes on areas initially without whelks but with recruits. Because of the movements of whelks and their predation on the barnacles, the final abundances of whelks and barnacles in each plot bore little relationship to their initial experimental treatments. Thus caution is needed in interpreting the causes of static patterns of abundance in the field, where the processes involved earlier are not definitely known. These results point to a need to incorporate variation in recruitment into models of biological interactions.