Recruitment vs. Postrecruitment Processes as Determinants of Barnacle Population Abundance

Determining the relative contributions of recruitment vs. postrecruitment processes to adult populations is an unresolved issue. The “recruit–adult” hypothesis suggests that the density of recruits is a good predictor of adult density when low but not when high. That is, the relative importance of recruitment vs. postrecruitment factors varies inversely with increasing density of recruits. In a rocky intertidal habitat at two Oregon coastal sites, a field experiment was done using two barnacle species to test this hypothesis. The relative impacts of these factors on adult barnacle abundance was determined using a reciprocal transplant design to manipulate both the density of barnacles established by recruitment and the postrecruitment conditions (tidal height, wave exposure) in which they lived. The relative contribution of recruitment to adult densities was strongly context dependent and species specific. While density of recruits clearly influenced density of adults for both species in most combinations of site, zone, and exposure, the effects of physical and biotic factors ranged from strong to weak. For Chthamalus, recruitment generally had a stronger impact on density of adults than did postrecruitment processes, while for Balanus, postrecruitment factors tended to have stronger effects than did recruitment. These differences were an apparent consequence of differential susceptibilities to postrecruitment processes. Chthamalus was more tolerant of both biotic and abiotic forces than was Balanus and usually had high survival during periods of high Balanus mortality. Heat and desiccation were identified as the primary postrecruitment mortality factors in the high zone, whereas biotic interactions (competition and predation) were the most likely postrecruitment mortality processes in the mid-zone. Mortality was generally density dependent at the site with the strongest effects of postrecruitment processes, and generally density independent at the site with the strongest effects of recruitment. To determine whether trends were more consistent with the recruit–adult hypothesis at larger scales, data for each species were pooled across exposures (zone scale) and across exposures and zones (site scale) and also were compared to literature data from sites around the world (global scale). Each analysis led to the conclusion that recruitment can be a strong determinant of density of adults, but that the magnitude of this relationship depends on context, varies with species, and can be strongly modified by postrecruitment processes. Even when density of recruits is low, postrecruitment factors can be important in determining density of adults. Thus recruitment is a necessary but sometimes insufficient determinant of adult population density. These results support earlier suggestions that predictive models must incorporate both recruitment and postrecruitment factors and will thus depend on understanding the coupling between benthic and pelagic processes.