Recruitment vs. postrecruitment processes as determinants of barnacle population abundance

Determining the relative contributions of recruitment vs. postrecruitment p rocesses to adult populations is an unresolved issue. The "recruit-adult" h ypothesis suggests that the density of recruits is a good predictor of adul t density when low but not when high. That is, the relative importance of r ecruitment vs, postrecruitment factors varies inversely with increasing den sity of recruits. In a rocky intertidal habitat at two Oregon coastal sites , a field experiment was done using two barnacle species to test this hypot hesis. The relative impacts of these factors on adult barnacle abundance was deter mined using a reciprocal transplant design to manipulate both the density o f 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 speci es specific. While density of recruits clearly influenced density of adults for both species in most combinations of site, zone, and exposure, the eff ects of physical and biotic factors ranged from strong to weak. For Chthama lus, recruitment generally had a stronger impact on density of adults than did postrecruitment processes, while for Balanus, postrecruitment factors t ended to have stronger effects than did recruitment. These differences were an apparent consequence of differential susceptibilities to postrecruitmen t processes. Chthamalus was more tolerant of both biotic and abiotic forces than was Balanus and usually had high survival during periods of high Bala nus mortality. Heat and desiccation were identified as the primary postrecruitment mortali ty factors in the high zone, whereas biotic interactions (competition and p redation) were the most likely postrecruitment mortality processes in the m id-zone. Mortality was generally density dependent at the site with the str ongest effects of postrecruitment processes, and generally density independ ent at the site with the strongest effects of recruitment. To determine whe ther trends were more consistent with the recruit-adult hypothesis at large r scales, data for each species were pooled across exposures (zone scale) a nd across exposures and zones (site scale) and also were compared to litera ture data from sites around the world (global scale). Each analysis led to the conclusion that recruitment can be a strong determinant of density of a dults, but that the magnitude of this relationship depends on context, vari es with species, and can be strongly modified by postrecruitment processes. Even when density of recruits is low, postrecruitment factors can be impor tant in determining density of adults. Thus recruitment is a necessary but sometimes insufficient determinant of adult population density. These resul ts support earlier suggestions that predictive models must incorporate both recruitment and postrecruitment factors and will thus depend on understand ing the coupling between benthic and pelagic processes.