The consequences of variation in initial settlement vs. post-settlement mortality in rocky intertidal communities

Abstract

Initial settlement of planktonic propagules may affect the structure of intertidal communities in some circumstances but not in others. Of the very few studies of initial settlement, most have been done on intertidal barnacles. Population density of barnacle settlers was sometimes limited by either planktonic larval supply or adverse water column factors, and at other times by space limitation on the shore. Mortality in the first weeks after settlement was independent of settler density for two species in four localities. Therefore the density of young recruits at the end of a few weeks of settlement on shore was a positive function of settler density in these studies. Mortality between juvenile and adult stages was independent of initial density when recruitment was light, but was density-dependent when recruitment was heavy. As a consequence, adult density was positively correlated with recruit density when recruitment was light, but uncorrelated when recruitment was heavy. The causes of variation in distribution of initial settlers have never been adequately studied, and most evidence is anecdotal. The vertical intertidal limit of juveniles was correlated with the distribution of planktonic larvae in two species and determined by post-settlement mortality in another species. The vertical limit of adults was determined by recruitment in some situations and by post-recruitment mortality in others. Horizontal variation was linked to planktonic supply in one location. In several species, recruitment at several sites could be consistently ranked over several years. Such site consistency argues that either settlement or early post-settlement mortality is determined by specific local physical conditions at a site, such as coastal morphology, internal waves, shore waves, currents, or by consistent abundances of natural enemies. Existing general models of community structure probably apply mainly to sites with high rates of settlement or recruitment, not to those where these rates are low.