A latitudinal gradient in northeast Pacific intertidal community structure: evidence for an oceanographically based...

Intertidal systems have been models for the study of the roles of competition, predation, and disturbance in determining community structure. These systems exhibit considerable regional variability in percentage cover and in the strength of interspecific interactions, which may be due largely to effects of varying larval supply. In Oregon and Washington, experimental studies of space allocation among sessile invertebrates have emphasized the role of benthic processes such as competition and predation. In contrast, studies in central California have emphasized the importance of larval supply. In this article, we identify a gradient in percentage cover in the middle and upper intertidal zone that is consistent with an oceanographically based explanation for these differences: percentage cover of mussels and barnacles is much higher in Oregon, where nearshore circulation promotes high recruitment, than in California, where strong offshore currents inhibit recruitment. A mathematical model incorporating larval transport and interspecific competition for space offers an explanation for the one violation of the hypothesis-higher percentage cover of Chthamalus spp. in California. The findings illustrate that attempts to synthesize regional differences in community structure and dynamics can benefit from considering both the benthic adult and pelagic larval environments.