Abstract

Aim: Our aim in this paper is to present the first broad-scale quantification of species abundance for rocky intertidal communities along the Pacific coast of North America. Here we examine the community-level marine biogeographical patterns in the context of formerly described biogeographical regions, and we evaluate the combined effects of geographical distance and environmental conditions on patterns of species similarity across this region. Location: Pacific coast of North America. Methods: Data on the percentage cover of benthic marine organisms were collected at 67 rocky intertidal sites from south-eastern Alaska, USA, to central Baja California Sur, Mexico. Cluster analysis and non-metric multidimensional scaling were used to evaluate the spatial patterns of species similarity among sites relative to those of previously defined biogeographical regions. Matrices of similarity in species composition among all sites were computed and analysed with respect to geographical distance and long-term mean sea surface temperature (SST) as a measure of environmental conditions. Results: We found a high degree of spatial structure in the similarity of intertidal communities along the coast. Cluster analysis identified 13 major community structure 'groups'. Although breaks between clusters of sites generally occurred at major biogeographical boundaries, some of the larger biogeographical regions contained several clusters of sites that did not group according to spatial position or identifiable coastal features. Additionally, there were several outliers - sites that grouped alone or with sites outside their region - for which localized features may play an important role in driving community structure. Patterns of species similarity at the large scale were highly correlated with geographical distance among sites and with SST. Importantly, we found community similarity to be highly correlated with long-term mean SST while controlling for the effects of geographical distance. Main conclusions: These findings reveal a high degree of spatial structure in the similarity of rocky intertidal communities of the north-east Pacific, and are generally consistent with those of previously described biogeographical regions, with some notable differences. Breaks in similarity among clusters are generally coincident with known biogeographical and oceanographic discontinuities. The strong correlations between species similarity and both geographical position and SST suggest that both geography and oceanographic conditions have a large influence on patterns of intertidal community structure along the Pacific coast of North America. © 2008 The Authors.