THE IMPORTANCE OF SECONDARY SUBSTRATUM IN A ROCKY INTERTIDAL COMMUNITY

The maintenance of diversity of the rocky mid-intertidal community of western North America has long been considered the result of mechanisms which prevent the mussel Mytilus californianus Conrad, the dominant competitor, from occupying all rock surfaces. Proposed mechanisms include preferential predation by starfish and physical disturbances like wave shear. However, an examination of mussel beds along the Californian and Oregonian coastline revealed that virtually all species facing competitive exclusion on the rock are present on the shells of M. californianus. Therefore, while diversity on the rock would decline, that of the community would change little, if at all, were mussels to occupy all rock surfaces. Although the same species are found on the two substrata, their abundances differ on them. However, for the commonest species the sign and magnitude of this difference is different on the two sides of Pt Conception, California. This suggests that the impact of mechanisms which govern the amount of area occupied by mussels will vary depending upon location. While there are several possible explanations for this geographical difference, data are presented suggesting it may be due to differences in the relative hardness of the mussel and rock, or to differences in abundance of the territorial limpet Lottia gigantea Sowerby.