Changes in barnacle population structure along an intertidal community gradient in the Florida Keys

Population distributions, densities and percentage deaths and individual size variations of the barnacles Chthamalus stellatus and Tetraclita squamosa, and the distribution of the vermetid snail Spirogluphus annulatus are measured to clarify determinants the Florida Keys (USA). The species composition of the pilings community and barnacle population densities correspond to physical gradients dependent on piling distance from shore. Interspecific competition for space is insignificant in determining C. stellatus presence or absence on pilings. Intraspecific crowding in highdensity zones of adult T. squamosa is evidenced by a significant decrease in basal diameter as density increases. The peak percentage of dead C. stellatus coincides with peak densities. The percentage of dead T. squamosa is at a low point at peak densities due to the ability of T. squamosa cyprids to settle and survive on adult shells in high-density regions. Feasible perturbation experiments for testing the importance of interspecific competition in determining the densities and vertical distributions of the species are discussed. The results of such experiments can be used to test the assumptions and predictions of the widely applied competition coefficient measure proposed by Levins (1968). Several theoretical deficiencies of Levins' measure are discussed.