Intraspecific Competition and Facilitation in a Northern Acorn Barnacle Population

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Abstract
The role of density—dependent mortality in shaping adult populations of the acorn barnacle, Semibalanus balanoides, was studied on a sheltered New England shore and revealed that high recruitment densities can have both positive and negative effects on barnacle survival. Recruitment greatly exceeded the capacity of the space to support adults in the three years of this study. At low tidal heights, recruits grew rapidly, experienced severe crowding, and rarely survived to reproduce. At higher tidal heights, individuals grew more slowly and were less affected by crowding, and many (°10%) survived to reproduce. Recruit density was manipulated to examine crowding effects on survival. At low tidal heights, no recruits survived to reproduce at any density, due to intraspecific crowding, algal overgrowth, and predation. At intermediate tidal heights, lower recruitment decreased intraspecific crowding and increased survivorship, while at high tidal heights, lower recruitment decreased survivorship. In the high intertidal habitat, S. balanoides survivorship was higher on boulders than on rock cobbles due to thermal buffering, and intraspecific crowding decreased rock and barnacle temperatures and facilitated survival. Shading high intertidal cobbles from solar radiation decreased maximum rock and barnacle temperatures, dramatically increased survivorship and eliminated the survivorship advantage of high densities. High recruitment of S. balanoides can lead to massive density—dependent mortality in physically benign habitats, but in physically stressful habitats high recruitment density may buffer individuals from physical stress and facilitate survival. Variation in the nature of density—dependent interactions along environmental gradients may be important in generating pattern in many natural populations.