Scale-Dependent Recruitment and Divergence of Intertidal Communities

Sheltered rocky bays in the Gulf of Maine support communities usually dominated either by the fucoid rockweed Ascophyllum nodosum or the mussel Mytilus edulis. In order to test whether clearing size determines the subsequent pattern of recruitment by intertidal organisms, we examined four common species in clearings in stands of A. nodosum. Circular experimental clearings of 1, 2, 4, and 8 m in diameter, which mimicked severe ice scour, and uncleared controls were established at 12 sites spread across four bays. Recruitment of A. nodosum and of three species not common in A. nodosum stands (mussels, barnacles, and the rockweed Fucus vesiculosus) varied with clearing size and the patterns did not vary among bays. Recruitment of A. nodosum declined exponentially with clearing size. Fucus and barnacles recruited in greater densities to large clearings (≥4 m) compared to small clearings (<4 m). Recruitment of F. vesiculosus was also site specific, reflecting patterns of local abundance, but that of A. nodosum was not. Mussels recruited onto artificial substrates in fewer numbers in large clearings, but in marginally greater numbers to rock underneath these substrates in large clearings. Recruitment densities of mussels and barnacles were strongly site specific. These results suggest that (1) the rate of succession varies site specifically, and (2) the trajectory of succession, either convergence toward or divergence from the surrounding community, is contingent upon the interplay between species interactions and the size of the clearing.