Scale-dependent recruitment and divergence of intertidal communities

Sheltered rocky bays in the Gulf of Maine support communities usually domin ated either by the fucoid rockweed Ascophyllum nodosum or the mussel Mytilu s edulis. In order to test whether clearing size determines the subsequent pattern of recruitment by intertidal organisms, we examined four common spe cies 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 uncle ared controls were established at 12 sites spread across four bays. Recruit ment of A. nodosum and of three species not common in A. nodosum stands (mu ssels, barnacles, and the rockweed Fucus vesiculosus) varied with clearing size and the patterns did not vary among bays. Recruitment of A. nodosum de clined exponentially with clearing size. Fucus and barnacles recruited in g reater densities to large clearings (greater than or equal to4 m) compared to small clearings (<4 m). Recruitment of F. vesiculosus was also site spec ific, reflecting patterns of local abundance, but that of A. nodosum was no t. Mussels recruited onto artificial substrates in fewer numbers in large c learings, but in marginally greater numbers to rock underneath these substr ates in large clearings. Recruitment densities of mussels and barnacles wer e strongly site specific. These results suggest that (1) the rate of succes sion varies site specifically, and (2) the trajectory of succession, either convergence toward or divergence from the surrounding community, is contin gent upon the interplay between species interactions and the size of the cl earing.