Survivorship of juvenile barnacles and mussels: spatial dependence and the origin of alternative communities

Mussels, barnacles, and rockweeds often form a distinct mosaic of patches on rocky intertidal shores, and it has been suggested that these communities may represent alternative community states. One way that alternative community states can arise is if early successional events are scale-dependent, but it is not known if juvenile survivorships of mussels and barnacles are, in fact, scale-dependent. Scale-dependence of barnacles (Semibalanus balanoides (L.)) and mussels (Mytilus edulis L.) was tested in the Gulf of Maine, USA. In winter 1997, clearings of 1, 2, 4 and 8 m in diameter and uncleared controls were made in stands of the rockweed Ascophyllum nodosum (L.) Le Jolis at 12 sites spread evenly across four bays on Swan's Island, ME. Summer and fall-winter survivorship of barnacles, which recruited in spring 1997, were estimated by tracking the 1997 cohort until late winter 1998. Survivorship of mussels was estimated from following the fate of transplanted juveniles over 4 days in late August 1997. Both barnacles and mussels showed better survival in 4 and 8 m clearings than in small clearings and controls. There was also significant variation in survivorship among sites. Densities of gastropods in the clearings did not reflect survivorship patterns of barnacles and mussels. Barnacle survivorship increased in fall and winter, and in large clearings was comparable to survivorship in barnacle-dominated habitats. Mussel survivorship was low in all clearing sizes suggesting that mussel beds develop slowly.