Broad‐scale geographic variation in the organization of rocky intertidal communities in the Gulf of Maine

Abstract

A major challenge facing ecology is to better understand how large‐scale processes modify local‐scale processes to shape the organization of ecological communities. Although the results of ecological experiments are repeatable on local scales, different results often emerge across broad scales, which can hinder the development of general predictions that apply across the geographical range of a community. Numerous studies in the southern Gulf of Maine have shaped our understanding of community organization and dynamics on New England rocky intertidal shores, where consumers strongly control recovery from disturbance on sheltered shores, and high invertebrate recruitment and competition for space dictate recovery on wave‐exposed shores. It is unclear, however, whether the effects of consumers and recruitment variation on resulting community organization in this region apply more broadly to rocky intertidal habitats throughout the Gulf.We characterized variation in rocky intertidal community structure at 34 sites throughout the Gulf of Maine and experimentally examined the influence of consumers (present, absent) and wave energy (wave‐exposed, sheltered) on community recovery from disturbance in the northern and southern Gulf. Our results reinforced previous work in the southern Gulf, because consumers dictated the recovery of fucoid algae and mussels on sheltered shores, whereas high barnacle and mussel recruitment and competition for space shaped recovery on wave‐exposed shores. However, on sheltered shores in the northern Gulf, neither consumers nor barnacle and mussel recruitment impacted recovery, which was dominated by fucoid algae. Moreover, recovery on wave‐exposed shores in the northern Gulf was quite distinct from that observed in the southern Gulf; barnacle and mussel recruitment was negligible and fucoid algae dominated recovery, including the long‐term establishment of Ascophyllum nodosum, which is largely absent from wave‐exposed shores in the southern Gulf. Thus, distinct community types emerged in the northern and southern Gulf despite their sharing of many of the same species. These patterns likely emerged because of regional differences in coastal oceanography that modulate the recruitment of barnacles and mussels. Hence, increased attention to regional factors should provide key insight into how rocky‐shore communities are organized in the Gulf of Maine and elsewhere.