Revisiting a classic paradigm: Assessing consumer control of rocky shores in the southern and northern Gulf of Maine

Background/Question/Methods   Rocky intertidal shores in the southern Gulf of Maine (GOM) are a model system for studying the effects of consumers and physical stress on community dynamics and have provided the foundation for one of the overarching theories in community ecology: the Menge-Sutherland Environmental Stress Model. Although intertidal communities throughout the GOM host a similar suite of species, the distribution and abundance of these organisms differ dramatically between northern and southern shores. Despite striking differences, northern GOM rocky shores have been subject to substantially less investigation than their southern counterparts. In a comparative study of northern (Lubec, ME) and southern (Nahant, MA) rocky shores, we sought to examine whether consumer control, a classic feature of southern GOM rocky shores, exerts a similar influence on northern shores. We assessed consumer density at northern and southern sites, and the impacts of location, wave action, clearing size and consumer exclusion on the recovery of disturbed patches on rocky shores in the GOM.

Results/Conclusions   Results indicate that predators (the dogwhelk, Nucella lapillus, and the crab, Carcinus maenas) and the generalist, herbivorous snail, Littorina littorea, were more abundant on southern shores than northern shores, and that these differences in abundance influenced patterns of invertebrate (barnacles, Semibalanus balanoides, and mussels, Mytilus spp.) and algal (the canopy forming rockweed, Fucus vesiculosus) recovery. Thus, strong consumer control was confirmed on southern GOM rocky shores, but in contrast, not supported the north. For example, F. vesiculosus recovered faster and Mytilus spp. were more abundant when mobile consumers were excluded in the south, but consumer exclusion had no impact in the north. In contrast to southern shores, clearing size and recruitment limitation drove recovery patterns in the northern GOM. Large clearings in the north, uninfluenced by the surrounding algal canopy, recovered faster than small clearings, though the mechanism behind this pattern remains unclear. In addition, low recruitment of sessile invertebrates (S. balanoides and Mytilus spp.) in the north indicates that nearshore oceanographic processes may play a larger role in GOM community dynamics than previously assumed. Consequently, further investigation into the impacts of disturbance size and bottom-up processes at a larger scale throughout the GOM will further our understanding of how these classic model systems operate and will inform community ecology theory.