Rock walls: small-scale diversity hotspots in the subtidal Gulf of Maine

The physical orientation of rocky substrate profoundly affects subtidal marine communities of sessile organisms. Anecdotal descriptions of dramatic differences between communities on rock walls and adjacent horizontal rocky bottoms abound in the literature and are common knowledge among scuba divers, yet these differences have rarely been quantified by ecologists. We sampled rock walls and adjacent horizontal rock bottoms at 8 subtidal sites across the Gulf of Maine. Species richness and abundance, in percent cover, of sessile invertebrates on vertical walls averaged 4 times higher than that on horizontal platforms, whereas abundance of macroalgae on horizontal rock was about 3 times that on vertical walls. Both macroalgae and sessile invertebrates were less abundant, particularly on horizontal surfaces, at sites with high sea urchin densities. The consistency of sessile invertebrate domination of vertical walls versus macroalgal domination of horizontal rock, combined with previous experimental results, suggests that competition for space between autotrophic and heterotrophic organisms drives this pattern. The partitioning of autotrophs (macroalgae) and heterotrophs (sessile invertebrates) between horizontal and vertical surfaces respectively implies that topographic heterogeneity plays an important role in the structure, composition and function of rocky subtidal ecosystems.