“Home Sweet Fucus”: The Role of Fucus distichus as a Community Habitat Former at Low and High Margins of its Intertidal Range

Fucus distichus is a common and ecologically important canopy-forming seaweed found on the rocky intertidal shores of the Northern Hemisphere. In addition to its role in primary production, F. distichus is a critical habitat former for many invertebrates. It provides both physical structure and protection from sunlight and high temperatures, preventing desiccation and heat stress in intertidal environments. In order to quantify the communities living on and under Fucus, we sampled quadrats along transects at the low and high ends of its intertidal zone at three sites on San Juan Island. We recorded counts of invertebrates and estimated percent cover of understory macroalgae and occasional epiphytes. We hypothesized that there would be differences in invertebrate assemblages occupying the low and high edges of Fucus and sought to compare these among sites. We hypothesized that there would be differences in species diversity and richness in the low and high assemblages. Across sites, we found that distinct groups of organisms occupy the low and high edges of Fucus and that Littorina scutulata, Chthamalus dalli, and Semibalanus cariosus contribute most to this dissimilarity. We also found that in general, the diversity of invertebrate and algal assemblages is higher in the low edge and lower in the high edge of the Fucus zone