Assessing a macroalgal foundation species: community variation with shifting algal assemblages

Abstract

Foundation species have strong, positive effects on local community structure; increasing biodiversity and species abundances by providing food and habitat. On coastal temperate and subpolar rocky reefs, canopy-forming kelps form three-dimensional habitats that support numerous fish, invertebrate, and algal species. Throughout the Aleutian Archipelago, unregulated sea urchin grazing has largely removed the foundation canopy-forming kelp, Eualaria fistulosa, and most subcanopy algae. Consequently, most nearshore rocky reefs have shifted from kelp to sea urchin (Strongylocentrotus spp.) dominated habitats. These latter habitats are either urchin barrens devoid of all fleshy macroalgae, or transition forests devoid of all fleshy macroalgae algae but E. fistulosa. These three distinct communities (kelp and transition forests, and urchin barrens) were used to test the influence of E. fistulosa and sea urchins on the associated communities. Contrary to initial expectations, in transition forest habitats where E. fistulosa is the lone macroalga, no differences in community structure or in the size structure of benthic invertebrates were seen relative to urchin barrens. In kelp forests, where E. fistulosa coexisted with subcanopy macroalgae and urchins were less abundant, faunal communities were more species rich with higher abundance, biomass, and percent cover of numerous filter feeders. These findings stress not only the strong negative impact which urchins can exert on the kelp forest communities, but also the context-dependent nature of foundation species.