Invasion-mediated shifts in the macrobenthic assemblage of a rocky subtidal ecosystem

Abstract

We tracked changes in community composition that occurred with state shifts in the rocky subtidal ecosystem in Nova Scotia, Canada, from 1992 to 2008. At the beginning of our study, a dense aggregation of sea urchins was destructively grazing a kelp bed, leaving coralline-algae- dominated barrens in its wake. In subsequent years, the system underwent a series of state shifts mediated by mass mortality of sea urchins due to amoebic disease, defoliation of kelp by an invasive bryozoan Membranipora membranacea, invasion of the green alga Codium fragile ssp. fragile, and finally decline of C. fragile and recolonisation by kelps. Using multidimensional scaling (MDS), we found 4 macroalgal assemblage types (dominated by kelp, coralline algae, C. fragile, or a transitional mixture of species), each associated with a distinct invertebrate assemblage. Sea stars and kelp-grazing gastropods were associated with the kelp-dominated state; sea urchins, chitons and scale worms were most abundant in the barrens state; and small bivalves and amphipods were most abundant in the C. fragile-dominated state. Invertebrate diversity remained relatively constant despite dramatic shifts in the algal assemblage. Although the system was vulnerable to multiple perturbations between 1992 and 2002, the partial recovery of kelp beds and associated fauna by 2008 demonstrates some resilience in the longer term.