Density-dependent sea urchin grazing: differential removal of species, changes in community composition and alternative community states

The grazing activity of consumers causes shifts between alternative states in a variety of terrestrial and marine ecosystems. One of the best examples of a consumer-driven shift occurs on temperate marine reefs, where grazing by high densities of sea urchins results in a shift from a foliose algal- to a crustose algal-dominated state. In this study, we focussed on 2 largely untested but important issues during the transition from a foliose algal- to a crustose algal-dominated state: (1) whether sea urchins impact foliose algal community composition by differentially removing species and (2) whether any impacts of grazing vary with 2 different densities of aggregating sea urchins. We tracked the movement of a high-density front of the sea urchin Heliocidaris erythrogramma and then performed experimental manipulations of H. erythrogramma at 2 unusually high but naturally occurring densities. Non-metric multidimensional scaling (nMDS) followed by analysis of similarities (ANOSIM) showed differences in the foliose algal community composition, and therefore differential removal of species, between permanent plots before and during grazing (surveys), and between grazed and ungrazed plots (experiment). Of the 6 abundant foliose algae, 2 had relatively low survivorship (Amphiroa anceps and Zonaria diesingiana), while 2 had relatively high survivorship (Delisea pulchra and Corallina officinalis) when grazed by high densities of sea urchins. Grazing by different densities of H. erythrogramma resulted in differences in the foliose algal community composition and for the chemically-defended D. pulchra there appeared to be a threshold sea urchin density required before its removal. Our results show that an intermediate community state composed of grazer-resistant foliose algae and crustose algae can occur, which may have important consequences for community composition.