Influence of starfish grazing on lagoonal microalgal communities: non-competitive mechanisms for unimodal effects on diversity

Abstract

We quantified the effects of grazing by the starfish Parvulastra (=Patiriella) exigua on microalgal assemblages in a lagoon on the South African west coast. This starfish is common on rocky shores in South Africa and Australia, but rarely occurs on soft sediments. Although grazing is well known to be a dominant structuring agent of marine systems, the influence of grazing starfish is poorly documented. Our results indicate that at natural densities, P. exigua plays an important role in structuring soft-sediment microalgal assemblages, having no significant effect on microalgal bio- mass, but reducing microalgal abundance probably through consumption. Interestingly, concentra- tions of extracellular polymeric substances (EPS) increased with densities of P. exigua, indicating a potentially important ecological role of this starfish in enhancing carbohydrate levels on the sediment either through mucus addition or stimulation of bacterial biomass. Taxonomic richness and diversity of microalgae showed classical hump-shaped responses to increasing starfish density. Our findings are discussed in the context of the intermediate disturbance hypothesis, which proposes that distur- bance promotes diversity at intermediate intensities by eliminating dominant species and allowing poor competitors to co-exist. We also present evidence for a mechanism based on non-competitive interactions to explain the promotive effects of this starfish on microalgal diversity, arising from the ability of this species to enhance sedimentary EPS levels. Our results collectively indicate that P. exigua plays an important role in regulating diversity and richness of microalgal assemblages and concur with previous assertions that this starfish plays an underestimated role in structuring intertidal systems.