Starfish are significant determinants of community structure in marine ecosystems. The starfish Parvulastra (Patiriella) exigua is normally found on intertidal rocky shores. We examined a major outbreak of this species in Langebaan Lagoon, a marine embayment on the west coast of South Africa, and the only known soft-sediment ecosystem where P. exigua flourishes. We document this outbreak and describe subsequent field caging experiments testing the influence of this starfish on soft-sediment community structure. Grazing by P. exigua resulted in a reduction in microalgal biomass that was linearly related to starfish density, but it promoted bacterial density. Macrofaunal communities at intermediate densities of P. exigua were different and more diverse to those at high and low starfish densities, with Shannon-Wiener and Fischer's α indices peaking at intermediate density of P. exigua. It is possible that P. exigua at intermediate densities promotes co-existence of macrofauna by reducing the competitive edge held by dominant species. However, this effect may also be due to the balance between (1) negative effects of microalgal depletion and incidental grazing on invertebrate recruits at high densities of P. exigua, and (2) enhancement of bacteria by P. exigua leading to increased biofilm coverage on the sediment, which promotes macrofaunal recruit diversity in soft sediments. We suggest a need for caution when citing results from marine soft sediments as evidence for the intermediate disturbance hypothesis without knowledge of the promotive and inhibitory effects of the target organism.KEY WORDS: Population outbreak · Disturbance · Recruitment · Biofilms · Macrofauna · Promotive effects · Inhibitory effects Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 398: [173][174][175][176][177][178][179][180][181][182] 2010 Lagoon on the west coast of South Africa between 1975 and1983. We also present results from a manipulative field experiment testing the effects of this starfish on community structure at more natural densities following the outbreak. P. exigua is relatively small (5 cm in diameter) and is very common on intertidal rocky shores in South Africa and Australia (Branch & Branch 1980, Jackson et al. 2009). It rarely occurs in intertidal lagoonal sandflats or other soft-sediment ecosystems, although the population in Langebaan Lagoon has been established there for at least the last 50 yr (Day 1959).Starfish are important structuring agents of invertebrate communities in rocky and coral reef ecosystems. Paine (1974) identified the keystone role of the starfish Pisaster ochraceus in intertidal rocky communities, demonstrating that its predation on mussels reduces their competitive edge, leading to increased diversity of co-occurring rock-occupying species. The coraleating crown-of-thorns starfish Acanthaster planci is a major determinant of coral reef community structure in the Indo-Pacific notorious for its devastating effects, and is regarded as a gre...

No abstract