Unexpected effects of starfish grazing on sandflat communities following an outbreak

Abstract

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...

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

“…One possible explanation for this is that increasing grazing pressure caused a shift in size of microalgae, with individuals becoming larger. The lack of an effect on microalgal biomass is surprising, as several studies have reported grazer-induced reductions in chl a levels (Jackson et al 2009, Pillay et al 2009, 2010a.The most significant finding arising from the present study was the enhancement of EPS levels on the sediment surface with increasing densities of Parvulastra exigua. This ability of P. exigua may be due to 2 mechanisms.…”

“…The addition of bacteria to faeces of P. exigua may also enhance bacterial growth on sediment (Hargrave 1970). Microalgal exudates are unlikely to contribute to the elevation of EPS levels recorded in the present study, as P. exigua had negative effects on microalgal abundance and no effect on biomass.The hump-shaped unimodal relationship recorded between densities of Parvulastra exigua and microalgal richness and diversity has also been recorded in experimental tests of this starfish on soft-sediment macrofauna (Pillay et al 2010a). Unimodal effects on diversity have been recorded in studies of various other grazers in marine ecosystems (Paine & Vadas 1969, Lubchenco 1978.…”

“…Through consumption, grazers remove competitively dominant species that monopolise habitats, thereby allowing sub-dominants to establish. However, the role of non-competitive mechanisms by which grazers promote diversity are poorly understood and reported in the literature, even though such pathways are potentially significant (Pillay et al 2010a).From a benthic perspective, non-competitive interactions are likely to be of importance in promoting diversity in aquatic soft-sediment ecosystems, as competition and competitive exclusion have rarely been shown to be important structuring forces in these habitats (Peterson 1979, Wilson 1990. Soft sediments are 3-dimensional ecosystems, allowing organisms to partition space vertically, by burrowing, thereby potentially minimising competition for space and food (Wilson 1990, Seitz 1998, Little 2000.…”

“…Substrate-associated EPS in turn play important ecological roles; they are food for invertebrates (Decho 1990), stabilise sediments, promote laminar flow of over-lying water (Paterson & Hagerthey 2001) and are important in the settlement and recruitment of benthic species (Lam et al 2005, Pillay et al 2007). All of these effects have important implications for the structuring of soft-sediment assemblages.The starfish in question (Parvulastra exigua) is a small grazing species (roughly 5 cm across arms) that is common in Australian and South African intertidal rocky-shore habitats (Branch & Branch 1980, Jackson et al 2009), but rarely occurs in soft-bottomed systems such as estuaries and lagoons (Pillay et al 2010a). Starfish play important roles in structuring marine benthic communities, but most studies have been based on their roles as predators (Paine 1974, McClintock & Lawrence 1985, Stevenson 1992.…”

“…Wave action is low and the lagoon is sheltered from the effects of oceanic swell, although disturbance can occur in the face of southeasterly winds (Day 1959). were not employed as these would have allowed starfish to escape and confounded interpretation of cage effects (Virnstein 1978, Jackson et al 2009, Pillay et al 2010a. In this experimental design, any cage effect would have been held constant across all treatments and all patterns observed would have been attributable to the effects of P. exigua.…”