Patiriella exigua: grazing by a starfish in an overgrazed intertidal system

Abstract

Intertidal rocky shores in south-eastern Australia are dominated by a diverse assemblage of grazing invertebrates that feed on micro-algal biofilms. This resource is spatially variable and frequently over-grazed, causing strong inter-and intra-specific competition among grazers. Most studies on intertidal grazing are about gastropod molluscs. We observed, however, damaged patches in intertidal biofilms that appeared to be associated with the herbivorous asterinid starfish Patiriella exigua (Lamarck). In contrast with predatory starfish, there have been few ecological studies about herbivorous starfish, even though they are often abundant. We demonstrated that these patches were caused by grazing by this starfish. We then used field-based remote-sensing methods to demonstrate that amounts of chlorophyll were reduced inside grazing marks, quantified these changes and measured their longevity. In experiments, starfish could graze up to 60% of the epilithic micro-algae beneath their everted stomach during a single feeding event lasting on average 22 min. Over 5 d, 2 caged starfish could remove nearly half of the available micro-algae from areas of 144 cm 2 . Changes to the amounts of chlorophyll in grazing marks were persistent, remaining visible on sandstone substrata for several weeks. Amounts of chlorophyll on grazed areas of substratum were significantly different from ungrazed areas up to 28 d after being grazed. Recovering areas were demonstrated to be bio-equivalent to ungrazed areas after a similar period. Thus, P. exigua can have a large effect on the temporal and spatial distribution of micro-algal food, with numerous possible consequences for intertidal assemblages. The ecological importance of herbivorous starfish may be greater than previously perceived.KEY WORDS: Herbivore · Intertidal · Echinoderm · Micro-algae · Spatial heterogeneity · Chlorophyll · Food 376: 153-163, 2009 species, persistence of poorer competitors may be dependent on physical heterogeneity in the distribution of resources (Fletcher & Underwood 1987, Fransen et al. 2001. Heterogeneous dispersion of resources can also support greater densities of organisms (Doncaster 2001). Resale or republication not permitted without written consent of the publisher OPEN PEN ACCESS CCESSMar Ecol Prog SerSettlement of the dispersive larvae of many sessile invertebrates (e.g. barnacles, oysters) is also influenced strongly by amounts and types of biofilm (Keough & Raimondi 1995, 1996, Wieczorek & Todd 1998. Thus, grazers altering the amounts, types and distributions of biofilms also affect patterns of settlement and subsequent assemblage structure.Grazing on micro-algae also removes the early stages of development of macro-algae, preventing the growth of foliose macro-algae, which has numerous effects on the diverse assemblages in these habitats (reviewed in Hawkins & Hartnoll 1983). Where the early microscopic phases of life history are not removed by grazing, fleshy, branched algae grow, smothering sessile animals, such as barnacles (B...

Unexpected effects of starfish grazing on sandflat communities following an outbreak

“…At low P. exigua density, diversity of recruits may be limited by low biofilm coverage, but enhanced as density of P. exigua and biofilm coverage increase. As P. exigua exceeds intermediate levels, its grazing may outweigh any promotive effects of biofilms, selecting against species with slow recovery times.Our results revealed novel effects of the starfish Parvulastra exigua on community structure in intertidal sandflats and concur with the notion of Jackson et al (2009) that grazing by this echinoderm may be an underestimated force in intertidal systems. Its promotive effects on bacteria and macrofauna were unanticipated, but importantly suggest a need for caution when using studies from soft sediments as support for the intermediate disturbance hypothesis without a full appreciation of the promotive effects of the target organism.…”

“…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).…”

“…Many studies have reported intricate effects involving positive or negative effects depending on grazer density and species (Hargrave 1970, Flint & Goldman 1975, Morrisey 1988 with both direct and indirect consequences (Anderson 1999). Grazing may affect resource availability and habitat heterogeneity (Jackson et al 2009), rates of succession and recruitment (Levinton & Stewart 1982) and the abundance and composition of algal and invertebrate assemblages (Morrisey 1988, Keleher et al 2003.Until recently, grazing by starfish has not been considered a major determinant of algal and invertebrate community structure on rocky shores, where limpets and other grazing molluscs have generally been shown to be of primary importance (Hawkins & Hartnoll 1983, Jackson et al 2009). However, Jackson et al (2009 have noted that starfish grazing on rocky communities may be underestimated, particularly in terms of its effects on microalgal biofilms.…”

Chlorophyll a and intertidal epilithic biofilms analysed in situ using a reflectance probe

“…Field spectrometry has enabled the abundance and composition of intertidal micro algae on rocky (Murphy et al 2005a) and soft sedimentary (Paterson et al 1998, Meleder et al 2003, Murphy et al 2005b) substrata to be determined in situ, allowing specific hypotheses to be tested about the roles of micro algae in ecological processes (as exemplified in detail by Murphy et al 2008, Jackson et al 2009, Murphy & Tolhurst 2009, Ive$a et al 2010. The advantages of field spectrometry over other methods are that it is not destructive or intrusive, less laborious and less expensive.…”

Patchiness in resource distribution mitigates habitat loss: insights from high-shore grazers

“…Habitat loss often entails habitat fragmentation at organism-relevant scales within a population or metapopulation, exacerbating its effects (Lande 1987, Hanski et al 1996, Hill and Caswell 1999. Habitat structure can stabilize population fluctuations (Murdoch 1977), facilitate the persistence of poor competitors (Fletcher andUnderwood 1987, Fransen et al 2001), and moderate predation efficiency (Ellner et al 2001) and grazing pressure (Jackson et al 2009). At the community scale, spatial heterogeneity raises species richness (MacArthur and MacArthur 1961) and fractal landscapes influence species packing (Johnson et al 2003).…”