Effects of Patch Size on the Abundance of Sessile Marine Invertebrates

Abstract

The effects of patch size on the abundance of sessile invertebrates were investigated for two kinds of patchiness in a subtidal habitat in southern Australia: discrete pieces of substratum that were surrounded by water (isolated patches), and areas that were cleared within a background of other sessile organisms (nonisolated patches). They species composition of nonisolated patches following reoccupation was independent of patch size. Vegetative growth of organisms adjacent to the patches was the major means of reoccupation. Most larvae that recruited into the patches were overgrown, so that the species composition of the patches reflected the outcomes of interference competition among residents of patches. As a result, sponges and colonial tunicates were the most common taxa, with bryozoans and serpulid polychaetes being excluded. In contrast, the occupation of isolated patches was strongly size—dependent. On small substrata, species' abundances reflected their ability to colonize substrata by planktonic larvae, with few interactions among residents of patches. Bryozoans were the most abundant taxon, with a small number of patches containing tunicates, which have the potential to monopolize such patches. Larger substrata had a higher probability of being colonized by tunicates and sponges, in addition to bryozoans. Species compositions and abundances in these large patches were influenced most strongly by interference competition, rather than by recruitment. Bryozoans recruited preferentially onto small substrata (i.e., more recruits per square centimetre), while tunicate recruits occurred disproportionately on large substrata. Previously noted differences between the abundances of sessile invertebrate taxa on pier pilings and on the shells of the bivalve Pinna bicolor at Edithburgh can be ascribed to: (1) Differences in substratum size, which, when combined with the life histories of sessile invertebrates, result in variations in the relative importance of recruitment and interference competition for space. (2) Differential recruitment of bryozoans and tunicates that may be interpreted as evolved responses to patch—size—dependent rates of mortality (bryozoans) or size—dependent mean fitness (tunicates). (3) Predation by monacanthid fish, which reduces the incidence of tunicates (and thus potential competition) on Pinna shells and reduces the potential role of recruitment on pier pilings. (4) Differences between the total recruitment rates of some taxa on Pinna beds and beneath piers.

Why do floating structures create novel habitats for subtidal epibiota?

“…While understanding and accounting for patterns of abundances of epibiota within urban structures has provided considerable insights to marine ecology as a discipline (e.g. Osman 1977, Sutherland & Karlson 1977, Keough 1984, the capacity for such research to account for the ecology of assemblages on rocky reefs is unclear and questionable (Glasby & Connell 2001).…”

Epibenthic colonization of concrete and steel pilings in a cold-temperate embayment: a field experiment

“…The current regime around a cylindrical structure is diVerent from commonly used panels or complexed constructed artiWcial reefs in colonization studies and will inXuence the epibenthic development (Mullineaux and Garland 1993;Lemire and Bourget 1996;Guichard et al 2001). In addition, diversity in marine invertebrate assemblages has been shown to be scaledependent (Keough 1984;Sousa 1984;Eggleston et al 1999).…”

Effects of regular and irregular temporal patterns of disturbance on biomass accrual and species composition of a subtidal hard-bottom assemblage

“…In contrast to the regular disturbance regime, relatively larger patches of cleared or recently cleared substratum were opened in irregular disturbance regimes, due to clumping of disturbance events. Larger, contiguous patches are microhabitats that diVer from smaller patches (Keough 1984) and could thereby aVect recruitment in several ways. First, neighborhood eVects--detrimental eVects of sweeping algal fronds on larval settlement and early recruitment have been reported to decrease when patch size increases (Jenkins et al 1999).…”