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.