The Roles of Compensatory Mortality, Physical Disturbance, and Substrate Retention in the Development and Organization of a Sand-Influenced, Rocky-Intertidal Community

Manipulative experiments (using fenced exclosures, fence controls, and unmanipulated controls), as well as successional and community structure studies were performed near Dutch Har- bor, San Nicolas Island, California to examine the importance of the anemone Anthopleura elegan- tissima, the sand tube worm Phragmatopoma californica, and macroalgae, in the structure and development of a sand-influenced community. Following the removal of Anthopleura elegantissima, the cover of opportunistic algae such as Cladophora columbiana, Chaetomorpha linum, and Ralfsiaceae increased during the 1 st 6 mo, where- as Corallina vancouveriensis and Hydrolithon decipiens suffered bleaching and decreased in abun- dance as a result of aerial exposure. The density of grazing molluscs (Tegula funebralis, littorines, limpets, and chitons) also increased during the 1st 6 mo, then declined markedly, coincident with the recruitment and growth of the sand tube building polychaete, Phragmatopoma californica. In the upper-intertidal area (above + 1. 1 m relative to mean lower low water), Phragmatopoma californica showed little recruitment ( 70% cover after 20 mo). Anthopleura elegantissima again regained preremoval abundance (26% cover) in the succes- sional studies after 2 yr. At low levels (below +0.5 m), where sand burial did not occur, P. californica persisted even when contiguous with A. elegantissima aggregations. Successional data indicated that P. californica achieved preremoval levels in <12 mo, whereas A. elegantissima was unsuccessful at recruiting into these regions. It is suggested that the stress of exposure to air (desiccating and thermal effects) and heavy sand inundation, as well as differential abilities to sequester and dominate available free space, are re- sponsible for the developmental and structural patterns in this community. At desiccated and ther- mally stressed upper-intertidal levels, Anthopleura elegantissima facilitates colonization and subse- quent survival of coralline algae and small Phragmatopoma californica aggregations while simultaneously inhibiting the recruitment of opportunistic macroalgae. Phragmatopoma californica can rapidly colonize and retain newly available free space in low- to mid-intertidal regions with reduced aerial exposure stress. At mid-levels, the upper-intertidal patterns prevail, but when released from A. elegantissima inhibition by disturbance, P. californica colonies show extensive development. However, in mid-intertidal regions, compensatory mortality of P. californica caused by sand depo- sition prevents these large aggregations from persisting and A. elegantissima becomes dominant. In the low-intertidal zones, with little sand-related stress, P. californica inhibits both A. elegantissima and macroalgae, while it may enhance recruitment and survival of its own juvenile stages in a self- facilitating strategy of larval selectivity. Therefore, the importance of A. elegantissima depends on compensatory mortality of the superior competitor P. californica, caused by periodic stresses (e.g., exposure to air and sand inundation) and physical disturbance.