Spatial Relationships among Encrusting Marine Organisms in the New England Subtidal Zone

Competition for substratum by encrusting marine organisms usually occurs by direct overgrowth at zones of contact. The documented mechanisms of competitive displacement are numerous, but the relative importance of species in a competing assemblage has been difficult to quantify. This study of the encrusting community on subtidal vertical rock walls in northern Massachusetts used 2 yr of photographic sampling of permanent quadrats to examine the competitive role of each common space occupier. The surface area gained or lost by individuals or colonies of each species in interactions with others was measured over intervals of ≥ 30 d. Four quantitative indices were used to describe competitive roles: the first measured the overgrowth activity of each species within a given assemblage (overgrowth index); the second assessed the relative importance of each species as an overgrowth competitor in the assemblage (weighted overgrowth index); the third described each species' ability to resist overgrowth (resistance index); and the fourth measured each species' growth rate when advancing over other organisms. Analysis of percent cover on subtidal rock walls over a 2—yr period showed that the ascidian Aplidium pallidium, the sponge Halichondria panicea, the octocoral Alcyonium siderium, a red fleshy crustose alga, and a mat of amphipod tubes covered most of the rock surface continuously, although the relative abundances of the species varied across sites and seasons. Several of the common species gained space by overgrowing other species directly or by lateral growth pushing the adjacent species away (lateral movement). This group included several colonial and solitary ascidian species, calcareous tubeworms, and encrusting bryozoans. A few species, including the fleshy red crustose alga and the crustose coralline alga Phymatolithon, gained space from others when an overlying organism receded. The various indices of spatial interaction showed that species which were common and frequent overgrowers in the system were usually not good at resisting overgrowth themselves. Species good at resisting overgrowth were generally large and slow—growing, and were not the most active overgrowers of other species. The interactions among species in this study were basically hierarchical, with the larger thicker colonies and individuals being the winners in competition for space. However, there were a number of reversals of competitive success, especially among the thick crustose forms, and there were numerous "standoffs" where borders did not change during the 30—d interval (19—79% of all encounters, depending on the species involved; 41% overall). The indices of spatial interaction (growth, overgrowth, resistance, overgrowth weighted by abundance) were significantly correlated between exposed and protected sites but were not well correlated between warm and cold seasons. Thus, species tended to have the same competitive roles in slightly different communities but their importance varied as their growth rate changed between warm and cold periods. Overgrowth did not always result in death of the organism overgrown. Species such as the fleshy red crustose alga and the coralline alga Phymatolithon were often encountered alive under invertebrates that had been in position for at least several months. Thus, poor competitors in this system were maintained by two processes: some are early recruits that grow rapidly but later get overgrown, while others are able to withstand overgrowth until predation or some other disturbance removes the overgrower.