Field studies demonstrate that the population structure of the barnacle Balanus glandula differs between locations of high and low larval settlement rate. These observations, together with results from a model for the demography of an open, space-limited population, suggest that the settlement rate may be a more important determinant of rocky intertidal community structure than is presently realized. Locations with a low larval settlement rate exhibit a generally low abundance of barnacles that varies slightly within years and greatly between years, reflecting yearly differences in settlement. Locations with a high-settlement rate exhibit a generally high abudance of barnacles. However, the abundance varies greatly within years with a significant oscillatory component (period, 30 weeks) and only slightly between years regardless of yearly differences in settlement. At the low-settlement location mortality of barnacles is independent ofthe area occupied by barnacles. At the high-settlement location mortality is cover-dependent due to increased predation by starfish on areas of high barnacle cover. In both locations the coverindependent component of mortality does not vary with age during the first 60 weeks. As assumed in the demographic model, the kinetics of larval settlement can be described as a process in which the rate of settlement to a quadrat is proportional to the fraction of vacant space within the quadrat. Generalizations that the highest species diversity in a rocky intertidal community is found at locations of intermediate disturbance, and that competition causes zonation between species of the barnacle genera Balanus and Chthamalus, seem to apply only to locations with high-settlement rates.Many members of ecological communities in the marine intertidal zone have a life history consisting of pelagically dispersed larvae and sessile, space-limited adults. Familiar examples include barnacles and mussels (1-5). Larval phases as short as 2-3 weeks may lead to transport oflarvae in excess of 100 km (6-10). As a result, recruitment to a local section of the shore is from larvae that likely originated at other sites. Hence, a local section of shore is an open population that is not satisfactorily treated by the models of primarily closed populations applied to terrestrial ecological communities over the last decade (11)(12)(13) This study also confirms a key assumption of the openpopulation demographic model, that settlement to vacant space can be treated as a process in which the rate of settlement in a quadrat is proportional to the fraction of vacant space in it, with a constant of proportionality specific to location and time (including season). Further, this study reveals that disturbance (mortality that removes space-occupying organisms) is a cover-dependent process for barnacles subject to predation by the starfish Pisaster ochraceus and that the cover-independent component of survivorship is independent of age for at least the first 60 weeks of life.Finally, we note that the importance of t...

Menge and Sutherland (1976) predicted that in physically benign habitats: (1) community structure will be most strongly affected be predation, (2) the effect of predation will increase with a decrease in trophic position in the food web, (3) trophically intermediate species will be influenced by both predation and competition, and (4) competition will occur among prey species which successfully escape consumers. These predictions were tested in a tropical rocky intertidal community on the Pacific coast of Panama. The most abundant mobile species included fishes and crabs, which occupied the top trophic level, and predaceous gastropods and herbivorous molluscs, which occupied intermediate trophic levels. The most abundant sessile organisms were encrusting algae, foliose algae, barnacles, and bivalves. Diets were broad and overlapping, and 30.3% of the consumers were omnivorous. Each consumer group had strong effects on prey occurring at lower trophic levels: (1) Fishes and crabs reduced the abundance of predaceous snails, herbivorous molluscs, foliose algae, and sessile invertebrates. (2) Predaceous gastropods reduced the abundance of herbivorous molluscs and sessile invertebrates. (3) Herbivorous molluscs reduced the abundance of foliose algae and young stages of sessile invertebrates, and altered relative abundances of the encrusting algae. The encrusting algae, although normally the dominant space occupiers, proved to be inferior competitors for space with other sessile organisms when consumers were experimentally excluded. However, the crusts escaped consumers by virtue of superior anti-herbivore defenses and competed for space despite intense grazing. Observations do not support the hypothesis that the trophically intermediate species compete. Hence, with the exception of this last observation, the predictions of the Menge and Sutherland model were supported. Although further work is needed to evaluate other predictions of the model in this community, evidence from this study joins an increasing body of knowledge supporting the model. Contradictory evidence also exists, however, indicating that aspects of the model require revision.

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ABSTRACT. The availability of shells has been shown to limitThe few studies of hermit crab recruitment to date