Larval settlement rate: A leading determinant of structure in an ecological community of the marine intertidal zone

Abstract

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...

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Patterns of demography for rocky-shore, intertidal invertebrates approaching their geographical range limits: tests of the abundant-centre hypothesis in south-eastern Australia

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Experimental studies on predation and cannibalism of the settlers of Chasmagnathus granulata and Cyrtograpsus angulatus (Brachyura: Grapsidae)

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