Spatial pattern in a low-density population of the barnacle Balanus amphitrite Darwin

The spatial distribution of an intertidal population of the barnacleBalanus amphitrite was studied close to a sewage outfall nearQuequén, Argentina (38° 34S,58° 38 30 W). All individuals within an areaof 0.8 m² were mapped during 12 censuses, from September 1990to May 1993. Population density within the study area varied between 401 and99 ind m^-2. Spatial pattern was analysed using mean distancesto the nearest neighbour (NND), goodness of fit tests between observed andexpected frequency distributions of NND, and analysis of crowding. Thespatial pattern of the population was clumped during most of the studyperiod. This trend persisted, although not reaching statisticalsignificance, when the population density decreased as a consequence ofmortality and failure of successive annual recruitments. The cohortrecruited in summer 1992 (49 individuals) was randomly distributed relativeto adult barnacles already present within the study area. No consistentrelationship was observed between mortality and NND.