The effects of density on the zoanthid Palythoa caesia

1. Few studies have examined density dependence or the mechanisms of population regulation in clonal marine organisms. Clonal organisms living on coral reefs have been particularly neglected. I examine the response of the tropical intertidal zoanthid Palythoa caesia to a density manipulation, where I decreased density from natural levels of 80-90% cover to a range of values down to 0% cover. 2. I found major differences in fission and fusion rates with density, with fission decreasing and fusion increasing as density increased. The growth rate of large colonies decreased substantially as density increased, although small colonies were unaffected. Mortality, on the other hand, was independent of density. Recruitment rates peaked at intermediate densities, but were low overall. 3. While density accounted for less than 10% of the variation in the demographic rates measured, it explained 62% of the variation in changes in cover within a given aggregation. Change in cover was greatest (30% in 2 years) for quadrats with 23-33% cover, and became negative for quadrats with over 80% cover. The highest density quadrat (90% cover) experienced an 18% decrease in cover over 2 years. 4. When the data on colony level dynamics for each density were integrated into population dynamics models, I found a steady decrease in population growth rates with density, along with an increase in the proportion of individuals in the largest size class. However, the proportional contribution of each size class to future generations (reproductive value) did not change with density. 5. These results suggest that during the study period the population size of Palythoa was being regulated, probably primarily by fission and/or fusion, with all quadrats approaching an equilibrium cover of ≃80%.