Colony size-frequency distributions of scleractinian coral populations: spatial and interspecific variation

Colony size is an important characteristic for clonal modular organisms such as corals be- cause life-history processes, e.g. reproduction and mortality, are strongly related to size. These pro- cesses are affected by the environment, and size-frequency distributions of coral populations provide information on the responses of populations to environmental conditions. We hypothesised that the size-frequency structure of coral populations in degraded environments would differ from those in a more pristine environment. We used log-transformed colony-size data of 13 coral species from the fringing reefs of Curacao, Netherlands Antilles. Data were collected at 4 sites, representing 2 environ- ments: a heavily urbanised coastal area with degraded reefs, and an upstream control area. Population structure was dependent on species and site. Size-frequency distributions of the same species from dif- ferent sites were twice as similar as distributions of different species, indicating the strong effects of various life-history traits among species. Site effects were indicated by significant differences between distributions of 11 species at 4 sites. Mean colony size varied up to an order of magnitude between spe- cies, and much less between sites; however, in 10 species mean colony size was also significantly dif- ferent among the 4 sites. The pattern of the differences was not consistent among species, indicating that mean colony size in degraded sites was higher for some species and lower for others. Parameters describing the shape of the population appeared to reflect a general response to reef condition. In the degraded area, frequency distributions tended to show increased negative skewness, occasionally extreme positive kurtosis, and smaller standard deviations: 11 populations were generally more neg- atively skewed, with the bulk of the population concentrated in the larger size classes; kurtosis was on average higher and extremely peaked; and standard deviations indicated that colony size varied less. This evidence suggests lower recruitment and higher partial mortality in larger colonies in this area. Lower recruitment decreased the influx into the smaller size classes of the populations, and higher par- tial colony-mortality decreased the proportion of colonies in the higher size classes while simultane- ously increasing the proportion of colonies in the medium-sized classes. Species that attained large sizes (indicative of great age) appeared to have size-frequency distributions skewed to the left, while small species were more skewed to the right. The data indicate a general dichotomy in coral life- history strategies with respect to colony size, with small species generally having a shorter lifespan and reproduction being relatively frequent and successful. Thus, new input into smaller size classes occurs continuously. Species that attain large sizes live longer and are less dependent on frequent re- cruitment, and consequently populations tend to become 'impoverished' in small colonies, resulting in size-frequency distributions that are skewed to the left. Based on changes in population structure, this research indicates that some species, such as Colpophyllia natans and Diploria labyrinthiformis, are relatively sensitive to environmental conditions.