The population genetics and demography of soft corals, the second most abundant benthic invertebrate group on the Great Barrier Reef (GBR), were investigated to identify the proceses that determine their abundance and distribution. The study focusses particularly on the interplay between the demographic proceses and life history strategies of soft corals that commonly lead to their dominance in many coral reef communities. My general objective was to identify the mechanisms that allow soft coral species, specifically Sinularia flexibilis and Clavularia koellikeri, to dominate nearshore communities and to determine the likely time scales involved.

One potential mechanism to attain high cover is rapid colonisation of newly opened substrata, thus pre-empting space and preventing recruitment by potential competitors. The mortality caused by the 1998-bleaching event constituted an opportunity to evaluate the role that recruitment plays in the dynamics of recovering soft coral assemblages. Living cover declined by half at study sites on nearshore reefs in the Palm Island group (central GBR) that had been dominated by soft corals prior to the bleaching. In contrast to the common expectation that soft corals rapidly colonise substrata, a slow recovery was documented in the three years following the mortality, with soft coral cover increasing by only 16% between 1998 and 2000. The slowness of this recovery indicates that high cover is neither the result of rapid recruitment through sexual nor asexual recruits, at least in the time frame of this study.

Colonies of Sinularia flexibilis (Alcyoniidae) had size-dependent growth and mortality rates, and a high population turnover mostly derived from asexual replication. Small colonies generally increased in area by three-fold per year, whereas large colonies decreased in size mainly by binary fission. A matrix modelling study showed that population growth was variable among localities and time intervals, but all cases leading to increasing populations. Also, this study indicates that changes in the rates of colony growth, fission and stasis all have the potential to contribute equally to population growth. This finding is in stark contrast to studies of most other clonal species, which have found that stasis, especially of the largest sizes, largely controls their demography. Thus, although most vital rates in Sinularia flexibilis were characteristic of a clonal species, the finding that demographic processes in all size classes contribute similarly to population growth is novel.

Despite the larger contribution of asexual compared to sexual reproduction to population increases found in the demographic study, population genetic structures were not highly clonal at small spatial scales. Small-scale mapping of genotypes indicated that more than 60% of genotypes were unique, with the largest genet being represented by only nine daughter colonies. A high genetic diversity was also characteristic of populations of this species surveyed on 12 reefs and of Clavularia koellikeri (Clavulariidae) on six reefs, including both inshore and midshelf reefs along the length of the GBR. For both species, the population genetic structure was in agreement with that of a sexually reproducing species, when species were sampled at intervals ³5 m for S. flexibilis and ³3 m for C. koellikeri.

Genetic differentiation among populations of the larval brooder Clavularia koellikeri was four to thirty times that found for the gamete broadcaster Sinularia flexibilis, depending on the spatial scale compared.