Influences of habitat and natural disturbances on contributions of massive Porites corals to reef communities

We compared densities, distributions and size frequencies of massive corals in the genus Porites on five relatively exposed, mid-shelf reefs (50 km offshore) in the central Great Barrier Reef with those on a sheltered inshore reef (10 km offshore). Data included various transect and mapping studies between 1984 and 1990, estimates of size-dependent damage from the crown-of-thorns starfish Acanthaster planci, estimated densities of herbivorous sea urchins (potential predators of juveniles), and observations of size-specific effects of tropical cyclones. Assemblages of Porites spp. on mid-shelf reefs were dominated by small colonies (2 to 10 cm diam) established either from planula larvae or from small tissue remnants that had survived A. planci predation in the early to mid-1980s. Large colonies (up to 10 m diam) were scarce, except for localized aggregations on terraces at the base of reef slopes (6 to 12 m deep). Extensive space suitable for settlement by coral larvae can be attributed to recurrent cyclones and A. planci outbreaks. Despite low sea urchin predation, the slowly growing Porites juveniles are likely to die from overgrowth by numerous, much faster growing corals. On the sheltered inshore reef, the coral community was dominated by very large (>5 m diam) Porites colonies, several centuries old; recruitment was mainly by fragmentation of large colonies; there was little space available for settlement, and probabilities of juvenile mortality from grazing urchins were high. Differences in settlement and early survival of Porites spp. are exacerbated by different regimes of storm damage. A model is proposed that links wave climate with the size and age reached by corals before dislodgement by storm waves, and which is consistent with observed densities and size-frequency distributions of Porites in sheltered and exposed areas.