Density-dependent dynamics of soft coral aggregations: the significance of clonal growth and form

In clonal plants and animals, stolons and runners often promote rapid directional growth and escape from crowded microhabitats. Here we evaluate the effects of density on clonal growth and dispersal by stolons, on colony mortality, and on recruitment in the soft coral Efflatounaria sp. This colonial organism forms dense aggregations on mid-shelf and outer reefs of the Great Barrier Reef, Australia, where it is subjected to frequent physical and biological disturbances. Stolonal growth and asexual recruitment of new colonies (by budding) were enhanced by experimentally reducing local density. Within unmanipulated aggregations of Efflatounaria, per-capita rates of asexual recruitment were higher at low density, but colony survivorship was lower. Furthermore, the effect of density on stolonal growth and dispersal of daughter colonies varied as a function of a colony's history of disturbance. Disturbance was simulated by detaching from the substrate a newly budded colony that was still connected by a stolen to the parent colony. At low density, these pairs of partially detached colonies moved apart, while at high density, stolonally connected colonies moved closer together.\ud \ud Our results suggest that Efflatounaria employs a plastic life-history strategy that promotes recovery from injuries and the formation of dense aggregations. At low density, stolons facilitate rapid directional growth, asexual recruitment, and aggregation. At high density, clonal growth is inhibited, and mortality rates are greatly reduced. Enhanced survival within aggregations provides the adaptive context for interpreting the influence of density and disturbance history on the population dynamics of this clonal organism