Density-size associations and shading in octocoral canopies
One of the most well documented cross-community scaling relationships in terrestrial plant and intertidal communities is self-thinning. Self-thinning, a form of direct density dependence, refers to a reduction in population density caused by competitively induced losses within a cohort of growing organisms. As a result of this phenomenon, a negative relationship between the density of organisms and mean organism size is often demonstrated in these communities.
As stony corals continue to decline in abundance, there is interest in the potential of other macroinvertebrates, such as octocorals, to play a stronger role in the structure and function of future reefs. Octocoral communities can form dense underwater canopies that are frequently referred to as "animal forests," however, there has been little quantification of whether these communities are subject to similar density-dependent processes as terrestrial forests. The purpose of this study is investigate the applicability of classic forest ecology density-size associations to octocoral communities.
Competition in these underwater canopies can occur through competition for space, food, nutrients, and/or light. Like many shallow-water reef inhabiting corals, most gorgonians possess zooxanthellae, so at least some portion of their metabolic requirements is met by photosynthates transferred from these symbionts. Therefore, the shade produced by larger octocoral colonies could exclude or inhibit the growth of surrounding octocoral colonies. In addition to quantifying the relationship between octocoral density and average colony size, we are investigating the role of shading (i.e. light interference) as a possible mechanism for density-size associations in octocoral communities.
This study is led by Hannah Nelson (me) with Peter Edmunds (CSUN). The work is conducted in the Virgin Islands National Park at the Virgin Islands Environmental Resource Station. This research is part of the St. John Long-Term Coral Reef Dynamics (NSF-LTREB) project.