Regulation of population size of arborescent octocorals on shallow Caribbean reefs

The coral reef crisis is defined by declining cover of scleractinians, but on Caribbean reefs it also is associated with increasing abundances of octocorals. The demographic causes of these increases are not understood, but parallels between marine ‘forests’ of octocorals, and terrestrial forests of trees (e.g. formation of a 3-dimensional framework providing habitat), suggests that insights into causation might be found by comparing the 2 types of forest. This study describes communities of octocorals in St. John, US Virgin Islands, and evaluates their dynamics for goodness of fit to a construct reflecting 2 processes structuring terrestrial forests: self-thinning and density-dependent recruitment. Octocorals (>5 cm tall) were censused on 6 reefs (7−9 m depth) from 2014−2017, thus revealing differences among years in their density and height. At 5 sites, this variation was inconsistent with self-thinning, but at the 6th, there was a trend for densities to decline and height to increase, with these effects largely attributed to Eunicea spp.; this pattern is consistent with several processes, one of which is self-thinning. For recruits (≤5 cm tall), densities differed among sites and times, but neither density nor per capita recruitment were density-dependent. For Eunicea spp., the recruit density was positively associated with adult density in 2014, and per capita recruitment was inversely related to adult density in 3 of 4 years. These results highlight the challenges of inferring that common mechanisms structure animal and plant forests, and they underscore the complexity of processes contributing to the recent population growths of Caribbean octocorals.