Regulation of population size of arborescent octocorals on shallow Caribbean reefs

Abstract

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 selfthinning, 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.

High ecological resilience of the sea fan Gorgonia ventalina during two severe hurricanes

“…Addressing mechanisms promoting octocoral success has benefitted from the analogy with terrestrial forests ( Rossi et al, 2017 ), which has motivated consideration of the emergent consequences of dense stands of octocorals. These include creating a unique sub-canopy flow regime ( Guizien & Ghisalberti, 2016 ), modified recruitment ( Privitera-Johnson, Lenz & Edmunds, 2015 ), and a context in which self-thinning (sensu Yoda et al, 1963 ) might play a role in regulating population size ( Edmunds & Lasker, 2019 ).…”

Resilience of Octocoral Forests to Catastrophic Storms

“…The shallow reefs on the south shore of St. John, U.S. Virgin Islands, have recently undergone a shift in benthic community structure that favors octocorals 21,22,[41][42][43][44] . The decline in abundance of scleractinians has been recorded in detail [45][46][47] .…”

Figure 1: Study sites at Yawzi Point (9-m depth) (A) and Tektite (14-m depth) (B) in 2016.

“…Evidence of density-dependent effects were not detected in the present study, even though densityassociated recruitment was reported from this locality for G. ventalina over and 2014(Privitera-Johnson, Lenz & Edmunds, 2015, and Eunicea spp. in 3 of 4 years from 2014-2017 (Edmunds & Lasker, 2019), and signs of self-thinning were found for Eunicea spp. in shallow water (9-m depth) adjacent to Tektite (Edmunds & Lasker, 2019).…”

“…Addressing mechanisms promoting octocoral success has benefitted from the analogy with terrestrial forests (Rossi et al, 2017), which has motivated consideration of the emergent consequences of dense stands of octocorals. These include creating a unique sub-canopy flow regime (Guizien & Ghisalberti, 2016), modified recruitment (Privitera-Johnson, , and a context in which self-thinning (sensu Yoda et al, 1963) might play a role in regulating population size (Edmunds & Lasker, 2019).…”

“…Using the results from the main objective as a context, changes in the stands of G. ventalina were explored for signs that they conformed to the changes expected from underlying density-dependent mechanisms (recruitment and self-thinning (Yoda et al, 1963;Westoby, 1984)) that can regulate population sizes of plants and animals (Westoby, 1984;Fréchette & Lefaivre, 1995). These mechanisms appear to modulate the population size of some octocorals (Privitera-Johnson, Linares et al, 2008;Edmunds & Lasker, 2019), but exploring for population regulation in species assemblages is problematic, especially for self-thinning (White, 1981). By studying G. ventalina at two sites in St. John, it was possible to reduce these effects and use this system to explore: (1) the association between recruitment and density of adult colonies (i.e., density-dependent recruitment), and (2) the association between biomass (using colony height as a proxy) and density (i.e., self-thinning).…”