Density Dependence Drives Habitat Production and Survivorship of Acropora cervicornis Used for Restoration on a Caribbean Coral Reef

Abstract

Coral restoration is gaining traction as a viable strategy to help restore degraded reefs. While the nascent field of coral restoration has rapidly progressed in the past decade, significant knowledge gaps remain regarding the drivers of restoration success that may impede our ability to effectively restore coral reef communities. Here, we conducted a field experiment to investigate the influence of coral density on the growth, habitat production, and survival of corals outplanted for restoration. We used nursery-raised colonies of Acropora cervicornis to experimentally establish populations of corals with either 3, 6, 12, or 24 corals within 4m 2 plots, generating a gradient of coral densities ranging from 0.75 corals m −2 to 12 corals m −2. After 13 months we found that density had a significant effect on the growth, habitat production, and survivorship of restored corals. We found that coral survivorship increased as colony density decreased. Importantly, the signal of density dependent effects was context dependent. Our data suggest that positive density dependent effects influenced habitat production at densities of 3 corals m −2 , but further increases in density resulted in negative density dependent effects with decreasing growth and survivorship of corals. These findings highlight the importance of density dependence for coral restoration planning and demonstrate the need to evaluate the influence of density for other coral species used for restoration. Further work focused on the mechanisms causing density dependence such as increased herbivory, rapid disease transmission, or altered predation rates are important next steps to advance our ability to effectively restore coral reefs.

Clonal diversity impacts coral cover in Acropora cervicornis thickets: Potential relationships between density, growth, and polymorphisms

“…Experimental manipulation of genotypic diversity in A. cervicornis did not influence growth or partial mortality in previous work (Ladd et al, ), but the authors point out that nonrandom or artificial distribution of genets or limited diversity may have influenced this outcome. Experimental manipulation of species diversity in a restoration project was also inconclusive (Cabaitan, Yap, & Gomez, ), and a meta‐analysis found weak negative influence of diversity on resistance and resilience in coral reefs (Zhang et al, ).…”

“…Studies of diversity effects are typically confounded by density (Stachowicz et al, 2007), and we assume that coral cover is related to colony density in the present study. Coral density negatively impacts growth and branching in experimentally manipulated A. cervicornis (Griffin et al, 2015;Ladd et al, 2016), correlates with growth anomalies (Aeby et al, 2011), and influences associated invertebrate communities in Caribbean Acroporids (Baums, Miller, & Szmant, 2003a, 2003b. Acropora corals are the preferred but not the only prey of gastropod coral predators which leads to the aggregation of gastropods on coral recruits and remnants of previous thickets causing colony death (Baums, Miller, & Szmant, 2003b;Knowlton, Lang, & Keller, 1990).…”

Near‐term impacts of coral restoration on target species, coral reef community structure, and ecological processes

“…For example, while outplant survival and growth can be maximized by outplanting at moderate densities (Ladd et al ), finding ways to achieve higher densities of restored coral could benefit restoration by aggregating fishes and initiating positive feedbacks (Huntington et al ). Beyond promoting coral growth and herbivory (Shantz et al ), concentrating schooling fishes like grunts in restored areas could increase the abundance of fishes that prey on corallivores (Ladd & Shantz ), potentially helping to reduce the high incidence of partial mortality and disease we observed in restored areas.…”

“…The effects of restoration may depend on the density of restored corals, which could influence important processes like disease dynamics and the aggregation of fishes (Ladd et al ; Huntington et al ). We used linear mixed effects models to test how the mean density (cm TLE m −2 ) and the percent cover of restored A .…”

Harnessing ecological processes to facilitate coral restoration

“…For example, outplanting A cervicornis at moderate densities can promote positive density dependence, maximize habitat production, and minimize the spread of coral diseases and coral mortality (Ladd et al . ).…”