Density‐dependent effects on initial growth of a branching coral under restoration

Abstract

Coral reef restoration aims to help threatened coral ecosystems recover from recent severe declines. Here we address whether coral fragments should be out‐planted individually or in larger aggregations. Theory suggests alternative possible outcomes: whereas out‐plants within aggregations might suffer from heightened negative interactions with neighbors (e.g. competition for space), they may alternatively benefit from positive interactions with neighbors (e.g. buffering wave disturbances). On a degraded reef in the Caribbean (St. Croix, USVI), using out‐plants of the critically endangered staghorn coral Acropora cervicornis, we experimentally tested how aggregation density (1–20 out‐planted coral fragments spaced at approximately 5 cm) influenced initial coral growth (over 3 months). Coral growth declined as a function of aggregation size, and out‐plants within larger aggregations had fewer and shorter secondary branches on average, indicative of horizontal competition for space. Our results therefore suggest that wide spacing of individuals will maximize the initial growth of out‐planted branching corals.

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

“…The longer (13 month) duration of our study allowed us to elucidate the effect of the density of restored corals over timescales more relevant to coral reef community recovery. Similar to Griffin et al (2015), we found that growth rates in our 24-colony treatment were lower than the others at the end of the experiment. However, we show that density dependence can influence the success of A. cervicornis outplanted for restoration, and that the strength and direction of density dependence changes with coral density.…”

“…Our findings suggest that outplanting A. cervicornis for restoration in moderate densities (3 corals m −2 -our 12-colony treatment) maximizes growth rates and habitat production while minimizing tissue loss and coral mortality. To our knowledge, the only other study that manipulated the density of A. cervicornis colonies found a negative relationship between coral density and linear extension (Griffin et al, 2015). However, this study only tracked corals for a period of 3 months, had limited replication (n = 1 per density), and potentially confounded density effects with genotype effects.…”

“…To address these declines, coral restoration has gained increasing attention as a viable strategy to help degraded reefs recover, with large-scale restoration efforts now underway across the globe (Young et al, 2012). Current restoration efforts are primarily focused on restoring ecosystem engineers by outplanting nursery-raised corals to degraded reefs (Lirman et al, 2014;Cabaitan et al, 2015;Griffin et al, 2015). These projects have shown that coral size (Garrison and Ward, 2008), genotype (Lirman et al, 2014), and source location (Forrester et al, 2013) all influence the success of restored corals.…”

“…Similarly, on coral reefs outplant density and arrangement will likely affect important responses such as growth rates, habitat production, disease dynamics, and, ultimately, coral survivorship. Indeed, Griffin et al (2015) found that short-term growth rates of restored Acropora cervicornis over 3 months in the US Virgin Islands were inversely related to outplant density. In contrast, Shaish et al (2010) found no differences in mortality or bleaching of restored Montipora digitata in high density, low density, or "patchy" arrangements of nursery-raised corals in the Philippines after 15 months.…”

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

“…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).…”