Introduction

Background & Rationale

Coral reef function under multi-scale stressors

The damaging effects of interacting stressors on ecosystem function have become increasingly apparent under climate change (McCarty 2001). To sustain ecosystem function, management strategies must account for the effects of global-scale stressors, namely climate change, and local-scale stressors, such as species harvesting (Grimm et al. 2013; Gissi et al. 2021). On coral reefs, previous studies suggest that a threshold of >10% live coral cover and functionally diverse coral communities may be critical for maintaining reef growth and resilience under future climate stress (Perry et al. 2013; Darling et al. 2019; Vercelloni et al. 2020). However, coral assemblages have declined and simplified under the increasing impacts of both climate stressors (e.g. high sea surface temperatures, acidification; Figure 1) generated at global scales and other human stressors (e.g. coastal development, fishing pressure) generated at local or regional scales (Burman et al. 2012; Estrada-Saldivar et al. 2019; Briand et al. 2023). The resulting sparse, homogenized coral communities often lack functional traits that serve important ecological roles, such as habitat provision, leading to reduced ecosystem functioning and resilience (Darling et al. 2017).

Figure 1. Bleached coral caused by high thermal stress.

Recovering coral reef function through restoration

Protective management approaches, such as restrictions on species harvest within Marine Protected Areas, may effectively conserve ecosystem functioning in coral reefs exposed to less intense and frequent climate stress (Strain et al. 2019). However, scientists and conservation practitioners have more recently turned towards reef restoration as a means to bolster ecosystem resilience in ways that can mitigate rapid changes generated by significant, chronic climate stress (Fidelman et al. 2019; Shaver et al. 2022). In recent years, restoration initiatives have commonly sought to facilitate the recovery of coral reefs by replanting nursery-reared Scleratinian (hard) coral species (Figure 2; Table 1), which build the complex reef framework inhabited by a diversity of secondary species, including fish essential to local fisheries (Coker et al. 2014). To meet these goals, recent work has called on scientists and practitioners to conduct restoration in areas that are less likely to experience future climatic and local stress, target sites that represent a variety of reef habitats (e.g. coral cover, oceanographic conditions), and restore a diversity of coral morphologies and functional roles (Seraphim et al. 2020; Shaver et al. 2022). Despite the growing number of reef restoration initiatives, few have explicitly accounted for these criteria in project design and site selection (Boström-Einarsson et al. 2020) due to the paucity of work quantifying restoration outcomes and the scale at which factors affecting success operate (NOAA 2022).

Table 1. Phases of asexual propagation of corals through fragmentation, a common method of coral reef restoration.

Figure 3. Coral reef recovery through restoration (NOAA).

Figure 2. Nursery growing coral fragments to be replanted on reef structure.

The current & future scope of coral reef restoration

Recent advances in remote sensing, restoration monitoring, and data accessibility have opened doors to quantifying and refining restoration targets using globally rich databases (Madin et al. 2019; Boström-Einarsson et al. 2020). Previous analyses have used global coral cover and climate thresholds to project the most effective management strategies, including recovery or restoration, for >2,000 reefs (Darling et al. 2019; Figure 4). However, such frameworks apply simplified approaches to delineate reefs based on only two variables. A more advanced, multivariate approach could better define where restoration is currently occurring, as well as predict how and where restoration would be most suitable. A better understanding of predictors of restoration success will provide conservation practitioners with the insight to effectively rebuild coral populations above minimum (e.g. 10% cover) thresholds in degraded coastal habitats, as well as enhance ecosystem resilience against climate change.

Figure 4. Management strategy framework proposed by Darling et al. (2019) for >2,000 reefs based on coral cover (10%) and climate (4 degree heating weeks) thresholds.

Research Objectives

The aim of this study is to evaluate the current scope of restoration, as well as how and where restoration can succeed under the multi-scale impacts of stressors across heterogenous reef habitats, equipping practitioners with the information needed to design restoration projects that support reef functioning and resilience. In particular, I will answer and adopt the following questions and objectives:

(1) Where is restoration occurring?

(2) What determines restoration success?

(3) Where will restoration be successful?

Within this framework, (1) I predict that coral reef restoration is currently taking place in locations of convenience and where there are funds to support initiatives, such as in developed regions. Developed coasts are often subject to higher amounts of local stress due to human activity (e.g. fishing or water pollution) than more remote or offshore reefs. Additionally, (2) I predict that restoration will be most successful when restoring a diversity of coral morphologies and functional roles, rather than just a single species, as well as under low stress conditions. Finally, (3) I predict that restoration will be most suitable (i.e. successful and beneficial) when applied to reefs that are less likely to experience future climatic and local stress and occur across a variety of reef habitats. However, current efforts within this developing field are most likely not targeting the reefs and applying the methods necessary to facilitate restoration success. Results from this analysis will provide insight on whether current projects are effectively targeting reefs for restoration based on recommended criteria and identify predictors of restoration success to help guide restoration planning and design to optimize functional recovery in coral reefs globally.