UC Santa Barbara

Coral reefs are the ecological, economic, and social backbone of tropical coastal communities. Yet, more than half of the world’s tropical reefs are gone. With further declines predicted, there is an urgent need to develop effective solutions to resurrect degraded reefs. Part of the challenge of reef restoration is that coral reefs are extremely complex ecosystems. As such, they are often studied via a reductionist approach, whereas rebuilding an ecosystem requires a comprehensive plan to reconstruct the ecological processes necessary for a functioning reef ecosystem. Unfortunately, while we have a mechanistic understanding of many factors driving coral reef decline, there is a dearth of information available to guide us on how to restore degraded coral reefs and recover their ecosystem functions.

To this end, my dissertation examines how harnessing key ecological processes on coral reefs can facilitate the pace and success of coral reef restoration. In Chapter 1, I investigated the role of restored coral density on habitat production, and explored mechanisms contributing to density dependence in coral restoration. I found evidence of a unimodal relationship among restored Acropora cervicornis colonies suggesting positive density dependence at intermediate densities. Importantly, these findings highlight the fundamental role that basic restoration design elements, like outplant density, play in the success or failure of coral restoration. In Chapter 2, I assessed the importance of genotypic identity and diversity in restoration outcomes. Using a field experiment, I identified a tradeoff between thermal-tolerance and growth rates among A. cervicornis genotypes, suggesting genotypic identity is a critical factor to incorporate into restoration planning.

While restoring individuals to rebuild coral populations is an important first step in coral restoration, outplanted corals do not exist in isolation when transplanted to a degraded reef. Non-scleractinian invertebrates like sponges, gorgonians, and zoanthids are increasing on reefs, yet there is a paucity of data on interactions between these increasingly common organisms and corals. In Chapter 3, using observational surveys I found that competitive interactions were pervasive on Florida reefs, with 60% of sessile benthic invertebrates interacting with at least one other invertebrate. Further, results from a common garden competition experiment demonstrated that non-scleractinians like sponges and zoanthids consistently outcompeted the common species Porites porites and Siderastrea siderea, suggesting competition may limit the success of these coral species and is likely to remain an important process structuring contemporary coral reef communities.

Chapter 4 addresses our knowledge gap on the effects of coral restoration on reef communities and important ecosystem functions. To do so, I conducted surveys of sites in the Florida Keys that had undergone coral restoration paired with unmanipulated control sites. I found that coral restoration enhanced coral populations, increasing coral cover 4-fold, but manifested in limited differences in coral and fish communities. Interestingly, damselfishes, whose territorial behavior may deter important processes like herbivory, were the only group of fishes that positively responded to coral restoration. These findings suggest that additional considerations beyond outplanting corals will likely be necessary to effectively restore coral reefs in a time of increasingly frequent and intense disturbances.

Lastly, in Chapter 5 I synthesized literature on coral restoration and reef ecology to identify key drivers of recovery and propose a path forward to improve coral restoration. Specifically, restoration practitioners can manipulate factors such as the density, diversity and identity of transplanted corals and leverage existing ecological processes on coral reefs to restore positive feedback processes, or disrupt negative feedback processes, and facilitate restoration success. Importantly, the results of this dissertation can be directly applied to inform how coral reef restoration is conducted and improve our ability to effectively restore degraded coral reefs.