Coral reefs are degrading under global stressors that are increasing in frequency and severity as the Anthropocene accelerates. My thesis contributes to our scientific understanding of the dynamics that govern degraded coral reef states. More specifically, I contribute to our understanding of feedback processes on degraded coral reefs in conceptual and experimental ways by confronting both ecological and social-ecological feedbacks in ways that may have merit in triggering coral recovery.
My four presented studies (Chapters 1-4) pursue the following research questions: 1. Which habitat drivers best predict juvenile coral densities following bleaching?
2. Can macroalgae-reinforcing feedbacks be weakened through shading?
3. Can sea urchins effectively weaken macroalgal feedbacks given their current natural densities?
4. Can red and green loops uncover missing social-ecological feedbacks?
Juvenile corals are a critical life history stage representing survival and growth of new recruits into the population. Chapter 1 compares juvenile coral densities from before the 2016 bleaching event with those after and identifies abiotic and biotic habitat drivers collected in the inner Seychelles that predict juvenile coral densities. Following the 2016 bleaching event, juvenile coral densities were significantly reduced by about 70 %, with a particularly severe decline in juvenile Acropora corals. Macroalgae present a major obstacle to survival of juvenile corals shortly following mass bleaching, but their influence varies as a function of herbivore biomass, reef structure, and reef type. In contrast, increasing structural complexity on granitic reefs is a strong positive predictor of juvenile coral density. Macroalgae can maintain and increase their dominance with effective self-reinforcing feedback
mechanisms and can significantly compromise ecosystem function.
Chapter 2 assesses shading as a management tool in an experimental confrontation of macroalgal feedbacks, aiming to maximise the benefit of habitat mosaic reefscapes in the inner Seychelles. Shading reduces the algae’s ability to photosynthesise by 29 % to the point where macroalgal cover can be reduced by 51 % and turf algal growth can be reduced by 82 % within six weeks of shading. After removal of shading structures, herbivore grazing rates decreased at shading plots, and algal beds recovered quickly, almost completely regrowing within three months.
Tropical sea urchins are often considered as macroalgal grazers, but this assumption relies heavily on geographically limited observations of select species. Chapter 3 addresses these gaps for a common urchin species in the Seychelles, Echinothrix calamaris, using a combination of survey and experimental approaches in the inner Seychelles. Habitat driver models revealed patch-reef types as the best positive predictor and macroalgae as the best negative predictor of urchin densities. Experimentally penning urchin densities (maximum 4.44 urchins m-²) resulted in a reduction of macroalgal cover by only 13 %. Therefore E. calamaris at current densities in Seychelles (mean: 0.02 urchins m-2, maximum: 0.16 urchins m-2) are unlikely to perform significant macroalgae controlling functions.
People use their local ecosystems and can retrieve signals about how their actions affect ecosystem health. Capturing, interpreting, and responding to signals that indicate changes in ecosystems is key for their sustainable management and breaks in this signal-response, called missing feedbacks, will allow ecosystem health to degrade unnoticed. Chapter 4 applies an existing concept from sustainability science, the red-loop green-loop (RL-GL) model, to uncover missing feedbacks between reefs and people of Jamaica from the year 600 until now. This allowed the factors responsible for missing feedbacks to be identified – a main factor in Jamaica was seafood exports. An intervention to move Jamaica back to more sustainable dynamics between people and reefs could be to gradually move away from seafood exports and build ownership and management capacity in local seafoods.
Overall, my thesis emphasises the importance of habitat for coral recruitment following severe coral bleaching as well as for urchin density and function in Seychelles. Furthermore, I cover management approaches to confront reinforcing feedbacks of expanding macroalgal fields, especially for a mosaic reefscape setting. I test the first method to reduce macroalgal cover via the alteration of the light regime. My thesis also includes the first study to apply the RL-GL concept to a coral reef social-ecological system and I advocate for its practicality in uncovering missing feedbacks and in gaining an understanding of past, present, and future sustainability that can be of use in other systems.
