Top-down control in marine coastal ecosystems
Keywords: Humanities and Sciences.
Biology.

Consumers, both predators and herbivores, play integral roles in structuring communities and overall ecosystems through top-down control on their prey. Yet, consumers are being rapidly lost from ecosystems worldwide from human induced stressors including hunting, land use change, and heat stress events. This global loss of consumers can have dramatic consequences for community structure, ecological function, and ecosystem services. In my dissertation, I combined observational data and manipulative field experiments across the highly diverse, and economically and ecologically important ecosystem - coral reefs - to evaluate the role of consumers and the possible consequences of their decline. In Chapter 1, we evaluated if there is congruence in the diversity patterns of major coral reef benthic groups (i.e., corals, algae, sponges, and gorgonians) using observational surveys across a regional spatial scale in the Bahamian Archipelago. We found that beta diversity is correlated between coral reef benthic groups, demonstrating that changes in the composition of benthic taxa may have cascading effects to overall community composition, and that future research of biodiversity trends should account for this non-independence. In Chapter 2, we used a field experiment in Palmyra Atoll to evaluate how large fishes influence community variability and turnover during coral reef benthic succession. We found that these large fishes canalize benthic succession on coral reefs and suggests that the loss of large fishes from stressors like overfishing may lead to benthic communities that are more variable across space and time. In Chapter 3, we again used this field experiment in Palmyra Atoll, and evaluated if and how large fishes influence coral recruitment directly through grazing and indirectly through grazing scars. We found that the positive and negative effects of fishes varied through time, and the role of fishes decreased over time, suggesting that fishes are important in promoting initial coral recovery after disturbances like global bleaching events that are increasing in frequency. Finally, in Chapter 4 we used a meta-analysis of published research and an ecosystem scale experiment in a large marine protected area (MPA) in the Chagos Archipelago to evaluate if and how large fishes influence coral recruitment and if there are cascading trophic interactions down from large predators. We found that large fishes typically promote coral recruitment in most regions of the world, but these effects are highly variable, and in the Chagos Archipelago, fishes did not promote coral recruitment. However, in both our meta-analysis and in our field experiment, fishes do play strong roles in structuring the benthic composition, decreasing algae and promoting CCA. Finally, in the Chagos, there was no evidence for cascading trophic interactions from large predators. Our results suggest that promoting herbivorous fish biomass is important in many regions around the world to enhance coral recruitment, and that there is high functional redundancy in the fish and benthic community inside a highly complex food web in a remote and large marine protected area. Overall, my dissertation demonstrates that consumers can play important roles in structuring coral reef ecosystems, but this top-down control varies across space and through time and is context dependent. These results have important implications for the conservation and management of fish and shark populations and overall coral reef resilience as reefs face dramatically increasing disturbances from climate change and local human stressors.