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Understanding the role of herbivorous fishes in top-down and bottom-up processes on coral reefs


A foundational goal in ecology is to understand the top-down and bottom-up forces that regulate communities. By ingesting organic matter and excreting inorganic nutrients, consumers play a critical role in multiple pathways that influence ecosystem function. As ecosystems change under the threat of anthropogenic stressors, understanding the impacts to consumer assemblages is crucial, though few studies have quantified how alterations to consumers affect multiple ecosystem processes. In this study, we used a 13-year time series from coral reefs in the South Pacific that have either undergone shifts to algal dominance or remained in the coral state in order to scale individual-level processes to herbivorous fish populations in the different reef states. By collecting empirical data on organismal traits to partition various top-down and bottom-up functions, we emphasize the importance of understanding the different processes that consumers can influence. We show that reefs in the coral state host large-bodied parrotfishes that contribute up to 50% more space-clearing herbivory (turf and macroalgae), 88% higher rates of bioerosion, and excrete a higher ratio of N:P. In contrast, reefs that have undergone transitions to algal states are characterized by abundant detritivorous fishes and small-bodied parrotfishes that influence 56% more detritivory and 29% higher rates of herbivory on epibionts rather than turf or macroalgae. By scaling individual traits to ecosystem processes, our study highlights multiple consumer-mediated mechanisms that may alter ecosystem function. Ultimately, our results provide a better understanding of how changes to ecosystem state impact top-down and bottom-up functions with direct implications for ecosystem recovery and resilience.

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