Drivers of fine-scale space use and grazing patterns of juvenile parrotfish on western Atlantic reefs

Grazing of macroalgae is critical for coral reef resilience. While the importance of herbivory is well documented, grazing variation has mostly been studied at relatively large scales (e.g., inter-reef or inter-habitat). Within a habitat patch, however, grazing varies because of factors including predation risk and food availability, partly driven by three-dimensional structure. This patchiness of herbivorous foraging is not well described or understood. We observed in situ movement and grazing of juvenile Scarus iseri (striped parrotfish) on six Florida Keys patch reefs to investigate spatial drivers of a foraging herbivore. We created virtual three-dimensional reconstructions of the patch reefs using photogrammetry and extracted fine-scale habitat metrics at 1 m 2 scales including surface area, grazeable surface area, and viewshed, the area visible given the surrounding structure. Using generalized linear mixed-effect hurdle models, we regressed space use and grazing in each 1 m 2 grid cell against structural complexity metrics, grazing surface area availability, and predator presence. Space use and grazing exhibited complex relationships with metrics associated with high complexity and significant but variable correlations with group size and predator activity suggested predation risk affects grazer behavior. Both space use and grazing were concentrated in a small number of cells distributed across the reefs, with 75% of space use occurring in 24% of cells and 75% of grazing in 22%. Understanding the fine-scale effects of complexity on grazers is critical to predicting the consequences of continued structural loss within habitat patches and on the functioning, resilience, and ecosystem services of Western Atlantic reefs.