A spatial numerical model for seagrass-herbivore interactions and the formation of reef halos.

Reef halos are circular patterns of bare sand surrounding patch reefs, formed by herbivorous fish grazing near their reef refuges. These formations serve as indicators of ecological processes, providing insights into interactions among herbivores, vegetation, and predators. Their size and prevalence are influenced by predator and herbivore densities, fishing pressure, and temperature, making them valuable proxies for assessing the impact of anthropogenic stressors on reef ecosystems. Halos can also be monitored using satellite imagery and artificial intelligence tools, offering a scalable method for evaluating ecosystem health. In this study, we present the first spatially explicit agent-based model to explore reef halo formation. By integrating seagrass growth models with herbivorous fish behavior, we capture the spatial complexity of halo dynamics. Our model reproduces observed field patterns, including halo size variability driven by temperature. In addition, the model uncovers new insights into the mechanisms behind the formation of sand corridors-vegetation-free pathways that link isolated halos-an aspect of halo dynamics that was previously unresolved. We propose that these corridors are shaped by limitations in rhizome growth rather than shifts in herbivore foraging behavior. These findings advance our understanding of the ecological processes driving halo formation and enhance the predictive value of halos as indicators of coral reef ecosystem health. The model offers a deeper insight into how reef systems respond to environmental pressures, providing a powerful tool for monitoring and managing reefs amid climate change and anthropogenic impacts.

Supplementary information: The online version contains supplementary material available at 10.1007/s00338-025-02729-3.