Seeing Halos: Spatial and Consumer-Resource Constraints to Landscapes of Fear.

AbstractVegetation-free space, or "halos," surrounding habitat patches are visually striking spatial phenomena observed in various ecosystems. These halos are linked to the landscape of fear hypothesis, where risk-averse herbivores concentrate grazing near safe shelters within their habitat. We develop theory demonstrating how habitat distribution shapes trophic interactions, leading to alternative stable states in spatial patterns. Using coral reefs as a model system, we investigate the relationship between halo patterns and predator populations. Specifically, we address the inconsistency between theoretical predictions and empirical observations, where halos are absent in some protected reefs and their sizes are uncorrelated with predator abundance. Our findings reveal that long-term coral distribution patterns influence trophic interactions, supporting the landscape of fear hypothesis. When coral patches are dispersed, herbivore shelter from predators is more evenly distributed across the seascape, facilitating overgrazing and halo oscillation. When coral patches are clustered, limited shelter stabilizes halos, but reduced herbivore limitation can also drive critical transitions to cycles with low vegetation that are difficult to reverse. Discordance between theory and observations may therefore arise from differences in underlying spatial shelter distribution, with broad implications for how landscapes of fear emerge from patchy ecosystems to signal resilience.