Critical thresholds of adult patch density and spacing during coral fertilization.

Climate change and other anthropogenic stressors have severe impacts on the ecological functioning of marine ecosystems by causing widespread declines in population sizes and, for surviving individuals, limiting the capacity for population recovery through sexual reproduction. Ecological theory suggests that affected populations can suffer local extinction because of Allee effects, where reduced population densities prevent gamete encounters, resulting in reproductive failure. Without understanding the relationship between the density or spacing of spawning individuals and fertilization success, coral reefs may unknowingly pass a critical population threshold, further complicating conservation efforts. In this study we conducted a series of independent manipulative field experiments using three common simultaneous hermaphroditic spawning Acropora species in two locations (One Tree Island, Great Barrier Reef, and Ngermid Bay, Palau) to assess evidence of Allee effects in small populations. Experimental 'patches' of corals were structured with mean intercolonial distances ranging from 1 m to 2 m, resulting in low but measurable fertilization success (1.2-8.7%). We developed a mechanistic coral fertilization model and validated its predictions against this empirical data, finding close alignment. Depending on the species and their colony size, the model predicts that adult coral densities need to exceed 13-50 colonies per 100 m for reefs to ensure 10% fertilization success.