Critical thresholds of adult patch density and spacing during coral fertilisation

Extreme climate events have severe impacts on the ecological functioning of marine ecosystems by causing wide-spread declines in population sizes and, for surviving individuals, limiting the capacity for population recovery through sexual reproduction. Ecological theory suggests that impacted populations can suffer local extinction due to Allee effects that occur during reproduction ^1-3 : large distances between corals prevents gamete encounters, resulting in reproductive failure ^4-6 . Corals are particularly vulnerable to climate impacts; however, without understanding the relationship between the spacing of spawning individuals and fertilisation success, reefs may pass a critical population threshold before effective conservation measures can be implemented. To assess the influence of adult patch characteristics on fertilisation success, we conducted a series of manipulative field experiments using three common broadcast-spawning Acropora species in two countries (One Tree Island, GBR; and Nikko Bay, Palau). Experimental coral populations ranged in mean intercolonial distance from 1 to 2 m and resulted in low but notable fertilisation success, ranging from 1.2 to 8.7%. We developed an independent mechanistic coral fertilisation model whose predictions closely aligned with the empirical data. The model predicts that in absence of strong convergence zones, adult coral densities need to exceed 13 – 50 colonies per 100 square meters for reefs to remain 10% reproductively functional.