Distance decay among coral assemblages during a cycle of disturbance and recovery

Abstract

The characterization of distance decay in similarity among plant or animal communities both extends ecosystem description and provides insights into formative ecological events and processes. Here, we examine distance decay among coral communities in a common habitat on northwestern Australian reefs, seeking to better understand the roles of disturbance and coral life history strategies in the changing reefscape. In established communities in 1997, when coral cover and generic richness were uniformly high, there was high similarity (~81 %) and negligible distance decay, both within sets of 15 contiguous 50-m transects and among 250-m sites separated by 500 km. Following a 75 % reduction in coral cover and a comparable loss of generic richness to mass bleaching in 1998, similarity declined to ~67 % and there was strong distance decay at <25 km. By 2010, pre-disturbance coral cover and generic richness had been restored and similarity had returned to ~80 %, but weak distance decay had persisted in the community. Among assemblages with contrasting life histories, the disturbance increased distance decay most in the brooding corals, and it remained strong until 2010. In contrast, broadcast spawning corals and the more resistant regenerating corals had largely reverted to their pre-disturbance state of high mean similarity and weak distance decay by 2010. These differences among life history groups reflect the greater vagility of the broadcast spawning corals, the resistance of the regenerator assemblages to the disturbance and their recovery from uniformly distributed remnants, and the susceptibility of the brooding species combined with their limited capacity to disperse beyond the local site. Beyond a spatial extent of 25 km, distance decay was absent in all years and for all coral groups, indicating the qualitatively different source-sink dynamics when reefs are separated by 10s of km of open ocean.