Thresholds of Coral Cover That Support Coral Reef Biodiversity

Abstract

Global environmental change, such as ocean warming and increased cyclone activity, is driving widespread and rapid declines in the abundance of key ecosystem engineers, reef-building corals, on the Great Barrier Reef. Our ability to understand how coral associated species, such as reef fishes, respond to coral loss can be impeded by uncertainty surrounding natural spatio-temporal variability of coral populations. To address this issue, we developed a semi-parametric hierarchical Bayesian model to estimate long-term trajectories of habitat-forming coral cover as a function of three spatial scales (sub-region, habitat and site) and environmental disturbances. The relationships between coral cover trajectories and fish community structure were examined using posterior predictive distributions of estimated coral cover from the statistical model. In the absence of direct observations of fish community structure, we used the probability of coral cover being above some ecological threshold values as a proxy for potential disruptions of fish community structure. Threshold values were derived from published field studies that estimated changes in the structure of coral-reef fish communities and coral cover after major disturbances. In these studies, fish community structure did not change where post-disturbance coral cover was > 20%. Disruptions in the structure of these communities were observed when coral cover dropped to between 10–20% and declines in fish diversity were typical where coral cover ranged from between 5 and 10%. Based on these thresholds values, posterior probabilities of coral cover being above 20% and between 10 and 20% and between 5 and 10% were calculated across spatial scales on the Great Barrier Reef (GBR) from 1995 to 2011. At the GBR scale, probabilities of coral cover being above these thresholds remained relatively stable through time. Across years, probabilities of coral cover being at least > 20% remained null for the sub-regions of Cairns, Townsville, Whitsundays and Swain but highly variable between reef sites within these sub-regions, with the exception of Townsville. In the Townsville area, probabilities of coral cover being between 10–20% and 5–10% declined from 0.75 to 0 during the study period. This finding highlights potential sub-regional fish community structure disruptions which have not yet been observed at this spatial scale. As frequency and intensity of disturbance events continue to rise, and consequently, as coral cover declines further, the probabilistic Bayesian approach presented in this chapter could be used to help provide early warnings of major ecological shifts at management relevant scales in the absence of direct observations.