Responses of reef fish communities to coral declines on the Great Barrier Reef

Abstract

The functional roles of certain reef fishes are considered to facilitate recovery of reef ecosystems following coral mortality. Maintenance of high fish species diversity and associated functional diversity are thought to represent an 'ecological insurance' against ecosystem degradation. We examined responses of reef fish communities to varied levels of coral decline on 22 individual reefs of the Great Barrier Reef over an 11 yr period. Using 7 measures of species diversity, we found that fish diversity rarely decreased due to coral declines, even on 7 reefs that suffered massive coral losses (cover decreased by >75%). However, maintenance of fish diversity on those 7 reefs belied major changes in fish communities that involved increases in abundance of large herbivores and decreases in abundance of both coral-dependent fishes and species with no obvious dependence on coral. The magnitude of change in species abundances increased linearly with the magnitude of coral decline. While the proportion of species that increased or decreased in abundance varied considerably among reefs, 45 to 71% of fish species decreased in abundance on some reefs. Ecological function is related to abundance, so such decreases are likely to indicate reduced ecosystem function. Our results suggest that: (1) reef fish diversity may not be a reliable indicator of reef resilience and (2) predicted declines in coral cover due to global warming are likely to cause changes in the structure of reef fish communities, but the nature of these changes and associated capacity of reef fishes to assist ecosystem recovery will vary among reefs