Hawaiian reefs through the resilience lens

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With the world’s coral reefs endangered by various threats such as global warming, ocean acidifi cation, overfishing and pollution, there is an urgent need to anticipate and prevent further losses of coral and to reverse shifts in already degraded reefs. Avoiding regime shifts in coral reefs is important because they are among the most biologically diverse ecosystems on earth and harbour approximately 25% of all marine species.

Such challenges require a better understanding of the resilience of these complex reef systems and of the human and natural factors threatening them. This is the conclusion of a study in a Philosophical Transactions special issue on marine shifts around the world, led by Jean-Baptiste Jouffray.

Novel approach reveals the picture
Using one of the most comprehensive coral reef datasets available, Jean-Baptiste Jouffray and colleagues developed new methodology for detecting, visualising and defi ning multiple reef regimes across 302 sites in the Hawaiian archipelago. The study offers a promising avenue to use this novel approach for analysing other ecosystems too.

While previous scientific debate has largely been centred around the shift between coral-dominated reefs and the undesired large fl eshy algae-dominated reefs, Jouffray and co-workers were surprised to fi nd that over half the reefs studied in Hawaii belong to a third state dominated by turf algae. This raises the question of whether turf-dominated reefs are stable or transitional states moving towards macroalgae dominance or, conversely, towards coral recovery. Time-series data showing changes in the reefs over a period of time will be required to investigate the question further.

Like fish, like reef
Higher numbers of herbivorous (algae-eating) fish proved to be the strongest indicator of reef status throughout the Hawaiian islands. The greater the number of herbivorous fish found at a study site, the healthier the coral and the smaller the algal presence. However, deeper analysis of the fish distribution data revealed that type of herbivore is also important. This opens up an avenue for more refi ned and effi cient management of reefs.

Furthermore, the study found that sewage and other effluents from human settlements were the second greatest factor in reef decline in Hawaii, confi rming findings elsewhere. The study was based on a collaboration with researchers from Stockholm Resilience Centre, the Center for Ocean Solutions at Stanford University, the National Oceanic and Atmospheric Administration, Scripps Institute of Oceanography and Conservation International-Hawaii.