Coral Reef Ecosystem’s Response to Climate Change and Over-Fishing

The coral reefs in our ocean have some of the largest and most dynamic ecosystems on the planet. Predicting how these ecosystems respond to climate change and over-fishing could let scientists inform and influence policy makers to make the right decisions regarding our oceans. Graham et al. (2011) examined coral reef fish and developed a model to predict their extinction vulnerability due to climate change. They used both climate change variables and variables that integrated over-fishing in their model. The authors found that while most reef fish are not at extreme risk of extinction from climate change, when pressures from climate change and over-fishing are both applied to the coral ecosystem, the entire fish community is extremely vulnerable. These fish are a key part of the thriving coral ecosystems and transport nutrients through it. A promising discovery of this paper was that over-fishing seems to have a larger impact on these species than does climate change, making commitment to fishery programs a foreseeable solution.—Connor O’Boyle

Graham, N.,Chabanet, P., Evans, R., Jenning, S., Letourneur, Y., MacNeil, M., McClanahan, T., Ohman, M., Polunin, N., Wilson, S., 2011. Extinction vulnerability of coral reef fishes. Ecology Letters 14, 341—348.

Graham et al. developed a two variable approach to coral reef fish extinction vulnerability. This system set climate vulnerability against extinction risk, with each fish species being evaluated for four climate change variables. These four variables were diet specialization, habitat specialization, recruitment specialization for live coral, and body size. Diet specialization evaluates a species dependency on a coral diet. Habitat specialization influences the amount of population decline when there is a coral loss. Recruitment specialization expresses a species’ dependency on coral during settlement and early life. The inclusion of body size as one of the variables represents a trend that small-bodied fish are often closely associated with the reef structure/matrix and therefore are more prone to predation if there is coral loss. The four variables were quantified for 134 species of coral reef fish, including juvenile and adult stages, by expert knowledge/FishBase.

The extinction risk index was based on four indicators of extinction risk, geographical range size, occupancy, numerical rarity, and depth range. The species’ geographical ranges were calculated using polygons over coastal waters/reefs where the species were present. Occupancy measured the number of fish occupying a single area based on 66 surveys of coral reef locations across the Indian Ocean. Numerical rarity was calculated from fish density data from a 1990s dataset for geographical locations. Depth ranges represent the higher risk of shallow coral habitats to climate change and over fishing. The authors obtained data for depth ranges by using FishBase and current literature. The four risk variables were combined with the four climate variables and weighted by a panel of experts; the authors called this the “expert ranked weighting scheme” or Analytic Hierarchy process (AHP). The AHP weighted the variables against each other, making a matrix, and applied to the scaled data for each variable. These weights were then used to calculate the final indices for every species.

The four main groups of coral reef fish the authors were studying were surgeonfishes, parrotfishes, butterflyfishes, and wrasses. All four groups had sufficient data and varied significantly in the assessed variables. The authors assessed how well their model worked by using it with the data collected from the inner Seychelles islands (a mass coral bleaching event). They calculated the percent change in abundance for the species present between 1994 and 2005, the coral bleaching event happened in 2008. The model fit was assessed using Baynesian goodness-of-fit, where deviations showed either good or bad model fitness. In addition to testing the model the scientists assessed the combined effects of climate change and over-fishing using a fuzzy logic expert system (which is a collection of functions and rules that are used to reason about data).

Graham et al. found that obligate corallivores (butterflyfish and other fish that need live coral polyps to feed) were the most impacted by the climate changes followed by facultative corallivores (more lenient in terms of diet than obligate corallivores) and micro-invertivores (fish that eat the invertebrates living in the coral). Fifty-six out of the 134 species investigated showed a high vulnerability to climate change. When used for the coral bleaching event in the Seychelles islands, the model produced a negative relationship that had a well-fit correlation. The authors found a negative and convex relationship in species vulnerabilities to both over-fishing and climate change. This means that species are either highly susceptible to over-fishing or climate change, rarely both.

Overall the authors found that certain functional groups of reef fish were more vulnerable to climate change. However, no single functional group was more prone to global extinction. Obligate corallivores were the most vulnerable, however most obligate corallivores have broad dispersal so if local extinctions take place the overall species will most likely survive by spreading into new habitats. This suggests that adjacent reefs may support recovery from climate change alterations. The fish that were least vulnerable to climate change, the roving grazers and scraper/excavators, were the most vulnerable to over-fishing. The species most vulnerable in the authors’ model was the tubelip wrasse, L.unilineatus. The recovery of these species will depend on their specific habitat regeneration and connectivity to unaffected populations. Thirty seven percent of the 56 highly vulnerable species had multiple attributes that made them susceptible, showing that species face multiple facets of climate change threats. Every fish species was sensitive either to over-fishing or to climate change. This means that in a world where both over-fishing and climate change are in progress, current communities of fish could be under severe risk. Because we have more control over fishing than climate change the authors propose stricter limitations on coral reef fishing in the short term and continued pressure to alter our global climate impact in the long term.

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