Herbivores are 'critical' to survival of reefs

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Herbivorous fish are key to the long-term survival of coral reefs in the face of increasing stress from anthropogenic disturbances and climate change, according to a study published in a recent issue of the journal Ecology Letters.

Andrew Hoey and David Bellwood have discovered that herbivorous fish such as tangs (surgeonfish) and parrotfishes are able to hold macroalgae at bay and prevent them from overrunning coral reefs only up to a certain point.  Once the algae reach a critical density, the fish are unable to control them and the coral will be overrun.

The main aim of the study was to examine the influence of the physical structure of Sargassum stands on the feeding behaviour of herbivorous coral reef fishes.  The authors experimentally manipulated macroalgal density, and used remote underwater video cameras to record the feeding activities of both grazing and browsing fishes.

The experiments were conducted in the Great Barrier Reef, with the Sargassum being planted at four different densities in two study sites. Eight hours of video footage were taken per session, and the number of bites taken by each fish species from both the Sargassum and the algal-turf covered substratum recorded.

After recording more than 70,000 bites from 30 fish species that removed an average of 10 kg of macroalgae a day, the authors found that browsing activity was dominated by only two species: the Brassy chub (Kyphosus vaigiensis) and the Unicorn tang (Naso unicornis). Both species were found to feed more intensively on the Sargassum within the lower density patches at first, but began shifting to the higher-density patches as the low-density Sargassum stands were depleted.

The authors also recorded more than 15,000 bites from 28 fish species on the substrate of the experimental patches. Scraping and excavating parrotfishes, primarily the Rivulated parrotfish (Scarus rivulatus) and the Steephead parrotfish (Chlorurus microrhinos) respectively were the dominant grazers.  

The authors found that the grazers, in contrast to the browsers, consistently stayed away from the patches with high macroalgal density. The authors hypothesise that the fish could be avoiding the dense algae patches because of a higher density of predators within (they found a weak positive correlation between algal density and predator biomass), or because the large algal stands were simply more unpalatable.

The results of the study indicate that there is a critical threshold at which the fish are able to control the macroalgae (i.e. when the rate of algal removal is higher than growth), after which they move on to more open areas to feed and allow the algae to proliferate and take over the reef.  This area could be as small as 75–115 square metres, according to the authors’ calculations.

Given that many herbivorous reef fishes are often caught for food, it is not surprising to find a fundamental change in the reef community from a coral-dominated one to an algae-dominated one in many heavily-fishes areas. 

According to lead author Andrew Hoey: "How herbivores respond in areas of the world where they are still heavily fished may be absolutely critical to the survival of large areas of reef in Asia and the Pacific – and hence to the human communities who depend on them for food, tourism and other resources.”

For more information, see the paper: Hoey, AS and DR Bellwood (2011) Suppression of herbivory by macroalgal density: a critical feedback on coral reefs? Ecology Letters 14, pp. 267–273.