How a lack of predators affects the reef 'landscape'

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When the big predators are away, the small fish come out to play. This emboldening of marine grazing fish that are normally preyed on by sharks and other large predators can significantly alter the landscape of the coral reef.

In a paper published in the most recent issue of the journal American Naturalist, Elizabeth Madin and co-authors demonstrate how the removal of large predators (such as sharks, groupers and snappers) through fishing can dramatically alter the distribution of the primary producers on a coral reef though a cascading effect.  

The authors sought to answer two questions: (1) Can removal of predatory fish through fishing lead to cascading changes in coral reef benthic community structure by altering the behavioral response of the prey? (2) What are the expected patterns of macroalgal distribution under different predation risk scenarios?

The authors conducted field studies in the Northern Line Islands, situated in the central Pacific Ocean, focusing on 11 sites within three of the archipelago’s atolls (Palmyra, Tabuaeran and Kiritimati). Whereas Palmyra Atoll has never been fished, Tabuaeran and Kiritimati atolls are under severe fishing pressure.

This provided a basis for the comparison of the effects of the removal of large predators on the behaviour of the herbivorous fishes, of which the following prey species were studied: Blackbar damselfish (Plectroglyphidodon dickii), Bullethead parrotfish (Chlorurus sordidus) pictured above, Whitecheek surgeonfish (Acanthurus  nigricans) and the Bicolor chromis (Chromis margaritifer).  

The authors quantified predation risk, foraging excursion area, feeding rates, macroalgal distribution, and shelter distribution at the study sites.

The data obtained from the surveys agreed with the predictions made by the models, i.e. the presence of large predators caused the herbivorous fish to make shorter feeding excursions. This in turn caused them to feed at only a few selected spots near their shelters. Because of this patchy grazing, reef habitats where predators were present were more heterogeneous (having different characterisics or qualities).  

By comparison, sites where the large predators have been removed through fishing were more homogenous (of the same kind) consisting largely of grazed areas with cropped turf algae. This was because the herbivorous fishes could make larger and longer feeding excursions in the absence of the predators, significantly altering the landscape in the process.

This study indicates that fishing may dramatically alter the competitive balance among key benthic organisms and in turn influence the spatial structure of the benthic community in the reef.  

Understanding how humans can instigate such a trophic cascade may better equip conservation workers and resource managers in using the full suite of potential ecological interactions and impacts to guide policy decisions.

For more information, see the paper: Madin, EMP, SD Gaines, JS Madin and RR Warner (2010) Fishing indirectly structures macroalgal assemblages by altering herbivore behavior. American Naturalist 176, pp. 785–801.