The effects of top–down versus bottom–up control on benthic coral reef community structure
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While climate change and associated increases in sea surface temperature and ocean acidification, are among the most important global stressors to coral reefs, overfishing and nutrient pollution are among the most significant local threats. Here we examined the independent and interactive effects of reduced grazing pressure and nutrient enrichment using settlement tiles on a coral-dominated reef via long-term manipulative experimentation. We found that unique assemblages developed in each treatment combination confirming that both nutrients and herbivores are important drivers of reef community structure. When herbivores were removed, fleshy algae dominated, while crustose coralline algae (CCA) and coral were more abundant when herbivores were present. The effects of fertilization varied depending on herbivore treatment; without herbivores fleshy algae increased in abundance and with herbivores, CCA increased. Coral recruits only persisted in treatments exposed to grazers. Herbivore removal resulted in rapid changes in community structure while there was a lag in response to fertilization. Lastly, re-exposure of communities to natural herbivore populations caused reversals in benthic community trajectories but the effects of fertilization remained for at least 2 months. These results suggest that increasing herbivore populations on degraded reefs may be an effective strategy for restoring ecosystem structure and function and in reversing coral–algal phase-shifts but that this strategy may be most effective in the absence of other confounding disturbances such as nutrient pollution.
KeywordsAlgae Nutrients Herbivory Phase-shifts Restoration
This research was supported by the Hawai’i Coral Reef Initiative grant no. NA870A0381. We thank Dr David Bellwood and two anonymous reviewers for valuable comments that have helped to improve the manuscript. We would also like to thank Randall Kosaki, Brent Carman, William Walsh, Peter Hendricks, Robert Nishimoto, Lisa Wedding, Linda Preskitt, Anne Creason, Steven Cotton and Brian Zgliczynski for boat and dive support and help in the field.
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