Interactive effects of herbivory and substrate orientation on algal community dynamics on a coral reef
Herbivory is a significant driver of algal community dynamics on coral reefs. However, abiotic factors such as the complexity and orientation of the benthos often mediate the impact of herbivores on benthic communities. We experimentally evaluated the independent and interactive effects of substrate orientation and herbivorous fishes on algal community dynamics on a coral reef in the Florida Keys, USA. We created horizontal and vertical substrates, mimicking the trend in the reduction of vertical surfaces of coral reefs, to assess how algal communities developed either with herbivory (open areas) or without herbivory (herbivore exclosures). We found that substrate orientation was the dominant influence on macroalgal community composition. Herbivores had little impact on community development of vertical substrates as crustose algae dominated these substrates regardless of being in exclosures or open areas. In contrast, herbivores strongly impacted communities on horizontal substrates, with upright macroalgae (e.g., Dictyota spp., articulated coralline algae) dominating herbivore exclosures, while filamentous turf algae and sediment dominated open areas. Outside of exclosures, differences between vertical and horizontal substrates exposed to herbivores persisted despite similar intensity of herbivory. Our results suggest that the orientation of the reef benthos has an important impact on benthic communities. On vertical surfaces, abiotic factors may be more important for structuring algal communities while herbivory may be more important for controlling algal dynamics in flatter areas. Thus, the decline in structural complexity of Caribbean coral reefs and the flattening of reef substrates may fundamentally alter the impact that herbivores have on benthic community dynamics.
We appreciate the valuable comments and suggestions provided by several anonymous reviewers as well as Alastair Harborne and Andy Shantz that helped to improve the manuscript. Additionally, we want to thank Mark Ladd and Andy Shantz for their help in the field setting up the experiment. We also want to thank members of the FIU marine macroalgae research lab and the Burkepile Lab at UCSB that collaborated with field data collection, in particular, Amanda Brannon and Alexandra Simmons. This is contribution #105 from the Center for Coastal Oceans Research in the Institute of Water and Environment at Florida International University.
This work was facilitated by Grant OCE 1130786 from the National Science Foundation to D.E. Burkepile. The funders had no role in study design, data collection, and analysis, decision to publish, or preparation of the manuscript.
Compliance with ethical standards
The authors declare that all applicable international, national and/or institutional guidelines for sampling, care and experimental use of organisms for the study have been followed and all necessary approvals have been obtained.
Conflict of interest
The authors declare that they have no conflict of interest.
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