, Volume 159, Issue 1, pp 181–190 | Cite as

Grazing intensity influences the strength of an associational refuge on temperate reefs

  • Stuart Levenbach
Community Ecology - Original Paper


Recent studies have emphasized the role of positive interactions in ecological communities, but few have addressed how positive interactions are mediated by abiotic stress and biotic interactions. Here, I investigate the effect of a facilitator species on the abundance of macroalgae over a gradient of herbivory. Grazing by sea urchins can be intense on temperate reefs along the California coast, with benthic macroalgae growing exclusively in physical refuges and interspersed within colonies of the strawberry anemone, Corynactis californica. Field experiments indicated that the net effect of C. californica on turf algae was strongly nonlinear over a gradient in density of sea urchins. At low intensities of urchin grazing, the anemone and macroalgae competed for space, with algae capable of overgrowing C. californica. At intermediate grazing intensities, C. californica provided a refuge for turf algae but not for juvenile kelp. Neither turf algae nor kelp benefited from the presence of C. californica at the highest levels of grazing intensity, as sea urchins consumed nearly all macroalgae. The hump-shaped effect observed for C. californica contrasts with the prevailing view in ecological theory that positive interactions are more common in harsh environmental conditions. The results reported here qualify this view and underscore the need to evaluate positive interactions over a range of abiotic stress and consumer pressure.


Facilitation Competition Sea urchin Kelp forest Associational defense 



The field work would not have been possible without the following dedicated volunteer divers: M. Beritzhoff, M. Cabanne, A. Craig, Q. Do, J.M. Ecker, R. Fisher, C. Kane, M. Kay, S. Lucato, C. Pierre, A. Parsons-Field, B. Pitterle, J. Phillips, A. Ray, M. Saxter, C. Simpson, C. Smith, J. Sprague, S. Thomas–Bignami, T. Welche, and lab assistants D. Combs and L. Levenbach. This research benefited from conversations with K. Arkema, A. Brooks, T. Even, H. Lenihan, S. Holbrook, S. Murawski, A. Rassweiler, D. Reed, and R. Schmitt. Special thanks go to the generous financial and logistical support from the Santa Barbara Coastal LTER (NSF grant no. OCE-9982105), and the Nuevo Energy Company for access to oil platforms. This research was also supported by graduate student fellowships from the University of California Toxic Substances and Teaching Program, the University of California Marine Council’s Coastal Environmental Quality Initiative, UCSB Affiliates Graduate Dissertation Fellowship, and a Regent’s Fellowship from the University of California at Santa Barbara. The experiments complied with the current laws of the country in which they were performed.


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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  1. 1.Department of Ecology, Evolution, and Marine BiologyUniversity of CaliforniaSanta BarbaraUSA

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