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Predicting understorey structure from the presence and composition of canopies: an assembly rule for marine algae


Assembly rules provide a useful framework for predicting patterns of community assembly under defined environmental conditions. Habitat created by canopy-forming algae (such as kelps) provides a promising system for identifying assembly rules because canopies typically have a large and predictable influence on understorey communities. Across >1,000 km of subtidal South Australian coastline, we identified natural associations between assemblages of understorey algae and (1) monospecific canopies of Ecklonia radiata, (2) canopies comprised of E. radiata mixed with Fucales (Cystophora spp. and Sargassum spp.), and (3) gaps among canopies of algae. We were able to recreate these associations with experimental tests that quantified the assembly of understorey algae among these three habitat types. We propose the assembly rule that understorey communities on subtidal rocky coast in South Australia will be (1) monopolised by encrusting coralline algae beneath monospecific canopies of E. radiata, (2) comprised of encrusting corallines, encrusting non-corallines, and sparse covers of articulated corallines, beneath mixed E. radiata-Fucales canopies, and (3) comprised of extensive covers of articulated corallines and filamentous turfs, as well as sparse covers of foliose algae and juvenile canopy-formers, within gaps. Consistencies between natural patterns and experimental effects demonstrate how algal canopies can act as a filter to limit the subsets of species from the locally available pool that are able to assemble beneath them. Moreover, the subsets of species that assemble to subtidal rocky substrata in South Australia appear to be predictable, given knowledge of the presence and composition of canopies incorporating E. radiata.

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P. Keddy, L. Benedetti-Cecchi, and J. Estes gave insightful suggestions on this manuscript and their advice and encouragement were greatly appreciated. We also thank C. Körner and three anonymous reviewers for their useful and constructive comments. We thank M. Anderson for an advance copy of her PERMANOVA software, and T. Elsdon, B. Russell, J. Stehbens, A. Bloomfield, T. Greig, and C. Pale for field support. This research was supported by grants from the Australia and Pacific Science Foundation and the Nature Foundation of South Australia to A.D.I and S.D.C., and an Australian Research Council Discovery grant to S.D.C. The research in this manuscript complies with the current laws of Australia.

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Correspondence to Andrew D. Irving.

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Communicated by Christian Koerner

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Irving, A.D., Connell, S.D. Predicting understorey structure from the presence and composition of canopies: an assembly rule for marine algae. Oecologia 148, 491–502 (2006).

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  • Association
  • Benthos
  • Ecklonia radiata
  • Habitat heterogeneity
  • Kelp