REPORT The importance of structural complexity in coral reef ecosystems N. A. J. Graham • K. L. Nash Received: 29 May 2012 / Accepted: 9 November 2012 / Published online: 26 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012 Abstract The importance of structural complexity in coral reefs has come to the fore with the global degradation of reef condition; however, the limited scale and replica- tion of many studies have restricted our understanding of the role of complexity in the ecosystem. We qualitatively and quantitatively (where sufficient standardised data were available) assess the literature regarding the role of struc- tural complexity in coral reef ecosystems. A rapidly increasing number of publications have studied the role of complexity in reef ecosystems over the past four decades, with a concomitant increase in the diversity of methods used to quantify structure. Quantitative analyses of existing data indicate a strong negative relationship between structural complexity and algal cover, which may reflect the important role complexity plays in enhancing herbivory by reef fishes. The cover of total live coral and branching coral was positively correlated with structural complexity. These habitat attributes may be creating much of the structure, resulting in a collinear relationship; however, there is also evidence of enhanced coral recovery from disturbances where structural complexity is high. Urchin densities were negatively correlated with structural com- plexity; a relationship that may be driven by urchins eroding reef structure or by their gregarious behaviour when in open space. There was a strong positive relationship between structural complexity and fish density and biomass, likely mediated through density-dependent competition and refuge from predation. More variable responses were found when assessing individual fish fam- ilies, with all families examined displaying a positive relationship to structural complexity, but only half of these relationships were significant. Although only corroborated with qualitative data, structural complexity also seems to have a positive effect on two ecosystem services: tourism and shoreline protection. Clearly, structural complexity is an integral component of coral reef ecosystems, and it should be incorporated into monitoring programs and management objectives. Keywords Coral reef ecology  Coral reef fishes  Ecosystem function  Diversity  Marine protected areas  Ecological methods Introduction Structural complexity can be defined as the physical three- dimensional structure of an ecosystem. Much of this structure can be provided by the physical shape and com- plexity of living organisms, such as trees, grasses, kelp and corals, often termed ecosystem engineers or foundation species (Jones et al. 1994; Bruno and Bertness 2001). However, structural complexity can be provided by other structural elements of the environment, such as geological features and underlying dead matrices formed by organ- isms (Kleypas et al. 2001). Structural complexity in eco- systems creates manifold microhabitat types and is expected to lead to a greater diversity and abundance of associated organisms (MacArthur and MacArthur 1961; Crowder and Cooper 1982). Indeed, the effects of structural Communicated by Ecology Editor Prof. Mark Hay Electronic supplementary material The online version of this article (doi:10.1007/s00338-012-0984-y) contains supplementary material, which is available to authorized users. N. A. J. Graham (&)  K. L. Nash ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, QLD 4811, Australia e-mail: nick.graham@jcu.edu.au 123 Coral Reefs (2013) 32:315–326 DOI 10.1007/s00338-012-0984-y