Measuring Structural Complexity on Coral Reefs

Structural complexity on coral reefs has been shown to positively influence several measures of biodiversity, and is thus an important ecological variable. The dominant field method used by reef scientists to measure structural complexity is the chain-and-tape method, which produces a measure of rugosity calculated as the ratio of contour–following vs. straight distance between two points on the reef. Expanding on this method, we developed simple and easy-to-use tools to measure rugosity at four spatial scales for a range of typical coral reef structures, and also used the data to calculate fractal dimensions at three intermediate scales. We show that measures of structural complexity change unpredictably across spatial scales, and illustrate that typical coral reef structures are too complex for any single measure to function as a comprehensive index of structure over a range of scales. This illustrates that considerations of spatial scale are important when measuring structural complexity, and that the smallest scale obtainable with current remote sensing technology and methods is not directly related to the scale used in most studies of fish ecology. We also illustrate that the fractal dimension measure is more closely related to human intuitive perception of structural complexity than is rugosity, though we are unable to test its value in fish ecology with our current dataset. Future research will relate fish body size to the scale of reef structural complexity, and develop remote sensing-based methods to map structural complexity over large spatial extents. Introduction The influence of structural complexity on the biodiversity of fish assemblages has received much attention in the fish ecology literature (McCormick 1994; Szmant 1997). Most studies have concluded that high structural complexity is spatially correlated with overall fish species richness (Chabanet et al., 1997), or the abundance of specific trophic guilds (Hixon and Beets 1993) or mobility guilds (Friedlander and Parrish 1998). Other studies have found that the fish biodiversity variables are temporally correlated with structural complexity, declining as reef frameworks disintegrate after mass bleaching events (Jones et al., 2004; Wilson et al., 2006) or experimental manipulation (Syms and Jones 2000). However, the in-situ quantification of structural complexity has received surprisingly little attention, and there is neither consensus on the best measure of structural complexity, nor on the most relevant spatial scale at which to measure or assess it. Table 1 illustrates approaches taken to quantify structural complexity in studies of coral reefs and other near-shore environments, and the different scales at which the complexity has been measured. The variety of methods seen in the table exists despite the acknowledged importance of structural complexity for coral reef biodiversity and ecology. Few studies have investigated what measure of structural complexity best predicts a positive effect on fish biodiversity (McCormick, 1994; Gratwicke and Speight, 2005), and none have determined the best spatial scale at which to measure it. In this study, we investigated the influence of spatial scale on measures of structural complexity. We did this by measuring rugosity, the most commonly used