Morphological Composition Influences Redundancy, Complementarity and Ecological Relevance of Habitat Complexity Metrics in Simulated Coral Communities

Earth's most complex and biodiverse ecosystems are characterised by high habitat complexity. On coral reefs, habitat complexity is influenced by the diverse morphology and composition of hard corals, shaping reef structure and shelter provision for many species. Various metrics are used to quantify reef complexity, yet, it remains unclear how well these metrics capture ecological functions such as shelter provision. We used a published dataset of 13 distinct coral community types generated using a 3D functional-structural model to investigate the redundancy, complementarity and ecological relevance of 11 habitat complexity metrics (seven ecologically meaningful shelter metrics and four more general structural metrics). We were especially interested in the extent to which structural metrics predicted shelter metrics, potentially reducing the need for more complicated and direct shelter measurements. We used Pearson's correlations to compare metrics in (i) one pooled analysis from all community types, and (ii) 13 individual analyses for each community type. In the pooled analysis, structural metrics were strongly correlated, while the shelter metrics formed two distinct groups-'pelagic' and 'benthic.' Within these groups, the metrics were highly correlated indicating redundancy, but between groups, they showed weak correlations indicating complementarity. For individual community types, the redundancy or complementarity of these metrics varied with coral morphology. Structural metrics were useful predictors of shelter metrics for certain community types; for example, surface rugosity was a strong predictor of shelter volume for tabular and digitate coral communities but not for other communities. Fractal dimension was highly complementary to other metrics, but further investigation is needed to identify its ecological relevance. We highlight that there is no universal metric, and it is important to consider a range of suitable habitat complexity metrics and morphological community composition for ecosystems with morphologically distinct biogenic habitat formers.