Do greater coral cover and morphological diversity increase habitat complexity?

Abstract Coral reefs are complex three-dimensional habitats where the sizes, shapes, diversity and composition of hard corals influence the overall reef structure, and thus the provision of shelter for reef-associated species such as fish. Various metrics have been used to quantify ‘habitat complexity’ on coral reefs, yet debate persists regarding the most informative metric and how effectively the available metrics capture ecologically meaningful functions such as shelter provision. We used Coralcraft , a three-dimensional mechanistic model, to investigate how the morphological diversity and composition of coral communities influences habitat complexity at local (metres) scales. We developed new metrics of shelter to capture the mechanisms by which structure is likely important to reef species, accounting for factors such as the size of predator and prey and different hunting strategies. We simulated the growth of 13 coral community types with varying compositions of 10 common coral morphologies, calculating coral cover and 10 habitat complexity metrics (six novel and four well-established) over a five-year period. We found that more diverse coral communities did not always have the greatest structural complexity and shelter, in part due to certain morphologies having disproportionate influence on the resulting habitat complexity. Communities with lower structural complexity did not necessarily provide less shelter. The relationship between coral cover and habitat complexity metrics varied widely between different communities and was often nonlinear. We conclude that accounting for the morphological composition of coral communities can vastly improve the ability to predict or infer habitat complexity—both structural complexity and shelter provision—from measures of coral cover.