Quantifying the shelter capacity of coral reefs using photogrammetric 3D modeling: From colonies to reefscapes

Structural complexity plays a key role in the functioning of coral reef ecosystems. Reef-building corals are major contributors to this complexity, providing shelter and food for numerous invertebrates and fish species. Both structural complexity and shelter capacity of reefscapes are determined by several components such as spurs and grooves, slope, caves and holes, vegetation and coral colonies. Quantifying the shelter capacity from coral colonies to reefscapes is a fundamental step to estimating ecosystem potential to support biodiversity. Here, we applied underwater photogrammetry to quantify shelter volumes provided by individual coral colonies. Overall, 120 3D models of coral colonies from branching, massive, columnar and tabular growth forms were studied. Three reefscapes were also 3D modeled. The study encompasses three Indo-Pacific Islands: Reunion, Europa and New Caledonia. At the colony level, measurements of diameter, planar area, surface and shelter volume were computed. At the reefscape, the diameter and planar area of each colony were extracted from orthomosaics and then used to estimate shelter capacity. Linear models had high accuracy for predicting shelter volume (a 3D metric) from 2D metrics: the diameter= 83.1%, R²= 0.95; the planar area= 87.5%, R²= 0.95 and the colony surface= 87.3%, R²= 0.96. The surface complexity and the shelter volume of the colonies allowed inferring the size of shelters provided by coral growth forms. Quantitative descriptors (i.e. relative percentage of shelter by growth form, the abundance of coral colonies, “Shannon-Shelter Index”) revealed reefscape-scale shelter differences.Our major finding is that planar area and diameter of coral colonies are satisfactory proxies for estimating shelter volume. These new proxies allow 2D metrics to quantify 3D shelter provision, which can support scientists and managers in conservation actions since such metrics are widely used in monitoring programs. Future investigations on the relationships between shelter provision and reef biodiversity will improve the understanding of these complex ecosystems.