Quantifying Multiscale Habitat Structural Complexity: A Cost-Effective Framework for Underwater 3D Modelling

Abstract

Coral reef habitat structural complexity influences key ecological processes, ecosystem biodiversity, and resilience. Measuring structural complexity underwater is not trivial and researchers have been searching for accurate and cost-effective methods that can be applied across spatial extents for over 50 years. This study integrated a set of existing multi-view, image-processing algorithms, to accurately compute metrics of structural complexity (e.g., ratio of surface to planar area) underwater solely from images. This framework resulted in accurate, high-speed 3D habitat reconstructions at scales ranging from small corals to reef-scapes (10s km 2 ). Structural complexity was accurately quantified from both contemporary and historical image datasets across three spatial scales: (i) branching coral colony (Acropora spp.); (ii) reef area (400 m 2 ); and (iii) reef transect (2 km). At small scales, our method delivered models with <1 mm error over 90% of the surface area, while the accuracy at transect scale was 85.3%˘6% (CI). Advantages are: no need for an a priori requirement for image size or resolution, no invasive techniques, cost-effectiveness, and utilization of existing imagery taken from off-the-shelf cameras (both monocular or stereo). This remote sensing method can be integrated to reef monitoring and improve our knowledge of key aspects of coral reef dynamics, from reef accretion to habitat provisioning and productivity, by measuring and up-scaling estimates of structural complexity.

Cost and time-effective method for multi-scale measures of rugosity, fractal dimension, and vector dispersion from coral reef 3D models

“…These accuracies are on par with the accuracies of 85.3 ± 0.6% [33] and 89% [56], the only other studies to compare linear rugosity from a SfM 3D model (albeit using different methods) to chain-and-tape measurements.…”

Habitat structural complexity metrics improve predictions of fish abundance and distribution

“…The use of swath acoustics to derive digital elevation models with horizontal spatial resolutions typically on the order of a few squared meters (Cameron et al 2014) can allow the estimation of various parameters including slope, aspect, curvature and surface rugosity (the ratio between surface area and planar area) across broad spatial extents (Rattray et al 2009). Recent advances in computer vision also allow the quantification of habitat structural complexity of the seafloor across large spatial extents and at centimeter resolution (McKinnon et al 2011, Williams et al 2012, Burns et al 2015, Figueira et al 2015, Leon et al 2015, Ferrari et al 2016a). This provides an important new approach to investigate the role of complexity and related metrics on fish communities and marine ecosystems at very fine scales (Harborne et al 2012, Ferrari et al 2016b.…”

The assessment of fishery status depends on fish habitats

“…First, models based on meta‐analysis of experimental trawling may allow global estimation of the impacts of trawling on soft‐sediment habitats (Hiddink et al., ). Second, video monitoring and automated image analysis are enhancing capacity to observe large areas of habitats, particularly in areas that are difficult to survey with conventional methods (e.g., Ferrari et al., ).…”