Coral demographic performances in New Caledonia, a video transect approach to operationalize imagery-based investigation of population and community dynamics
Authors: Kayal Mohsen , Mevrel Eva , Ballard Jane 
Published: May 2023

Demographic studies that quantify species performances for survival, growth, and reproduction are powerful means to understand and predict how species and communities respond to environmental change through the characterization of population dynamics and sources of demographic bottlenecks. However, demographic studies require fine-scale surveys of populations in the field, and are often too effort-intensive to be replicable at large scale and in the long-term. To surpass this obstacle, we developed a digital approach for extracting demographic data on species abundances, sizes, and positions within video-transects, facilitating back-from-the-field data acquisitions on population and community dynamics from video surveys. The approach is based on manual coral identification, size-measurements, and mapping along video-transects, mimicking what is traditionally performed in the field, thought it can be automated in the future with the deployment of artificial intelligence. We illustrate our approach with the characterizations of species demographic performances using surveys of a reef-building coral community in New Caledonia recorded with underwater cameras, therefore optimizing time spent in the field. The results provide quantitative measures of coral community composition and demographic performances as key ecological indicators of coral reef health, shed light on species life strategies and constraints to their demographics, and open paths for further quantitative investigations. Key findings include the diversity of species life strategies in terms of relative investment in survival, growth, and reproduction found among taxa dominating the coral community, indicating the diversity of demographic paths to ecological success and that several species have adapted mechanisms to prevail under limiting hydrodynamic environments. Our approach facilitates image-based extractions of demographic data, helping to accelerate empirical endeavors in ecology and ecosystem management.

Author summary

Sustainable ecosystem management requires comprehension of key ecological processes that affect species resilience. Accurate and reoccurring measurements of species helps us understand how they are responding to various environments and predict what might happen in the future. We developed a digital approach that mimics measurements traditionally performed in the field to measure species abundance, size, and distributions using video records of the ecosystems. This transition to imagery-based surveys helps researchers and managers acquire fine-scale ecological data while optimizing time spent in the field, particularly for studying remote and extreme environments where access is limited. We illustrate the application of our approach by characterizing the dynamics of a coral community in the vast tropical reef system of New Caledonia, where such evaluations of demographic processes controlling coral resilience are inexistent but necessary.