High-latitude marginal reefs support fewer but bigger corals than their tropical counterparts

Aim Despite the awareness that climate change impacts are typically detrimental to tropical coral reefs, the effect of increasing environmental stress and variability on the population size structure of coral species remains poorly understood. This gap in knowledge limits our ability to effectively conserve coral reef ecosystems because size specific dynamics are rarely incorporated. Our aim is to quantify variation in the size structure of coral populations along a tropical-to-subtropical environmental gradient. Location 20 coral populations along a latitudinal gradient on the east coast of Australia (∼23°S to 30°S). Time Period Between 2010 and 2018. Major taxa studied Scleractinian corals. Methods We apply two methods to quantify the relationship between environmental covariates and coral population size structure along a latitudinal environmental gradient. First, we use linear regression with summary statistics, such as median size as response variables; a method frequently favoured by ecologists. The second method is compositional functional regression, a novel method using entire size-frequency distributions as response variables. We then predict coral population size structure with increasing environmental stress and variability. Results Compared to tropical reefs, we find fewer but larger coral colonies in marginal reefs, where environmental conditions are more variable and stressful for corals in the former. Our model predicts that coral populations may become gradually dominated by larger colonies (> 148 cm2) with increasing environmental stress. Main conclusions With increasing environmental stress and variability, we can expect shifts in coral population size structure towards more larger colonies. Fewer but bigger corals suggest low survival, slow growth, and poor recruitment. This finding is concerning for the future of coral reefs as it implies populations may have low recovery potential from disturbances. We highlight the importance and usefulness of continuously monitoring changes to population structure over large spatial scales. Data availability Data is supplied in the supplementary information, or upon request. Once accepted for publication it will be made openly available on Dryad. ### Competing Interest Statement The authors have declared no competing interest.