A functional-structural coral model

Abstract

In biological systems, structural complexity is recognized as an important driver of coexistence and species diversity. This is particularly true for coral reefs, where some of the most biodiverse life on Earth coexists. A key contributor to reef structural complexity is the varied morphologies into which reef corals can grow. A large number of coral species of different forms can be found on a typical reef system, and, as well, individual species may show high plasticity in growth morphology in response to the local environmental conditions. As environmental conditions and disturbances regimes shift with climate change, future coral reef assemblages are in question. Many corals respond to light in a comparable ways to plants, due to the presence of symbiotic algae in the cells of the animals that photosynthesise and it is well established that corals respond to their environment (e.g. light) by adjusting their morphology. Corals have very slow growth rates and field observations capturing growth and competition are therefore difficult. As such, a modelling approach provides a much needed opportunity to explore changes in coral structure and functioning. Here, we adapt a functional-structural plant modelling approach, commonly used in plant sciences, to represent coral colonies. Functional-structural modelling combines functional components, such as photosynthesis, growth rates, transport of resources and responses to environmental parameters, with a dynamic representation of the 3D structure or architecture of the modelled plant(s), or in this case, corals. The aim was to create a 3D functional-structural model where structure, function and response to local environmental factors are specified by a set of ‘morpho-functional’ parameters, and determine whether this model could represent some of the major coral growth forms seen on coral reefs. Understanding the growth, competition and mortality of organisms at a three-dimensional (3D) level is important in understanding an organism’s role as an engineering species and the mechanisms that lead to the maintenance of structural integrity. The results show that the model can simulate corals with distinct morphologies by varying the simple set of morpho-functional parameters, including resources required for growth, self-avoidance and resource sharing. From varying these parameters, coral morphologies emerge that match with observed coral shapes in nature that are known to have different growth rates and structural fragility. These include hemispherical, encrusting, columnar and tabular forms. The full diversity of morphologies is not yet captured, and further investigation into other parameters is required. There are many potential future applications of this functional-structural coral model, including matching model output at a coral community level to field measurements from a real coral community. If the model can represent real morphological assemblages for different environmental conditions it could be used to predict future assemblages under different climatic disturbance regimes.