Theoretical insights from functional-response modeling of green sea turtle aggregation on seagrass

Long-term conservation efforts have contributed to the recovery of green sea turtle (Chelonia mydas) populations in many regions, highlighting the need to reassess conservation plans within a broader ecosystem context. In some areas, localized high turtle densities can impose strong grazing pressure on seagrass meadows. Two measures have been suggested to mitigate such impacts: (i) enhancing seagrass growth through nutrient or symbiotic algae supplementation, and (ii) managing turtle populations by predator protection or localized commercial use. However, model-based studies focusing on seagrass-turtle interactions remain limited, and the resulting effects of the measures on system dynamics are not well understood. To address the gap, the author developed a simple seagrass-turtle model that incorporates turtle aggregation in seagrass patches. By assuming that turtles move to the most seagrass-rich patch and applying simple distributions for seagrass shoot numbers per patch and the number of accessible patches, turtle aggregation was modeled as a simple functional response. The constructed model revealed that turtle aggregation causes positive density dependence in seagrass and negative density dependence in turtles, leading to destabilization through increased seagrass growth and stabilization through turtle population management. Extending the model to incorporate additional ecological factors, such as alternative turtle food resources or spatially heterogeneous environments, alongside empirical assessments of turtle mortality and its causes, could inform future management strategies.