12. Resilience and Dynamics of Coral Reefs Impacted by Chemically Rich Seaweeds and Unsustainable Fishing

Coral reefs are globally threatened by numerous natural and anthropogenic impacts. The proliferation of seaweeds in coral reefs is one of the most common and significant reasons for the decline of healthy corals. Some seaweeds release chemicals that are harmful to corals. The chemicals released by toxic seaweeds damage corals in areas of direct contact. While herbivorous reef fish play an important role in preventing the overgrowth of seaweeds on corals, unsustainable fishing of herbivores disrupts the ecological balance in coral reefs. This induces changes in the community structure from the dominant reef-building corals to one by seaweeds. We have considered a mathematical model of interactions between coral, toxic seaweeds, and herbivores to investigate the phase shifts from coral- to seaweed-dominated states. We investigate how seaweed toxicity and overfishing trigger negative effects on the ecological resilience of coral reefs through trophic cascades. It is observed that in the presence of seaweed toxicity and unsustainable fishing, the system can exhibit an irreversible dynamics through hysteresis cycles. Further, we employ Mawhin’s coincidence degree theory to investigate the existence of a unique positive almost periodic solution of the nonautonomous version of our model by incorporating synchronous or asynchronous seasonal variations in different parameters. The results from computer simulations have potential applications to control the overgrowth of seaweeds in coral reefs as well as to prevent coral bleaching.