Heat Waves Promote, but Cyclones Reverse, Coral-to-Seaweed Regime Shifts by Reshaping Reef Resilience

Marine heat waves and cyclones disrupt coral reef ecosystems, altering their resilience by leaving distinct material legacies. Heat waves kill coral tissue, leaving behind their skeletons that gradually erode, while cyclones physically scour away the biota on the reef. Field experiments revealed that these material legacies significantly influence shifts between coral-dominated and seaweed-dominated states on tropical reefs.

Dead coral skeletons protect seaweeds from herbivory, enabling a transition to a seaweed-rich state that becomes self-sustaining after the skeletons disappear. In fact, 73% of plots added with heat wave skeletons transitioned to seaweeds within four years, while none of the scouring (cyclone) plots managed to transition to seaweeds, as they lacked the protective skeletons. Remarkably, 87% of seaweed-rich plots that were scoured to simulate a cyclone remained seaweed-free after four years.

Reef trajectories following natural disturbances echoed these findings: four years after a heat wave-induced coral mortality event, 63% of time-series plots had shifted to seaweeds, with no plots recovering to coral-dominated states. Similarly, four years after a cyclone removed skeletons from a previous disturbance, none of the plots had transitioned to seaweeds, and 60% had reverted back to coral dominance.

Disturbances not only alter ecosystem states but also reshape the resilience landscape by creating or removing material legacies. The findings suggest that successive disturbances do not necessarily lead to cumulative negative effects. In fact, after a reef is driven into a seaweed-rich state, a cyclone can act as a rescue disturbance by removing established seaweeds.

When structural refugia are also removed, the seaweed stability domain is significantly reduced, illustrating how disturbance type and sequence play a critical role in determining ecosystem trajectories.