Global conditions are changing, leading to novel settings in which ecosystems must operate. In particular, ecological disturbances that were once rare are becoming commonplace, like heatwaves both on land and in the ocean that cause widespread mortality of species and leave behind legacies that pose new challenges to the resilience of contemporary ecosystems. Coral reefs, for example, have historically been afflicted mainly by tropical storms that create reef-scouring waves, but in recent decades, episodes of coral bleaching caused by marine heatwaves have become an additional, prevalent source of mass coral mortality. Tropical storms tend to remove corals from the reef entirely, whereas marine heatwaves that cause coral bleaching kill corals but leave their stony skeletons intact – a type of material legacy. When these structures are left in place, this creates a fundamentally different post-disturbance environment in which reef recovery must take place and poses an uncertain future for these incredibly important ecosystems.
In my dissertation research, I explored how shifting disturbance regimes on coral reefs affect the capacity for these ecosystems to rebound from disturbance and regain their pre-disturbance identity, structure, and function. Through a combination of mathematical modeling, remote sensing, and field experimentation, I compared the impacts of tropical storms and coral bleaching on reef resilience, investigating how dead coral skeletons modify key ecological processes and influence trajectories of community assembly after disturbance. I found that dead coral skeletons impair key ecological processes that underpin coral resilience, like top-down control of macroalgae (spatial competitors of coral) by herbivores, and the successful recruitment of new coral colonies. The impairment of these two critical processes can erode coral resilience and ultimately increase the occurrence of transitions from reefs dominated by corals to those dominated by macroalgae. More encouragingly, however, I found that removing dead skeletons after bleaching can mitigate these negative impacts, offering a promising avenue to explore for the management of coral reefs that have been perturbed by disturbances that leave in place large standing stocks of dead coral skeleton. As material legacies become more prominent ecological features, a critical need emerges to understand the powerful roles they play in altering ecosystem resilience, as well as the roles we may be able to play in leveraging these legacies for desirable management outcomes.
