Dead Coral Skeletons May Actually Harm Reef Recovery by Sheltering Competing Algae

Our research reveals something counterintuitive about how reefs recover from disasters. We, led by Kai Kopecky at UC Santa Barbara, wanted to understand why some reefs bounce back quickly from disturbances while others seem to get stuck in degraded states dominated by algae instead of coral.

We focused on a key distinction between types of disasters. Some disturbances, like powerful storms, completely remove dead coral structures, leaving flat surfaces behind. Others, like marine heatwaves that cause bleaching, kill the coral tissue but leave the calcium carbonate skeletons standing. Using mathematical modeling, they tracked how benthic space holders change over time following these different disturbance types.

The model revealed that dead coral skeletons could substantially diminish coral resilience if they provided macroalgae refuge from herbivory. The material legacy of dead skeletons broadened the range of herbivore biomass over which coral and macroalgae states are bistable. Those standing skeletons weren't just neutral—they were actively making it harder for herbivorous fish and sea urchins to keep algae in check by creating hiding spots for vulnerable young algae.

The skeletons effectively reduced the herbivores' capacity to control macroalgae without actually changing how many herbivores were present in the system. This refuge effect could be powerful enough to tip entire reef systems into algae-dominated states.

Our research is particularly relevant because disturbance regimes are changing. Structure-removing events like tropical storms appear to be increasing in intensity, while structure-retaining events like marine heat waves are becoming more widespread, frequent, and severe. Our findings suggest that reefs hit by bleaching events might be more vulnerable to permanent shifts than those hit by storms, even if the immediate coral mortality is similar. This could help explain some of the tremendous variability in resilience observed across coral reef ecosystems.

Citation

Kopecky, Kai L.; Stier, Adrian C.; Schmitt, Russell J.; Holbrook, Sally J.; Moeller, Holly V. (2023). Material legacies can degrade resilience: Structure‐retaining disturbances promote regime shifts on coral reefs. Ecology.

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This paper is Open Access.