Chemical ecology, ecosystem tipping points, and the death spiral of coral reefs

Coral reefs are in precipitous global decline. In the last 3 – 4 decades, coral cover has declined by 80% throughout the Caribbean and 50% throughout the tropical Pacific, with seaweeds commonly replacing corals. Much of the decline and lack of recovery can be attributed to alterations in fundamental biotic interactions that are mediated via bioactive secondary metabolites. Experiments on Fijian reefs demonstrate that herbivory by specific mixes of herbivorous fishes is critical for suppressing chemically-rich seaweeds that damage corals on contact via allelopathic lipids on their surfaces. Of equal importance is how coral and fish larvae respond to chemical cues from overfished areas dominated by seaweeds versus no-take marine protected areas (MPAs) dominated by corals. Recruiting fishes and corals chemically sense and are attracted to coral dominated areas protected from fishing while being chemically repulsed by seaweed dominated areas that are overfished. Attraction and repulsion are cued by odors from specific corals and seaweeds that best predict reef quality. Both recruiting fishes and coral larvae refused to settle in overfished, seaweed dominated areas, but recruited readily to immediately adjacent reefs where fishing was banned and corals dominated. These chemically-cued behaviors can close the open nature of marine populations, suppress larval export from coral dominated marine protected areas to degraded reefs, and prevent recovery of coral and fish populations once reefs degrade and become dominated by seaweeds.