Indirect effects of overfishing on Caribbean reefs: Sponges overgrow reef-building corals

Consumer-mediated indirect effects at the community-level are difficult to demonstrate empirically. Here, we show an explicit indirect effect of overfishing on competition between sponges and reef-building corals from surveys of 69 sites across the Caribbean. Removal of sponge-eating angelfishes and parrotfishes resulted in > 3 fold increase in overgrowth of corals by sponges, with coral-sponge contact increasing from 11.0% to 25.6%, and these sponges were mostly species palatable to sponge predators. Palatable species have faster rates of growth or reproduction than defended sponges, which instead make metabolically expensive chemical defenses. On average, overfished sites had lower macroalgal cover, contrary to prevailing assumptions about seaweed control by herbivorous fishes. Coral-sponge competition provides an additional and unambiguous justification for marine protected areas (MPAs) in the Caribbean, where the conceptual model of sponge community ecology and defense trade-offs is notable for the clarity of top-down control and indirect effects across a broad geographic region.