In most reefs, fleshy algae are kept in check by grazers like surgeonfish and parrotfish, which nibble them down to well-trimmed lawns. But humans kill the grazers with spears, hooks, and nets. We also kill top predators like sharks, leading to population explosions of medium-sized predators, which then take out the grazers. Either way, we give the algae an advantage. The well-trimmed lawns become overgrown fields, and the neighbouring corals start to die.
