People blast reefs with dynamite, and the crown of thorns, and even their swimming larvae, are attracted to the metabolytes reseased by damaged corals. People break coral when walking on reefs, to get shells for tourists, to get coral rocks for building material. Reefs are overfished. At least two species of large fish eat the crown of thorns. They are gone from most reefs near people.
People poison the reefs with various chemicals to kill or stun fish.
Agricultural chemicals applied to island gardens or used to control mosquitos wind up in the sea, and in the corals. This weakens the corals. The predators of the crown of thorns, including triton shells and a beautiful little shrimp, may die or encounter breeding problems from the concentrated poisons in the starfish's flesh. Increased concentrations of carbon dioxide in the sea water reduces calcium deposition, slowing coral growth. And then there is the triton.