Extinction?
If the parrot fish became extinct there would be a lot more algae living on the coral. This is because the parrot fish would normally eat the algae off the coral. So in a way the parrot fish help control the amount of algae in the reef. Also the predators of the reef would lose one of there prey items thus lessening there population. Also if the number of predators became less there would be more fish in the reef because of the lack of predators.