Modern corals are restricted to growing in warm, clear, well-lit, shallow water. over the last two million years, coral reefs have undergone periods of great growth as well as mass extinctions. all but one catastrophic event has contributed to the changes in reefs over the years. that is

The answer is Climate change. It will take decades for the coral reef to recover from the catastrophic climatic changes including El Nino in 1997-98. Climate Change is the main and undeniable threats to the loss of the corals. It has negative effects on coral populations in some ways. This includes the Color Bleaching, Coral Disease, and Ocean Acidification. 

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms