Reefs provide food for many types of fish which, in turn, provide food for others. The reef also acts as a very good filter by absorbing bacteria from the surrounding waters. By acting as a barrier, reefs also serve to protect coastlines from storm damage. Also, they are major tourist attractions.
As sea temperatures change and the coral reefs' largest predators (sharks) migrate to other food sources, a reef system will change dramatically within a short time. Huge reef systems that took hundreds of years to form may shrink or disappear within decades. Reefs are as volatile as the seas themselves and are hardly a stable ecosystem.