Why is the coral reef an ecosystem?
I think it would be an ecosystem because coral is a living thing, and fish and other water animals live nearby. Maybe if you check your textbook or some worksheets, it might even say that they get something from the coral.
Hope I helped!
The following is a definition of an ecosystem.
http://en.wikipedia.org/wiki/Ecosystem
Below are explanations of coral reefs as ecosystems.
http://www.reefrelief.org/reef.shtml
http://www.seaworld.org/animal-info/info-books/coral/reef-ecosystem.htm