An ecosystem consists of biotic and abiotic component which interacts with each other to maintain an equilibrium in an environment. Coral reefs are the oldest ecosystem. They are diverse underwater ecosystems, held together by calcium carbonate structures secreted by corals. Coral reefs consist of colonies of tiny animals found in marine waters, that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups.