Example of ecological balance: coral reefs

• Coral reefs provide a habitat for a huge number of marine organisms (about a quarter of all marine species although they cover less than 1 % of the sea floor).
• Predatory fish help to maintain the balance of the reef by controlling the populations of small organisms such as sea urchins, which can destroy the reefs if their numbers increase abnormally.
• (Sea urchins and herbivorous fish graze on algae that would otherwise smother the corals; but if urchins explode in number they also erode coral.) The reef is therefore a good example of how predators, herbivores, algae and corals keep each other in check.

The reef also depends on abiotic conditions you already know: warm water within a narrow temperature range (Module 3), light for the symbiotic algae (Module 7), and a pH that allows calcification (Module 7).