We know that plants and animals make up the coral reefs, and that they all have an important and special place in the community. They each eat certain kinds of foods, and each is also a source of food for others. Certain plants and animals have special relationships with other species so that they cannot live without them. All coral reef communities are systems that are balanced. All of the parts of the system have a special place and function. In a system all things are related, and all of the plants and animals of the reef depend on one another and its surroundings.

All members of the community can be classified as either producers, organisms that produce energy, generally from photosynthesis (these are plants and plant-like organisms), and consumers,organisms that consume energy by eating other organisms (these are animals and fungi).

Producers are very important and are the base of the food chain. This is because all of the energy comes from them. On coral reefs there are three major types of producers: blue-green algae, seaweeds, and reef-building corals.

Reef-building or stony corals have a very special relationship with tiny plant-like organisms. These are called zooxanthellae. The zooxanthellae live in the corals' tissues, and share mutual benefits. So, the corals are producers because the zooxanthellae provide nutrients through photosynthesis to the corals. As a result, many corals live entirely off sunlight.

The producers are eaten by the consumers. You can find two types of primary consumers on the coral reefs. They are herbivores, animals that eat plants, and carnivores, animals that eat animals. There are some special carnivores that eat corals. These carnivores are called corallivores.

The sea urchin is an example of a herbivore as a consumer:

Sea urchins are the major herbivores on reefs and control the abundance of seaweeds by constantly grazing on them off rocky surfaces. There are many different species of sea urchins in Hawaiian reefs. The most common are rock boring sea urchins.