A community consists of a number of different populations that interact with one another. Because environments vary considerably in size and complexity, so too do the communities that occupy them. Within any community, different animals interact with one another. Some of these interactions benefit both partners, others benefit only one. However, intense competition between species has no advantage to either animal.
Some relationships only benefit one of the animals, and are harmful to the other. Deer ticks are parasites that live by sucking the blood of deer. They also suck the blood of other creatures, including humans, and can pass on diseases to the animals they live off.
A relationship between two species from which both benefit is known as mutualism. The oxpecker bird feeds on the lice and ticks that infest African gazelles. The bird has the benefit of a constant food supply. The gazelle gains from having insects removed that would otherwise suck its blood.
Coral reefs are formed by many corals living together. They provide a very rich environment for a multitude of animals. However, the corals, which form the basis of the reef, need more food than is available around them to survive. They obtain the additional food they need from microscopic algae, which live inside them. The algae need sunlight to manufacture their food so coral reefs are only found in clear, shallow water.