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 to