Every ecosystem consists of a large set of relationships. Some relationships are easier to determine than others, and some effects are evident whereas others are unseen. Although most ecological relationships occur when there is some sort of interaction between two species, there are some instances in which a relationship is built on nothing at all except being in the same area.
Mutualism is an ecological relationship that occurs between two species that both benefit from the partnership. The things gained by each party are usually different. These partnerships can be based around food, such as cases in which one species gains a meal and the other gains a health boost. Wild birds in grasslands often follow herds of hoofed animals such as zebras and antelope, then feed off the pests carried in their fur. The birds get a meal, and the larger animals get protection from mites and fleas. In other cases there may be a reproductive element involved as in the partnership wherein bees spread pollen.
The relationship between a predator and its prey is perhaps one of the most evident and clearly visible in the natural world. Predators can be at the top of the food chain or in the middle, and not all predators are animals or feed on other animals; the relationship simply means that one life form consumes another.
Traditional examples of a predator and prey relationship include a fox and a rabbit or an owl and a mouse. This type of ecological relationship can also be applied to situations wherein the deer is the predator and the grass is the prey or the pitcher plant is the predator and the fly is the prey.
A parasitic relationship is one in which two species live in close contact with each other and one benefits whereas the other is harmed. In most cases the host is not killed, as they are more valuable to the parasite when they are alive.
Insects are some of the most common parasites. Fleas and ticks infest both wild and domestic animals, feeding off their blood while causing bites, sores, scabs and itching. Other parasitic relationships involve creatures like tapeworms living inside a host and stealing nutrients. Parasites attack plants as well as humans and animals, sucking out juices and water from leaves and stems.
Neutralism is a type of ecological relationship that occurs when two species live in the same area and neither has a positive or negative effect on the other.
For example, two species of birds can share the same tree; one species builds nests and eats mainly seeds whereas the other lives in the hollows of the tree and eats mainly insects. There is no competition for food or for shelter, and neither bird relies on the other for any kind of support.
Commensalism is a symbiotic relationship in which one party has a definite benefit from its relationship with another species while that other species is not harmed or helped by the partnership. This often occurs when one species is providing protection, a home or transportation to another without gaining anything in return. A tree provides a home for birds with no return benefit, much as sharks provide a place for barnacles to adhere to. In some cases, such as the example with the shark and barnacle, the benefit to one species can be twofold. Not only do barnacles have a place to adhere to that keeps them safe from predators, but they can also feed off the shark's leftovers.