Interactions among organisms.
Any relationship in which there is a close and permanent association between organisms of different species is symbiosis, or "living together".There are several types of symbiosis. They are predation, parasitism, commensalism, and mutualism.

Predation:
A relationship in which one organism preys on another as a source of food. An example is an owl hunting a field mouse.

Mutualism:
A relationship in which both organisms benefit from each other. An example of a mutualistic relationship is that of the ant and the acacia tree. The ant protects the tree from herbivores from eating the tree as well as clearing vegetation away from the tree that may compete for resources. The tree provides a home for the ants.

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Ant and Acacia Tree

Parasitism:
A relationship in which one organism derives benefit at the expense of the other. An example of parasitism is a tapeworm living in the intestines of dogs. The tapeworm takes nourishment from the dog in which it lives.

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Tapeworm

Commensalism:
A relationship in which one organism derives benefit with neither benefit nor harm to the other. An example of a commensal relationship would be that of a sea anemone and the clown fish. The clown fish is covered in a secretion that makes it immune to a sea anemone's attack. The clown fish can find protection within the anemone, while bringing the anemone no benefit or harm.

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Clownfish in Sea Anemone