Mutualism

Mutualism is an interspecific interaction in which both species involved derive a benefit from the association -- represented as (+, +) in the plus/minus notation. Unlike predation or parasitism, neither partner is harmed; instead, each supplies the other with something -- food, shelter, transport, or a service such as pollination or seed dispersal -- that improves its own chances of survival and reproduction.

A textbook example of an extremely close, obligate mutualism is the lichen, which is not a single organism at all but a stable partnership between a fungus and a photosynthetic partner, usually a green alga or a cyanobacterium. The fungal partner provides a protective physical structure, anchors the lichen to rock or bark, and absorbs water and minerals; the algal or cyanobacterial partner uses sunlight to photosynthesise and shares the sugars it produces with the fungus. Neither partner can normally survive and grow in the same way on its own outside the partnership, which makes the lichen relationship one of the tightest mutualisms known.

Plant-pollinator mutualisms are another very important category. A flowering plant depends on an animal -- commonly an insect such as a bee, moth, or the specialised fig wasp, or a bird such as a sunbird -- to carry its pollen from flower to flower, enabling fertilisation and seed production; in exchange, the plant rewards its pollinator with nectar or pollen as food. In some cases this relationship has become extraordinarily specific through coevolution: a particular fig species is pollinated by only one particular species of tiny fig wasp, and that wasp species can complete its own life cycle -- laying eggs and having its larvae develop -- only inside the fruit (technically an inflorescence called a syconium) of that one fig species. Each depends entirely on the other's continued existence. …