Species can interact in ways that differ in how each partner's fitness is affected.

A useful sign notation describes the direction of the effect:

• mutualism $(+,+)$: both species benefit;
• parasitism $(+,-)$: the parasite benefits while the host is harmed;
• commensalism $(+,0)$: one species benefits while the other has little measurable effect.

A parasite typically obtains resources from a host over an extended period without immediately killing it. Parasites can reduce host growth, survival or reproduction and can therefore regulate host populations.

In a mutualism, each partner provides something that increases the other's fitness. For example, a pollinator obtains food while a plant gains pollen transfer. Some mutualisms are facultative, while others are so interdependent that one partner cannot persist without the other.

Commensalism is harder to establish experimentally because demonstrating a truly negligible effect on one species can be difficult. The same interaction can also change sign with environmental conditions.

These categories describe net fitness effects, not moral cooperation or conflict. An interaction classified as mutualistic in one environment can become costly in another if resource availability changes.

Long-term close associations between species are often called symbioses. Symbiosis is a description of close association; it does not mean mutualism specifically.