A symbiotic relationship in which both species benefit is called mutualism. In a mutualistic relationship, both species survive and thrive more effectively than if the species lived on their own. Both members do, however, incur costs from such a relationship.
Many mutualistic associations actually begin with a negative relationship between two species, such as one species preying on the other. The prey species either has to fight off the enemy or learn to live with it. Instances of mutualism are most common when the environment is stressful. Examples of this are places lacking water or food, filled with toxic air, or containing poor soil.
Mutualism can be either obligatory or facultative. In obligatory associations, neither species can live without the other. An example is the close living arrangement between coral and algae species. In facultative mutualism, both species can live without the other, but find it useful to live in a relationship. As an example, facultative mutualism occurs when smaller fish called wrasses pick parasites off larger fish.