Mutualism is a positive relationship between two individuals of different species where both individuals have a gain in fitness. Many mutualistic relationships are symbiotic in nature. Symbiosis is a relationship where the two organisms live together in close physical association, such as in a lichen, which is made up of an algae and a fungi.

Obligate mutualisms are pairings where neither individual can live independently without the other. Again this is true in the case of lichens; neither the algae nor the fungi can survive without the other. Despite a large number of obligate mutualisms, the majority of mutualisms are facultative. Facultative mutualism is where the partners may coexist without a reliance on each other and only form partnerships in an opportunistic way.