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.
Gil Wojciech, Polish Forest Research Institute, www.forestryimages.org
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.
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