Mutualism in a facultative, nonsymbiotic system is shown to require that the benefit gained from fitness of mutualists exceeding that of conspecific nonmutualists be greater than the costs incurred by failure of some individuals to establish mutualism and the loss of fitness when mutualism is not established: pwMS + qwMU > wNM. The general model is applied to ant-extrafloral nectary mutualism, and found to require p [H(1 - D)A] > IA. That is, mutualism at EFNs is found to be favored by (1) a high frequency of establishment of the interaction (p), (2) low costs of nectar and nectaries (IA), and (3) high herbivory (H), high ant effectiveness in defense (A), and ineffective alternate defenses (D). Existing information on extrafloral nectaries is considered in light of this result.
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Kathleen H. Keeler (1981) studied this question.
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