Predatory mutualisms: The association between Melissotarsus ants and armored scale insects

When described in the context of mutualisms, predation is commonly attributed with the role of disruption and destabilization. Predation between mutualists is expected to result in antagonism, and is therefore not expected to contribute to the establishment and/or maintenance of mutually beneficial interactions. The role that predation may play in the evolution of mutualisms is underappreciated if it is assumed that predation only serves to disrupt mutualisms. We know of at least a few examples of mutualism where predation plays a central role in the “contract” between interacting species, this metaphorical contract being the means by which each species receives net benefit(s) from otherwise costly actions. One such association is the relationship between Melissotarsus ants and armored scale insects (Hemiptera: Diaspididae). New research into these associations indicates that Melissotarsus ants tend to armored scale insect populations, providing protection and other services, in exchange for meat. Armored scale insects appear to be the primary (and perhaps sole) prey of their mutualist partners. In spite of this, armored scale insect populations experience enormously increased abundance when attended by these ants. The Melissotarsus/diaspidid association, as well as a few other examples, can be recognized as a particular form of mutualism, predatory mutualism, in which predation upon one partner by the other serves a crucial function in the maintenance of species interactions and the stability of association. Further studies into these relationships can help us appreciate the diversity of roles that predation fulfills in the evolution of mutualisms.