Ant-plant interactions

Ant-plant interactions

Ant-plant interactions are extremely abundant. They involve seed and fruit dispersal (myrmecochory), ant-protection associations (myrmecophyty), as well as seed robbing by harvester ants or the use of leaf material for fungus culture by leaf cutting ants. Ant-protection associations range from facultative and generalized to obligate and highly specialized. In facultative, generalized associations, plants secrete small volumes of nectar from nectaries outside the flowers, so-called extra-floral nectaries (EFNs). In specialized ant-plant associations non-pathogen cavities in hollow stems, branches, petioles, leaf pouches or rhizomes are provided by the plants serving as nesting space for the inhabiting ant colony. Very often, protein- and lipid-rich food is produced in specialized food bodies, famous examples are the Beltian bodies of the bull-horn Acacias, or the Müllerian bodies of the Cecropia species. In most cases, the ant-plant association is known to be a multispecies community with additional partners such as ant-tended coccids or fungi.

Specialized ant-plant associations are considerably rarer than generalized ones, but still are found in over 100 genera of tropical angiosperms (Davidson & McKey, 1993) and at least 20 plant families (O'Dowd, 1982). The specialized myrmecophytic ants have evolved in five of 12 subfamilies in the Formicidae. As Bronstein et al (2006) noted, growing evidence suggests that generalized ant-plant associations involve evolution primarily on the part of the plants, whereas in the more specialized myrmecophytic systems evolutionary processes are more often bilateral. Host selection mechanisms or olfactory cues for a quick acquisition of the defending ants evolved several times with different partners (e.g. Acacia-Pseudomyrmex, Cecropia-Azteca, Hirtella-Allomerus, Macaranga-Crematogaster, Piper-Pheidole). The evolutionary history and the traits under selection in both partners are interesting examples for the development and maintenance of mutualisms.