Azteca instabilis ants and the defence of a coffee shade tree: An ant-plant association without mutual rewards in Chiapas, Mexico

Ants (Hymenoptera: Formicidae) are important predators of herbivorous insects on plants (Rosumek et al. 2009). Ant removal or absence may result in negative indirect effects on plants, as herbivore abundance and herbivory increase and plant growth and reproduction decline (Rosumek et al. 2009, Schmitz et al. 2000). Ant presence on plants often results from a mutualistic interaction. For example, strong highly coevolved ant-plant mutualisms are found on myrmecophytic plants that house ants in domatia (specialized nesting sites). Weaker mutualistic associations are found with myrmecophilic plants that only offer extra-floral nectaries (EFNs) or food bodies to attract ants, or on other plants hosting honeydew-producing hemipterans (indirect ant-plant interactions) that mediate ant abundance (Hlldobler & Wilson 1990). However, in most cases, plants and arboreal ants form more passive associations, where ants nest in the natural cavities of branches or bark, or construct carton nests on plant substrates (Hlldobler & Wilson 1990) and the only reward plants offer these ants is the use of their substrates. In these situations the indirect effect of ants on plants is merely by chance, a byproduct of ant presence (byproduct association).