Optimal Defense Theory in an ant‐plant mutualism: extrafloral nectar as an induced defense is maximized in the most valuable plant structures

Optimal Defense Theory (ODT) predicts that to maximize the benefits of defense against herbivores while minimizing its costs, plants will invest in defenses to structures according to their value and to the likelihood that they will be attacked. Constitutive defenses are expected in structures of high value, whereas induced defenses are expected in structures of low value. Regarding the biotic defense mediated by extrafloral nectaries (EFNs) and based on ODT, we predicted that under control conditions EFNs on higher-value structures would produce more nectar than would EFNs on lower-value structures, attracting more ants; however, when damaged, EFNs on higher-value structures would not increase the production of extrafloral nectar (since constitutive defenses should be employed in this region), whereas EFNs on lower-value structures would so (since induced defenses should be employed in this region), at a level commensurate with the extent of damage. Here we test these predictions in a Brazilian ant-plant mutualism. Qualea multiflora (Vochysiaceae), a savanna tree, presents EFNs on both lower-value structures (leaves) and higher-value structures (inflorescences). We simulated herbivory by cutting 10% or 40% of the leaves, or 10% of the flowers, then monitoring extrafloral nectar production and ant attendance. Extrafloral nectar volume and calorie content, as well as ant abundance, were higher in EFNs of inflorescences compared to EFNs of leaves both before and after simulated herbivory, consistent with one of our predictions. However, EFNs on both leaves and inflorescences, not leaves only, were induced by simulated herbivory, a pattern opposite to our prediction. Plants subjected to higher levels of leaf damage (i.e., more damage to lower-value tissues) produced more and higher-calorie extrafloral nectar, but showed similar ant abundance, partially consistent with our prediction. Our results show that extrafloral nectar production before and after simulated herbivory, as well as the ant recruitment, vary according to the plant structure on which EFNs are located. Our study is unique showing that ant recruitment via extrafloral nectar follows predictions from Optimal Defense Theory, and that the ant foraging patterns may be shaped by the level and region damaged in the plant.