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.