High levels of direct anti-herbivore defense are associated with low levels of indirect defense in a facultative ant-plant mutualism

Plants utilize a wide variety of anti-herbivore defenses. There has been some controversy over whether indirect defenses, in which a plant defends itself against herbivores by attracting their natural enemies, are associated with decreased levels of direct anti-herbivore defenses (such as noxious chemicals or structural deterrents). I tested the hypothesis that plants with higher levels of direct defenses have lower levels of indirect defenses in a previously unknown ant-plant mutualism. Buttonwood (Conocarpus erectus: Combretaceae) possesses extrafloral nectaries (EFNs) that attract ants by secreting nectar. Excluding ants from C. erectus plants increases leaf damage, indicating that EFNs function as an indirect defense by attracting ants. Plants with more EFNs attract more ants, so the number of EFNs is a reasonable proxy for the level of indirect defense. I found a negative association between the number of EFNs per plant and the density of leaf trichomes, a direct defense against herbivores. This pattern could be due to limited resource availability, negative genetic correlations between defenses, or redundancy of function of different defenses. Ongoing experimental work will determine if leaf trichomes and extrafloral nectaries are redundant anti-herbivore defenses.