Plant phenotype influences the effect of ant mutualists on a polymorphic mangrove

Summary 1. This study examines the effect of ant mutualists on two morphs of the mangrove Conocarpus erectus. ‘Silver’C. erectus plants have a dense layer of leaf hairs, or trichomes, while ‘green’ plants have very few trichomes. Ants are attracted to extrafloral nectaries (EFNs) produced on the foliage of both high‐ and low‐trichome morphs. 2. High‐trichome plants produced fewer and smaller EFNs. Conocarpus erectus plants with fewer and smaller EFNs produced less nectar and attracted fewer ants. 3. Ant exclusion increased herbivory and decreased growth in low‐trichome plants, but had no detectable effect on herbivory and growth in high‐trichome plants. 4. Synthesis. This study demonstrates that intraspecific variation in a direct anti‐herbivore defence influences the magnitude of the beneficial effect of ants on plants. Individuals with low trichome densities benefitted from ant mutualists while individuals with high trichome densities did not. This likely creates stronger selection for the maintenance of EFN production in low‐trichome plants, a conclusion supported by the fact that low‐trichome plants produced more and larger EFNs than high‐trichome plants, and suggests that the two morphs are on divergent evolutionary trajectories with regard to anti‐herbivore defence.