Herbivory Weakens the Stability of a Floral Polymorphism

AbstractHerbivores have profound impacts on the maintenance of plant diversity. While simple optimal foraging models predict that herbivores should selectively attack common plant phenotypes, models that allow for differential plant resource acquisition can predict a preference for rarer phenotypes, leading to the destabilization of intraspecific polymorphism. We show that herbivores may play such a destabilizing role in the dimorphic Hesperis matronalis system by weakening negative frequency dependence. Across 40 patches of varying morph ratios, locally rarer H. matronalis morphs grew larger in the early season but received disproportionately more herbivory in the mid- and late season, leading to lower female fitness and survival. We rescued the advantage of being rare by experimentally reducing later herbivory through a plant immunity induction treatment. Feeding assays with a mustard-feeding caterpillar Trichoplusia ni confirmed that herbivores had higher survival on locally rare morphs and preferred to feed on rarer morphs. Our results are consistent with the hypothesis that herbivores may have evolved to use rarity as a cue to track resources, potentially weakening the advantage of being rare, a critical component of stable coexistence. We argue that this phenomenon of insect herbivores weakening the maintenance of plant diversity may be common in stable polymorphic systems.