Herbivory alters the expression of a mixed-mating system.

The direct and indirect effects of vegetative herbivory on the mating system of Impatiens capensis were analyzed through a survey of herbivory in natural I. capensis populations and manipulation of leaf damage in the field. Across 10 wild populations of I. capensis proportion of cleistogamous flowers had a significant positive exponential relationship with natural levels of herbivory. Similarly, experimental leaf damage increased the proportion of flowers and seeds that were cleistogamous. Leaf damage also reduced the biomass of cleistogamous progeny more severely relative to that of chasmogamous progeny. The cumulative effect of leaf damage was to increase plant reliance on fitness derived from cleistogamous progeny. Leaf damage indirectly affected mating system traits by reducing chasmogamous flower size, leading to a reduction in pollinator visitation. Under these experimental conditions, herbivory did not significantly reduce the number of simultaneously open flowers and potential for geitonogamy, nor did it result in significant changes in the composition of the pollinator fauna. These findings are among the first to demonstrate that herbivory has consequences for mating system and should be considered a factor shaping mating system evolution.