OVERCOMPENSATION IN RESPONSE TO MAMMALIAN HERBIVORY - FROM MUTUALISTIC TO ANTAGONISTIC INTERACTIONS

Studies of natural and simulated herbivory were conducted to assess the effects of secondary herbivory and plant association on the reproductive success of Ipomopsis aggregata. Over the 5-yr period of this study 77% of all plants were browsed by ungulate herbivores at some time during the flowering season. Of these, 33% were subsequently browsed. Removal of the single inflorescence stimulated the production of, on average, five new flowering stalks from dormant lateral buds along the remaining portion of the plant's stem. Although regrowth shoots were initially avoided by ungulates following the removal of scarlet gilia's single inflorescence, plant tips were secondarily browsed following stem elongation and flower bud formation. Secondary herbivory had no effect on the compensatory outcome. Plants that were naturally browsed produced significantly higher numbers of flowers and fruits than plants that were not eaten, even when plants were secondarily browsed. Because there were no significant differences in numbers of seeds produced per fruit or in seed mass, an increase in total fruits produced by browsed plants resulted in an increase in fitness through seed production. Observational and experimental results indicate that I. aggregata switches from a "mutualistic" to an "antagonistic" interaction with its ungulate herbivores in order to achieve its greatest fitness. Results of experimental clipping showed that high levels of secondary herbivory on I. aggregata would be detrimental, decreasing fitness by almost-equal-to 70%. An apparent change in plant quality following the initial bout of herbivory, however, deters high levels of subsequent herbivory, restricting tissue removal to the tips of the plant. When plants were found in close association with either pine or grasses (to add in the potential negative effects of competition), browsed plants still outperformed control plants, producing significantly more flowers and fruits than uneaten control plants. As in a previous study, these results support the contention that mammalian herbivores can benefit plants, enhancing plant fitness.