The continuum of plant responses to herbivory: the influence of plant association, nutrient availability, and timing

In a single population of Ipomopsis arizonica (Polemoniaceae), we show a continuum of compensatory responses to vertebrate herbivory. We demonstrate experimentally that the degree of herbivore impact depends on plant association, nutrient availability, and timing of grazing. From 1985 to 1987, the most common response to vertebrate herbivory was equal compensation, whereby grazed plants set numbers of fruits and seeds equal to controls within the same growing season. However, we also observed cases of significant overcompensation and undercompensation. In 1985 and 1987, overcompensation occurred in vertebrate-grazed plants that were supplemented with nutrients and growing free of competition. These plants produced 33% to 120% more fruit than control, ungrazed plants. Cases of undercompensation occurred in groups where I. arizonica grew in association with grasses or where nutrients were not supplemented. Grazed and clipped plants in these groups produced from 28% to 82% as many fruits as did ungrazed controls. Our studies indicate that the compensatory response of plants to grazing is probabilistic when three external factors are considered. The probability of compensation for herbivory decreases as competition with other plants increases, as nutrient levels decrease, and as the timing of herbivory comes later in the growing season.