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

In a single population of Ipomopsis arizonica (Polemoniaceae), there is a continuum of compensatory responses to vertebrate herbivory. From 1985-1987, 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, but there were also cases of significant over- 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. The compensatory response of plants to grazing is probabilistic when 3 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. -from Authors