How herbivores consume plants: The implications of fixed versus proportional damage

Background/Question/Methods: Evolutionary ecologists measure plant damage in a variety of ways. In most cases, damage is standardized as the proportion of tissue lost to herbivory, but in some cases, it may be reported as an absolute amount. We argue that the way herbivory is distributed in the world, whether in a fixed manner, independent of plant size, or proportional to plant growth, may have implications for the distribution of plant defense traits. We modify the growth-rate model of plant defense (Coley et al 1985) to relax the assumption that herbivory is a fixed trait in the environment, and to make the benefits of defense proportional to the rate of herbivory. We infer that these changes alter implicit assumptions about the relationship between growth and tolerance to herbivory, and we use common milkweed, Asclepias syriaca, to test some of these predictions.

Results/Conclusions: We find that when herbivory is a function of the plant’s realized growth rate, the model shows that maximal growth rate can no longer explain variation in optimal defense. Regardless of whether herbivory is proportional or fixed, variation in herbivory can explain variation in optimal defense. For A. syriaca, prior studies report evidence more consistent with growth-rate rather than herbivory as a driver of geographic distribution of resistance. However, we did not find a relationship between growth and tolerance to proportional damage. Consequently, if damage is proportional rather than fixed, we cannot yet explain the relationship between growth and resistance in this species.