Herbivory promotes plant production and reproduction in nutrient-poor conditions: Effects of plant adaptive phenology

Many studies have demonstrated positive effects of herbivory on plant performance, and these encompass two categories of effects: enhancement of primary production and enhancement of reproductive success. These positive responses of plants to herbivory have been called "grazing optimization." One possible mechanism of these paradoxical phenomena is the nutrient cycling promoted by herbivory. This article models the nutrient cycling hypothesis and analyzes the evolution of plant production and reproduction enhanced by herbivores, using dynamic optimization of plant phenology. Especially when there is nutrient competition among plant individuals or nutrient transportation by herbivores, we can apply the concept of evolutionary stability for the dynamic optimization. Two types of plant responses, long-term and short-term, are examined. Long-term response is an adaptive response for a given level of herbivory pressure, while short-term response is a nonadaptive one to various levels of herbivory, different from the level to which the plant is adapted. The analysis shows that both long-term and short-term grazing optimizations in primary production can occur under poor nutrient conditions and high nutrient recycling rates. However, grazing optimization in reproduction occurs under the same conditions but requires further conditions. In particular, long-term reproductive grazing optimization occurs only when nutrient competition exists among plant individuals. Accordingly, the present analysis revealed the following points concerning grazing optimization: poor nutrient condition is necessary, nutrient competition between plant individuals can promote optimization, and the native condition of the plant is important in the short-term response.