Theory of grazing optimization in which herbivory improves photosynthetic ability

Herbivory had been generally considered to have a negative effect on plants, but a lot of studies have recently indicated that continuous herbivory pressure has a positive effect on plant performance, known as “grazing optimization.” Based on field observations, we analytically examined a hypothesis of grazing optimization in which herbivory improves the photosynthetic ability of individual plants. We examined plant performance under various herbivory pressures and considered the evolution of plant phenology in response to a given herbivory pressure. First, we compared plant performances measured under their native conditions with specific herbivory levels. This was called the long-term response. Second, we examined the performances of plants adapting to a certain level of herbivory pressure under a non-native herbivory intensity. This was called the short-term response. According to numerical analysis, in realistic situations, grazing optimization is unlikely to be observed as a long-term response. However, grazing optimization can occur as short-term response if a plant is adapted to a certain level of herbivory pressure and the photosynthetic ability decreases significantly with the increasing size of vegetative parts. Our results suggest that improved photosynthetic ability by herbivory can result in grazing optimization, although it is constrained by the functional form of photosynthetic ability, native conditions, and experimental design.