Effect of herbivory and plant species replacement on primary production

Grazing optimization occurs when herbivory increases primary production at low grazing intensities. In the case of simple plant-herbivore interactions , such an effect can result from recycling of a limiting nutrient. However, in more complex cases, herbivory can also lead to species replacement in p lant communities, which in turn alters how primary production is affected b y herbivory. Here we explore this issue using a model of a limiting nutrien t cycle in an ecosystem with two plant species. We show that two major plan t traits determine primary production at equilibrium: plant recycling effic iency (i.e., the fraction of the plant nutrient stock that stays within the ecosystem until it is returned to the nutrient pool in mineral form) and p lant ability to deplete the soil mineral nutrient pool through consumption of this resource. In cases where sufficient time has occurred, grazing opti mization requires that herbivory improve nutrient conservation in the syste m sufficiently. This condition sets a minimum threshold for herbivore nutri ent recycling efficiency, the fraction of nutrient consumed by herbivores t hat is recycled within the ecosystem to the mineral nutrient pool. This thr eshold changes with plant community composition and herbivore preference an d is, therefore, strongly affected by plant species replacement. The quanti tative effects of these processes on grazing optimization are determined by both the recycling efficiencies and depletion abilities of the plant speci es. However, grazing optimization remains qualitatively possible even with plant species replacement.