GRAZING OPTIMIZATION AND NUTRIENT CYCLING - WHEN DO HERBIVORES ENHANCE PLANT-PRODUCTION

In a general theoretical ecosystem model, we investigate the condition s under which herbivores increase primary production and lead to grazi ng optimization through recycling of a limiting nutrient. Analytical a nd simulation studies of the model lead to several general results. Gr azing optimization requires that (1) the proportion of nutrient lost a long the herbivore pathway be sufficiently smaller than the proportion of nutrient lost throughout the rest of the ecosystem; and that (2) i nputs of nutrient into the system be greater than a threshold value, w hich depends on the sensitivity of plant uptake rate to an increase in soil mineral nutrient. An increase in nutrient turnover rate is not s ufficient to explain grazing optimization in the long term. When a nut rient is the single limiting factor, plant biomass and productivity at equilibrium are determined only by the balance of ecosystem inputs an d outputs of nutrient. Processes that do not have an impact on these i nputs or outputs have no effect on primary producers. On the other han d, turnover rates are important for the transient dynamics of the syst em, and the equilibrium analysis is relevant only if it can be reached in a reasonable time scale. The equilibrium is not reached by a compa rtment with a very slow turnover rate, such as the resistant soil orga nic matter, before several centuries. On a small time scale, such a co mpartment can be considered constant, and the trend of the system is p redicted with a simplified system. The results at equilibrium are inse nsitive to the functional form used to describe herbivore consumption: the results obtained for simple, linear, donor-controlled herbivory a lso apply to most forms of more realistic, recipient-controlled herbiv ory. We conclude that grazing optimization is most likely to occur in systems with large losses of the limiting nutrient during recycling of plant detritus, or where herbivores bring nutrient from outside the e cosystem considered (which acts to reduce, or even make negative, the fraction of nutrient lost along the herbivore detritus pathway).