Grazing optimization and nutrient cycling: Potential impact of large herbivores in a savanna system

Using a model, we test the prediction that herbivory can result in grazing optimization of primary production in a nitrogen-limited system where large losses of nitrogen occur in annual fires. The model is based on the nitrog en budget of the humid savanna of Lamto, Ivory Coast, estimated from field data. At present, the ecosystem contains few herbivores, but buffalo and ko b populations are increasing. We show that grazing optimization through recycling of nitrogen would occur at Lamto in the short term (i.e., several decades) if the percentage of ni trogen lost from the system out of the amount ingested by herbivores is <24 %, and in the long term (i.e., several centuries) if it is <19%. When 25% o f nitrogen is lost by herbivores, primary production is maintained at a hig h level up to very high consumption rates. Because losses due to herbivores are likely to be lower than these values in this particular ecosystem, we conclude that grazing optimization is likely to occur in the Lamto savanna.