Plant productivity and soil nitrogen as a function of grazing, migration and fire in an African savanna

1 Herbivores can play a key role in affecting ecosystem function, but their direct and indirect effects are often confounded with each other and have rarely been dissected. Predictions for open systems, i.e. those with cross-habitat nutrient fluxes and dispersal, may differ from those expected in closed systems, where no such transfers occur, but these differences have only recently begun to be characterized. 2 We present a theoretical model of plant productivity and soil nitrogen (N) based on the Serengeti ecosystem in order to investigate the interplay among herbivore movement, nutrient transport across habitats (spatial subsidies), the fire regime and the effects of herbivores on N cycling. 3 Model results suggest that the fire regime and herbivore migration are key determinants of primary productivity and fertility, and that the impact of these factors depends on grazing intensity. At low intensity, high grass biomass leads to extensive fires and N volatilization, but this effect is reduced at high grazing intensity. Without migration, the model predicts that primary productivity and fertility initially increase, then decline with increasing grazing intensity. Conversely, seasonal migration decouples the growing and grazing seasons, leading to a monotonic increase in productivity with grazing intensity. Cross-habitat N transport has a relatively modest effect on N dynamics, and the magnitude of the seasonality effect outweighs the spatial subsidy effect. 4 Our model suggests that herbivores and fire may play key interactive roles in regulating producers and determining ecosystem functional properties in grazing systems, both through consumption and via indirect effects on nutrient availability. The direction and magnitude of these effects could potentially vary greatly among ecosystems with resident vs. migratory herbivores, suggesting that spatial coupling through animal movement can be important at both the population and ecosystem levels. 5 This study suggests that the degree to which trophic interactions influence ecosystem function may be strongly modulated by whether systems are open or closed. It also suggests that direct and indirect effects of consumers on their resources (top-down effects) may be far more important than nutrient subsidies in open terrestrial systems.