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

Abstract

Summary1 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.

Short‐term effects of elevated precipitation and nitrogen on soil fertility and plant growth in a Neotropical savanna

“…Insect herbivory, which we also did not manipulate, can also have profound effects on ecosystem nutrient cycling in the Cerrado. For example, the effects of leafcutter ants on both plant and soil N (Sternberg et al 2007, Costa et al 2008, Mundim et al 2009) are analogous to the impacts that large migrating ungulates have on N availability in African savannas (Augustine 2003, Holdo et al 2007, Cech et al 2008. How both fire and herbivory interact with global change factors could be crucial to predicting soil-plant feedbacks with climate change and rising N deposition rates in the Cerrado in the coming decades.…”

Annual mass drownings of the Serengeti wildebeest migration influence nutrient cycling and storage in the Mara River

“…The Serengeti wildebeest (Connochaetes taurinus) migration is the largest remaining terrestrial migration (3), and a great deal of research has been done on the migration's role in facilitating terrestrial nutrient cycling and promoting vegetation, prey, and predator biomass in the savanna grasslands of the Serengeti Mara Ecosystem (4)(5)(6). This migration also may impact the ecology of aquatic ecosystems in the region, particularly through annual mass drownings that occur during river crossings when the migration is in the northern portion of its route (7) (Fig.…”

Invasive Plant Species and Biomass Production in Savannas

“…Its acknowledged that grazing ecosystems consisting of savannas and grasslands support more herbivore biomass than any other terrestrial habitat and that there is a long history of coevolution of plants and herbivores due to their coexistence of tens of millions of years from the late Mesozoic (Frank et al, 1998). The stability of such coexistence has been attributed to the regular migration of large ungulate herbivores in response to spatial and temporal variation in resources as well as the positive feedback of grazing intensity and fire on primary productivity and fertility (Holdo et al, 2007;Frank et al, 1998).…”