Effects of Grazing and Browsing on Tropical Savanna Vegetation

In this Chapter we review studies that model the dynamics of tropical savannas and the effects of grazing and browsing on the vegetation. Many empirical studies illustrate the large impact that grazers and browsers can have on savanna vegetation, both directly and indirectly. We summarize this understanding in a simple model to capture the dynamics of trees, grasses, herbivores and fire in tropical savannas. This model is based on the assumption that herbivores can trigger a positive feedback mechanism between fuel load (grass biomass), fire intensity and damage of fire to trees. The positive feedback mechanism can lead to savanna vegetation where either both trees and grasses co-occur (savanna), or a state where only trees occur (i.e. woodland). As a direct relation exists between vegetation and the density of herbivores, in this model we explore what the effect is that dynamic interactions between vegetation and herbivore populations can have on the stability of savanna vegetation. By including the interactions between herbivore population growth and forage availability a negative feedback is introduced, that modifies the positive feedback resulting in reduction of the possibility that savanna vegetation shifts from tree-grass coexistence to tree-dominated vegetation. In this Chapter we demonstrate the importance of grazing and browsing in this process in tropical savannas, especially when populations of grazers and browsers are free-ranging, without movement limitations, so that herbivores can migrate to prevent food shortage during the dry season. Limitations on movement may lead to overexploitation of the vegetation by both grazers and browsers and, hence, to shifts from tree-grass coexistence to tree-dominated vegetation. The model presented in this Chapter brings together numerous studies on the influence of grazing and browsing on tropical savanna vegetation and generates predictions to be tested by future studies.