Herbivore culling influences spatio-temporal patterns of fire in a semiarid savanna.
Abstract
Fire and herbivory are competitive processes in grassy ecosystems as they both consume above-ground biomass. Despite this interaction between fire and grazing, herbivore and fire management plans in conservation areas are usually perceived, and hence managed, independently. This study aims to demonstrate the importance of acknowledging and integrating the interdependence between herbivore and fire management. We compare average proportional area burnt during years when large-scale culling kept herbivore biomass within a narrow range within the Kruger National Park, South Africa, against subsequent years of no culling when herbivore densities increased to effectively double the biomass. Furthermore, to improve our mechanistic understanding, we utilize natural gradients in rainfall (north to south) and grazing (distance from rivers) to test a conceptual model of how herbivory influences fire occurrence under different rainfall regimes. Fires were more prevalent during the culling period than thereafter, and the fire suppression effect of increasing grazing biomass was most pronounced in areas closer to rivers (50% reduction within 1 km of rivers). The "grazer effect" (i.e. difference between% area burnt between low and high grazer biomass years) seems to be more pronounced in arid landscapes than in semiarid landscapes, which in turn is more pronounced than in the mesic landscapes. Despite elephants being mixed feeders, elephant density influences fire occurrence more than the density of buffaloes or hippopotami. Synthesis and applications: Herbivore and fire management actions should be integrated, as events that influence herbivore densities go on to influence fire occurrence in subsequent years. In areas with 400-600 mm MAP it is particularly important because changes in herbivore numbers - due to drought, predation, disease, or human interventions - can result in switches between fire- and herbivore-dominated systems, with cascading consequences for biodiversity and ecosystem function. Furthermore, grazing gradients (in this case distance to closest perennial river) are critical as it allows both fire and herbivore-dominated systems to occur within the same system along the gradient. This increases management possibilities, as management actions that induce grazing gradients can have positive implications for maintaining biodiversity in protected areas.