Grazing herbivores reduce fire activity via fuel reductions across broad-scale savanna gradients

Grazing herbivores shape grass biomass in African savannas. However, it is unclear if the magnitude of these impacts simply is driven solely by grazer abundance, or also depends on environmental conditions (e.g., rainfall and soil conditions). Understanding the interplay among grazing, rainfall, and soils is essential because changes in grass biomass cascade to affect fire regimes and other ecosystem properties. Reductions in fire frequency and intensity as a result of intensive grazing pressure have been demonstrated locally, but it is unclear if these effects scale up broadly. To examine the effects of grazing herbivores on grass biomass and fire regimes across savannas, we combined data on herbivore exclosures (6 reserves) with remotely sensed fire activity and grazer density data (31 reserves) in sub-Saharan Africa. Grazing herbivore density strongly reduced grass biomass, with effect sizes approaching those of annual rainfall. Moreover, the effect of grazing herbivore density on grass biomass did not strongly depend on rainfall or soil conditions. Across landscapes, the suppression of grass biomass by grazers reduced fire potential, with larger burned area reductions in reserves with more grazers. Grazing herbivore effects on grass biomass and fire activity are substantial, strongly tied to grazing herbivore densities, and generally consistent across different environments.