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Abstract Climate models predict increases in the frequency and intensity of extreme‐weather events. The impacts of these events may be modulated by biotic agents in unpredictable ways, yet few experiments cover sufficient spatiotemporal scales to measure the interactive effects of multiple extreme events.We used 15 years of a 28‐year experiment spanning several significant droughts to investigate how rainfall, large herbivores, and soil‐engineering termites affect understorey vegetation in a semi‐arid savanna.Herbivory was the dominant influence on community structure—decreasing cover, increasing species richness, and favouring occurrence of annuals relative to perennials—but these effects were contingent on rainfall and termitaria in non‐additive (hence unpredictable) ways.A separate experiment showed that resource enrichment, mimicking the effects of termitaria, does not straightforwardly compensate for top‐down effects of herbivory.Synthesis. Our study highlights the potency of top‐down forcing in African savannas. It suggests impressive robustness to drought and underscores the value of multi‐decadal experiments for studying interactions among multiple drivers of ecosystem dynamics. 

Abstract Fire and herbivory interact to alter ecosystems and carbon cycling. In savannas, herbivores can reduce fire activity by removing grass biomass, but the size of these effects and what regulates them remain uncertain. To examine grazing effects on fuels and fire regimes across African savannas, we combined data from herbivore exclosure experiments with remotely sensed data on fire activity and herbivore density. We show that, broadly across African savannas, grazing herbivores substantially reduce both herbaceous biomass and fire activity. The size of these effects was strongly associated with grazing herbivore densities, and surprisingly, was mostly consistent across different environments. A one‐zebra increase in herbivore biomass density (~100 kg/km²of metabolic biomass) resulted in a ~53 kg/ha reduction in standing herbaceous biomass and a ~0.43 percentage point reduction in burned area. Our results indicate that fire models can be improved by incorporating grazing effects on grass biomass.