Disentangling disturbances in drylands: Interactions among herbivory, drought, and termite activity in savanna plant communities

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 a 28-year experiment spanning several significant droughts to investigate how rainfall, large herbivores, and soil-engineering termites affect understory vegetation in a semi-arid savanna. Herbivory was the dominant influence on community structure – decreasing cover, increasing species richness, and favouring occurrence of annuals more than perennials – but these effects were contingent on rainfall and termitaria in non-additive (hence unpredictable) ways that contrasted with expectations from previous work. A separate experiment affirmed that resource supplementation does not straightforwardly compensate for herbivory effects. Our study highlights the potency of top-down forcing in African savannas, suggests impressive robustness to drought, and underscores the value of multi-decadal experiments for disentangling multi-disturbance interactions.