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

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 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 relative to perennials—but these effects were
contingent on rainfall and termitaria in non-additive (hence
unpredictable) ways. A separate experiment affirmed that resource
supplementation does not straightforwardly compensate for herbivory
effects. 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.