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ABSTRACT Whistling thorn acacia (Acacia(Vachellia)drepanolobium) forms nearly monospecific stands among woody species in black cotton soils in East Africa arid highlands. The tree defends itself against large mammal herbivores with spinescence and symbiotic ants. While these defenses have been extensively studied, little is known about the extent to whichA. drepanolobiumdefense may benefit other plants growing in close association. We examined variation in herbaceous vegetation height, biomass, and composition between areas underneathA. drepanolobiumcanopies and the adjacent matrix in both fenced herbivore exclosures and unfenced areas. In unfenced areas, there was more tall herbaceous vegetation and biomass underneath tree canopies than away from tree canopies, while these differences were not significant in fenced exclosures. Both height and biomass of understory vegetation were negatively correlated withA. drepanolobiumcanopy height. Species richness was higher underneath tree canopies in both fenced and unfenced locations. In the unfenced locations, species evenness was lower underneath tree canopies than in the surrounding matrix, but the opposite was true in the fenced herbivore exclosures. The differences in herbaceous vegetation composition (Bray–Curtis dissimilarity index) between underneath tree and off tree locations were more pronounced in the unfenced areas than within the fenced herbivore exclosures. Our findings suggest that highly defended trees may moderate herbivore effects on herbaceous vegetation. To the extent that herbaceous vegetation underneath trees experiences protection from herbivory, such refugia microhabitats may serve as recolonization nuclei in attempts to restore chronically overgrazed systems. 

Abstract Irruptions in plant and animal populations are not uncommon, but the factors underlying irruptions are rarely explored quantitatively. In addition, it has been suggested that these irruptions may be reduced by predators or herbivores, but there is a paucity of controlled experimental evidence. Using data from the Kenya Long‐term Exclosure Experiment (KLEE), we show that populations of perennialHibiscusspp. (primarilyHibiscus flavifolius) show multiple short‐term irruptions a year after rainy periods, increasing in abundance in some cases by more than an order of magnitude before declining in ensuing months and years. We demonstrate that these irruptions are largely limited to experimental plots from which large mammalian herbivores have been excluded, particularly megaherbivores (elephants, mostly). This represents a rare controlled, replicated experimental demonstration of top‐down regulation of irruptions. African elephants and giraffes are often at greater risk of local extirpation than other large mammals, and their absence appears to destabilize this African savanna ecosystem. 

Abstract Fire and herbivory have profound effects on vegetation in savanna ecosystems, but little is known about how different herbivore groups influence vegetation dynamics after fire. We assessed the separate and combined effects of herbivory by cattle and wild meso‐ and megaherbivores on postfire herbaceous vegetation cover, species richness, and species turnover in a savanna ecosystem in central Kenya. We measured these vegetation attributes for five sampling periods (from 2013 to 2017) in prescribed burns and unburned areas located within a series of replicated long‐term herbivore exclosures that allow six different combinations of cattle and wild meso‐ and megaherbivores (elephants and giraffes). Vegetation cover (grasses, mainly) and species richness were initially reduced by burning but recovered by 15–27 months after fire, suggesting strong resilience to infrequent fire. However, the rates of recovery differed in plots accessible by different wild and domestic herbivore guilds. Wildlife (but not cattle) delayed postfire recovery of grasses, and the absence of wildlife (with or without cattle) delayed recovery of forbs. Herbivory by only cattle increased grass species richness in burned relative to unburned areas. Herbivory by cattle (with or without wildlife), however, reduced forb species richness in burned relative to unburned areas. Herbivory by wild ungulates (but not cattle) increased herbaceous species turnover in burned relative to unburned areas. Megaherbivores had negligible modifying effects on these results. This study demonstrates that savanna ecosystems are remarkably resilient to infrequent fires, but postfire grazing by cattle and wild mesoherbivores exerts different effects on recovery trajectories of herbaceous vegetation. 

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.