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Herbivory is a major driver of ecosystem functioning in semi-arid African savannas, yet its impact on herbaceous biomass dynamics has been poorly understood. Using a 72 ha animal exclosure established 3 years prior, we measured the distribution of herbaceous biomass and species composition in the presence and absence of herbivory in a semi-arid savanna toposequence along the Sabie River in Kruger National Park, South Africa. We found that, in the presence of large herbivores in the reference site, the biomass of herbaceous plant species generally increased from the hillcrest to the lower, wetter riparian zone, while species richness did not change. However, after only 2 and 3 years of herbivore exclusion, the annual maximum biomass in the riparian area increased threefold to similar to 600 g m(-2). Simultaneously, plant species richness declined by 55% and 64% from 10.2 to 9.2 species per m(2) in 2004 and 2005, respectively. In the absence of herbivores, fast-growing grasses overtopped and shaded out forbs of lower stature, resulting in a reduction in the richness of herbaceous species. We conclude that large herbivores are essential in reducing herbaceous competition and maintaining species diversity and ecosystem functioning in African savanna ecosystems, especially in riparian corridors. Published by Elsevier Ltd.