In areas with diverse herbivore communities such as African savannas, abiotic forces like fire frequency and gradients in soil fertility may alter the top-down role of different herbivore species on plant community dynamics. Differences in herbivore traits such as size, feeding guild, and dietary requirements may result in different impacts of diverse herbivore guilds across abiotic gradients. In two seven-year experiments in the Kruger National Park, South Africa, we used two types of exclosures (plus open access controls) to examine the impacts of different herbivores on plant community dynamics across fire and productivity gradients. Full exclosures excluded all herbivores >0.5 kg (e.g. elephant, zebra, impala) while partial exclosures allowed access only to animals with shoulder heights ≤0.85 m (e.g. impala, steenbok).
Fire frequency (annual burns, triennial burns, or unburned areas) drove habitat selection by different groups of herbivores. For example, annual burns attracted a diverse suite of herbivores, especially zebra and wildebeest, which exerted strong top-down control on plant abundance. In contrast, unburned areas attracted the least diverse suite of herbivores, dominated by impala, which strongly impacted plant richness and diversity. Similarly, when we established these exclosures across productivity gradients in Kruger, we also showed heterogeneous impacts of different herbivore groups across productivity. Both larger (e.g. elephant, buffalo, zebra) and smaller (e.g. impala, warthog) herbivores had significant effects on plant diversity at higher productivities, with herbivore exclusion leading to decreases in diversity. Yet, at lower productivity, smaller herbivores had the most dramatic impacts on plant communities, with their exclusion leading to increases in diversity. These changes in plant diversity were strongly influenced by plant dominance, with diversity decreasing sharply as dominance increased and increasing as plant dominance decreased. Ultimately, our work shows that abiotic forcing can result in differential consumer pressure across a landscape and result in heterogeneous patterns in top-down control of community dynamics.