Ecosystems managed for wildlife and livestock: Contrasting herbivore impacts in a heavily grazed African savanna

Abstract

Replacement of diverse wild herbivore assemblages by livestock across expansive regions of African savanna grassland is predicted to modify tree-grass interactions. Yet, despite the ubiquity of this herbivore regime change, consequences of shifting from wildlife to livestock for savanna ecosystems are poorly resolved. We assessed the effects of livestock grazing on the herbaceous community of a nutrient rich, semi-arid savanna. Direct and indirect impacts - moderated via differences in tree cover - of intensive cattle grazing in communal rangelands, on the herbaceous community were contrasted with heavily utilized wild herbivore grazing systems. Communal rangelands showed a more than two-fold increase in woody cover compared to wildlife areas. Herbaceous differences between wildlife and livestock areas were driven primarily by changes in the area beneath trees, and differences in how densely wooded areas are used by wildlife and livestock. In wildlife systems, grazing intensity was much lower below than away from tree canopies, whereas in livestock systems grazing intensity was high irrespective of tree canopy presence. Away from trees, structure and functional traits of herbaceous vegetation were markedly similar between the two herbivory systems. In our study area, communal rangelands used by livestock, predominantly cattle, are not a degraded subset of wildlife areas but support near equivalent diversity, although with different grass and forb species, and warrants consideration when evaluating their contribution to biodiversity conservation in the region. Nonetheless, higher bare ground cover and the lack of tall-grass reserves mean that communally managed rangelands with low grazer diversity might be more sensitive to intra- and inter-annual climate variability since they are associated with lower forage variability.