Shrub layer dynamics under different herbivore densities in an arid savanna, Botswana.
A grazing experiment in an arid savanna in Botswana examined heavy and moderate grazing by cattle. The dynamics of the shrub layer were followed for 5 years. In the areas with no and moderate grazing, shrub densities fluctuated but showed no consistent change, while density increased with heavy grazing. At the end of the experiment, density, cover, total mass, shoot and leaf mass and leaf area of shrubs were higher in the area under heavy grazing than in those under no or moderate grazing. There were no differences in shrub abundance between ungrazed and moderately grazed areas. The increase in shrub abundance with heavy grazing was accounted for by 2 of the 5 recorded species, Acacia mellifera and Grewia flava. Both are shallow rooted suggesting that they were favoured by an increase in water availability in the surface soil following overgrazing of the grass layer.