Introduction of giraffe changes acacia distribution in a South African savanna

Large mammal herbivores can have significant effects on the structure and c omposition of plant communities. We studied the impacts of an introduced gi raffe population on Acacia species at Ithala Game Reserve in South Africa. Browse intensity and Acacia mortality were assessed in field transects, and in road transects covering the reserve road network. Several Acacia specie s occurring in high-density giraffe areas had high levels of mortality. Pop ulations of Acacia davyi were extinct in areas accessible to giraffe. Most A. caffra trees within giraffe browse height were dead and A. karroo, the m ost common species, was also heavily affected. Some species, including A. t ortilis, showed no or very low mortality attributable to giraffe browsing. Healthy populations of sensitive species occurred in areas within, and adja cent to, the reserve in areas with low or no giraffe browsing. Areas too st eep for giraffe access formed spatial refuges for these trees. The differen tial mortality that is occurring as a consequence of giraffe browsing is al tering species composition and species distribution in this savanna landsca pe.